SUMMARY AND CONCLUSIONS

My findings are these:

  1. There is a well-defined limit to temporal rates of increase in global mean temperature for periods of warming during the past 850,000 years. It can be expressed in the relationship ΔT = 0.38 Δy0.27, where Δy is the number of years during which sustained warming has occurred.

  2. Natural fluctuations of climate are several orders of magnitude larger than both the secular increase of temperature predicted by current climate models and the probable actual increase over the next century.

  3. The results of comparisons of predicted rates of global temperature increase with past observed rates of increase depend greatly on the time periods chosen. Unless the time periods are carefully selected to represent comparable periods (of rapid warming, for example), comparisons can be so skewed as to be misleading or meaningless. Even if the IPCC-1 model predictions were correct, the rate of global temperature increase over the next century would probably be no more than the maximum rates of increase experienced at various times in the past 8,000 years. At worst, they would be double those rates. Even these predicted increases in temperature will be so small by comparison with natural climatic fluctuations, and spread over such a long time scale, that their short-term impact on human behavior and welfare is likely to be minimal and probably not measurable. It thus seems reasonable to conclude that the long-term secular temperature change will not produce dramatic and disruptive changes in the earth's biological systems.

Does this mean that we as scientists should not be interested in the scientific issues entailed by enhanced greenhouse effects and their potential impacts? Not at all: What happens to the earth's climate because of human activities, and what happens to the earth's ecosystems as a result, should be foci of intense scientific scrutiny. We have only one earth to lose, and we should not lose it by default. Let us put our effort into devising productive research approaches, not waste it in chasing chimeras.

Commentary on the Paper of Reifsnyder

SOROOSH SOROOSHIAN

University of Arizona

In his analysis, Dr. Reifsnyder has used temperature as the sole indicator of climate variability, in both space and time. I should like to bring up two points related to this approach.

First, the data Dr. Reifsnyder has used in his analysis come from the published literature and from analyses made by numerous researchers. Some of them are instrumental observations; others result from the analysis of proxy records. They come from many sources, various time periods, and locations worldwide. Not only does each type carry an individual potential for error, but combining them may risk an apples-and-oranges comparison. When data from varied sources are used, as Dr. Reifsnyder and some others have done, one must ask, "To what extent do such uncertainties affect the results and the conclusions drawn from them?"

Even if we ignore the temporal and spatial uncertainties associated with the temperature data, data can still be biased because of changes in instrument technology or in the manner in which measurements are taken. A fair degree of confidence can be placed in data that have been corrected for sources of error; when we are talking about deducing several degrees of temperature change over hundreds or thousands of years from proxy records, we need to acknowledge the magnitude of the associated uncertainties. Let me adduce a similar situation with precipitation data. In collaboration with the Agricultural Research Service, we at Arizona have been monitoring precipitation amounts over a small, well-instrumented experimental watershed. We have a very high density of sophisticated rain gauges, yet the analysis of our data shows that if the measurement errors are not taken into account, the information value may be limited. Conclusions drawn from those data need to be questioned as well.

Dr. Reifsnyder's paper also deals with the impact of temperature change on vegetation. I will not take issue with his position that vegetation will have the ability to tolerate the changes in global mean temperature estimated by various models. I will, however, argue that not only temperature is involved. Even small changes in mean global temperature can have an impact on the general circulation of the atmosphere and the oceans. A temperature change not only might increase the severity of droughts and floods but might expand the regions of the globe affected by them as well. Trees might tolerate a temperature change, but could they also survive a sustained drought?

I should like to endorse a third point that Dr. Reifsnyder makes, although indirectly: Scientists need to be responsible



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