Past Climates

The climate of the Arctic is known to have undergone great variations in the past. PAGES, the Past Global Changes project of the International Geosphere-Biosphere Program (IGBP, 1992), addresses two major time scales or time streams: (1) the last 1000-2000 years, the period of human impact on the planet, which includes climatic features like the Little Ice Age and the Medieval Warm Interval, and (2) the last few glacial/interglacial cycles that cover several hundreds of thousands of years, when major changes happened to the climate/biosphere system.

The Little Ice Age and Medieval Warm Period were the most recent examples of cooler and warmer climates, respectively. The North Atlantic region was unusually mild when the Norse first settled in Greenland in the 10th century AD, and then plunged into a 500-year cold spell known as the Little Ice Age, starting about AD 1300. By 1500, archaeological and historical evidence shows that the Norse settlers had completely disappeared. The changing climate was undoubtedly a factor in the disappearance of the Norse settlements and illustrates the potential impacts of climate change on humans and human activities.

Historical climate records generally do not go back more than 2000 years, but past climates can be reconstructed from many different proxy indicators, including tree rings (1000 years), ice core records (100,000 years), lake sediments (million years), and marine sediments (10 million years), among others. The ice core record shows details of past climatic variability. At the end of the last Ice Age, about 13,000 years ago, the climate was beginning to warm during a period called the B0lling-Aller0d when it suddenly plunged back to Ice Age conditions. This 1300-year-long cold period is named the Younger Dryas because the polar wild-flower Dryas octopetala had a resurgence in Europe during this time. Temperatures in Greenland dropped by about 7°C back to full ice-age conditions. At the end of the Younger Dryas, the climate returned to warmer and wetter interglacial conditions.

Ice ages occur at roughly 100,000-year intervals and are thought to be caused primarily by changing amounts and distribution of sunlight on the planet due to long-term variations in the Earth's orbit and the inclination of its spin axis to the sun. These are the so-called Milankovich cycles after the Yugoslav mathematician who computed them 75 years ago. During ice ages when ice sheets covered large parts of the Northern Hemisphere continents, the Bering Strait between Asia and North America was above sea level and allowed humans to migrate from one continent to the other. Glaciers also temporarily covered other straits between Svalbard and Novaya Zemlya. Continental runoff was probably considerably reduced, particularly if an ice cap existed that dammed the big northward-flowing Siberian rivers.

Results obtained from the deep ice cores recovered by European and American researchers from the Greenland ice sheet and from Antarctica have produced remarkable new insights into short- and long-term climatic changes, with the abrupt changes revealed by the ice cores being particularly surprising. During the last Ice Age, the ice core record revealed unexpected abrupt climate shifts that had not been seen before. These oscillations, called Dansgaard-Oeschger cycles, lasted from centuries to millennia, jumping abruptly from cold to warm climates before slowly reverting to cold conditions again. Many of these oscillations seem to be associated with the collapse of big ice sheets, which sent numerous icebergs into the North Atlantic, the so-called Heinrich events. While a full understanding of these abrupt climate changes is lacking at present, it seems likely that the collapse of ice sheets played a major role.

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