Maximum Rates of Projected and Actual Increases in Global Mean Temperature as Compared with Bioclimatic Fluctuations



Predictions have been made and promulgated by authoritative world bodies that the rate of model-predicted global climate warming will be 10 to 40 times that observed following the end of the last ice age (IPCC, 1990b; Schneider, 1989). If this rate of global warming were to actually occur, the effects on earth ecosystems could be dramatic, perhaps catastrophic.

A survey of published rates of increase in global mean temperature for various periods of rapid warming during the past 850,000 years shows that there appears to be an upper limit represented by the relationship ΔT = 0.38 Δy0.27, where Δy is the period during which ΔT shows sustained warming.

Analysis shows that the actual rate of warming during the millennium marking the end of the last ice age was approximately 2.5°C. Furthermore, the IPCC-I "most likely" model-estimated rate of global temperature increase in the past century (IPCC, 1990b) is 2 to 3 times the observed rate for the same period. My analysis shows that for a predicted "business-as-usual" emissions scenario, the climate change that can be expected by the end of the next century is at most twice the maximum rates observed since the last ice age. But since the models used by IPCC appear to over-predict global climate change for the past century by a factor of at least 2, one can assume that in actuality the rate of global warming over the next century will be no more than past maximum rates.

Last, contemporary measured temporal and spatial variations of climate and microclimate are shown to be several orders of magnitude greater than the secular increase of temperature predicted to result from anthropogenic global warming. This relationship may have important consequences for prediction of ecological change in the next century.


Professor Emeritus, School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut

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