Quaternary oceans and climate change: lessons for the future?
There is much interest in ice-age studies in recent decades, in the context of global warming. The relevant findings are these: large regular changes in climate occurred within the last million years, especially in the northern North Atlantic. Extreme conditions were similar, suggesting strong negative feedback at the edges of the range of variation. The nature of the periods of climate variation suggests orbital forcing by modulation of internal oscillations involving lagged negative feedback on ice buildup. Transitions from cold to warm were rapid and they were not readily reversed, indicating that ice dynamics underlies abrupt climate change. Accelerated rates of ice decay upon warming correspond to a sea-level rise of one to two m/century. Millennial-scale abrupt disturbances known as “Dansgaard-Oeschger” and “Heinrich” Events occur when large ice masses are present in the northern hemisphere. They may be considered experiments on the ocean’s response to massive meltwater input. When using results from ice-age studies to project future developments, one must be aware that the future will be largely outside of experience with regard to the recent geologic past. Also, there are as yet no generally accepted explanations for striking changes in the past, such as the rapid rise of carbon dioxide during deglaciation, demonstrating a profound lack of understanding of climate dynamics. This is, so far, the lesson from ice-age studies: they say much about the deficiencies in our level of understanding, and not so much about what is ahead.