GREENLAND ICE CORE RECORDS AND RAPID CLIMATE-CHANGE

Long ice cores from Greenland yield records of annually resolved clima te change for the past ten to twenty thousand years, and decadal resol ution for one hundred thousand years or more. These cores are ideally suited to determine the rapidity with which major climate changes occu r. The termination of the Younger Dryas, which marks the end of the la st glacial period, appears to have occurred in less than a human lifet ime in terms of oxygen isotopic evidence (a proxy for temperature), in less than a generation (20 years) for dust content and deuterium exce ss (proxies for winds and sea-surface conditions), and in only a few y ears for the accumulation rate of snow. Similarly rapid changes have b een observed for stadial-interstadial climate shifts (Dansgaard-Oeschg er cycles) which punctuate the climate of the last glacial period. The se changes appear to be too rapid to be attributed to external orbital forcings, and may result from internal instabilities in the Earth's a tmosphere-ocean system or periodic massive iceberg discharges associat ed with ice sheet instability (Heinrich events). In contrast, the Holo cene climate of the Arctic appears to have been relatively stable. How ever, the potential for unstable interglacials, with very rapid, short lived climatic deteriorations, has been raised by results from the low er part of the GRIP ice core. These results have not been confirmed by other ice cores, notably the nearby GISP2 core. Evidence from other r ecords of climate during the Eemian interglacial have yielded mixed re sults, and the potential for rapid climate change during interglacial periods remains one of the most intriguing gaps in our understanding o f the nature of major Quaternary climate change.