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

Studies from a variety of disciplines document recent change in the northern high-latitude environment. Prompted by predictions of an amplified response of the Arctic to enhanced greenhouse forcing, we present a synthesis of these observations. Pronounced winter and spring warming over northern continents since about 1970 is partly compensated by cooling over the northern North Atlantic. Warming is also evident over the central Arctic Ocean. There is a downward tendency in sea ice extent, attended by warming and increased areal extent of the Arctic Ocean's Atlantic layer. Negative snow cover anomalies have dominated over both continents since the late 1980s and terrestrial precipitation has increased since 1900. Small Arctic glaciers have exhibited generally negative mass balances. While permafrost has warmed in Alaska and Russia, it has cooled in eastern Canada. There is evidence of increased plant growth, attended by greater shrub abundance and northward migration 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 together, 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 quality, or provide limited spatial coverage. The recent high-latitude warming is also no larger than the interdecadal temperature range during this century. Nevertheless, the general patterns of change broadly agree with model predictions. Roughly half of the pronounced recent rise in Northern Hemisphere winter temperatures reflects shifts in atmospheric circulation. However, such changes are not inconsistent with anthropogenic forcing and include generally positive phases of the North Atlantic and Arctic Oscillations and extratropical responses to the El-Nino Southern Oscillation. An anthropogenic 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.