5 - Changes in Climates of the Past: Lessons for the Future

Summary

Editor's Introduction

The record of climate change over the geologic past is one of the most often-cited, but least-understood, pieces of evidence supporting the argument that human activities may have an important effect on future climate. Michael B. McElroy notes that other factors, judging from the record of past changes, may also be important to the future climate. For example, changes in the intensity of solar radiation (caused by the periodic changes in the position of the Earth's orbit relative to the sun) have been a critical driving force in the long-term pattern of climate change over millions of years.

But from the distribution of various atomic isotopes in mud and ice samples that have been laid down over hundreds of thousands of years, McElroy concludes that future human emissions may have a comparable effect on the climates to come as changes in solar intensity and orbital position have had in the past. By studying the natural archives gleaned from mud and ice, he suggests that we can improve our understanding of the dynamics of climate change. With these data, scientists may be able to unravel the puzzling mechanisms that shift ocean currents and atmospheric circulation, possibly triggering the onset of rapid climate change. Emphasizing the complexity of this set of closely coupled, non-linear systems, McElroy observes that human activities are causing changes in atmospheric composition that are on the same scale as those associated with major climate changes (ice ages and sudden warmings) of the geologic past. Despite the potential implications of this analysis, he is careful to avoid even the appearance of sensationalism.