FIGURE 2.4 Climatic records of the past 1000 years. (a) The 50-year moving average of a relative index of winter severity compiled for each decade from documentary records in the region of Paris and London (Lamb, 1969). (b) A record of δ18O values preserved in the ice core taken from Camp Century, Greenland (Dansgaard et al., 1971). (c) Records of 20-year mean tree growth at the upper treeline of bristlecone pines, White Mountains, California (LaMarche, 1974). At these sites tree growth is limited by temperature with low growth reflecting low temperature. (d) The 50-year means of observed and estimated annual temperatures over central England (Lamb, 1966).

References:

Lamb, H. H. (1969). Climatic fluctuations, in World Survey of Climatology, 2, General Climatology, H. Flohn, ed., Elsevier, New York, pp. 173–249.

Dansgaard, W. S., S. J. Johnsen, H. B. Clausen, and C. C. Langway, Jr. (1971). Climatic record revealed by the Camp Century ice core, in The Late Cenozoic Glacial Ages, K. Turekian, ed., Yale U.P., New Haven, Conn., pp. 37–56.

LaMarche, V. C., Jr. (1974). Paleoclimatic inferences from long tree-ring records, Science 183, 1042.

Lamb, H. H. (1966). Climate in the 1960s, Geog. J. 132, 183.

which was most pronounced in the Atlantic sector of the sub-Arctic. A rapid worldwide retreat of mountain glaciers and a poleward extension of the ranges of many flora and fauna took place then.

There is considerable evidence that, between the 1940’s and about 1970, the climatic changes of the earlier part of this century had tended to undergo a reversal. Temperatures had mostly fallen, especially in the Arctic and the Atlantic sub-Arctic, where sea ice has been increasing. The circulation of the northern hemisphere appears to have shifted in a manner suggestive of an increasing amplitude of the planetary waves and of greater extremes of weather conditions in many areas of the world. (The situation in the southern hemisphere has not been so well documented.) These events have culminated, at times in the last several years, in the emergence of anomalous conditions in the monsoon belt of the tropics and in widespread drought in the Sahel zone of Africa and in northwest India (see Figure 2.6). To what extent these calamitous recent events are related to each other as manifestations of a globally coherent fluctuation of climate is not clear. In any event, they dramatize the fact that climatic variability, whether globally coherent or not, is to be expected no less on time scales of months and years than on time scales of centuries and millennia. An evident faltering of these tendencies of climate, in just the last five or ten years, attests also to the ephemeral nature of all climatic “trends.” Such is the nature of climate and climatic variations.

2.3
ORIGINS OF CLIMATE VARIABILITY

Mindful as we are of the remarkable variability of climate in the remote past, the lesser variability of present-day climate catches us by surprise only in the sense that we lack a satisfactory explanation for it, and we do not know how to

FIGURE 2.5 Recorded changes of annual mean temperature of the northern hemisphere as given by Budyko (1969) and as updated after 1959 by H. Asakura of the Japan Meteorological Agency (unpublished results).

Reference:

Budyko, M. I. (1969). The effect of solar radiation variations on the climate of the earth, Tellus 21, 611.



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