Arctic Climate Change, Variability, and Extremes

Global warming over the past half century has been amplified in the Arctic, especially in the cold season. Other Arctic indicators, especially those of the cryosphere, show signals consistent with the warming of the past half century. This Arctic amplification of the warming arises from a number of processes in the climate system, including the feedbacks associated with the loss of sea ice and snow, the increase of atmospheric moisture, and the vertical temperature structure of the Arctic atmosphere. Ocean heat fluxes into the Arctic from the North Atlantic and North Pacific also appear to have contributed to the Arctic warming through a reduction of sea ice. Internal variability, which played a major role in Arctic warming during the early twentieth century, appears to have been a minor contributor to the more recent warming, which has also been associated with unprecedented extremes of Arctic temperature and sea ice. There is evidence for increased moisture content of the Arctic atmosphere and corresponding impacts on episodes of extreme warmth. The recent variations of Arctic temperature and associated variables fit well with the simulations of Arctic climate by global and regional climate models. Projected changes include a continued warming of the Arctic even under moderate mitigation scenarios, and an increase of Arctic precipitation consistent with the higher temperatures and atmospheric humidities.