Observational evidence of recent change in the northern high-latitude environment

Studies from a variety of disciplines document recent change in the norther n high-latitude environment. Prompted by predictions of an amplified respon se of the Arctic to enhanced greenhouse forcing, we present a synthesis of these observations. Pronounced winter and spring warming over northern cont inents since about 1970 is partly compensated by cooling over the northern North Atlantic. Warming is also evident over the central Arctic Ocean. Ther e is a downward tendency in sea ice extent, attended by warming and increas ed areal extent of the Arctic Ocean's Atlantic layer. Negative snow cover a nomalies have dominated over both continents since the late 1980s and terre strial precipitation has increased since 1900. Small Arctic glaciers have e xhibited generally negative mass balances. While permafrost has warmed in A laska and Russia, it has cooled in eastern Canada. There is evidence of inc reased plant growth, attended by greater shrub abundance and northward migr ation of the tree line. Evidence also suggests that the tundra has changed from a net sink to a net source of atmospheric carbon dioxide. Taken togeth er, these results paint a reasonably coherent picture of change, but their interpretation as signals of enhanced greenhouse warming is open to debate. Many of the environmental records are either short, are of uncertain quali ty, or provide limited spatial coverage. The recent high-latitude warming i s also no larger than the interdecadal temperature range during this centur y. Nevertheless, the general patterns of change broadly agree with model pr edictions. Roughly half of the pronounced recent rise in Northern Hemispher e winter temperatures reflects shifts in atmospheric circulation. However, such changes are not inconsistent with anthropogenic forcing and include ge nerally positive phases of the North Atlantic and Arctic Oscillations and e xtratropical responses to the El-Nino Southern Oscillation. An anthropogeni c effect is also suggested from interpretation of the paleoclimate record, which indicates that the 20th century Arctic is the warmest of the past 400 years.