NOAA
Why Does Wind Blow?
The Short Answer:
Gases move from high-pressure areas to low-pressure areas.
And the bigger the difference between the pressures, the faster the air will move from the high to the low pressure.
That rush of air is the wind we experience.
Wind is a part of weather we experience all the time, but why does it actually happen?
The air will be still one day, and the next, powerful gusts of wind can knock down trees.
What is going on here?
The main cause of wind is a little surprising.
It’s actually temperature.
More specifically, it’s differences in temperature between different areas.
How would temperature differences make the wind blow?
The gases that make up our atmosphere do interesting things as the temperatures change.
When gases warm up, the atoms and molecules move faster, spread out, and rise.
That’s why steam coming off a pot of boiling water always goes upward.
When air is colder, the gases get slower and closer together.
Colder air sinks.
The sun warms up the air, but it does so unevenly.
Because the sun hits different parts of the Earth at different angles, and because Earth has oceans, mountains, and other features, some places are warmer than others.
Because of this, we get pockets of warm air and cold air.
Different temperatures lead to different pressures
Since gases behave differently at different temperatures, that means you also get pockets with high pressure and pockets with low pressure.
In areas of high pressure, the gases in the air are more crowded.
In low pressure zones, the gases are a little more spread out.
You might think that the warm air would lead to a higher pressure area, but actually the opposite is true.
Because warm air rises, it leaves behind an area of low pressure behind it.
Here comes the wind!
Now we’re getting to the part where wind happens.
Gases move from high-pressure areas to low-pressure areas.
And the bigger the difference between the pressures, the faster the air will move from the high to the low pressure.
That rush of air is the wind we experience.
But why does the air move at all?
You might be wondering why the air would move from high pressure to low pressure in the first place.
This is something that happens in nature all the time: things always try to even out.
It’s called diffusion.
Even people do it!
When people get onto a bus, do they all sit on the same side of the bus first?
Do strangers sit next to each other when there are plenty of open seats?
No way.
People want to spread out as much as possible.
Next time you feel the wind blow, think about where it’s going, and what temperatures and pressures are causing it to do that.
Site last updated: October 1st, 2019
https://scijinks.gov/wind/
LISTEN TO THE SCIENCE, PEOPLE!
Re: LISTEN TO THE SCIENCE, PEOPLE!
NOAA
National Centers For Environmental Information
Mid-Holocene Warm Period – About 6,000 Years Ago
Paleoclimatologists have long suspected that the "middle Holocene," a period roughly from 7,000 to 5,000 years ago, was warmer than the present day.
Terms like the Altithermal or Hypsithermal or Climatic Optimum have all been used to refer to this warm period that marked the middle of the current interglacial period.
Today, however, we know that these terms are obsolete and that the truth of the Holocene is more complicated than originally believed.
What is most remarkable about the mid-Holocene is that we now have a good understanding of both the global patterns of temperature change during that period and what caused them.
It appears clear that changes in Earth's orbit have operated slowly over thousands and millions of years to change the amount of solar radiation reaching each latitudinal band of Earth during each month.
These orbital changes can be easily calculated and predict that the Northern Hemisphere should have been warmer than today during the mid-Holocene in the summer and colder in the winter.
The combination of warmer summers and colder winters is apparent for some regions in the proxy records and model simulations.
There are some important exceptions to this pattern, however, including colder summers in the monsoon regions of Africa and Asia due to stronger monsoons with associated increased cloud cover during the mid-Holocene, and warmer winters at high latitudes due to reduction of winter sea ice cover caused by more summer melting.
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today during summer in the Northern Hemisphere.
In some locations, this could be true for winter as well.
Moreover, we clearly know the cause of this natural warming, and we know without doubt that this proven "astronomical" climate forcing mechanism cannot be responsible for the warming over the last 100 years.
https://www.ncdc.noaa.gov/global-warmin ... arm-period
National Centers For Environmental Information
Mid-Holocene Warm Period – About 6,000 Years Ago
Paleoclimatologists have long suspected that the "middle Holocene," a period roughly from 7,000 to 5,000 years ago, was warmer than the present day.
Terms like the Altithermal or Hypsithermal or Climatic Optimum have all been used to refer to this warm period that marked the middle of the current interglacial period.
Today, however, we know that these terms are obsolete and that the truth of the Holocene is more complicated than originally believed.
What is most remarkable about the mid-Holocene is that we now have a good understanding of both the global patterns of temperature change during that period and what caused them.
It appears clear that changes in Earth's orbit have operated slowly over thousands and millions of years to change the amount of solar radiation reaching each latitudinal band of Earth during each month.
These orbital changes can be easily calculated and predict that the Northern Hemisphere should have been warmer than today during the mid-Holocene in the summer and colder in the winter.
The combination of warmer summers and colder winters is apparent for some regions in the proxy records and model simulations.
There are some important exceptions to this pattern, however, including colder summers in the monsoon regions of Africa and Asia due to stronger monsoons with associated increased cloud cover during the mid-Holocene, and warmer winters at high latitudes due to reduction of winter sea ice cover caused by more summer melting.
In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today during summer in the Northern Hemisphere.
In some locations, this could be true for winter as well.
Moreover, we clearly know the cause of this natural warming, and we know without doubt that this proven "astronomical" climate forcing mechanism cannot be responsible for the warming over the last 100 years.
https://www.ncdc.noaa.gov/global-warmin ... arm-period
Re: LISTEN TO THE SCIENCE, PEOPLE!
UCAR
Ocean on the Move: Thermohaline Circulation - A trip through the ocean on the path of thermohaline circulation, also known as the great ocean conveyor
The currents flowing through the ocean, a process called thermohaline circulation, can have an impact on climate.
What is thermohaline circulation?
Cold water, in general, is denser than warm water.
Likewise, water with a high salinity is denser than water that contains less salt.
Surface ocean currents are primarily driven by winds.
Deep ocean currents, on the other hand, are mainly a result of density differences.
The thermohaline circulation, often referred to as the ocean's "conveyor belt", links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans.
Multiple mechanisms conspire to increase the density of surface waters at high latitudes.
Cold winds blowing over the oceans chill the waters beneath them.
These winds also increase evaporation rates, further removing heat from the water.
These chilled waters have increased densities, and thus tend to sink.
Formation of sea ice also helps to increase the density of water near Earth's poles.
As seawater freezes, salt is forced out of the ice in a process called "brine exclusion".
The ice is essentially not salty.
The excluded salt increases the salinity of the cold water immediately below the ice, making it denser still.
The salty, cold water near the poles sinks toward the ocean floor.
Just as rivers on land flow downhill towards the sea, deep density-driven currents in the oceans move along submarine valleys towards the deepest parts of the ocean.
The cold, salty waters that drive the thermohaline circulation form in the Arctic Ocean, the North Atlantic, and the Southern Ocean.
The shallow ocean floor along the Bering Straight prevents deep currents from flowing out of the Arctic Ocean into the Pacific.
Dense water on the floor of the North Atlantic moves southward, eventually joining the sinking waters of Southern Ocean in the far South Atlantic.
Once again, a shallow section of the ocean floor blocks the flow from moving into the Pacific.
In this case the Drake Passage, between the Antarctic Peninsula and the southern tip of South America, prevents the current from flowing westward.
So the thermohaline circulation turns to the east.
Here the current splits; some flows northward along the east coast of Africa into the Indian Ocean, while the rest continues eastward along the southern coast of Australia and finally, veering northward, makes it into the vast Pacific basin.
At this point the two branches of the thermohaline circulation finally begin to mix with the lighter, warmer waters above and work their way back to the surface.
Scientists estimate that the trip from the North Atlantic to the deep water upwelling sites in the Pacific takes about 1,600 years.
To balance the flow of deep water into the Indian and Pacific basins, surface water must flow back out.
Warm surface waters from the Pacific flow through the Indonesian Archipelago into the Indian Ocean, where they join with other currents that have risen from the depths.
This combined flow works its way westward around the southern tip of Africa into the South Atlantic.
Next, the surface flow moves northward through the Atlantic.
Aided by a nudge from the warm Gulf Stream surface current, this water makes its way once again to the extreme North Atlantic, where the cycle begins again.
This global circulation pattern mixes the waters of the world's oceans, turning the ocean reservoirs into a single, vast, interconnected system.
Thermohaline circulation plays an important role in supplying heat to the polar regions.
Therefore, it influences the rate of sea ice formation near the poles, which in turn affects other aspects of the climate system (such as the albedo, and thus solar heating, at high latitudes).
Water's long trip through the ocean's depths on the great ocean conveyor belt, far from surface water influences and contact with the atmosphere, contributes to the lag time between climate forcings and our planet's reactions to them.
Heat and dissolved carbon dioxide, carried to the oceans depths by the thermohaline circulation, may remain "buried" in the abyss for centuries.
This "burial" may forestall initial effects of global climate change; but like zombies in a horror flick, may come back to haunt us much later when they arise from the depths.
UCAR
Postal Address: P.O. Box 3000, Boulder, CO 80307-3000
Shipping Address: 3090 Center Green Drive, Boulder, CO 80301
This material is based upon work supported by the National Center for Atmospheric Research, a major facility sponsored by the National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.
National Center for Atmospheric Research (NCAR)
University Corporation for Atmospheric Research (UCAR)
https://scied.ucar.edu/ocean-move-therm ... irculation
Ocean on the Move: Thermohaline Circulation - A trip through the ocean on the path of thermohaline circulation, also known as the great ocean conveyor
The currents flowing through the ocean, a process called thermohaline circulation, can have an impact on climate.
What is thermohaline circulation?
Cold water, in general, is denser than warm water.
Likewise, water with a high salinity is denser than water that contains less salt.
Surface ocean currents are primarily driven by winds.
Deep ocean currents, on the other hand, are mainly a result of density differences.
The thermohaline circulation, often referred to as the ocean's "conveyor belt", links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans.
Multiple mechanisms conspire to increase the density of surface waters at high latitudes.
Cold winds blowing over the oceans chill the waters beneath them.
These winds also increase evaporation rates, further removing heat from the water.
These chilled waters have increased densities, and thus tend to sink.
Formation of sea ice also helps to increase the density of water near Earth's poles.
As seawater freezes, salt is forced out of the ice in a process called "brine exclusion".
The ice is essentially not salty.
The excluded salt increases the salinity of the cold water immediately below the ice, making it denser still.
The salty, cold water near the poles sinks toward the ocean floor.
Just as rivers on land flow downhill towards the sea, deep density-driven currents in the oceans move along submarine valleys towards the deepest parts of the ocean.
The cold, salty waters that drive the thermohaline circulation form in the Arctic Ocean, the North Atlantic, and the Southern Ocean.
The shallow ocean floor along the Bering Straight prevents deep currents from flowing out of the Arctic Ocean into the Pacific.
Dense water on the floor of the North Atlantic moves southward, eventually joining the sinking waters of Southern Ocean in the far South Atlantic.
Once again, a shallow section of the ocean floor blocks the flow from moving into the Pacific.
In this case the Drake Passage, between the Antarctic Peninsula and the southern tip of South America, prevents the current from flowing westward.
So the thermohaline circulation turns to the east.
Here the current splits; some flows northward along the east coast of Africa into the Indian Ocean, while the rest continues eastward along the southern coast of Australia and finally, veering northward, makes it into the vast Pacific basin.
At this point the two branches of the thermohaline circulation finally begin to mix with the lighter, warmer waters above and work their way back to the surface.
Scientists estimate that the trip from the North Atlantic to the deep water upwelling sites in the Pacific takes about 1,600 years.
To balance the flow of deep water into the Indian and Pacific basins, surface water must flow back out.
Warm surface waters from the Pacific flow through the Indonesian Archipelago into the Indian Ocean, where they join with other currents that have risen from the depths.
This combined flow works its way westward around the southern tip of Africa into the South Atlantic.
Next, the surface flow moves northward through the Atlantic.
Aided by a nudge from the warm Gulf Stream surface current, this water makes its way once again to the extreme North Atlantic, where the cycle begins again.
This global circulation pattern mixes the waters of the world's oceans, turning the ocean reservoirs into a single, vast, interconnected system.
Thermohaline circulation plays an important role in supplying heat to the polar regions.
Therefore, it influences the rate of sea ice formation near the poles, which in turn affects other aspects of the climate system (such as the albedo, and thus solar heating, at high latitudes).
Water's long trip through the ocean's depths on the great ocean conveyor belt, far from surface water influences and contact with the atmosphere, contributes to the lag time between climate forcings and our planet's reactions to them.
Heat and dissolved carbon dioxide, carried to the oceans depths by the thermohaline circulation, may remain "buried" in the abyss for centuries.
This "burial" may forestall initial effects of global climate change; but like zombies in a horror flick, may come back to haunt us much later when they arise from the depths.
UCAR
Postal Address: P.O. Box 3000, Boulder, CO 80307-3000
Shipping Address: 3090 Center Green Drive, Boulder, CO 80301
This material is based upon work supported by the National Center for Atmospheric Research, a major facility sponsored by the National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.
National Center for Atmospheric Research (NCAR)
University Corporation for Atmospheric Research (UCAR)
https://scied.ucar.edu/ocean-move-therm ... irculation
Re: LISTEN TO THE SCIENCE, PEOPLE!
Introduction: The Ocean’s Meridional Overturning Circulation
Andreas Schmittner 1, John C.H. Chiang 2, and Sidney R. Hemming 3
The meridional overturning circulation is a system of surface and deep currents encompassing all ocean basins.
It transports large amounts of water, heat, salt, carbon, nutrients and other substances around the globe, and connects the surface ocean and atmosphere with the huge reservoir of the deep sea.
As such, it is of critical importance to the global climate system.
This monograph summarizes the current state of knowledge of this current system, how it has changed in the past and how it may change in the future, its driving mechanisms, and the impacts of its variability on climate, ecosystems and biogeochemical cycles.
1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.
2 Department of Geography and Center for Atmospheric Sciences, University of California, Berkeley, California, USA.
3 Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA.
The surface waters of the Earth’s oceans are dense enough to sink down to the abyss at only a few key locations (Plate 1).
These sites of deep water formation are located at high latitudes because the density of seawater is strongly temperature dependent, colder ocean water being denser than warmer water.
However, the density of seawater also depends on its salt content.
This is why deep water is presently formed in the North Atlantic, which is salty, but not in the North Pacific, which is fresher.
Subduction in the North Atlantic is fed by northward flow at the surface, transporting tropical and subtropical water masses into the subpolar and polar North Atlantic.
The Gulf Stream and North Atlantic Drift are part of these northward flowing warm and salty surface currents.
In winter, the warm current prevents excessive sea ice formation in the subpolar North Atlantic, and its heat is released into the atmosphere.
The net result is relatively warm conditions over the greater North Atlantic region compared to similar latitudes of the North Pacific; this exemplifies the climatic importance of the Atlantic overturning circulation.
Newly formed deep water in the North Atlantic, called North Atlantic Deep Water, flows southward as deep western boundary currents along the eastern margin of the Americas, crosses the equator, and eventually enters the Antarctic circumpolar current (ACC) of the Southern Ocean.
There, it mixes with other deep water masses like Pacific Deep Water to form a new identity, the Circumpolar Deep Water; as such, the circumpolar current is sometime referred to as a “giant mixmaster”.
Some of this deep water then penetrates northwards, filling the deep waters into the Pacific and Indian oceans.
Ultimately, these deep waters have to return to the surface.
However, where and how exactly the ocean upwells is poorly understood.
Presently it is believed that most deep water returns to the surface in the high latitude Southern Ocean by mechanical uplift driven by strong westerly winds there (see e.g. chapter by Gnanadesikan et al. in section 2), but it might be possible that some deep water resurfaces at low latitudes, owing to vertical (diapycnal) mixing processes.
The second area of deep and bottom water formation is the Antarctic coast, including the marginal Ross and Weddell Seas (R and W in Plate 1, respectively).
There, processes associated with sea ice formation (e.g. brine water rejection) are important in creating the densest waters of the world ocean.
This deep water, called Antarctic Bottom Water, is distinctly colder and fresher than North Atlantic Deep Water, and flows northward underneath it in the Atlantic below 4000m in depth.
The current system as sketched above and in Plate 1 is popularly called “the great conveyor belt” and sometimes “thermohaline circulation”.
The latter term points to density differences, controlled by temperature and salinity changes, in driving the flow.
However, the interior density distribution is not determined only through buoyancy (heat and freshwater) fluxes at the surface, but also by internal mixing processes as well as the flow itself, and hence also depends on forcing by winds and tides.
In fact, the wind-driven ocean circulation, which is not included in Plate 1, dominates the strong current systems in the upper few hundred meters of the ocean, such as the subtropical and subpolar gyres, and interacts nonlinearly with the buoyancy-driven flow.
Moreover, as pointed out in the chapter by Wunsch, the ocean is a turbulent fluid, and mesoscale transient eddies (the ocean weather) lead to complex and chaotic flow trajectories of individual water parcels.
The interaction of these eddies with the mean flow is not well understood.
Deep water production, and hence the overturning circulation, is sensitive to perturbations of surface buoyancy fluxes.
The modeled Atlantic overturning exhibits nonlinear hysteresis behavior with the possibility of rapid transitions between different states triggered by small freshwater perturbations.
This behavior was first shown by Henry Stommel in the 1960’s, using a box model analysis, and subsequently was reproduced by more complex two- and three-dimensional ocean and coupled ocean–atmosphere models.
The sensitive nature of the Atlantic overturning circulation is supported by the paleoclimate record.
Analysis of data from various paleoclimate archives, such as ice cores from Greenland and Antarctica, sea and lake sediments, and speleothems, draws a fascinating picture of substantial and abrupt fluctuations in climate during the last ice age that is consistent with repeated transitions between different states of the overturning circulation, as described in sections 5 and 6.
Inferences from the past also raise the possibility that future anthropogenic global warming might seriously weaken the circulation or even lead to an abrupt slowdown (section 7).
In fact, model projections of future climate show that buoyancy input through warming and freshening of North Atlantic surface waters will likely lead to a reduction of the circulation.
However, how much of a weakening to expect for a particular forcing scenario, or the likelihood of a complete shutdown, are currently not known and a subject of intense research.
This monograph brings together different perspectives on the ocean’s overturning circulation and its impacts, with authors ranging from physical oceanographers studying the modern system and the recent past, paleoceanographers with their view of changes in the distant past, and climate modelers trying to understand its global impacts and future evolution.
Together the studies form a comprehensive description of the variability of the overturning circulation on all time scales from interannual to millennial.
The book is aimed not only at active researchers and experts in the field but is intended also for students and everyone with an interest in climate change and the oceans.
It contains significant educational aspects and a well-balanced mix of overview papers and research papers.
The authors, acknowledged experts in their areas of research, range from world-renowned senior pioneers to young scientists with fresh ideas.
The book begins with an historic account by Longworth and Bryden on the quantification of the flow in the Atlantic and how our perception of it changed during the last 50 years, influenced by important progress in measurements and theory.
Despite significant advances in our theoretical understanding of the overturning circulation, it is still very much an active area of research as demonstrated by the papers in section 2.
Gnanadesikan, de Boer, and Mignone review the theoretical concepts relating the ocean’s density structure to the flow, and highlight the importance of the Southern Ocean in the return flow of deep water to the surface and its role for the Atlantic overturning.
Marchal et al. examine the role of sub-grid scale vertical mixing on the circulation.
Numerical models play an important role in this research and their fidelity has improved in recent years.
However, despite success in reproducing many features of the large-scale circulation, major issues remain, as pointed out in the perspective by Carl Wunsch, one of the great pioneers in physical oceanography.
He also highlights the difficulties in quantitatively estimating the flow field from present-day observations, let alone from the much sparser paleoclimate data set.
His critical assessment of the paleoclimate literature reveals many unanswered questions and cautions us not to mistake even well-established hypotheses as facts.
Measurements provide the basis for our understanding of the present-day circulation.
The papers in section 3 demonstrate that many elements of the current system in the North Atlantic are now known in unprecedented detail.
Quadfasel and Kaese summarize observations from the past decade of the flows of dense water from the Nordic Seas over the ridges between Greenland and Scotland.
These overflows link deep water formation in the polar North Atlantic and Arctic oceans with the circulation to the south of the ridges.
The observed state and variability during the last decade of the flow in the subpolar North Atlantic, including convection in the Labrador and Irminger Seas, are described in great detail by Schott and Brandt.
Smethie and coauthors review the use of anthropogenic tracer (chlorofluorocarbon) measurements in estimating the overturning circulation and its variability.
Section 4 summarizes our knowledge on decadal to centennial variability of the circulation and its climatic impacts.
The review by Delworth, Zhang, and Mann suggests that fluctuations of the meridional overturning circulation are involved in multidecadal to centennial variability of North Atlantic climate, and that their impacts are global, including modulations of tropical rainfall and possibly influencing Atlantic hurricane activity.
Latif and coauthors examine the very different mechanisms responsible for decadal versus multidecadal variability and suggest a high degree of predictability of the Atlantic overturning and hence North Atlantic climate on decadal time scales.
The papers in section 5 demonstrate the abundance of new paleoproxy information on millennial time scale variability during the last glacial and deglacial periods, and the increasing convergence between observed changes and the hypothesis that the Atlantic meridional overturning circulation is central to those changes.
Clark and coauthors provide a synthesis of multi-millennial time scale variability during the last glacial period, using empirical orthogonal functions analysis of proxy observations together with model results, and propose a new mechanism of multi-millennial oscillations of ocean circulation and sea level.
Came and coauthors demonstrate the consistency of proxy measurements of deglacial temperature and salinity variations at intermediate depths in the western tropical Atlantic, with changes simulated in a coupled model in response to freshening of the high North Atlantic; together, they provide evidence of significant slowdown of the Atlantic meridional overturning during deglaciation.
Rial and Yang present intriguing evidence from models exhibiting spontaneous Dansgaard/Oeschger-like climate oscillation to suggest that the timing of abrupt climate changes may be modulated by longer frequency insolation changes.
Sarnthein and coauthors develop a novel method that they call “C14 plateau-tuning” to determine paleowater mass reservoir ages at four key Pacific and Atlantic locations, and the new information is used to infer changes in ocean circulation and ventilation during deglaciation.
Finally, Skinner, Elderfield, and Hall present a synthesis of deep water temperature, oxygen isotope and carbon-13 measurements to shed light on links between overturning circulation perturbations, sea level, and interhemispheric climate changes on millennial timescales.
Section 6 focuses on the impact of changes in the overturning on global climate, ecosystems and biogeochemical cycles.
Wallace Broecker, one of the great pioneers of paleoceanography, presents a very personal account on his thinking about the “great conveyor belt” and its impact on climate.
Many lines of paleoproxy evidence from the tropics now convincingly show that abrupt climate changes are strongly manifest there, especially in precipitation.
Wang and coauthors use a new precipitation proxy record from speleothems to infer how millennial oscillations during the last glacial were associated with north-south shifts in the Intertropical Convergence Zone.
This is consistent with the study of Cheng, Bitz, and Chiang, who analyze the short-term climatic response of a coupled climate model to an abrupt reduction in the Atlantic overturning.
They show a fast coupling of the high latitudes with the tropics through adjustments of the atmospheric circulation.
Schmittner, Brook, and Ahn use a coupled climate carbon cycle model to suggest that changes in Southern Ocean stratification caused by a reduction of North Atlantic Deep Water formation are important in understanding the impact of changes in the overturning on atmospheric CO2.
A similar mechanism is invoked by Sigman, de Boer, and Haug as part of a new hypothesis linking changes in the overturning to the deglaciation.
The final section (section 7) looks to future projections.
Bryan and coauthors examine the response of the Atlantic overturning over the next few centuries to CO2 stabilization scenarios in a coupled climate model.
Saenko investigates possible effects of projected changes in Southern Ocean winds on upwelling and the associated conversion of dense deep waters to light surface waters.
Lastly, Swingedouw and Braconnot show in their climate model that the melting of the Greenland ice sheet, not incorporated in most other model projections, can push the system over the edge and lead to a collapse of the circulation, even in the moderate 2×CO2 stabilization scenario.
The interdisciplinary nature of the problem of the ocean’s overturning circulation and its impacts, as evidenced by the range of topics and expertise of contributors, is one of the fascinating aspects of the research.
We believe we can learn a lot from each other and hope this book contributes to bringing the different disciplines together.
REFERENCE
Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf, On the driving processes of the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, 2007, doi:10.1029/2004RG000166.
J. C. H. Chiang, Department of Geography and Center for Atmospheric Sciences, 547 McCone Hall, University of California, Berkeley, California 94720, USA.
S. R. Hemming, Lamont-Doherty Earth Observatory, Department of Earth and Environmental Sciences, Columbia University, New York, New York 10025, USA.
A. Schmittner, College of Oceanic and Atmospheric Sciences, 104 Ocean Administration Building, Oregon State University, Corvallis, Oregon 97331, USA. (aschmittner@coas.oregonstate.edu)
Andreas Schmittner 1, John C.H. Chiang 2, and Sidney R. Hemming 3
The meridional overturning circulation is a system of surface and deep currents encompassing all ocean basins.
It transports large amounts of water, heat, salt, carbon, nutrients and other substances around the globe, and connects the surface ocean and atmosphere with the huge reservoir of the deep sea.
As such, it is of critical importance to the global climate system.
This monograph summarizes the current state of knowledge of this current system, how it has changed in the past and how it may change in the future, its driving mechanisms, and the impacts of its variability on climate, ecosystems and biogeochemical cycles.
1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.
2 Department of Geography and Center for Atmospheric Sciences, University of California, Berkeley, California, USA.
3 Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA.
The surface waters of the Earth’s oceans are dense enough to sink down to the abyss at only a few key locations (Plate 1).
These sites of deep water formation are located at high latitudes because the density of seawater is strongly temperature dependent, colder ocean water being denser than warmer water.
However, the density of seawater also depends on its salt content.
This is why deep water is presently formed in the North Atlantic, which is salty, but not in the North Pacific, which is fresher.
Subduction in the North Atlantic is fed by northward flow at the surface, transporting tropical and subtropical water masses into the subpolar and polar North Atlantic.
The Gulf Stream and North Atlantic Drift are part of these northward flowing warm and salty surface currents.
In winter, the warm current prevents excessive sea ice formation in the subpolar North Atlantic, and its heat is released into the atmosphere.
The net result is relatively warm conditions over the greater North Atlantic region compared to similar latitudes of the North Pacific; this exemplifies the climatic importance of the Atlantic overturning circulation.
Newly formed deep water in the North Atlantic, called North Atlantic Deep Water, flows southward as deep western boundary currents along the eastern margin of the Americas, crosses the equator, and eventually enters the Antarctic circumpolar current (ACC) of the Southern Ocean.
There, it mixes with other deep water masses like Pacific Deep Water to form a new identity, the Circumpolar Deep Water; as such, the circumpolar current is sometime referred to as a “giant mixmaster”.
Some of this deep water then penetrates northwards, filling the deep waters into the Pacific and Indian oceans.
Ultimately, these deep waters have to return to the surface.
However, where and how exactly the ocean upwells is poorly understood.
Presently it is believed that most deep water returns to the surface in the high latitude Southern Ocean by mechanical uplift driven by strong westerly winds there (see e.g. chapter by Gnanadesikan et al. in section 2), but it might be possible that some deep water resurfaces at low latitudes, owing to vertical (diapycnal) mixing processes.
The second area of deep and bottom water formation is the Antarctic coast, including the marginal Ross and Weddell Seas (R and W in Plate 1, respectively).
There, processes associated with sea ice formation (e.g. brine water rejection) are important in creating the densest waters of the world ocean.
This deep water, called Antarctic Bottom Water, is distinctly colder and fresher than North Atlantic Deep Water, and flows northward underneath it in the Atlantic below 4000m in depth.
The current system as sketched above and in Plate 1 is popularly called “the great conveyor belt” and sometimes “thermohaline circulation”.
The latter term points to density differences, controlled by temperature and salinity changes, in driving the flow.
However, the interior density distribution is not determined only through buoyancy (heat and freshwater) fluxes at the surface, but also by internal mixing processes as well as the flow itself, and hence also depends on forcing by winds and tides.
In fact, the wind-driven ocean circulation, which is not included in Plate 1, dominates the strong current systems in the upper few hundred meters of the ocean, such as the subtropical and subpolar gyres, and interacts nonlinearly with the buoyancy-driven flow.
Moreover, as pointed out in the chapter by Wunsch, the ocean is a turbulent fluid, and mesoscale transient eddies (the ocean weather) lead to complex and chaotic flow trajectories of individual water parcels.
The interaction of these eddies with the mean flow is not well understood.
Deep water production, and hence the overturning circulation, is sensitive to perturbations of surface buoyancy fluxes.
The modeled Atlantic overturning exhibits nonlinear hysteresis behavior with the possibility of rapid transitions between different states triggered by small freshwater perturbations.
This behavior was first shown by Henry Stommel in the 1960’s, using a box model analysis, and subsequently was reproduced by more complex two- and three-dimensional ocean and coupled ocean–atmosphere models.
The sensitive nature of the Atlantic overturning circulation is supported by the paleoclimate record.
Analysis of data from various paleoclimate archives, such as ice cores from Greenland and Antarctica, sea and lake sediments, and speleothems, draws a fascinating picture of substantial and abrupt fluctuations in climate during the last ice age that is consistent with repeated transitions between different states of the overturning circulation, as described in sections 5 and 6.
Inferences from the past also raise the possibility that future anthropogenic global warming might seriously weaken the circulation or even lead to an abrupt slowdown (section 7).
In fact, model projections of future climate show that buoyancy input through warming and freshening of North Atlantic surface waters will likely lead to a reduction of the circulation.
However, how much of a weakening to expect for a particular forcing scenario, or the likelihood of a complete shutdown, are currently not known and a subject of intense research.
This monograph brings together different perspectives on the ocean’s overturning circulation and its impacts, with authors ranging from physical oceanographers studying the modern system and the recent past, paleoceanographers with their view of changes in the distant past, and climate modelers trying to understand its global impacts and future evolution.
Together the studies form a comprehensive description of the variability of the overturning circulation on all time scales from interannual to millennial.
The book is aimed not only at active researchers and experts in the field but is intended also for students and everyone with an interest in climate change and the oceans.
It contains significant educational aspects and a well-balanced mix of overview papers and research papers.
The authors, acknowledged experts in their areas of research, range from world-renowned senior pioneers to young scientists with fresh ideas.
The book begins with an historic account by Longworth and Bryden on the quantification of the flow in the Atlantic and how our perception of it changed during the last 50 years, influenced by important progress in measurements and theory.
Despite significant advances in our theoretical understanding of the overturning circulation, it is still very much an active area of research as demonstrated by the papers in section 2.
Gnanadesikan, de Boer, and Mignone review the theoretical concepts relating the ocean’s density structure to the flow, and highlight the importance of the Southern Ocean in the return flow of deep water to the surface and its role for the Atlantic overturning.
Marchal et al. examine the role of sub-grid scale vertical mixing on the circulation.
Numerical models play an important role in this research and their fidelity has improved in recent years.
However, despite success in reproducing many features of the large-scale circulation, major issues remain, as pointed out in the perspective by Carl Wunsch, one of the great pioneers in physical oceanography.
He also highlights the difficulties in quantitatively estimating the flow field from present-day observations, let alone from the much sparser paleoclimate data set.
His critical assessment of the paleoclimate literature reveals many unanswered questions and cautions us not to mistake even well-established hypotheses as facts.
Measurements provide the basis for our understanding of the present-day circulation.
The papers in section 3 demonstrate that many elements of the current system in the North Atlantic are now known in unprecedented detail.
Quadfasel and Kaese summarize observations from the past decade of the flows of dense water from the Nordic Seas over the ridges between Greenland and Scotland.
These overflows link deep water formation in the polar North Atlantic and Arctic oceans with the circulation to the south of the ridges.
The observed state and variability during the last decade of the flow in the subpolar North Atlantic, including convection in the Labrador and Irminger Seas, are described in great detail by Schott and Brandt.
Smethie and coauthors review the use of anthropogenic tracer (chlorofluorocarbon) measurements in estimating the overturning circulation and its variability.
Section 4 summarizes our knowledge on decadal to centennial variability of the circulation and its climatic impacts.
The review by Delworth, Zhang, and Mann suggests that fluctuations of the meridional overturning circulation are involved in multidecadal to centennial variability of North Atlantic climate, and that their impacts are global, including modulations of tropical rainfall and possibly influencing Atlantic hurricane activity.
Latif and coauthors examine the very different mechanisms responsible for decadal versus multidecadal variability and suggest a high degree of predictability of the Atlantic overturning and hence North Atlantic climate on decadal time scales.
The papers in section 5 demonstrate the abundance of new paleoproxy information on millennial time scale variability during the last glacial and deglacial periods, and the increasing convergence between observed changes and the hypothesis that the Atlantic meridional overturning circulation is central to those changes.
Clark and coauthors provide a synthesis of multi-millennial time scale variability during the last glacial period, using empirical orthogonal functions analysis of proxy observations together with model results, and propose a new mechanism of multi-millennial oscillations of ocean circulation and sea level.
Came and coauthors demonstrate the consistency of proxy measurements of deglacial temperature and salinity variations at intermediate depths in the western tropical Atlantic, with changes simulated in a coupled model in response to freshening of the high North Atlantic; together, they provide evidence of significant slowdown of the Atlantic meridional overturning during deglaciation.
Rial and Yang present intriguing evidence from models exhibiting spontaneous Dansgaard/Oeschger-like climate oscillation to suggest that the timing of abrupt climate changes may be modulated by longer frequency insolation changes.
Sarnthein and coauthors develop a novel method that they call “C14 plateau-tuning” to determine paleowater mass reservoir ages at four key Pacific and Atlantic locations, and the new information is used to infer changes in ocean circulation and ventilation during deglaciation.
Finally, Skinner, Elderfield, and Hall present a synthesis of deep water temperature, oxygen isotope and carbon-13 measurements to shed light on links between overturning circulation perturbations, sea level, and interhemispheric climate changes on millennial timescales.
Section 6 focuses on the impact of changes in the overturning on global climate, ecosystems and biogeochemical cycles.
Wallace Broecker, one of the great pioneers of paleoceanography, presents a very personal account on his thinking about the “great conveyor belt” and its impact on climate.
Many lines of paleoproxy evidence from the tropics now convincingly show that abrupt climate changes are strongly manifest there, especially in precipitation.
Wang and coauthors use a new precipitation proxy record from speleothems to infer how millennial oscillations during the last glacial were associated with north-south shifts in the Intertropical Convergence Zone.
This is consistent with the study of Cheng, Bitz, and Chiang, who analyze the short-term climatic response of a coupled climate model to an abrupt reduction in the Atlantic overturning.
They show a fast coupling of the high latitudes with the tropics through adjustments of the atmospheric circulation.
Schmittner, Brook, and Ahn use a coupled climate carbon cycle model to suggest that changes in Southern Ocean stratification caused by a reduction of North Atlantic Deep Water formation are important in understanding the impact of changes in the overturning on atmospheric CO2.
A similar mechanism is invoked by Sigman, de Boer, and Haug as part of a new hypothesis linking changes in the overturning to the deglaciation.
The final section (section 7) looks to future projections.
Bryan and coauthors examine the response of the Atlantic overturning over the next few centuries to CO2 stabilization scenarios in a coupled climate model.
Saenko investigates possible effects of projected changes in Southern Ocean winds on upwelling and the associated conversion of dense deep waters to light surface waters.
Lastly, Swingedouw and Braconnot show in their climate model that the melting of the Greenland ice sheet, not incorporated in most other model projections, can push the system over the edge and lead to a collapse of the circulation, even in the moderate 2×CO2 stabilization scenario.
The interdisciplinary nature of the problem of the ocean’s overturning circulation and its impacts, as evidenced by the range of topics and expertise of contributors, is one of the fascinating aspects of the research.
We believe we can learn a lot from each other and hope this book contributes to bringing the different disciplines together.
REFERENCE
Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf, On the driving processes of the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, 2007, doi:10.1029/2004RG000166.
J. C. H. Chiang, Department of Geography and Center for Atmospheric Sciences, 547 McCone Hall, University of California, Berkeley, California 94720, USA.
S. R. Hemming, Lamont-Doherty Earth Observatory, Department of Earth and Environmental Sciences, Columbia University, New York, New York 10025, USA.
A. Schmittner, College of Oceanic and Atmospheric Sciences, 104 Ocean Administration Building, Oregon State University, Corvallis, Oregon 97331, USA. (aschmittner@coas.oregonstate.edu)
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene
Introduction
If you want to know what the climate was like in the Holocene, simply take some outerwear and go out into nature and look around.
Holocene refers namely to the present since the end of the Weichsel glaciation.
The interglacial Holocene has supported the development and growth of human civilizations, it has been the cradle of civilization, not to say their uterus.
It started around 11,700 years before present with a sudden warming from the cold period called Younger Dryas.
In only ten years time the temperature in Greenland rose with an impressive 8 degrees, which corresponds to that North Europe's climate was replaced with a Mediterranean climate.
It is not known, what caused this rapid rise in temperature.
Cenozoic, Tertiary and Quaternary
Cenozoic is the period of the mammals, which followed the Mesozoic that was the period of dinosaurs.
Tertiary is that part of Cenozoic, where no humans existed , and Quaternary means the part of Cenozoic, where humans exist.
Quaternary is composed of Pleistocene and Holocene.
Pleistocene is the period that we in common language call the Ice Age.
Holocene represents the present, which basically is a Pleistocene interglacial period.
Holocene is represented by the thin red line on the far left.
The climate of the Holocene is the subject of this Article.
During the following one thousand years, the temperature increased so that climate became several degrees warmer than today.
About 8,000 years before present, in Hunter Stone Age, occurred the hottest period throughout the Holocene.
This initiated the warm period called the Holocene Optimum, which lasted almost until about 4,500 years before present, whereafter the temperature continued to drop through bronze age, iron age and historical time until it reached a low point in "The Little Ice Age" in the years 1600- 1700.
Within the last few hundred years, the temperature has again increased, but not to such heights as in Hunter Stone Age.
The temperature in the Holocene
This graph is taken from Wikipedia.
It shows eight different reconstructions of Holocene temperature.
The thick black line is the average of these.
Time progresses from left to right.
On this graph the Stone Age is shown only about one degree warmer than present day, but most sources mention that Scandinavian Stone Age was about 2-3 degrees warmer than the present; this need not to be mutually excluding statements, because the curve reconstructs the entire Earth's temperature, and on higher latitudes the temperature variations were greater than about equator.
Some reconstructions show a vertical dramatic increase in temperature around the year 2000, but it seems not reasonable to the author, since that kind of graphs cannot possibly show temperature in specific years, it must necessarily be smoothed by a kind of mathematical rolling average, perhaps with periods of hundred years, and then a high temperature in a single year, for example, 2004 will be much less visible.
The trend seems to be that Holocene's highest temperature was reached in the Hunter Stone Age about 8,000 years before present, thereafter the temperature has generally been steadily falling, however, superimposed by many cold and warm periods, including the modern warm period.
However, generally speaking, the Holocene represents an amazing stable climate, where the cooling through the period has been limited to a few degrees.
The general decline in temperatures since 8000 years before present was overlaid by several cold and warm periods.
Thus we speak of five to seven cold periods in Holocene, including the Little Ice Age and several warm periods, including the Minoan, Roman, Medieval and Modern Warm Periods.
Temperature variations in the Holocene and the previous Weichsel glaciation
Temperature variations in the Holocene compared with the previous Weichsel ice age based on analysis of Greenland ice cores.
The vertical scale on the left shows the temperature on the surface of the ice, and the horizontal scale is years before present.
Time progresses from left to right.
It appears that the climate of the Holocene really has been very stable and the temperature has only varied a few degrees.
The most dramatic event so far has been the 8,200 cold period and the ensuing Holocene maximum in the Stone Age.
By contrast, the climate in the previous ice age was not nearly as stable.
The temperature often varied more than 20 degrees during a few hundred years or perhaps less.
The Holocene cold and warm periods, however, represent only small temperature changes compared to both the glaciation periods and the other interglacial periods.
This unique climatic stability made the development of agriculture possible, it created the basis for the development of civilizations and enabled eventually the industrial revolution and consequently the modern world with its technique and myriads of people.
Had we not had a window of about 10,000 years of stable climate with only small temperature variations, civilization would not have been nearly as developed, if at all existent, and Earth's population would have been only a fraction of the current.
Holocene temperature variations in comparison with the preceding interglacials
Temperature variations in Holocene compared with the preceding interglacials.
The vertical scale shows the temperature on the ice surface, and the horizontal time scale is in thousands of years before present.
It can be seen the Holocene temperature graph has a different shape than the previous interglacial periods.
Holocene has a nearly flat top, which represents a fairly stable climate through ten thousand years, while the preceding interglacials are generally pointed, that is the temperature has risen to a maximum and then declined again, maybe after only a few hundred years.
Only Holocene could offer a stable climate for a long time, during which agriculture and civilization could develop.
Note moreover almost all heating periods characteristic shape.
The heat comes suddenly, perhaps in a few decades and then decreases slowly.
This is also the case for the Holocene, except that temperature has dropped much slower than in the other interglacials and warming periods.
Many believe that the declining Milankovitch insolation is the cause of the general cooling trend during the Holocene.
The Milankovitch insolation is the theoretical insolation (received energy from the Sun) at 65 degrees northern latitude in June.
Its variation in the Holocene is mainly due to changes in the axis tilt, such that the northern hemisphere, in the beginning, received a big June insolation, as it turned more directly against the sun during the summer, while the axis in the present is more upright, and therefore the northern hemisphere receives not so much solar radiation in summertime.
Temperature and insolation in Holocene
The upper reddish graph represents the temperature in Celsius on the ice surface in Greenland.
The curve is generally falling through the Holocene but overlaid by many cold and hot periods.
Many operate with six cold spells, of which the best known are the 8,200 cold period and the Little Ice Age.
The most famous hot periods are the Minoan, the Roman and the Medieval warm periods.
The Norwegian Axel Blytt and the Swede Rutger Sernander developed in the 1800's the Blytt-Sernander period breakdown of the Holocene climate based on studies of Danish peat bogs.
It includes the periods Preboreal, Boreal, Atlantic, Subboreal and Subatlantic that are shown at the bottom of the figure.
Preboreal is also known as the Birch-pine period.
There are many different opinions about when the various Blytt-Sernander periods begin and end.
Today, some believe that this classification is outdated and prefer other divisions, which includes the Holocene Climatic Optimum, Postglacial and Neoglacial that all are shown at the top of this figure in colors.
The yellow-green graph shown below the temperature curve represents the theoretical Milankovitch insolation in Holocene in Watts per m2.
The Milankovitch insolation is solar radiation on 65. northern latitude in the month of June.
It can be seen that insolation-maximum occurred about 10,000 years before present with about 470 W/m2, and since then insolation has been declining steadily down to today's low value that is slightly less than 430 W/m2.
In the very early Holocene, northern Europe became vegetated by an open and light birch forest mixed with aspen, willow, mountain ash and pine.
The ever milder climate caused average summer temperatures to rise to 18-20 degrees, while winter temperature stood at just below freezing.
The composition of the forest trees changed; pine pushed back birch; hazel, elm, oak, ash, alder, fir and linden immigrated.
The 8,200 cold period
About 8,200 years ago, there was a sharp cooling in the Northern hemisphere.
It has been attributed to an excessive supply of cold glacial meltwater from glaciers in the Hudson Bay area.
Data from Disko Bay show that here too was a large production of melt water.
Samples taken from the ocean floor at Spitsbergen indicate that here the Arctic waters pushed further south already 8,800 years ago.
Reconstructed air temperature from analysis of sediments from the bottom of a Swedish lake
From HOCLAT - A web-based Holocene Climate Atlas (see link below).
Reconstructed summer air temperature from pollen analysis of sediments from the bottom of a Swedish lake at 58.55 northern latitude and 13.67 eastern longitude, which is a small Swedish lake between Vänern and Vättern.
The original data is the very thin line in the diagram at the top.
It has been smoothed with a form of mathematical rolling average over 500 years, it is the blue line.
In addition, the original data also have been mathematically smoothed over 3,000 years, it is the red line.
The figure at the bottom shows how much the blue line deviates from the red line, which is a measure of climate change.
The blue areas thus show how much the temperature of a cold period deviates from the more average temperature in this age, and the red areas show how much the temperature of a warming period exceeds the more average temperature in this age.
It is seen that the 8,200 cold spell represents a very severe climate change.
It has been a sudden change for Stone Age hunters.
Moreover, it looks as if the cold periods come at regular intervals.
The cold Period 5,900 years before present took place at the transition from Hunter-Stone Age to the Peasant Stone Age.
The cold period 3,500 was the beginning of the Bronze Age.
The small cold period around 1,800 occurred a few hundred years after birth of Christ, perhaps it was at that time, the Goths left the island of Skanza (Scandinavia).
The Little Ice Age seems to come somewhat early.
Many explain the 8,200 cold period as a result of a large discharge of cold meltwater into the Atlantic from Lake Agassiz at the edge of the Laurentide ice sheet in North America.
It may seem paradoxical that a warmer weather in the Arctic, which caused the melting of ice caps and sea ice and thus the production of cold fresh water, caused a colder climate in Northern Europe and probably also in North America.
This is explained by that the large amounts of cold fresh water, that is lighter than salt water, disturbed the ocean currents, and a weakened Gulf Stream was the cause of colder weather along the North Atlantic coasts.
Many believe that such meltwater mechanism also caused The Younger Dryas cold period.
TO BE CONTINUED ...
Introduction
If you want to know what the climate was like in the Holocene, simply take some outerwear and go out into nature and look around.
Holocene refers namely to the present since the end of the Weichsel glaciation.
The interglacial Holocene has supported the development and growth of human civilizations, it has been the cradle of civilization, not to say their uterus.
It started around 11,700 years before present with a sudden warming from the cold period called Younger Dryas.
In only ten years time the temperature in Greenland rose with an impressive 8 degrees, which corresponds to that North Europe's climate was replaced with a Mediterranean climate.
It is not known, what caused this rapid rise in temperature.
Cenozoic, Tertiary and Quaternary
Cenozoic is the period of the mammals, which followed the Mesozoic that was the period of dinosaurs.
Tertiary is that part of Cenozoic, where no humans existed , and Quaternary means the part of Cenozoic, where humans exist.
Quaternary is composed of Pleistocene and Holocene.
Pleistocene is the period that we in common language call the Ice Age.
Holocene represents the present, which basically is a Pleistocene interglacial period.
Holocene is represented by the thin red line on the far left.
The climate of the Holocene is the subject of this Article.
During the following one thousand years, the temperature increased so that climate became several degrees warmer than today.
About 8,000 years before present, in Hunter Stone Age, occurred the hottest period throughout the Holocene.
This initiated the warm period called the Holocene Optimum, which lasted almost until about 4,500 years before present, whereafter the temperature continued to drop through bronze age, iron age and historical time until it reached a low point in "The Little Ice Age" in the years 1600- 1700.
Within the last few hundred years, the temperature has again increased, but not to such heights as in Hunter Stone Age.
The temperature in the Holocene
This graph is taken from Wikipedia.
It shows eight different reconstructions of Holocene temperature.
The thick black line is the average of these.
Time progresses from left to right.
On this graph the Stone Age is shown only about one degree warmer than present day, but most sources mention that Scandinavian Stone Age was about 2-3 degrees warmer than the present; this need not to be mutually excluding statements, because the curve reconstructs the entire Earth's temperature, and on higher latitudes the temperature variations were greater than about equator.
Some reconstructions show a vertical dramatic increase in temperature around the year 2000, but it seems not reasonable to the author, since that kind of graphs cannot possibly show temperature in specific years, it must necessarily be smoothed by a kind of mathematical rolling average, perhaps with periods of hundred years, and then a high temperature in a single year, for example, 2004 will be much less visible.
The trend seems to be that Holocene's highest temperature was reached in the Hunter Stone Age about 8,000 years before present, thereafter the temperature has generally been steadily falling, however, superimposed by many cold and warm periods, including the modern warm period.
However, generally speaking, the Holocene represents an amazing stable climate, where the cooling through the period has been limited to a few degrees.
The general decline in temperatures since 8000 years before present was overlaid by several cold and warm periods.
Thus we speak of five to seven cold periods in Holocene, including the Little Ice Age and several warm periods, including the Minoan, Roman, Medieval and Modern Warm Periods.
Temperature variations in the Holocene and the previous Weichsel glaciation
Temperature variations in the Holocene compared with the previous Weichsel ice age based on analysis of Greenland ice cores.
The vertical scale on the left shows the temperature on the surface of the ice, and the horizontal scale is years before present.
Time progresses from left to right.
It appears that the climate of the Holocene really has been very stable and the temperature has only varied a few degrees.
The most dramatic event so far has been the 8,200 cold period and the ensuing Holocene maximum in the Stone Age.
By contrast, the climate in the previous ice age was not nearly as stable.
The temperature often varied more than 20 degrees during a few hundred years or perhaps less.
The Holocene cold and warm periods, however, represent only small temperature changes compared to both the glaciation periods and the other interglacial periods.
This unique climatic stability made the development of agriculture possible, it created the basis for the development of civilizations and enabled eventually the industrial revolution and consequently the modern world with its technique and myriads of people.
Had we not had a window of about 10,000 years of stable climate with only small temperature variations, civilization would not have been nearly as developed, if at all existent, and Earth's population would have been only a fraction of the current.
Holocene temperature variations in comparison with the preceding interglacials
Temperature variations in Holocene compared with the preceding interglacials.
The vertical scale shows the temperature on the ice surface, and the horizontal time scale is in thousands of years before present.
It can be seen the Holocene temperature graph has a different shape than the previous interglacial periods.
Holocene has a nearly flat top, which represents a fairly stable climate through ten thousand years, while the preceding interglacials are generally pointed, that is the temperature has risen to a maximum and then declined again, maybe after only a few hundred years.
Only Holocene could offer a stable climate for a long time, during which agriculture and civilization could develop.
Note moreover almost all heating periods characteristic shape.
The heat comes suddenly, perhaps in a few decades and then decreases slowly.
This is also the case for the Holocene, except that temperature has dropped much slower than in the other interglacials and warming periods.
Many believe that the declining Milankovitch insolation is the cause of the general cooling trend during the Holocene.
The Milankovitch insolation is the theoretical insolation (received energy from the Sun) at 65 degrees northern latitude in June.
Its variation in the Holocene is mainly due to changes in the axis tilt, such that the northern hemisphere, in the beginning, received a big June insolation, as it turned more directly against the sun during the summer, while the axis in the present is more upright, and therefore the northern hemisphere receives not so much solar radiation in summertime.
Temperature and insolation in Holocene
The upper reddish graph represents the temperature in Celsius on the ice surface in Greenland.
The curve is generally falling through the Holocene but overlaid by many cold and hot periods.
Many operate with six cold spells, of which the best known are the 8,200 cold period and the Little Ice Age.
The most famous hot periods are the Minoan, the Roman and the Medieval warm periods.
The Norwegian Axel Blytt and the Swede Rutger Sernander developed in the 1800's the Blytt-Sernander period breakdown of the Holocene climate based on studies of Danish peat bogs.
It includes the periods Preboreal, Boreal, Atlantic, Subboreal and Subatlantic that are shown at the bottom of the figure.
Preboreal is also known as the Birch-pine period.
There are many different opinions about when the various Blytt-Sernander periods begin and end.
Today, some believe that this classification is outdated and prefer other divisions, which includes the Holocene Climatic Optimum, Postglacial and Neoglacial that all are shown at the top of this figure in colors.
The yellow-green graph shown below the temperature curve represents the theoretical Milankovitch insolation in Holocene in Watts per m2.
The Milankovitch insolation is solar radiation on 65. northern latitude in the month of June.
It can be seen that insolation-maximum occurred about 10,000 years before present with about 470 W/m2, and since then insolation has been declining steadily down to today's low value that is slightly less than 430 W/m2.
In the very early Holocene, northern Europe became vegetated by an open and light birch forest mixed with aspen, willow, mountain ash and pine.
The ever milder climate caused average summer temperatures to rise to 18-20 degrees, while winter temperature stood at just below freezing.
The composition of the forest trees changed; pine pushed back birch; hazel, elm, oak, ash, alder, fir and linden immigrated.
The 8,200 cold period
About 8,200 years ago, there was a sharp cooling in the Northern hemisphere.
It has been attributed to an excessive supply of cold glacial meltwater from glaciers in the Hudson Bay area.
Data from Disko Bay show that here too was a large production of melt water.
Samples taken from the ocean floor at Spitsbergen indicate that here the Arctic waters pushed further south already 8,800 years ago.
Reconstructed air temperature from analysis of sediments from the bottom of a Swedish lake
From HOCLAT - A web-based Holocene Climate Atlas (see link below).
Reconstructed summer air temperature from pollen analysis of sediments from the bottom of a Swedish lake at 58.55 northern latitude and 13.67 eastern longitude, which is a small Swedish lake between Vänern and Vättern.
The original data is the very thin line in the diagram at the top.
It has been smoothed with a form of mathematical rolling average over 500 years, it is the blue line.
In addition, the original data also have been mathematically smoothed over 3,000 years, it is the red line.
The figure at the bottom shows how much the blue line deviates from the red line, which is a measure of climate change.
The blue areas thus show how much the temperature of a cold period deviates from the more average temperature in this age, and the red areas show how much the temperature of a warming period exceeds the more average temperature in this age.
It is seen that the 8,200 cold spell represents a very severe climate change.
It has been a sudden change for Stone Age hunters.
Moreover, it looks as if the cold periods come at regular intervals.
The cold Period 5,900 years before present took place at the transition from Hunter-Stone Age to the Peasant Stone Age.
The cold period 3,500 was the beginning of the Bronze Age.
The small cold period around 1,800 occurred a few hundred years after birth of Christ, perhaps it was at that time, the Goths left the island of Skanza (Scandinavia).
The Little Ice Age seems to come somewhat early.
Many explain the 8,200 cold period as a result of a large discharge of cold meltwater into the Atlantic from Lake Agassiz at the edge of the Laurentide ice sheet in North America.
It may seem paradoxical that a warmer weather in the Arctic, which caused the melting of ice caps and sea ice and thus the production of cold fresh water, caused a colder climate in Northern Europe and probably also in North America.
This is explained by that the large amounts of cold fresh water, that is lighter than salt water, disturbed the ocean currents, and a weakened Gulf Stream was the cause of colder weather along the North Atlantic coasts.
Many believe that such meltwater mechanism also caused The Younger Dryas cold period.
TO BE CONTINUED ...
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene, continued ...
Analysis of oxygen-isotopes in stalagmites from Costa Rica
Analysis of oxygen isotopes in stalagmites from Costa Rica shows a dry period around 8,200 before present.
From "Tropical response to the 8200 yr B.P. cold event? --" by Matthew S. Lachniet with others.
Even more beyond comprehension is it that a team of American geologists from the University of Buffalo and other universities have found that during the cold period glaciers on Baffin Island increased.
One can only conclude that if there simultaneously was produced meltwater, precipitation must have been even very big in the region.
Analyses of oxygen isotopes in stalagmites from caves in Costa Rica have shown that there was a dry period about 8,200 years before present, caused by weaker monsoon and reduced precipitation in Central America.
It questions the melt-water Gulf Stream theory, as the climate in Costa Rica's is not dependent of a warm Gulf Stream, this region belongs indeed the area near the equator, which supplies the heat to the Gulf Stream.
The cold period cannot, however, be detected in the Southern Hemisphere, neither in drill cores from the ice sheet in Antarctica glaciers in Bolivia or in samples taken from the seabed off the mouth of Murray River in Australia.
This indicates that the cold period can have been a truly North Atlantic phenomenon, perhaps caused by variations in the sea currents.
But after a while, the sea currents in the North Atlantic found back to their old routes if that had been the cause, and about 8,000 years before present began the warmest period of the Holocene ever.
In sediments from the bottom of the lakes, Huelmo and Mascardi, in the Andes Mountains in respectively Chile and Argentina scientists have found evidence of a cold period on the Southern Hemisphere, which lasted 800 years and occurred between 11,400 and 10,200 years before present.
The Holocene Climatic Optimum
The hottest time in the Holocene occurred in the Stone Age about 8,000 years before present, it is called the Holocene Maximum.
This warm climate continued largely through 3,500 years until 4,500 before present, when it was Neolithic period in Northern Europe.
It is assumed that the average temperature was 2-3 degrees higher than today.
This is supported by the fact that plants such as mistletoe and the subtropical aquatic plant Trapa natans grew widespread in south Scandinavia.
Linden, elm, spruce and oak were the most common trees in northern Europe's dense forests, which closed the continent's interior into a big impenetrable forest.
In Denmark, scientists have studied Stone Age settlements from the Holocene Climatic Optimum's period and found bones of various terrestrial and marine animals, including swordfish, sturgeon, sardine and tuna, dalmatian pelican and pond turtle, all of which are species that today live in warmer climes.
Another testimony of warmer climate in the past can be found in Dartmoor in Southern England, though slightly later than the Holocene Optimum.
Here Bronze Age farmers cultivated the land in 450 meters above sea level, which should be compared with the absolute limit on agriculture today, that is an altitude of 300 meters.
A team of scientists from the University of Copenhagen have analyzed driftwood and beach ridges along the coast of north-eastern Greenland and thereby uncovered the extension of sea ice during the Holocene Optimum.
Driftwood, that end up on the coast of northeastern Greenland come from North America and Siberia.
It has used several years to complete its journey and would only reach the coast of Greenland, if it is encased in ice, since free driftwood will sink to the bottom during such a long journey.
By collecting and dating driftwood with the carbon-14 method, researchers could calculate the amount of sea ice in different time periods.
Svend Funder and his colleagues also examined beach ridges along the coast.
Today beach ridges are not formed along the coast of Northern Greenland, as sea ice shields the coast year round.
By the carbon 14 method, the beach ridges have been determined to originate from the Holocene Optimum, during which period the sea must have been ice-free, at least in the summertime.
It was concluded that sea ice reached a minimum between 8,500 and 6,000 years ago, when the limit for full-year sea ice was located 1.000 km further north than in the present, and in summertime it covered an area only half as large as the sea ice area in the summer of 2007, when sea ice in recent times had its minimum.
Some studies indicate that the sea surface temperature of the world's oceans was up to 5 degrees higher than today's surface temperature (Darby, 2001).
Throughout most of the first part of Holocene, most of Europe, Asia and North America was covered by forest.
A large part of the biosphere's carbon was tied up in the wood of the trees.
Agriculture was introduced and as the trees rotted away or were burned, the atmospheric concentration of carbon increased in the form of CO2.
The ice cap in Peary Land in northern Greenland was drilled in 1977.
The ice core contained distinct refrozen meltwater layers all way down to the bedrock, which indicates that it did not contain ice from the Weichsel glaciation.
That is to say that the world's northernmost ice sheet melted completely away during the Holocene Optimum and was only restored when the climate became colder about 4,500 years ago.
Since less water was bound at the poles as inland ice than nowadays, the World Sea surface level at that time was 3 meters above today's sea surface level.
At the end of Maglemose hunters' period, around 8,500 before present, the climate in northern Europe had evolved into a so-called Atlantic climate.
It was a mild and humid coastal climate with summer temperatures 2-3 degrees higher than today.
As sea surface level in the World Ocean rose, it caused the salty seawater to enter the Ancylus Lake (Baltic Sea), and the water in the Baltic Sea basin again became salt.
The new sea is called the Littorina Sea after the saltwater snail Littorina littorea.
It lasted several hundred years before the salt content reached its maximum.
TO BE CONTINUED ...
Analysis of oxygen-isotopes in stalagmites from Costa Rica
Analysis of oxygen isotopes in stalagmites from Costa Rica shows a dry period around 8,200 before present.
From "Tropical response to the 8200 yr B.P. cold event? --" by Matthew S. Lachniet with others.
Even more beyond comprehension is it that a team of American geologists from the University of Buffalo and other universities have found that during the cold period glaciers on Baffin Island increased.
One can only conclude that if there simultaneously was produced meltwater, precipitation must have been even very big in the region.
Analyses of oxygen isotopes in stalagmites from caves in Costa Rica have shown that there was a dry period about 8,200 years before present, caused by weaker monsoon and reduced precipitation in Central America.
It questions the melt-water Gulf Stream theory, as the climate in Costa Rica's is not dependent of a warm Gulf Stream, this region belongs indeed the area near the equator, which supplies the heat to the Gulf Stream.
The cold period cannot, however, be detected in the Southern Hemisphere, neither in drill cores from the ice sheet in Antarctica glaciers in Bolivia or in samples taken from the seabed off the mouth of Murray River in Australia.
This indicates that the cold period can have been a truly North Atlantic phenomenon, perhaps caused by variations in the sea currents.
But after a while, the sea currents in the North Atlantic found back to their old routes if that had been the cause, and about 8,000 years before present began the warmest period of the Holocene ever.
In sediments from the bottom of the lakes, Huelmo and Mascardi, in the Andes Mountains in respectively Chile and Argentina scientists have found evidence of a cold period on the Southern Hemisphere, which lasted 800 years and occurred between 11,400 and 10,200 years before present.
The Holocene Climatic Optimum
The hottest time in the Holocene occurred in the Stone Age about 8,000 years before present, it is called the Holocene Maximum.
This warm climate continued largely through 3,500 years until 4,500 before present, when it was Neolithic period in Northern Europe.
It is assumed that the average temperature was 2-3 degrees higher than today.
This is supported by the fact that plants such as mistletoe and the subtropical aquatic plant Trapa natans grew widespread in south Scandinavia.
Linden, elm, spruce and oak were the most common trees in northern Europe's dense forests, which closed the continent's interior into a big impenetrable forest.
In Denmark, scientists have studied Stone Age settlements from the Holocene Climatic Optimum's period and found bones of various terrestrial and marine animals, including swordfish, sturgeon, sardine and tuna, dalmatian pelican and pond turtle, all of which are species that today live in warmer climes.
Another testimony of warmer climate in the past can be found in Dartmoor in Southern England, though slightly later than the Holocene Optimum.
Here Bronze Age farmers cultivated the land in 450 meters above sea level, which should be compared with the absolute limit on agriculture today, that is an altitude of 300 meters.
A team of scientists from the University of Copenhagen have analyzed driftwood and beach ridges along the coast of north-eastern Greenland and thereby uncovered the extension of sea ice during the Holocene Optimum.
Driftwood, that end up on the coast of northeastern Greenland come from North America and Siberia.
It has used several years to complete its journey and would only reach the coast of Greenland, if it is encased in ice, since free driftwood will sink to the bottom during such a long journey.
By collecting and dating driftwood with the carbon-14 method, researchers could calculate the amount of sea ice in different time periods.
Svend Funder and his colleagues also examined beach ridges along the coast.
Today beach ridges are not formed along the coast of Northern Greenland, as sea ice shields the coast year round.
By the carbon 14 method, the beach ridges have been determined to originate from the Holocene Optimum, during which period the sea must have been ice-free, at least in the summertime.
It was concluded that sea ice reached a minimum between 8,500 and 6,000 years ago, when the limit for full-year sea ice was located 1.000 km further north than in the present, and in summertime it covered an area only half as large as the sea ice area in the summer of 2007, when sea ice in recent times had its minimum.
Some studies indicate that the sea surface temperature of the world's oceans was up to 5 degrees higher than today's surface temperature (Darby, 2001).
Throughout most of the first part of Holocene, most of Europe, Asia and North America was covered by forest.
A large part of the biosphere's carbon was tied up in the wood of the trees.
Agriculture was introduced and as the trees rotted away or were burned, the atmospheric concentration of carbon increased in the form of CO2.
The ice cap in Peary Land in northern Greenland was drilled in 1977.
The ice core contained distinct refrozen meltwater layers all way down to the bedrock, which indicates that it did not contain ice from the Weichsel glaciation.
That is to say that the world's northernmost ice sheet melted completely away during the Holocene Optimum and was only restored when the climate became colder about 4,500 years ago.
Since less water was bound at the poles as inland ice than nowadays, the World Sea surface level at that time was 3 meters above today's sea surface level.
At the end of Maglemose hunters' period, around 8,500 before present, the climate in northern Europe had evolved into a so-called Atlantic climate.
It was a mild and humid coastal climate with summer temperatures 2-3 degrees higher than today.
As sea surface level in the World Ocean rose, it caused the salty seawater to enter the Ancylus Lake (Baltic Sea), and the water in the Baltic Sea basin again became salt.
The new sea is called the Littorina Sea after the saltwater snail Littorina littorea.
It lasted several hundred years before the salt content reached its maximum.
TO BE CONTINUED ...
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene, continued ...
The development of the Baltic Sea
As the kilometers-thick Scandinavian ice sheet began to melt, it formed the freshwater lake, the Baltic Ice Lake.
It was a cold sea with drifting icebergs.
The lake surface was higher than the sea surface of the World's oceans.
Some believe that the ice-lake was emptied by a major flood disaster around year 9,600 BC, but most believe that it happened gradually.
The landscape of northern Europe was dominated by icy cold steppes and regular tundra roamed by a small number of reindeer hunters.
After the lake made connected with the World Sea, it became a brackish sea called the Yoldia sea, named after the mussel Yoldia arctica.
The Yoldia Sea had connection with the world's oceans through a strait that was located where the Swedish lakes and the Gøta river are today.
In the early hunter-Stone Age the tundra became vegetated of a birch forest mixed with aspen, willow, mountain ash and pine.
When Scandinavia was freed from the weight of the huge masses of ice, the land lifted, and the uplift cut off the Yoldia Sea's connection with the world's oceans, and it became once again a fresh water lake called the Ancylus Lake after the freshwater snail Ancylus fluviatilis.
Ancylus Lake maybe had drain through central Sweden at the Great Lakes.
As the climate became milder, and the summer average temperature rose to 18-20 degrees, and winter temperatures rarely fall below freezing, also the composition of the forest trees changed, pine replaced birch and hazel, elm, oak, ash, alder, fir and linden became common.
Around 7,000 before present, the climate of northern Europe was a so-called Atlantic climate.
It was a mild and humid coastal climate with summer temperatures 2-3 degrees higher than today.
The water level in the world's oceans increased after some time making the salty sea water enter the Ancylus Lake, and the water in the Baltic Sea basin again became salt.
The new sea is called the Littorina Sea after the saltwater snail Littorina littorea.
Because of land uplift the Littorina Sea's connection to the World Ocean during the past 2,000 years had become increasingly narrower and shallower, making it to the brackish sea, that we know today as the Baltic Sea.
In Australia scientists have analyzed sediments in the seabed off the mouth of the river Murray and found that from 17,000 to 13,500 years before present the Australian climate was wetter than it is in the present.
There have with certainty been found no indications of dry periods either in the Younger Dryas or 8,200 years before present, indicating that these cold periods are phenomena limited to the northern hemisphere.
TO BE CONTINUED ...
The development of the Baltic Sea
As the kilometers-thick Scandinavian ice sheet began to melt, it formed the freshwater lake, the Baltic Ice Lake.
It was a cold sea with drifting icebergs.
The lake surface was higher than the sea surface of the World's oceans.
Some believe that the ice-lake was emptied by a major flood disaster around year 9,600 BC, but most believe that it happened gradually.
The landscape of northern Europe was dominated by icy cold steppes and regular tundra roamed by a small number of reindeer hunters.
After the lake made connected with the World Sea, it became a brackish sea called the Yoldia sea, named after the mussel Yoldia arctica.
The Yoldia Sea had connection with the world's oceans through a strait that was located where the Swedish lakes and the Gøta river are today.
In the early hunter-Stone Age the tundra became vegetated of a birch forest mixed with aspen, willow, mountain ash and pine.
When Scandinavia was freed from the weight of the huge masses of ice, the land lifted, and the uplift cut off the Yoldia Sea's connection with the world's oceans, and it became once again a fresh water lake called the Ancylus Lake after the freshwater snail Ancylus fluviatilis.
Ancylus Lake maybe had drain through central Sweden at the Great Lakes.
As the climate became milder, and the summer average temperature rose to 18-20 degrees, and winter temperatures rarely fall below freezing, also the composition of the forest trees changed, pine replaced birch and hazel, elm, oak, ash, alder, fir and linden became common.
Around 7,000 before present, the climate of northern Europe was a so-called Atlantic climate.
It was a mild and humid coastal climate with summer temperatures 2-3 degrees higher than today.
The water level in the world's oceans increased after some time making the salty sea water enter the Ancylus Lake, and the water in the Baltic Sea basin again became salt.
The new sea is called the Littorina Sea after the saltwater snail Littorina littorea.
Because of land uplift the Littorina Sea's connection to the World Ocean during the past 2,000 years had become increasingly narrower and shallower, making it to the brackish sea, that we know today as the Baltic Sea.
In Australia scientists have analyzed sediments in the seabed off the mouth of the river Murray and found that from 17,000 to 13,500 years before present the Australian climate was wetter than it is in the present.
There have with certainty been found no indications of dry periods either in the Younger Dryas or 8,200 years before present, indicating that these cold periods are phenomena limited to the northern hemisphere.
TO BE CONTINUED ...
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene, continued ...
Samples of bottom sediments in the Australian Lake Frome and Lake Woods show that the climate in early Holocene between 9,500 and 8,000 years ago, and again 7,000 to 4,200 years ago, was considerably wetter than in the present.
The beginning of modern climatic conditions in Australia with periodic rainy seasons took place about 4,000 years ago.
Analyses of sediments from the Cariaco Basin in Venezuela indicates that the amount of water discharged into the basin during the Holocene Optimum was much greater than today, indicating that precipitation in the area must have been much larger in the first half of the Holocene than it is today. (Uriarte, Haug).
One of the geographical events in Europe, that most brings our thoughts to the Biblical account of the Flood, is perhaps the sudden flooding of the partially dried up Black Sea, which took place 5,500 years before present.
For reasons, we can only guess, the inland sea had lost its connection to the world's oceans and was partially dried out.
Its sea surface lay 150 m under the sea surface of the World Sea.
The Black Sea is fed by many large and water-rich rivers, just think of the Danube, Dnester, Dneieper and Don.
It is difficult to understand that it may have lost more water by evaporation than it received from the rivers.
It must be evidence that it really has been very hot during the Holocene Optimum.
It is assumed that the temperature during this period was 2-3 degrees higher than in present.
A marginal increase in the sea surface of the world's ocean 5,500 years before present created a small crack in the barrier of the Bosphorus, a negligible trickle of seawater into the Black Sea basin quickly evolved into a huge waterfall of salt water 200 times greater than Niagara.
It is assumed that the sea water gushed into the half dried up Black Sea and got sea surface level to rise by 15 cm. a day, and thus raised the water level the 150 meters up to the World Ocean surface level during about three years.
When the flood occurred, there was Neolithic time in Northern Europe and this there had surely been a long time in the area around the Black Sea.
The oceanographer Robert Ballard has examined the Black Sea bottom using an underwater robot and found evidence of human habitation.
Many peoples have the story of an initial flood among their old myths.
In the Genesis of the Bible God separated the waters and created Heaven and Earth.
The Bible has the story about the flood, and how Noah and his family survived.
In the Egyptian creation myth was in the beginning also a chaos of water, and the god Ra separated the waters and created the world.
In the Scandinavian mythology, the gods Odin, Vile, and Ve killed the original giant Ymer, in the flood that was created by Hrymer's blood all his children, the rimturses, drowned except for Bergelmer and his wife, and by these were the Jotuns.
Even the Australian Aborigines have an initial flood among their old myths.
TO BE CONTINUED ...
Samples of bottom sediments in the Australian Lake Frome and Lake Woods show that the climate in early Holocene between 9,500 and 8,000 years ago, and again 7,000 to 4,200 years ago, was considerably wetter than in the present.
The beginning of modern climatic conditions in Australia with periodic rainy seasons took place about 4,000 years ago.
Analyses of sediments from the Cariaco Basin in Venezuela indicates that the amount of water discharged into the basin during the Holocene Optimum was much greater than today, indicating that precipitation in the area must have been much larger in the first half of the Holocene than it is today. (Uriarte, Haug).
One of the geographical events in Europe, that most brings our thoughts to the Biblical account of the Flood, is perhaps the sudden flooding of the partially dried up Black Sea, which took place 5,500 years before present.
For reasons, we can only guess, the inland sea had lost its connection to the world's oceans and was partially dried out.
Its sea surface lay 150 m under the sea surface of the World Sea.
The Black Sea is fed by many large and water-rich rivers, just think of the Danube, Dnester, Dneieper and Don.
It is difficult to understand that it may have lost more water by evaporation than it received from the rivers.
It must be evidence that it really has been very hot during the Holocene Optimum.
It is assumed that the temperature during this period was 2-3 degrees higher than in present.
A marginal increase in the sea surface of the world's ocean 5,500 years before present created a small crack in the barrier of the Bosphorus, a negligible trickle of seawater into the Black Sea basin quickly evolved into a huge waterfall of salt water 200 times greater than Niagara.
It is assumed that the sea water gushed into the half dried up Black Sea and got sea surface level to rise by 15 cm. a day, and thus raised the water level the 150 meters up to the World Ocean surface level during about three years.
When the flood occurred, there was Neolithic time in Northern Europe and this there had surely been a long time in the area around the Black Sea.
The oceanographer Robert Ballard has examined the Black Sea bottom using an underwater robot and found evidence of human habitation.
Many peoples have the story of an initial flood among their old myths.
In the Genesis of the Bible God separated the waters and created Heaven and Earth.
The Bible has the story about the flood, and how Noah and his family survived.
In the Egyptian creation myth was in the beginning also a chaos of water, and the god Ra separated the waters and created the world.
In the Scandinavian mythology, the gods Odin, Vile, and Ve killed the original giant Ymer, in the flood that was created by Hrymer's blood all his children, the rimturses, drowned except for Bergelmer and his wife, and by these were the Jotuns.
Even the Australian Aborigines have an initial flood among their old myths.
TO BE CONTINUED ...
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene, continued ...
A Green Sahara
During a longer period of time, that roughly corresponds to the Holocene Optimum, Northern Africa experienced a time of a considerably more wet and rainy climate than that which now prevails in the region.
Many state the period to be 8,500 to 3,500 BC (10,500 to 5,500 years before present), but the dating seems to be uncertain.
Where now is barren and scorched desert, was then savannah with widespread grassland and some trees.
There lived lions, elephants, giraffes and other animals that are now characteristic of southern Africa.
The former professor of African history at London University Roland Oliver described the landscape as follows: The major mountain ranges Tibesti and Hoggar, which today are bare rocks, were then covered with forests of oak and walnut, linden, alder and elm.
The lower slopes, along with the smaller mountains - Tassili and Acacus to the north, Ennedi and Air to the south - were covered with olive, juniper and Aleppo pine.
Through the grasslands of the valleys' rivers were flowing teeming with fish.
Rock Art all over Sahara recalls a time when the country was greener and home to lions, elephants, giraffes, antelopes, hippos and crocodiles.
A picture from Tassili, which today is a scorched desert, shows men, who stand in boats sailing on water.
This shows that there existed lakes and rivers in places, where today cannot be found a straw of grass.
Most rock paintings in the Sahara are found in Algeria, Libya, Morocco and Niger, and to a lesser extent in Egypt, Sudan, Tunisia and some Sahel countries.
The Air Mountains in Niger, the Tassili-n-Ajjer plateau in southeastern Algeria and the Fezzan region in southwestern Libya are particularly rich in old rock paintings.
Lake Chad reached a maximum extent of about 400,000 square kilometers, which is larger than the modern Caspian Sea, with a surface level about 30 meters higher than in modern times.
(A) During last Glacial Maximum and late Pleistocene, that is 20,000 to 8,500 BC (22,000 to 10,500 years before present) the Sahara desert was devoid of any settlement outside the Nile Valley, and the desert stretched 400 km. farther south, than it does today.
(B) With the sudden onset of monsoon rain around 8,500 BC, the hyper-arid desert was replaced by savannah-like landscapes, which quickly became inhabited by prehistoric people.
In the early Holocene optimum southern Sahara and the Nile Valley were apparently too humid and dangerous for appreciably human settlement.
(C) Around 7,000 BC human settlements have been well established throughout eastern Sahara, where they created a cattle-nomadic culture.
(D) Decreasing monsoon rain caused a beginning drying out of the Egyptian part of the Sahara around 5,300 BC.
The prehistoric people were forced to seek into the Nile Valley, settling in oases or to emigrate to the Sudanese Sahara, where rainfall and surface water was still sufficient.
Sahara's return to actual desert conditions about 3,500 BC coincided with the initial stages of Egyptian civilization in the Nile Valley. - Kuper and Kröpelin (2006).
Around 3500 BC the desert again spread across North Africa, and the scattered cattle nomads moved to the Nile Valley, where they began tilling the soil, and where they created the first Dynasty and thus founded the famous Egyptian culture.
In pharaonic times, there were still lions in Egypt.
They lived on the border of the desert, where they were known as the keepers of the eastern and western horizon or guardians of the eastern and western descent to the underworld.
Sphinxes may depict a pharaoh as a lion figure with a human head.
There lived elephants in North Africa long after the desert had returned in the central Sahara.
North African forest elephants were somewhat smaller than both the Indian elephant and the African steppe elephant.
Its Latin name sounds Loxodonta Africana Pharaoensis, and it was exterminated in the second century; reportedly many have been killed in the Roman arenas.
It puzzled Cicero that when twenty elephants, an unprecedented number, were attacked by spearmen in the arena, their trumpeting of distress so harrowed the spectators that everyone in the theater began to weep.
The show was given by the great man Pompey.
Also, the Arabian desert in the Middle East and the Rajasthan desert between India and Pakistan experienced a wet period in the first part of Holocene.
In the dried-out lakes of the deserts have been found spores from plants, which are characteristic of a savannah vegetation.
Other studies indicate that Central Asia in the early Holocene experienced a wetter climate than today, while summer temperatures, that was from 2 to 3.5 degrees higher than today, prevailed.
In China rice could be planted almost a full month earlier than it usually is the case today.
Bamboo groves could be found three latitudes farther north than they are found in modern times. (Uriarte, Chu Ko-chen)
Many peoples have old myths about an original homeland, which they left in the distant past.
The ancient Doric Greeks immigrated from the north, the Scandinavian peoples remember Asgaard and Midgaard, according to their ancient myths the Romans originally came from Troy, and probably the best-known myth of this kind is the Biblical story of the expulsion from the Garden of Eden.
It is quite likely that the factors, which forced the peoples to emigrate, have been associated with such climate changes that took place in connection with the end of the Holocene Optimum.
TO BE CONTINUED ...
A Green Sahara
During a longer period of time, that roughly corresponds to the Holocene Optimum, Northern Africa experienced a time of a considerably more wet and rainy climate than that which now prevails in the region.
Many state the period to be 8,500 to 3,500 BC (10,500 to 5,500 years before present), but the dating seems to be uncertain.
Where now is barren and scorched desert, was then savannah with widespread grassland and some trees.
There lived lions, elephants, giraffes and other animals that are now characteristic of southern Africa.
The former professor of African history at London University Roland Oliver described the landscape as follows: The major mountain ranges Tibesti and Hoggar, which today are bare rocks, were then covered with forests of oak and walnut, linden, alder and elm.
The lower slopes, along with the smaller mountains - Tassili and Acacus to the north, Ennedi and Air to the south - were covered with olive, juniper and Aleppo pine.
Through the grasslands of the valleys' rivers were flowing teeming with fish.
Rock Art all over Sahara recalls a time when the country was greener and home to lions, elephants, giraffes, antelopes, hippos and crocodiles.
A picture from Tassili, which today is a scorched desert, shows men, who stand in boats sailing on water.
This shows that there existed lakes and rivers in places, where today cannot be found a straw of grass.
Most rock paintings in the Sahara are found in Algeria, Libya, Morocco and Niger, and to a lesser extent in Egypt, Sudan, Tunisia and some Sahel countries.
The Air Mountains in Niger, the Tassili-n-Ajjer plateau in southeastern Algeria and the Fezzan region in southwestern Libya are particularly rich in old rock paintings.
Lake Chad reached a maximum extent of about 400,000 square kilometers, which is larger than the modern Caspian Sea, with a surface level about 30 meters higher than in modern times.
(A) During last Glacial Maximum and late Pleistocene, that is 20,000 to 8,500 BC (22,000 to 10,500 years before present) the Sahara desert was devoid of any settlement outside the Nile Valley, and the desert stretched 400 km. farther south, than it does today.
(B) With the sudden onset of monsoon rain around 8,500 BC, the hyper-arid desert was replaced by savannah-like landscapes, which quickly became inhabited by prehistoric people.
In the early Holocene optimum southern Sahara and the Nile Valley were apparently too humid and dangerous for appreciably human settlement.
(C) Around 7,000 BC human settlements have been well established throughout eastern Sahara, where they created a cattle-nomadic culture.
(D) Decreasing monsoon rain caused a beginning drying out of the Egyptian part of the Sahara around 5,300 BC.
The prehistoric people were forced to seek into the Nile Valley, settling in oases or to emigrate to the Sudanese Sahara, where rainfall and surface water was still sufficient.
Sahara's return to actual desert conditions about 3,500 BC coincided with the initial stages of Egyptian civilization in the Nile Valley. - Kuper and Kröpelin (2006).
Around 3500 BC the desert again spread across North Africa, and the scattered cattle nomads moved to the Nile Valley, where they began tilling the soil, and where they created the first Dynasty and thus founded the famous Egyptian culture.
In pharaonic times, there were still lions in Egypt.
They lived on the border of the desert, where they were known as the keepers of the eastern and western horizon or guardians of the eastern and western descent to the underworld.
Sphinxes may depict a pharaoh as a lion figure with a human head.
There lived elephants in North Africa long after the desert had returned in the central Sahara.
North African forest elephants were somewhat smaller than both the Indian elephant and the African steppe elephant.
Its Latin name sounds Loxodonta Africana Pharaoensis, and it was exterminated in the second century; reportedly many have been killed in the Roman arenas.
It puzzled Cicero that when twenty elephants, an unprecedented number, were attacked by spearmen in the arena, their trumpeting of distress so harrowed the spectators that everyone in the theater began to weep.
The show was given by the great man Pompey.
Also, the Arabian desert in the Middle East and the Rajasthan desert between India and Pakistan experienced a wet period in the first part of Holocene.
In the dried-out lakes of the deserts have been found spores from plants, which are characteristic of a savannah vegetation.
Other studies indicate that Central Asia in the early Holocene experienced a wetter climate than today, while summer temperatures, that was from 2 to 3.5 degrees higher than today, prevailed.
In China rice could be planted almost a full month earlier than it usually is the case today.
Bamboo groves could be found three latitudes farther north than they are found in modern times. (Uriarte, Chu Ko-chen)
Many peoples have old myths about an original homeland, which they left in the distant past.
The ancient Doric Greeks immigrated from the north, the Scandinavian peoples remember Asgaard and Midgaard, according to their ancient myths the Romans originally came from Troy, and probably the best-known myth of this kind is the Biblical story of the expulsion from the Garden of Eden.
It is quite likely that the factors, which forced the peoples to emigrate, have been associated with such climate changes that took place in connection with the end of the Holocene Optimum.
TO BE CONTINUED ...
Re: LISTEN TO THE SCIENCE, PEOPLE!
Cenozoic III - End of Pleistocene, continued ...
Between the Holocene optimum and the Roman Warm Period
Around 5,500 to 5,000 years before present occurred the Piora cold period, which is named after the Val Piora valley in Switzerland, which was the first place, where it was identified by using pollen analysis.
The more heat-loving trees as elm and linden became rarer and never again regained their dominant position in the woods.
There have been found indications of this cold period in both Alaska, the Andes in Colombia and in the mountains of Kenya (Lamb).
Precipitation in the Rajasthani desert Harappa and Mohenjodaro
In the Indus Valley, where today Rajastan's arid Thar desert is spreading, the cities of Harappa and Mohenjodaro flourished between 4,600 and 3,900 years before present.
When their civilization was at its peak, it covered an area, which was larger than the Nile valley and Mesopotamia combined.
The inhabitants cultivated wheat, barley, melons, dates and perhaps cotton.
On the savannah and along the now dry river lived elephants, rhinos and water buffaloes.
The annual rainfall is estimated to have been between 400 and 800 mm.
In the Arabian desert has also been found evidence of human habitation from about 5,000 years before present.
Not much is known about the Minoan warm period beyond, what can be gauged from cores from boreholes in the ice sheet.
That the climate really was warmer then may be derived from that in the Minoan warm period, which occurred during the bronze age, millet was grown in southern Scandinavia.
Today Millet is grown in tropical and subtropical regions, it is an important crop in Asia, Africa and in the southern U.S.
The average annual temperature in Mississippi and Alabama is about 10 degrees, which should be compared with today's average annual temperature in Denmark, which is 8 degrees.
So maybe the climate in the Minoan warm period, was about 2 degrees warmer than present in southern Scandinavia.
As you may know, Rome is said to have been founded by Romulus and Remus in 753 BC
The Roman historian Livy tells us that in the city's early history occurred a few severe winters when there was ice on the Tiber, and the snow stayed for many days.
Before the Roman warm period beech trees is said to have been growing in mountains around Rome.
Climate changes have always taken place, it is documented in the Bible.
Jeremiah 18:14 in the Old Testament says: "Does the snow of Lebanon leave the crags of Sirion?"
"Do the mountain waters run dry, the cold flowing streams?" indicating that it was really relatively cool around the Mediterranean when Jeremiah lived around 600 BC.
In our days is no eternal snow on the mountains of Lebanon.
Extension of sea ice Pytheas travels
Around 310-300 BC the Greek explorer Pytheas traveled from Massalia (Marseille) along the shores of Western Europe.
He came to Scotland and Hebrides, where he saw the waves, which were "80 cubits high" (cubit is an ancient unit of length on 45.72 cm).
He sailed to the island of Thule, which was located 6 days and 6 nights sailing north of Berrice, which is assumed to be Shetland.
There is uncertainty about, whether Pytheas' Thule was the Faroe Islands, Iceland or western Norway.
The distance between Shetland and Faroe Islands is 150 nautical miles, and the distance between Shetland and Iceland is about 380 nautical miles.
On the journey to the Faroe Islands, he, therefore, should have kept a speed of 1 knot, which sounds pretty manageable, also for his time.
If Thule was Iceland, he should have kept a speed of 2.6 knots, which does not sound impossible with a good wind.
He describes Thule as an island located six days sailing north of Shetland, near the frozen sea.
There is no night at midsummer, he says, indicating that the location must be on the Arctic Circle and that he visited the island in the summer.
The frozen sea is one day's sailing north of the island, he says, which also indicates that the island must be Iceland, rather than the Faroe Islands.
However, in modern times the sea ice nearest Iceland in the summer is found north of Scorebysund on Greenland's east coast. the distance from Iceland to the north of Scoresbysund is more than 350 nautical miles.
Pytheas sailed maybe 2.6 knots, so it would have taken him almost 6 days and nights to get to the frozen sea - with the extension of sea ice in modern time.
But as he wrote that the sea ice was only a day's sailing to the north of Thule, we can conclude that the sea ice in the Arctic Ocean in the summer had much bigger extension in his time, 300 BC, than it has today.
Pytheas mentioned that the island was inhabited.
People lived on millet and other herbs and on fruits and roots, and where there were cereal and honey, they got their drink from it.
The country was rainy and lacked sunshine, he wrote.
This leads many to think that he, in fact, landed in Norway.
However, if the frozen sea was only one day's sailing away, it indicates only even bigger extension of sea ice.
TO BE CONTINUED ...
Between the Holocene optimum and the Roman Warm Period
Around 5,500 to 5,000 years before present occurred the Piora cold period, which is named after the Val Piora valley in Switzerland, which was the first place, where it was identified by using pollen analysis.
The more heat-loving trees as elm and linden became rarer and never again regained their dominant position in the woods.
There have been found indications of this cold period in both Alaska, the Andes in Colombia and in the mountains of Kenya (Lamb).
Precipitation in the Rajasthani desert Harappa and Mohenjodaro
In the Indus Valley, where today Rajastan's arid Thar desert is spreading, the cities of Harappa and Mohenjodaro flourished between 4,600 and 3,900 years before present.
When their civilization was at its peak, it covered an area, which was larger than the Nile valley and Mesopotamia combined.
The inhabitants cultivated wheat, barley, melons, dates and perhaps cotton.
On the savannah and along the now dry river lived elephants, rhinos and water buffaloes.
The annual rainfall is estimated to have been between 400 and 800 mm.
In the Arabian desert has also been found evidence of human habitation from about 5,000 years before present.
Not much is known about the Minoan warm period beyond, what can be gauged from cores from boreholes in the ice sheet.
That the climate really was warmer then may be derived from that in the Minoan warm period, which occurred during the bronze age, millet was grown in southern Scandinavia.
Today Millet is grown in tropical and subtropical regions, it is an important crop in Asia, Africa and in the southern U.S.
The average annual temperature in Mississippi and Alabama is about 10 degrees, which should be compared with today's average annual temperature in Denmark, which is 8 degrees.
So maybe the climate in the Minoan warm period, was about 2 degrees warmer than present in southern Scandinavia.
As you may know, Rome is said to have been founded by Romulus and Remus in 753 BC
The Roman historian Livy tells us that in the city's early history occurred a few severe winters when there was ice on the Tiber, and the snow stayed for many days.
Before the Roman warm period beech trees is said to have been growing in mountains around Rome.
Climate changes have always taken place, it is documented in the Bible.
Jeremiah 18:14 in the Old Testament says: "Does the snow of Lebanon leave the crags of Sirion?"
"Do the mountain waters run dry, the cold flowing streams?" indicating that it was really relatively cool around the Mediterranean when Jeremiah lived around 600 BC.
In our days is no eternal snow on the mountains of Lebanon.
Extension of sea ice Pytheas travels
Around 310-300 BC the Greek explorer Pytheas traveled from Massalia (Marseille) along the shores of Western Europe.
He came to Scotland and Hebrides, where he saw the waves, which were "80 cubits high" (cubit is an ancient unit of length on 45.72 cm).
He sailed to the island of Thule, which was located 6 days and 6 nights sailing north of Berrice, which is assumed to be Shetland.
There is uncertainty about, whether Pytheas' Thule was the Faroe Islands, Iceland or western Norway.
The distance between Shetland and Faroe Islands is 150 nautical miles, and the distance between Shetland and Iceland is about 380 nautical miles.
On the journey to the Faroe Islands, he, therefore, should have kept a speed of 1 knot, which sounds pretty manageable, also for his time.
If Thule was Iceland, he should have kept a speed of 2.6 knots, which does not sound impossible with a good wind.
He describes Thule as an island located six days sailing north of Shetland, near the frozen sea.
There is no night at midsummer, he says, indicating that the location must be on the Arctic Circle and that he visited the island in the summer.
The frozen sea is one day's sailing north of the island, he says, which also indicates that the island must be Iceland, rather than the Faroe Islands.
However, in modern times the sea ice nearest Iceland in the summer is found north of Scorebysund on Greenland's east coast. the distance from Iceland to the north of Scoresbysund is more than 350 nautical miles.
Pytheas sailed maybe 2.6 knots, so it would have taken him almost 6 days and nights to get to the frozen sea - with the extension of sea ice in modern time.
But as he wrote that the sea ice was only a day's sailing to the north of Thule, we can conclude that the sea ice in the Arctic Ocean in the summer had much bigger extension in his time, 300 BC, than it has today.
Pytheas mentioned that the island was inhabited.
People lived on millet and other herbs and on fruits and roots, and where there were cereal and honey, they got their drink from it.
The country was rainy and lacked sunshine, he wrote.
This leads many to think that he, in fact, landed in Norway.
However, if the frozen sea was only one day's sailing away, it indicates only even bigger extension of sea ice.
TO BE CONTINUED ...