Climate History Of The Earth

Before trying to decide whether humans are actually succeeding in changing the earth's climate at present, we must consider what sorts of climate change occurred before humans evolved, and what climate changes occurred before the major industrialization of the twentieth century. It is, unfortunately, beyond the scope of this chapter to discuss the various methods of calculating average temperatures at different times in the past; these methods are discussed in some of the references at the end of the chapter.

The variation of the mean surface temperature of the earth between the latitudes 40°N and 90°N as a nonlinear function of time from 500,000,000 b.c. to a.d. 1950 is shown in Figure 3-8. The time scale is arranged to emphasize recent years, for which more reliable data are available. More recent times are shown in more detail in Figure 3-9. Glacial periods occurred at intervals between 10,000 and 1,000,000 b.c. It looks, from Figure 3-8, as though there have also been major glacial episodes around 200,000,000 b.c.

FIGURE 3-8 Temperature variations in the Northern Hemisphere (40°N - 90°N) as a function of time. Redrawn, by permission of the publisher, from J. E. Oliver, Climate and Man's Environment. Copyright © 1973, John Wiley & Sons, Inc.

and 500,000,000 b.c. In general, the average temperatures before 1,000,000 b.c. were warmer than those that have occurred since that time. Since 1,000,000 b.c., the interglacial periods, although warmer than either the glacial periods or the present, were much cooler than the eras before that time. Notice that the difference in average temperatures between glacial and interglacial periods was only about 5°C; that is, comparatively small differences in average temperature have large effects on the total climate. Notice also that variations up to 2°C have occurred since a.d. 1000. This means that average temperatures in the Northern Hemisphere have varied appreciably before human activities could have had any effect.

Figures 3-8 and 3-9 deal with very long-term variations in average temperature. If we are interested in climate changes possibly attributable to human intervention, we shall have to focus on much shorter term fluctuations. Let us, however, start with the period from 4000 to 3000 b.c. (6000-5000 years before the present), where Figures 3-8 and 3-9 show a temperature maximum, the "Holocene maximum." During this time many glaciers had melted, and sea level was much higher than it is today, and major civilizations existed in Egypt. Between 3000 and 750 b.c. the world generally become somewhat cooler, glaciers advanced, and sea level dropped. Canals were constructed in Egypt to take care of problems associated with the general drop in water levels. Another warming trend culminated in about a.d. 1200, the "medieval warm period," the warmest climate in several thousand years in the Northern Hemisphere. Glaciers retreated so far that the Vikings were able to travel to and settle in Greenland and Iceland. Southern Greenland had average temperatures 2-4°C above present temperatures while Europe had about 1°C higher average temperatures than at present.

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FIGURE 3-9 Schematic diagrams of global temperature variations since the Pleistocene on three time scales: (a) the last million years, (b) the last ten thousand years, and (c) the last thousand years. Dotted line nominally represents conditions near the beginning of the twentieth century. Redrawn from J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds., Climate Change. The IPCC Scientific Assessment. Cambridge University Press, New York. Copyright © 1990. Used by permission of the Intergovernmental Panel on Climate Change.

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FIGURE 3-9 Schematic diagrams of global temperature variations since the Pleistocene on three time scales: (a) the last million years, (b) the last ten thousand years, and (c) the last thousand years. Dotted line nominally represents conditions near the beginning of the twentieth century. Redrawn from J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds., Climate Change. The IPCC Scientific Assessment. Cambridge University Press, New York. Copyright © 1990. Used by permission of the Intergovernmental Panel on Climate Change.

Between a.d. 1300 and 1450, there was a cooling trend that made it harder for ships to travel between Greenland and Iceland and made Greenland cool enough to cause the Vikings to abandon their settlements. A warming trend between 1450 and 1500 was followed by the "Little Ice Age," which did not end in some areas until 1850. Glaciers advanced and winters were very cold in this period. Since 1850, there has been a distinct warming trend, with precipitous retreats of glaciers that ended temporarily around 1940 (see Figures 2-1 and 3-10). Figures 2-1 and 3-10 are very similar, but they refer to somewhat different time intervals and the temperature averages were obtained from somewhat different sources. Both figures show that the warming trend resumed about 1975. The retreat of the glaciers is easy enough to document, but these very short-term recent temperature trends are very hard to evaluate. First of all, while glaciers are retreating in one part of the world, they often are advancing elsewhere. What is the average trend? Furthermore, climate and temperatures are variable in any one place from year to year, and it is not easy to find a general trend among fluctuations. The problem in the evaluation of short-term trends lies in variability in climate that occurs both from place to place and from year to year. The place-to-place variability can be taken care of by averaging temperatures over a large enough area; that is why we see a global mean land temperature in Figure 2-1 and a combined land and sea surface temperature in Figure 3-10. The year-to-year variability can be seen very clearly in both figures. The averages, however, still do not tell the whole story; in this period, they represent mostly an increase in the number of warm months during each year, not an overall increase in actual temperature. Furthermore, although the overall temperature changes have been similar in the

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FIGURE 3-10 Global-mean combined land-air and sea-surface temperatures, 1861-1989, relative to the average for 1951-1980. Redrawn from J. Jager and H. L. Ferguson, eds., Climate Change: Science, Impacts and Policy. Cambridge University Press, New York. Copyright © 1991. Used by permission of the Intergovernmental Panel on Climate Change.

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FIGURE 3-10 Global-mean combined land-air and sea-surface temperatures, 1861-1989, relative to the average for 1951-1980. Redrawn from J. Jager and H. L. Ferguson, eds., Climate Change: Science, Impacts and Policy. Cambridge University Press, New York. Copyright © 1991. Used by permission of the Intergovernmental Panel on Climate Change.

Northern and Southern Hemispheres, the magnitudes have not been the same, and the warming and cooling trends did not necessarily occur in the same year, or, in some cases, in the same decade.

Note that the variations shown in Figure 3-10 are generally less than 1°C, but, as we have seen, it takes only about 5°C to change from a glacial to an interglacial period. The greatest variability in average temperature appears generally to be near the poles, not near the equator.

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