the work of Dansgaard, Oeschger, and Heinrich, cores in the ice and seafloor can be extremely precise, revealing histories that can even record seasonal changes hundreds of thousands years ago. The most detailed records from cores extend to about 400,000 years ago. When changes in temperature, sea levels, and the amount of carbon dioxide in the atmosphere are plotted onto the same graph, the rise and fall of each variable are intimately linked; changes in one are linked to changes in each of the others. Temperatures have wiggled up and down about 6 degrees Celsius during this time. With the
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