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Post by thelivyjr »


There is no doubt that the most important changes which nature produces in the transparency of the atmosphere from time to time, over durations that directly concern us, are those due to variations in the amount of volcanic material present.

We shall return later in this chapter to effects on the atmosphere’s transparency produced by man's activities.

Massive volcanic explosions such as that of Mount St Helens in May 1980, pictured in fig. 111, put myriads of submicroscopic-sized rock particles and aerosol derived from sulphur dioxide into the stratosphere, where they are beyond the reach of the rain which washes such impurities out of the lower atmosphere.

The volcanic matter typically passes round the Earth in ten days to a few weeks, taking a different length of time at different heights owing to differences in the strength and sometimes also differences in the direction of the wind; these differences and diffusion processes, including convection and turbulence which transfer some of the material to somewhat higher and lower layers, gradually spread the material into an increasingly uniform veil which may cover the hemisphere concerned (or even the whole Earth) within about half a year.

The greater the height to which the exploded material is thrown by the eruption, the longer the veil will last.

The fall speeds of the minute particles are so small that they may take from twenty days to a year to fall one kilometre and are liable to stay for one to seven years, or more, in the stratosphere.

The effect of their partial interception of the solar (mainly shortwave) radiation, while the Earth’s outgoing (mainly long-wave) radiation passes nearly unhindered, is to warm the dust layer while at the Earth’s surface and in the lower atmosphere temperatures fall somewhat below what they otherwise would be.

The cooling, at its maximum in the first year, after various great eruptions reported in the past has been assessed (averaged over middle latitudes) at from 0.1 to around 1.0°C.

In 1783, when there were two very great eruptions — in Iceland and in Japan — in the same year, the combined effect may have been a cooling of the northern hemisphere by 1.3°C, gradually tailing away to zero over the following four or five years.

The Mount St Helens eruption of 1980 will probably not rank among the biggest eruptions in terms of stratospheric dust veils, in spite of the fact that one or two cubic kilometres of rock were blown into the air, because (unusually) a large proportion went in a nearly horizontal blast.

But it may be regarded as part of a significant global trend towards increased volcanic activity since about 1960 after a marked lull which in the northern hemisphere had lasted nearly fifty years.

On a scale which ranks volcanic dust veils in terms of the mass of material initially ejected and the duration and maximum spread of the veil, the great eruption of Krakatau in the East Indies in 1883 is ranked as 1000, and the total veil from various eruptions in the 1880s reached about 1500.

In 1902 a group of eruptions in the West Indies produced a new veil ranked as 1000, and this was renewed at least over the northern polar regions in 1912 by the great eruption of Katmai in Alaska.

After that there was no big injection of dust that seems to have affected the northern hemisphere until the eruption of Mount Agung in Bali in 1963 (dust veil index 800), which with eruptions of other volcanoes in the following years once more produced a veil rated at over 1000 by the late 1960s.

Two of the biggest eruptions in the early part of the nineteenth century had produced veils rated globally at 3000–4000 on the same scale, and it seems clear that any bunching in time of such great eruptions must produce significant coolings and related effects on the climate lasting for periods from some years to a decade or two.

Some prehistoric and early historic eruptions, such as that of Santorin in the Aegean in the time of Minoan Crete and Vesuvius in AD 79, can be assessed on the basis of surveys of the dust (and larger ejecta) deposited and still indentifiable.

Dust veil ratings from around 3000 to 10,000 seem probable for the Santorin eruption and between 1000 and 2000 for Vesuvius in AD 79.

The dust veil from the eruption of Öraefajökull in south Iceland in AD 1362 may be tentatively put at about 500, the lower rating arising partly because of the smaller extent of the globe affected by the spread of dust from high latitude eruptions.

One or two eruptions of Hekla in Iceland probably had a similar magnitude, e.g. in AD 1104 and about 750 BC.

A chronology of volcanic material, identified in the form of sulphuric acid in the year-layers of the Greenland ice-sheet by Professor W. Dansgaard of Copenhagen and his co-workers, C.U. Hammer and H.B. Clausen, shows a gratifying degree of agreement with the volcanic (global) dust veil chronology from AD 1500 referred to in these paragraphs.

(A correlation coefficient of 0.46, statistically significant at the 99.9 per cent confidence level, was obtained for the whole span of the dust veil index chronology, despite the fact that the dust veils from some parts of the world could not be expected to be fully represented in a deposit on the ice-sheet at latitude 71°N. Over the period 1770–1972, for which the dust veil chronology is presumably more reliable, being based on more nearly complete reporting, the correlation coefficent was 0.65.)

The measurements of the acid deposit in the Greenland ice have been carried as far as the ice-layer laid down in the year AD 553, and the comparison shown in fig. 112 between the successive half-century values of the acidity (little acidity upwards, much acidity downwards, in the diagram) and an index of northern hemisphere temperature shows an impressive parallelism.

It must surely be accepted that the variations of the amount of volcanic material carried in the atmosphere, and deposited by it, seem to have something to do with the climatic variations in the fourteen hundred years covered, even appearing as perhaps an important part of the causation of the Little Ice Age.

This thesis is supported by other approaches used by Bryson and Goodman of the University of Wisconsin, which indicate also that the cooling of the northern hemisphere since 1950 may be attributable to a doubling of the volcanic material in the northern hemisphere atmosphere over the same period.

Another volcanic chronology has studied the variations of eruptive activity over the hundred years since 1880 in different latitude zones and in the northern and southern hemispheres separately in terms of a simple classification of eruptions as great, moderate or small ash producers, and gives some weight to numerous moderate eruptions which were largely excluded by the dust veil index whose chronology we have discussed above.

There is a large measure of agreement between the two chronologies, but the new one reveals another feature which reinforces the apparent significance of volcanic dust for climate.

The fifty year lull in volcanic injections into the northern hemisphere stratosphere between 1912 and about 1960 was not matched in the southern hemisphere: in fact, the greatest peak of ash-producing volcanism in the southern hemisphere during the entire hundred-year span was between about 1925 and 1945.

And whereas there was a great warming of northern hemisphere climates during the fifty-year quiescence of the volcanoes after 1912, and particularly the great warming of the Arctic which took place between 1920 and 1940, southern hemisphere temperatures showed a dip during the 1930s (see fig. 91a).

There are some details which show quite clearly that volcanic variations were not the only cause of the climatic variations in these years.

For instance, the northern hemisphere cooling which set in about 1950 preceded any significant increase of volcanic activity; and the rising trend of temperatures which affected the southern hemisphere, as well as the northern hemisphere, for forty years after 1890 was not accompanied by a decrease of southern hemisphere volcanic activity (nor of the northern hemisphere volcanism before 1912).

Nevertheless, the evidence is strong that volcanic veils have played an important part in recent climatic history.

There have indeed been some studies — for example, a much longer, but less precisely dated, chronology of volcanic dust in the Antarctic ice-sheet — which seem to indicate greatly enhanced volcanic output during various main stages of the last major ice age.

But here the cause and effect relationship is by no means clear; and it is possible that the changes of stress on the Earth's crust, when enormous masses of water from the oceans were converted to ice on land, produced waves of volcanic activity.

Even if this be true, however, there may well have been a reaction — a ‘feed-back effect’ — of the dust veils in the atmosphere leading to a sharper cooling of the climate at the Earth's surface.

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Post by thelivyjr »


Much less can be said about the internal variations on time-scales from weeks to years in the heat economy, and the evolutions in the circulations, of atmosphere and oceans.

Most meteorologists believe it necessary at present to treat these in relation to longer-term forecasting as random in their occurrence.

There may nevertheless be some natural oscillation periods, such as one of thirty days (or very close to one month) which is prominent in the weather variations during the winter half of the year in middle latitudes of the northern hemisphere.

Hints have been found of associations with (a) various shorter-term cycles of solar activity, (b) variations in the tidal pull of the planets on the sun as their alignments change and which may have some effect on disturbances of the sun, and (c) cyclic variations of the combined tidal force of sun and moon acting upon the Earth and its atmosphere as well as on the oceans.

The varying activity of solar disturbance may itself be partly associated with the (predictable) tidal pull on the sun of the planets as their positions vary.

The likely period lengths are in many of these cases known but the correlations appear to be weak and unlikely to serve as a practicable basis for forecasting.

There may be an exception to this in the case of the complex of small wanderings of the Earth’s rotation axis (and hence of the poles) that are known collectively as the Chandler wobble.

This wobble is presumably related to readjustments of angular momentum (the momentum of spin) and of inertia between the solid Earth, and the fluid elements of its interior, and the atmosphere and oceans, at least partly under tidal forces.

The components of the wobble include an annual cycle of displacement of the poles by a few metres and oscillations of other period lengths ranging from about thirteen to fifteen months.

Several scientists in the United States and in Russia, notably I.V. Maksimov of the Main Geophysical Observatory, Leningrad, have been interested in the possible usefulness of the wobble in weather forecasting, since even such small displacements of the pole may produce enormously bigger displacements in the atmospheric circulation.

This is because of the effect of any momentum exchanges between the massive Earth and its thin atmospheric ‘skin’.

Lately Bryson and Starr in the United States have succeeded in resolving the wobble into discrete components, which facilitate prediction of it and seem to show useful associations with global weather development over some years ahead.

The hope is that this may open the way to some, at least partial, success in forecasting the weather season by season over periods from one to ten years ahead — a time-span of much practical importance for which it has hitherto seemed impossible to cater.

The naturally occurring changes in the surface of the Earth which affect the absorption of radiation and the flow of the winds and circulation of the oceans, and hence must alter the development of climate, are chiefly the very long-term changes associated with the drifting of continents and mountain-building over tens and hundreds of millions of years.

These do not concern us here.

Some changes, however, produced by the weather itself or the circulation of the oceans, or by accidents such as screening of the suns radiation by dense volcanic dust veils or blockage of certain channels by drifting polar sea ice, may have effects on the climate over a few months or a few years.

The greatest deviations of ocean surface temperature, amounting sometimes to 3°C over areas up to 1000 km across, which occur from time to time (a) in the tropics, as a result of changes in the amount of cold water upwelling (under the influence of the winds at continental coasts), and (b) in high latitudes, when an ocean current boundary is displaced, change the rate of heating of the overlying atmosphere.

The change in such cases is equivalent to a significant fraction of the solar heating available.

Comparable changes, with an even bigger immediate effect on prevailing temperatures, occur when the area of snow or ice is extended or reduced, particularly when extensions of these surfaces into middle latitudes are involved: in those latitudes in the absence of ice and snow the intake of solar radiation is substantial.

And the conversion of a desert or semi-desert area to a moist surface with grass cover, or the reverse change of savannah to desert, produces smaller but still significant changes of the heat absorption: the former increases it and the change to desert reduces it.

These changes seem to introduce self-perpetuating (or ‘positive feed-back’) tendencies, in that there is more convection and therefore tends to be more rainfall over the vegetation-covered than over the desert surface.

The effect of cloud cover in low latitudes is so great that over the Indian monsoon area in July there is actually a net loss of radiation - i.e. net outgoing radiation-from the Earth to space.

The effectiveness of extensive sea surface temperature anomalies of the scale mentioned above has been convincingly demonstrated both by theoretical modelling and observed correlations.

Thus, J. Namias has shown that the prevailingly cold weather of the 1960s and again the cold winters of the late 1970s over the eastern two-thirds of North America and over Europe were associated with a distortion of the circumpolar upper wind vortex producing outbreaks of cold polar surface winds, apparently induced by anomalous sea surface temperatures in the central part of the North Pacific Ocean.

A. Gilchrist and P.R. Rowntree of the United Kingdom Meteorological Office have shown that anomalously high sea surface temperatures in the tropical Atlantic near the Cape Verde Islands (latitude 17°N) tend to produce patterns of the atmospheric circulation which give cold winter weather in Europe.

Similar associations between anomalous warmth in the equatorial Pacific and cold winter weather over most of the United States were earlier demonstrated by theoretical modelling by Rowntree, following the observational studies of the late Professor Jakob Bjerknes.

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Post by thelivyjr »


We must now consider the range of effects, and possible effects, of human activities intruding upon the climatic regime.

The greatest change in the terrestrial environment so far produced by man is the clearing of the northern hemisphere’s temperate forest zone, which began on a small scale five thousand or more years ago, and its conversion to cultivation of the grasses which we use for grain crops and animal husbandry.

This must have increased the prevailing wind strengths but may not have had a great effect on the heat absorption of the lands in the latitudes concerned.

On the other hand, removal of the forest cover in low latitudes such as is now occurring in the Amazon basin, and either occurring or contemplated elsewhere, may be more serious: theoretical modelling studies suggest that the increase of surface albedo (the reflectivity of the Earth's surface) in this case would be likely to reduce heat absorption, and hence reduce convection and rainfall significantly.

About 34 per cent of the equatorial zone between latitudes 5°N and 5°S is at present covered by tropical rain-forest.

And it is estimated that complete deforestation might cool the Earth by 0.2–0.3°C and by a larger fraction of a degree in parts of the zone concerned.

One would also expect evaporation and rainfall to be reduced by several per cent in the tropical zone itself, possibly by 10 per cent in some part of the zone, with a smaller net decrease (about 1 per cent in the study reported) for the Earth as a whole.

Afforestation and deforestation on a merely local scale, as within a single river catchment, are unlikely to have any significant effect on climate save within the area of the forest itself where moisture is retained within the forest canopy.

Occasional exceptions may occur on showery and thundery days with light winds, such that the moisture is recycled and precipitated again within the same general region: but the general residence time of moisture in the atmosphere, reckoned to be about ten days, means that most rain deposits moisture far — even thousands of kilometres away — from the region where it was evaporated.

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Post by thelivyjr »


Deliberate attempts to modify the climate began on a local scale with the planting of shelter belts of trees, to reduce wind speeds and protect light soils from blowing away, among the agricultural improvements pioneered in Norfolk in the eighteenth century.

That practice has now been successfully introduced in many other places, notably near the coasts of Denmark and north Norway and on the plains of Russia.

Where modern irrigation projects have sought to modify the conditions for agriculture, if not the climate itself, over larger areas it has often been hoped that a general increase of moisture in the atmosphere over the artificially watered ground might result, particularly from the evaporation from reservoirs.

But in naturally arid regions, particularly in the case of the Aswan dam which is more or less at the axis of the Saharan desert zone, the evaporated moisture is likely to be dispersed in the atmosphere and carried far away by the winds, so representing a loss of water from the region.

Even in the lower Volga basin, in latitudes near 50°N, it seems that the extensive tapping off of the river water for irrigation has so increased the loss by evaporation as to reduce the flow into the Caspian Sea, contributing to the lowering of its level in recent decades.

The growth of population and industry in Soviet central Asia, and the need to cultivate the plains of that region so far as possible to grow grains and cotton, etc., have already for several decades placed excessive demands on the water resources of the region.

The levels of the lakes and rivers, and of the water table in the subsoil, have been falling.

The Aral Sea is liable to dry up completely and disappear by about the end of the century; and the Caspian Sea is becoming so much more saline, as its water level falls, that the supply of sturgeon — and hence the caviar — for which it is famed is threatened.

The Soviet Union has therefore been driven to consider what could be done to supplement the natural water supply.

The great rivers of Siberia and some in the northern part of European Russia flow north into the Arctic Ocean, and the needs of central Asia have given rise to a grandiose scheme which the Soviet authorities have often described as ‘reversing the flow of the rivers’.

What is contemplated is illustrated in fig. 113.

The full project would not only provide irrigation for the areas opened up for cultivation under Kruschev’s ‘virgin lands’ scheme in the 1950s and since, but could also include draining of the marshes in northwest Siberia.

The scheme has been under consideration for half a century and would undoubtedly be a triumph for what twentieth-century engineering can do, including the use of atomic power to blast rocky barriers away, provided that the side-effects on climate did not turn out to be serious.

This aspect has been the subject of much research and has caused hesitation.

Later reports that work has begun on the scheme suggest that a good deal of caution will be exercised — beginning, at least, by tapping off a very limited proportion of the flow of the rivers for pumping south to the dry regions near the Caspian and Aral Seas.

The dangers foreseen in the project outlined arise from the fact that it is the fresh water from the rivers concerned which forms a large proportion of the thin layer of low-salinity water that covers the surface of the Arctic Ocean.

The river Yenesei alone provides on average over 10 per cent of the total run-off of the northern continents into the Arctic Ocean.

It is in this layer that the Arctic sea ice is formed.

And if the layer were removed or seriously diminished, so that much of the polar sea became a salt-water ocean from the surface down, ice might not form on it — for the same reason that the Norwegian Sea and most of the Barents Sea remain open the year around.

When cooled, water with the salinity normal in the world's oceans becomes denser and does not reach its maximum density until near its freezing point, at about −2 °C.

By contrast, fresh water is densest at about +4°C (39°F), so that when the surface is cooled below that temperature the coldest water stays on top and at 0 °C ice is formed.

The salt water of the deep oceans, when cooled at the surface, goes into patterns of convection, the coldest and densest portions gradually sinking to the depths.

Hence the whole ocean — or a great depth of it — would need to be cooled to near its freezing point before ice formed and remained on its surface.

In practice, inhomogeneities and salinity (and density) differences might allow some ice to form, as occurs seasonally on the Southern Ocean around Antarctica; but it would be easily disturbed and destroyed by vertical mixing in rough weather and would doubtless be limited in extent to the colder and shallower regions near coasts.

It would probably always be much thinner than now, even in those regions where it still existed, and therefore likely to disappear in summer and be patchy in the transition seasons.

Altogether the climate of the areas of the Arctic converted into an open ocean north of 70–75°N would be, on average over the year, some 20–25°C warmer than now.

This huge change would be liable to shift the main thermal gradient of the hemisphere, and so alter the patterns of the large-scale wind circulation as to send the rain-and snow-giving cyclonic activity on new tracks, predominantly into the Arctic and sub-Arctic regions.

The consequence might well be to reduce the rainfall both in central Asia — the region designed to benefit from the engineering scheme — and, to a less extent, also over most of Europe.

The scheme discussed in these paragraphs is an example of how man might be able — whether inadvertently or intentionally — to alter the climate in a big way by disturbing the global regime at some point where it is delicately balanced.

In the case of most schemes, however, which have been suggested for deliberately modifying the climate, examination suggests that the global regime is extremely well buffered against upsets which might be caused by the relatively puny amounts of energy which are even now at human disposal.

The energy released by a one megaton nuclear explosion is of the order of one-hundredth of that disposed of by a single cyclone/depression over an hour or so or by a moderate-sized volcanic explosion.

(The indirect effects of nuclear explosions at the ground through the screening off of solar radiation by the dust injected into the stratosphere, as after volcanic eruptions, are likely to be the only significant effects on weather far from the scene.)

Around the 1950s and early 1960s there was some discussion, and in Moscow a conference was held, on the possibilities of modifying world climate deliberately with aims such as to increase as far as possible the total cultivable area of the Earth.

Since that time there has been a change of emphasis, probably due to a clearer understanding of the fact that the growth of population has already made the world community much more vulnerable to the dislocations that must result from any climatic shift and the wide-ranging year-to-year variations which would doubtless occur in the course of it.

Nowadays, the chief concern is over the possibility of a large-scale shift of world climate being brought about inadvertently, as a side-effect of human activities and their increasing scale.

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Post by thelivyjr »


The main worry about the impact that human activities are likely to have is related to the increase of the seemingly innocuous gas, carbon dioxide, in the global environment.

Carbon dioxide (CO2) is the end-product of the burning not only of wood but of all fossil fuels — coal, gas, oil, etc.

It is a very minor constituent of the atmosphere, only about 350 parts per million (ppm) by volume, but it is important because of its effects on the radiation passing through the atmosphere.

This applies particularly to the radiant energy going out from the Earth, because CO2 is not transparent to radiation at some of the long wavelengths most strongly represented in the emission from bodies at the temperatures prevailing at the Earth’s surface and in the atmosphere.

Hence, this radiation is absorbed on its way upward from the Earth by the CO2 in the atmosphere and re-radiated in all directions, so partly back to the Earth.

As a result of this, and the similar action of the water vapour in the atmosphere on radiation at a range of wave-lengths partly overlapping those which CO2 absorbs, the Earth's surface climate is some 35–40°C warmer than would be expected on a planet at this distance from the sun.

The action is reminiscent of a greenhouse, and the warming is sometimes spoken of as the greenhouse effect of carbon dioxide.

Similarly, the temperatures prevailing on other planets with CO2 in their atmospheres, notably Venus whose atmosphere consists largely of carbon dioxide, and also Mars, seem consistent with the magnitude of the expected carbon dioxide warming effect.

One would expect that an increase in the amount of CO2 in the Earth’s atmosphere would increase the greenhouse effect.

There is no doubt, from actual measurements, that the amount of carbon dioxide in the atmosphere has been increasing and that, with the increased rate of burning of fossil fuels,the rate of increase has become greater.

In 1880–90 the CO2 seems to have been around 290 ppm in the atmosphere.

Some who have studied the subject believe that before the massive clearing of forests for agriculture in the nineteenth century, the proportion (owing to assimilation of carbon from atmospheric CO2 by the vegetation) may have been as low as 270 ppm.

By 1950 the proportion had risen to about 310–15 ppm and by 1980 to 335–40 ppm.

These figures mean that the proportion of carbon dioxide in the atmosphere had increased 9 per cent by 1950 and 15–17 per cent by 1980 above the 1890 level, and over the last 150 years the increase may be as much as 26 per cent.

It has been calculated that this carbon-enriched atmosphere may have contributed to richer crop yields to the extent of a few per cent.

Carefully calculated estimates have been made as to the proportion of the extra CO2 generated by man's fuel-burning which stays in the atmosphere, how much is absorbed by the plant world (or elsewhere in the biosphere), and how much is dissolved in the oceans, where it may ultimately end up fixed as an increase of the calcium carbonate deposit on the ocean floor.

From these studies it seems that about half of the output of CO2 is remaining in the atmosphere, though the proportion varies slightly, as the oceans give up CO2 to the atmosphere when they warm up either seasonally or over longer periods and absorb more when they become cooler.

The most generally accepted calculations of the warming effect of increased carbon dioxide (through its increasing the effectiveness of the long-wave radiation trap) indicate that overall average temperatures at the surface of the globe should rise by about 1.9°C if the carbon dioxide concentration were doubled.

A more complete coverage of the expected scale of temperature change from the 1920 average (when CO2 was 300 ppm) for a wide range of concentrations is given in fig. 114.

There have been various other estimates of the warming of world climate to be expected from a doubling of the atmospheric carbon dioxide, since the case was first thoroughly argued through from physical principles by G.N. Plass in the 1950s.

Plass suggested a figure of 3.6°C temperature rise for a doubling of the CO2 and a 3.8°C fall if the CO2 were halved: on this basis it seemed that the entire warming of world climate (by about 1°C) since the industrial revolution in the eighteenth and nineteenth centuries might be explained in this way.

The CO2 warming thesis has always had a specially strong appeal to physical meteorologists as one element in the complex problems of atmospheric science which should be entirely predictable, since the effects of CO2 on radiation are clearly demonstrable and well understood in theory.

Nevertheless little was heard about the thesis in the 1960s, when it was discovered that world temperature was falling despite the more rapid increase of CO2 in the atmosphere than ever before.

Others professionally concerned with the CO2 problem suggested that the warming effect might be no more than one-tenth to one-fifteenth of Plass’s figure; but recent studies with the most sophisticated models, which not only allow for atmospheric transport of the CO2 and heat about the world but take at least some formal account of exchanges with the top layer of the ocean and possible effects on cloud cover, have pointed once more to a greater warming, between 2.0 and 3.5°C, for a doubling of the atmospheric carbon dioxide to 600 ppm.

These figures imply a steeper curve than that shown here in fig. 114; but they continue to be treated with scepticism by some climatologists and atmospheric modellers, because it has not yet been possible to include in a realistic dynamical way in the theoretical models the exchanges with the ocean (and to deeper layers within the ocean) or the effects on cloud and atmospheric humidity.

It is pointed out that a 1 per cent increase in mean cloudiness over the Earth, if such an increase occurred, would probably completely counteract the proposed CO2 warming effect — at least as regards the net effect over the globe.

Such an increase of cloud might come about through a real warming, of the ocean surface in low latitudes, leading to increased evaporation and input of moisture into the atmosphere, and hence more cloud over middle latitudes, producing lower temperatures than before there.

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Post by thelivyjr »


Besides carbon dioxide there are other substances which human activities are putting into the atmosphere, mostly in increasing amounts, which must also be supposed to affect the radiation balance.

The most discussed of these items has been the solid particulates in the form of dust and smoke, the latter from industrial and domestic fires and from agriculture, particularly the primitive slash and burn agriculture of the tropics.

At the same time the greatly increased area of the world that is tilled, often large-scale tillage of open prairies by tractor-drawn ploughs and other implements, seems to have increased the amount of dust in the lower atmosphere.

Atmospheric turbidity measurements at Washington, DC, in the United States and on the heights of the Alps at Davos agree in indicating substantial increases (by 50–80 per cent) from the beginning of this century.

The dust deposit measured in the ice-layers of glaciers on the high Caucasus shows an even greater increase but suggests that little change had occurred from the eighteenth century until around 1930.

Some have thought that the increase of dust haze should cool the Earth's surface (through interfering more with the incoming solar beam than with the Earth’s outgoing radiation) and may cancel the effect of carbon dioxide warming entirely.

Professor R.A. Bryson has likened the effect to that of volcanic dust — although the latter is in the stratosphere in those cases where its effect lasts for more than a few weeks, whereas the dust that man produces is maintained in the lower atmosphere — and has written of the ‘human volcano’.

One calculation suggested that increasing the suspended particulate matter by a factor of four should lower the Earth’s mean surface temperature by 3.5°C, but later modelling studies show that the effect must vary greatly with the prevailing size and absorptive properties of the particles and may in some cases be in the direction of warming.

Smoke trails, or ‘plumes’ as they are more usually called, from industrial or thickly inhabited areas commonly remain identifiable and reduce visibility over long distances down-wind from their source, particularly in air that is cool near the ground and warmer above so that convection is checked.

The industrial haze from the Ruhr and Belgium has long been liable to produce murky conditions in England in light easterly winds in winter (though less in recent decades with more efficient firing of domestic and industrial hearths, as also applies to the haze generated in British centres of population).

At the time of the great Fire of London in 1666 John Locke observed in Oxford some 80 km away that the unusual colour of the air ‘made the sunbeams of a strange red dim light’, later noting that they had heard nothing of the fire at that point, though it now seemed that it must have been due to the smoke.

And in the late sixteenth century the smoke from large-scale moorland fires in England was said to have ruined French vine crops in the bud — evidently indicating northerly winds in spring.

Other substances put into the atmosphere, which seem important in the same connection, are (a) the nitrous oxide produced by breakdown of nitrogen fertilizers in the soil and (b) methane and chlorofluoromethanes (more widely known as ‘freons’) — a range of chemical substances in which one or more chlorine or fluorine atoms replace some of the hydrogen atoms in methane — used in aerosol sprays and also by refrigerators.

If they get into the stratosphere, the latter substances, besides selectively absorbing long-wave radiation like carbon dioxide, destroy some of the ozone there.

The stratospheric ozone is important because it absorbs solar short-wave radiation, including some wave-lengths which would have lethal effects on living organisms, and thereby warms the stratosphere at the expense of the Earths surface and lower atmosphere.

The ozone therefore has a cooling effect so far as the surface climate is concerned.

There may also be significant contributions to the human disturbance of the radiation balance from sulphur dioxide (which is also harmful to human health, to vegetation and buildings, etc.), hydrogen sulphide, carbon monoxide and ammonia, even though the usual residence time of these in the atmosphere before removal by chemical action is much shorter.

The quantities of water vapour added by man (except very locally) are trivial compared to those naturally occurring.

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Post by thelivyjr »


The net effect of the increase of all these substances in the atmosphere as a result of man's activities is apparently in the direction of warming, and may in toto add about 50 per cent to the CO2 effect.

Flohn suggests that the simplest way of dealing with all these intrusions into the atmosphere is to consider a ‘virtual CO2 concentration’, which should have the same theoretical effect on temperature as the combined greenhouse effect of all the substances actually involved.

The reason most commonly advanced for why the carbon dioxide, or combined greenhouse effect, warming is not obvious at the present time is that it is not yet big enough to go beyond the range of the climatic fluctuations — sometimes, of course, in the opposite direction — produced by natural causes.

This range of natural climatic fluctuation is sometimes described as the ‘noise level’, which must of course make it difficult to identify any new trend — whether or not the trend were produced by man’s impact — before it had already reached a substantial amplitude.

Efforts have therefore been made to decide how soon the (assumed) further increase of carbon dioxide will produce a warming too strong to be offset or obscured by the natural variability of climate.

In such writing the natural variability is dismissed as unforecastable and therefore to be treated as random.

Those putting forward this view of the matter have taken +1°C as the approximate range of variation of the long-term temperature average produced by natural causes in the post-glacial world.

In consequence of this, they expect the warming by carbon dioxide, combined with the other substances contributing to an intensification of the greenhouse effect, to gain the upper hand and ‘swamp’ all other elements of climatic variation from the end of this century onwards and possibly from the 1980s on.

This view was strongly put in a statement approved by the executive committee of the World Meteorological Organization in 1976 (reported in The Times, London, 22 June 1976).

There is a fallacy in this part of the case, however, since it is impossible to define a figure for the range of natural variation of climate which is meaningful in this connection.

The record of prevailing temperatures, whether over the past few centuries or over the much longer-term record of ice ages and interglacial periods, shows that the range of variation is itself subject to variation.

We know, both from early thermometer records and from the indirect indications of tree ring sequences in Europe, that in the Little Ice Age climate, specifically in its later stages for some decades around AD 1700 and again between about 1760 and 1850 or later, the year-to-year variability (measured by the standard deviation) of these items was from 30 to 60 per cent greater than in the earlier twentieth century.

There are similar indications from the European tree ring studies regarding the last decades of the warmer climate of the high Middle Ages between AD 1280 and 1350.

And it is clear from isotope studies of the Greenland ice and other evidence that some much sharper changes took place in the later part of the last warm interglacial period.

One surely implausible suggestion that has been put forward on the basis of serious scientific arguments is that the undoubted strong warming that took place in high southern latitudes, south of about 45°S, between the 1950s and 1970s (amounting to rather more than 1°C in the overall average for the Antarctic south of 60°S) may be the first direct sign heralding the dominance of carbon dioxide warming.

The argument that the effect should be first noticed there, so far from all the manmade carbon dioxide sources, depends partly on heat storage and transfer by the oceans and partly on the freedom of high southern latitudes from the (increasing) contamination of the lower atmosphere by dust.

The authors of this suggestion take no cognizance of the evidence that high southern latitudes have a record of a partly antiphase relationship to the temperature variations affecting the rest of the world and enjoyed somewhat milder conditions during some of the sharpest phases of the recent Little Ice Age.

We shall return to some consideration of the distribution of temperature changes over the world, and their climatic consequences, in the next chapter.

There are other serious enigmas and difficulties which remain to be sorted out before we can be sure that the thermal effect of increasing carbon dioxide on the passage of radiation of different wave-lengths through the atmosphere, which appears straightforward in theory, should emerge clearly in the complexity of the world environment.

The effects involved in the oceans, and in the atmospheric moisture content and cloudiness, need further observation and study.

And proper allowance needs to be made for the reactions of the biosphere, since vegetation (including the plant life in the sea) growing more luxuriantly in a carbon-rich atmosphere may change colour sufficiently to affect the reflection and absorption of solar radiation.

It can be deduced that there was a much bigger change of the atmosphere's CO2 content far back in the geological past between the conditions before the development of the first vegetation cover and afterwards, but neither in this case nor with the later changes of vegetation extent does there seem to be the expected correlation with the development of climatic changes.

In theory the presumed changes of CO2 should have tended to stabilize the respective climatic regimes.

During the long warm periods of geological time, when there was little or no polar ice, the oceans must have had a smaller capacity to hold carbon dioxide dissolved and so the atmosphere should have been richer in CO2.

And during the ice ages the colder seas could dissolve so much more CO2 that its proportion in the atmosphere may have been much reduced.

A figure of 200 ppm has been suggested for the time of the ice age climax.

Contrary effects would, however, be implied by the changes of vegetation extent between warm periods and ice ages.

The most successful mathematical simulation of the variation of world temperature since AD 1600, and more specifically over the last hundred years (as in fig. 91), has been by an equation involving just three variables:

1 an index of the amount of volcanic material in the atmosphere;

2 warming latterly introduced, and increasing, through the continual addition of carbon dioxide to the atmosphere through the burning of fossil fuels;

and 3 an index of solar disturbance.

The fit was improved by adjusting the equation so as to double the effect of volcanic dust.

In a preliminary draft of their work the authors of the equation added a caution, which could apply equally to much more elaborate theoretical modelling work: ‘We are hesitant to try to improve the fit of our calculations to the observations by ‘tuning’ the model…."

"With so many free parameters to vary one could fit almost anything to anything…’.

They also punctiliously added that they had played down the computation performed of a correlation coefficient between their results and the four hundred year record of global surface temperatures used because of uncertainty about the reliability of that record.

The findings of Dansgaard and Hammer, and of Bryson and his co-workers, reported earlier in this chapter seem to reinforce the lesson of Schneider's experience and suggest that the effect of volcanic matter in the stratosphere in cooling the surface climate may bulk larger — and possibly a good deal larger — vis-a-à-vis the carbon dioxide warming effect than is commonly assumed today.

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Post by thelivyjr »


This chapter would not be complete without some further notice of man's activities as a cause of unintentional modification of climate on a local scale, particularly in cities and industrial areas and down-wind from them, and also in enclosed valleys and in waters with restricted circulation and outlet.

The artificial temperature rise in cities and in enclosed waters has been mentioned in earlier chapters.

These effects are sometimes referred to as urbanization and thermal pollution.

The artificially maintained warmth of some coastal inlets and backwaters near electric power stations and oil refineries, or other industrial complexes, may be able to support an exotic fauna and flora.

It was reported some years ago that a warm-water species of crayfish had established itself in Southampton Water, near the Fawley oil refinery, presumably introduced in bilge discarded by ocean liners approaching the port.

Despite some controversy it seems established that ‘urban heat islands’, the artificially warmed central parts of cities, tend to increase the activity of convection clouds, showers and thunderstorms over them and some way downwind from them.

This is liable to produce an increase of up to 10 per cent in the average yearly rainfall totals in the part most affected in big cities.

These conditions, and the relative freedom from frost and shelter from strong winds, may facilitate the cultivation of exotic plants in urban environments.

Perhaps the most important local effects produced by the activity in towns and industrial areas, and in a few other places (e.g. motorways, railways), are those due to smoke, steam and chemical pollution.

In this class perhaps the most widespread damage has been caused (to human health and to buildings and other property — cars, etc.) by sulphurous gases and sulphuric acid in the atmosphere, although in some places carbon monoxide, ozone and other gases, and in rural areas the substances in crop sprays and nitrogenous fertilizers, may also have serious effects.

Buildings and machines as well as human lungs are subject to corrosion and decay caused by these chemicals in the atmosphere.

Because lichens are particularly sensitive, their growth or absence may serve as an indicator of the cleanness of the air.

P. Brimblecombe of the School of Environmental Sciences in the University of East Anglia has traced the history of air pollution in London since the thirteenth century, and to some extent in other European cities, and of public attitudes to it, in a series of publications.

It is probable indeed that the smoke pollution and smells produced by industries such as the tanning of leather, pottery and lime production caused local complaint even in much earlier times in places where these industries were carried on in light winds and sheltered areas in and near towns, particularly where inversions of the usual vertical lapse of temperature with height developed in cold winter weather and prevented the escape and dispersal of the pollution upward into the atmosphere.

Such situations probably sometimes affected the choice of sites for industry, and the growth of the urban settlements near by, from ancient times.

Brimblecombe reports that coal was introduced to London for lime burning and smelting soon after AD 1200; and the results, together with the sewage problem associated with the building of privies over ditches and gutters, soon gave rise to unbearable stenches and many complaints.

In 1257 King Henry III’s queen was among the complainers, and from that time on commissions were appointed to consider the problem.

Matters became worse in the time of Queen Elizabeth I with the further introduction of coal, initially smoky Tyneside coals brought by sea from Newcastle (and therefore known as ‘sea-coals’), when wood was becoming difficult to secure, for domestic fires.

At first nice people refused to enter rooms where coal had been burnt.

And during the course of the seventeenth century the incidence of rickets, a children’s disease associated with deficiency of sunlight, increased sharply.

This disease ultimately became so rife that a survey in Leeds in 1902 found that half the children in the poorest districts were suffering from it.

And a similar incidence of it was characteristic in other cities of industrialized Europe.

But it was inevitable that coal should replace wood as fuel.

Much more complaint arose and a smokeless stove was invented before the seventeenth century was out, but nothing much seems to have been done about it.

Sir Christopher Wren’s new St Paul’s Cathedral, built in the last part of the century after the fire of London in 1666, is said to have been badly soiled already before completion.

The hanging of tapestries on the walls of rooms was largely given up by the early eighteenth century because they became so dirty and spotted, as reported in 1658 by Sir Kenelme Digby who became one of the early Fellows of the Royal Society.

More interestingly in the matters with which we are basically concerned, Brimblecombe has found it possible to illustrate the long — continued increase of pollution in the air over London (and neighbouring parts of the most densely inhabited and increasingly industrialized Europe) from a progressive change in the colour of the background skies painted by landscape painters, changing from the early dominance of blue to an increasing dominance of pinks and muddy shades of yellow-brown.

London fogs became widely known from this colour as ‘pea-soupers’.

Visibility at its worst sometimes fell so low that it was difficult for a person walking to see his feet.

And the darkness entailed using lamps throughout the day.

Dickens called these fogs ‘London particular’.

For as long as nothing was done to improve the efficiency of hearths and furnaces, right up to the present century, the growth of pollution of the city air was neatly paralleled — and could effectively be measured by — the increase of coal consumption.

Whatever measure we study, whether from the evidence of artists or the frequency of reports of fog or damage to buildings and measurement of the soot deposit on them, the pollution of London’s air was increasing all the time from before 1600 to about the beginning of the present century, the increases being most rapid in the seventeenth century and again after 1800.

Scientists at least from the time of Benjamin Franklin onwards pointed out that smoke from chimneys was unburnt fuel going out into the air to waste.

And smoke abatement legislation was first proposed in 1843 by a committee which reported to the British government, in the Mackinnon Report, but was not acted upon.

Improvement began in London and elsewhere with the adoption of more efficient furnaces in industry from early in the twentieth century, but the pollution from house fires continued.

It had earned Edinburgh the name of Auld Reekie (meaning the old smoky one).

Under the dark skies and surrounded by the smoke-soiled grasses, trees and buildings of the most industrialized areas of England (fig. 115) continental Europe and North America, some species of moth whose survival during the day depends on camouflage developed all-black forms.

(This melanism, first investigated in England, is a remarkable example of quick biological adaptation by the processes of mutation and selection.)

Substantial improvement was achieved in many more centres in Britain by the Clean Air Act, which at last became law in 1956.

Since that time the amounts of sunshine registered in British cities have increased.

(Rickets has now become much rarer, partly through advances in medical science but probably largely through better living conditions, including healthier air in cities.)

But the Act was against visible smoke, and the noxious fumes of sulphur dioxide (SO2) harmful both to lungs and to the stonework of buildings have continued.

These have been heavily implicated in several ‘smog’ disasters, in Belgium and the United States as well as in London, when in fogs formed under an inversion of temperature in winter, and with a lack of wind, sulphur dioxide concentration rose to lethal levels.

In the great London smog in December 1952 deaths in the Greater London district rose from 2062 in the week ended 6 December to 4703 in the following week.

Deaths from bronchitis and pneumonia showed a sevenfold increase.

There had been similar occurrences in ‘Cattle Show week’, 7–13 December, in 1873 and again in 1880, 1892 and 1948; and there was a recurrence in December 1962 which was somewhat less serious, perhaps already thanks to the new Clean Air legislation.

A similar great fog, ‘with a very sensible effect on the eyes’ and an acrid smell, extending all around London between 27 December 1813 and 2 January 1814 is recorded in the Annals of Philosophy.

If the situation has improved in London and Britain’s industrial areas thanks to legislation and the independent actions of industry and house-holders at last to burn fuel more efficiently, and elsewhere in the advanced countries in the temperate zone by similar moves, other pollution problems remain and not all are localized.

The sulphur dioxide from industrial and domestic chimneys continues as a threat to life, and seems to have increased.

Measurements of SO2 concentration in the air in Epping Forest (about 30 km northeast of London) show an increase from an average of 30 micrograms per cubic metre in 1784–96 to about 60 of the same units a century later, from 60 to 70 in 1909–19 and between 70 and 120 around 1970.

Not only human lungs are damaged, particularly in the old, but the animal world suffers and the leaves of vegetation.

For many years now there has been alarm and complaint in Scandinavia at the increasing acidity of the lakes and rivers there, and decline of the valued fish stocks, because of sulphur dioxide whose origin is attributed to the industrial areas and electrical power stations of Britain, the Benelux countries and Germany.

Los Angeles, despite the sunny warm climate general in California, has its own special smog problem.

The cool air off the sea tends to underlie warmer air overhead, and the inversion of temperature stops convection which might disperse the pollutants put into the city air; the situation is particularly common and at its worst in the warm seasons, summer and autumn.

The biggest contribution to the pollution seems to be the exhaust gases of motor vehicles, though industry is also implicated.

Concentrations of noxious fumes enough to make eyes smart are common, and there has been damage to vegetation a considerable distance inland.

A warning system and special controls on days of high pollution have been instituted.

New experiences of local pollution, sometimes of a serious order, arise in newly industrialized areas in the Third World, especially in enclosed valleys (fig. 116) and still air situations.

Even on camps and outstations, expedition sites and airfields established in the Arctic and Antarctic, smoke concentrations on some cold days with strong inversions of temperature may halt some activities.

At these places also the moisture put into the very cold air by aircraft and other vehicle exhausts on the ground may produce dense fog — a case of pollution by excess water in air which becomes saturated at very low water vapour concentrations.

Similarly fogs are occasionally formed or thickened by moisture from vehicle exhausts along motorways and from steam engines on railways in temperate latitudes in winter.

And it is a not uncommon sight in still, fair winter weather to see a layer of low stratus cloud formed by the steam from industrial cooling towers.

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Post by thelivyjr »



When we come to consider the possibilities of forecasting weather and climate to guide our planning for the future, we must take account of many different timescales.

The processes involved in the short and long-term developments, and the influences chiefly at work in each, the amount of detail which we may be able to foresee, and the degree of reliability attainable, are so different for each timescale that different handling methods and different ways of stating our conclusions are inevitable.

The first daily weather maps had been drawn by H.W.Brandes in 1820 from the observational data for 1783 then available after thirty-seven years!

It was the invention of the electric telegraph that by 1850 first made it possible to track the movement of storms before they arrived at the area of interest, and so made gale warnings and daily weather forecasts possible.

By the late nineteenth century these had attained a degree of success not altogether dissimilar from todays, though the recognition of fronts and the characteristics of different air-masses in the 1920s and 1930s introduced some details that were not understood earlier.

As upper air observations from balloon and aircraft became increasingly available, so it became possible to include adequate details of the upper winds and cloud conditions some hours ahead in forecasts for flying.

But it was the development of instrumentation capable of reporting the winds and temperatures in the upper air above the clouds and in all weathers — balloons carrying automatic radio—sounding apparatus and tracked by radar — from about 1940 onwards that first brought the circumpolar vortex under daily survey.

Up to this point forecasting had been more or less limited to plotting the travel and development of individual weather systems from their first appearance on the surface map day by day, steered by the winds aloft (as indicated by the surface winds in the warm sectors of the frontal cyclones) to their ultimate decay or absorption in another system.

Only when a slow-moving anticyclone settled over an area could ‘outlooks’ for two to three days fine weather ahead — e.g. for hay making and harvesting — be issued with satisfactory reliability.

But once the circumpolar vortex, and soon afterwards the jet stream, had been recognized, the principles governing the development and locus of formation of new weather systems — i.e. individual travelling cyclones and anticyclones on the surface weather map — could be better formulated and in ways adapted to the computer age and the numerically calculated forecasts for one, two, three and more days ahead.

It is in this realm that the main advance of daily weather forecasting since the 1930–50 period has come, in the greatly improved ability to indicate the weather development over several days ahead.

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Post by thelivyjr »


This type of forecasting, in all the detail that handling of the individual weather systems can give, runs into its ultimate limit as the errors resulting from any weaknesses of theory and coarseness of the specification of the situation existing at the outset on day zero, by observations at a necessarily limited network of points, build up when one computes the situation at successive intervals ahead.

Moreover, the atmospheric circulation uses up its existing store of energy in about five days, and the heating patterns by which the store is continually renewed themselves depend upon the weather that the circulation itself has produced.

Generally, it is found that the time taken for errors in the forecast situation to become doubled is from three to eight days.

And so at some number of days ahead — at present it looks like being on average around five to ten days ahead — the forecast map gradually ceases to bear a useful resemblance to the situation which actually emerges.

Forecasts for longer periods of the order of a season ahead depend on trying to spot by one technique or another the broad characteristics of that season, determined by the prevailing steering of the warm and cold surface winds and of the weather systems by the lay-out of the patterns of the circumpolar vortex.

This leads to statements about the probable departure of prevailing temperatures from the long-term average and about rainfall totals, rain or snow frequency, and storminess in relation to what is normally expected, and so on.

Because of the sharp local differences of rain and snowfall caused by most terrains, forecasts of these items are best expressed as a percentage (or range of percentages) of the long-term average.

At most only the boldest features of any time sequence of events within the season are likely to be specified, either on the basis of the gradual attainment of a climax of the regime characterizing that season and its subsequent weakening, or through recognition of some (often recurring) oscillation within the season.

Most success has been achieved by statistical approaches based on understanding (or perhaps just successful hypotheses) regarding the physical controls of the seasonal development of the atmospheric circulation in the particular year and those that determine the average course of the general run of years.

Rules have been recognized regarding developments associated with long-lasting anomalies of temperature in sequels to various developments of sea ice and snow cover distribution, to the formation of volcanic dust veils, and changes of solar activity, and so on.

Mistakes have probably been made through some ‘blind’ use of statistical associations not linked to correctly recognized physical processes, and through failing to recognize when such rules were merely duplicating each other (and so adding no weight to the argument for a particular forecast).

There seems to be room for the development of further statistical rules guided by the findings of theoretical modelling, but based, of course, on the use of observations from the real world.

Some help has been obtained from recognizing that the year tends to divide itself up into natural seasons, within which long spells of set weather type often prevail: towards the end of each natural season the controlling pattern of the circumpolar upper winds adjusted to the particular global heating pattern tends to break down, and the long spell of weather ends with it.

It is in high latitudes that the dates of the year which mark off these seasons seem to be most nearly fixed.

(With the declining sun in autumn within the Arctic circle nothing can stop the general freeze-up except a south wind, which cannot possibly occur everywhere.)

In middle latitudes there is rather more variation of the critical dates from year to year, but alike in the Soviet Union and at the Atlantic fringe of Europe it is recognized that there is a season from around the beginning of July through most of August when a persistent weather character tends to prevail.

Similarly, long spells of one character or another often develop within the winter season after Christmas or New Year until a change of pattern some time in mid-February or March.

The preceding late autumn or ‘forewinter’ period is different and is commonly marked by a quite different spell of weather.

In low latitudes, within the range of the seasonal migration of the equatorial rains, the spells of weather are much more alike in character from one year to another — as is recognized in the well-known monsoons and rainy and dry seasons, associated more or less with particular months of the year — but the dates of onset and ending of these seasons, and the occurrence or not of breaks within them, may differ widely from year to year.

Some use can also be made of the more or less biennial (two-year) cycle, which is a quite marked tendency in many (though by no means all) series of weather observations around the world.

Its effect is most marked in the summer and winter temperatures in some places, and rainfall in others, and is accompanied by a corresponding tendency for shifts of the large-scale wind circulation pattern.

Indeed, it seems that more or less regularly associated changes in the winds in the stratosphere — over both low and high latitudes — which, however, vary significantly in their timing from year to year, may serve as signals of the progress of the cycle and therefore of the surface weather likely to dominate the ensuing season.

Similarly, there is a suggestion of what amounts to a roughly 5 1/2 year cycle, which may also be used by the seasonal forecaster.

It was discovered, and tentatively explained, by the German pioneer of weather forecasting on this time-scale, the late Professor Franz Baur, whose judicious use of it contributed to his remarkable record of seasonal forecasting successes in the

It seems that the global wind circulation tends to produce maxima of the westerly winds in middle latitudes in the intermediate phases of the (rather variable) eleven-year sunspot cycles — i.e. when the sunspots are declining after a maximum and when they are rising towards the next maximum — such that Europe in particular gets more mild winters and rather dry (anticyclonic westerly) summers at these times.

Again it is important in forecasting practice to monitor the progress of the cycle.

How far it may ultimately be possible to exploit these tendencies to forecast more than one season, or more than one year, ahead — taking whatever account may be necessary of other influences working on that time-scale — cannot be adequately judged in the present state of knowledge.

There are suggestions that tidal forces, which affect the atmosphere and the Earth's crust itself as well as the oceans, and other forces associated with the alignment and occasional conjunctions of the planets, as well as the Chandler wobble of the Earth’s axis, may play some part.

Some have suggested that the last two items point to particularly disturbed years, with a climax of blocking, around the 1980s.

What is certain is that, in any attempt to foresee the prevailing weather development several years ahead, the reliability of specific detail must decline as the range of the forecast is extended.

It may, for instance, be — or it may become — possible to foresee a tendency for severe drought or the likelihood of a notably severe winter in a certain decade, or even about some particular year or years, but necessary to allow for a probable error of plus or minus one year (or, in some cases, several years) in its arrival.

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