Estimated Mean Global Temperature

Ma COLD WARM

Ma COLD WARM

Estimated Mean Global Temperature

FIGURE I. I Generalized temperature history of the Earth plotted as relative departures from the present global mean (from Frakes et al., 1992). Studying the proxy record of paleodimate is rather like looking through a telescope held the wrong way around; for recent periods there is evidence of short-term climatic variations, but these cannot be resolved in earlier periods.

It is abundantly clear from the paleodimate record that abrupt changes have occurred in the global climate system at certain times in the past. Nonlinear responses apparently have occurred as critical thresholds were passed. Our knowledge of what these thresholds are is completely inadequate; we cannot be certain that anthropogenic changes in the climate system will not lead us, inexorably, across such a threshold, beyond which may lie a dramatically different future climate state (Broecker, 1987). Only by careful attention to such episodes in the past can we hope to comprehend fully the potential danger of future global changes due to human-induced effects on the climate system.

0 0

Post a comment