The Rapidity Of Some Climatic Changes

Several climatologists, notably Bryson, Flohn and Manley, have drawn attention to the apparently great rapidity of a number of major climatic changes in the past. For example, three major coolings covering perhaps half, and in two cases more than half, the range between present (or interglacial) prevailing temperatures and the ice age climax temperatures seem to have taken place in and around the North Atlantic Ocean, in Europe and the Mediterranean, in the later stages of the last interglacial. These coolings, dated at roughly 115,000, 90, 000 and 70,000 years ago, took place within about one thousand years and possibly within a century or so.7 The first two seem to have lasted only two to five thousand years and were followed by rapid recovery to temperatures only a little below those previously prevailing in the ten thousand years of the warmest part of the interglacial. The third occurrence introduced fifty thousand years of colder climates, including the main phases of the last ice age. The rapidity of these coolings and warmings indicates that no great accumulations of ice were formed in the two abortive ice age onset events. The last recurrence phase of the ice age around 10,800 years ago was similarly abrupt: at that time the prevailing summer temperatures in England, which had become as warm as (or a little warmer than) today's, dropped 4-5 °C, probably within about fifty years, and small glaciers reappeared in the Lake District in northwest England. The colder climate lasted about six hundred years. In this case, however, the magnitude of the reduction of temperatures in so short a time is made more readily understandable by taking account of the nearness to England of a then still-extant ice-sheet in Scandinavia and a much smaller one in Scotland.

Well-dated evidence of shifts of the vegetation boundaries in post-glacial times, particularly in North America where there were no mountain barriers in the way of northward advance, make it clear that some of the warming episodes were also very rapid once the ocean had warmed up. The rapidity is especially clear when one makes the allowance that must be made for the time required for the vegetation to respond. There must be a lower limit of about a hundred years before standing pine forest can be replaced by forest dominated by oak, though the process may be speeded by wholesale disease and death of the old forest, followed by fire. Even so, some of these changes seem to have taken place so rapidly that the time elapsed was less than the error margin of the radiocarbon dates of organic matter from before and after the change.

One gets the impression that in many of the major climatic changes we have referred to the change of behaviour of the wind circulation is more or less instantaneous. The temperatures prevailing at the surface of the Arctic ice adjust themselves within a few years at most to either a calmer regime than before or to one with stronger winds and ocean currents which import more heat from other latitudes. Such changes—in both directions—have been observed within the present century. Shifts of the wind zones bring also displacements of the main belts of cloudiness. Through these and the changed pattern of wind and ocean heat transport the prevailing temperatures in other latitudes will also become adjusted to the new regime.

It therefore remains to explain the sudden changes of the wind regime which produce periods of perhaps ten to fifty or seventy years of great prevalence of blocking patterns, with wide-ranging meanders of the flow pattern of the circumpolar vortex aloft and stationary anticyclones and cyclones at the surface in middle latitudes. These in their turn produce persistent, or repeated, northerly and southerly surface winds, calms and sometimes easterly winds, dominating different sectors in middle latitudes. The result is a reduction of the frequency of mobile westerly situations, with their continual interchange of warmer and colder air-masses in those latitudes. The stationary features of a blocking situation may be so placed as to maintain an abnormal extension of snow cover over land or to 'waste' the snow in the ocean, where it has no effect in changing the surface characteristics and their response to solar radiation. Better understanding is needed of the controls which determine any long-term preference for particular positions and especially whether the blocking anticyclones are mostly over Greenland or Scandinavia. The position of such features in summer may either accelerate or hinder the melting of the previous winter's snow over broad regions in latitudes north of 45-60 °N. And the maintenance of cloud cover, or of abnormal upwelling or evaporation from the ocean surface, in lower latitudes may have similar effects upon the overall heating of the Earth. If they come at a critical stage of a long slow upward or downward trend of the energy budget, such periods of prevalent blocking patterns in the wind circulation may make the switch to a warmer or colder global climate very rapid.

There is a school of thought in meteorology, articulated in recent years by Lorenz, which maintains that such changes of the wind circulation need no external cause. It is suggested that various alternative regimes of wind and climate are possible without any change in the external influences, so that at any time the pattern may 'click over' from one regime to another. On this view, even the ice ages may have no external cause. This is a philosophy of pessimism, so far as further understanding and prospects of forecasting are concerned. It is an unnecessary pessimism in cases where external causes can be demonstrated,8 as seems clear at least in the cases of the effects of the Earth's orbital changes and the sequels to great volcanic outbursts. And there are hints that some variations in the frequency of stationary features in the large-scale wind circulation may be related to a number of variables in the motion of the Earth and to tidal forces on the sun associated with the alignments of the other planets, which have not hitherto been taken into account. Pioneer work by Bryson in the United States and Maksimov in the Soviet Union invokes the slow migration of the Earth's polar axis over distances of at most a few hundred metres under the influence of sun and moon and the even smaller 'Chandler wobble' of the Earth's polar axis. The latter is still unexplained but agreed to entail adjustments of the angular momentum of Earth and atmosphere. Unpublished work by Morth in England further invokes effects upon the angular momentum induced by dispositions of the other planets in the solar system, as well as variations of the Earth's magnetic field and its effects upon electrically charged droplets in the atmosphere. All these suggestions are still controversial, but attempts to prove or disprove them should be pursued because most of the variables in question are predictable or partly so.

The question about whether side-effects of man's activities may now, or soon, begin to modify the global climate—and how they have modified some local climates—is another story, to be dealt with in later sections of this book dealing with the world today and the outlook for the future. We shall also deal briefly with some of the things which have been, or may be, done in the way of deliberate action to modify climate either locally or on a global scale.

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