This is Section 2.8 of the Arctic Climate Impact Assessment
Lead Author: Gordon McBean. Contributing Authors: Genrikh Alekseev, Deliang Chen, Eirik Førland, John Fyfe, Pavel Y. Groisman, Roger King, Humfrey Melling, Russell Vose, Paul H.Whitfield
This chapter has described the arctic climate system; the region’s impact on the global climate system; recent climatic change depicted by the instrumental record; and the historical/paleoclimatic perspective on arctic climatic variability. Features of the arctic climate system that are unique to the region include the cryosphere, the extremes of solar radiation, and the role of salinity in ocean dynamics.
The climate of the Arctic is changing.Trends in instrumental records over the past 50 years indicate a reasonably coherent picture of recent environmental change in northern high latitudes.The average surface temperature in the Arctic increased by approximately 0.09 ºC per decade over the past century, and the pattern of change is similar to the global trend (i.e., an increase up to the mid-1940s, a decrease from then until the mid-1960s, and a steep increase thereafter with a warming rate of 0.4 ºC per decade). It is very probable that the Arctic has warmed over the past century, at a rate greater than the average over the Northern Hemisphere. It is probable that polar amplification has occurred over the past 50 years. Because of the scarcity of observations across the Arctic before about 1950, it is not possible to be certain of the variation in mean land-station temperature over the first half of the 20th century. However, it is probable that the past decade was warmer than any other in the period of the instrumental record.The observed warming in the Arctic appears to be without precedent since the early Holocene.
It is very probable that atmospheric pressure over the Arctic Basin has been dropping and it is probable that there has been an increase in total precipitation at the rate of about 1% per decade over the past century. It is very probable that snow-cover extent around the periphery of the Arctic, sea-ice extent averaged over the Arctic (during at least the past 40 years), and multi-year sea-ice extent in the central Arctic have all decreased. These climate changes are consistent with projections of climate change by global climate models forced with increasing atmospheric GHG concentrations, but definitive attribution is not yet possible.
Natural climate variability in the Arctic over the past two million years has been substantial. In particular, the past 20,000-year period is now known to have been highly unstable and prone to large rapid changes, especially temperature increases that occurred quickly (within a few decades or less). It is clear that between 400 and 100 years BP the climate in the Arctic was exceptionally cold, and there is widespread evidence of glaciers reaching their maximum post-Wisconsinan positions during this period.
Changes in the Arctic are very likely to have significant impacts on the global climate system. For example, a reduction in snow-cover extent and a shrinking of the marine cryosphere would increase heating of the surface, which is very likely to accelerate warming of the Arctic and reduce the equator-to-pole temperature gradient. Freshening of the Arctic Ocean by increased precipitation and runoff is likely to reduce the formation of cold deep water, thereby slowing the global thermohaline circulation. It is likely that a slowdown of the thermohaline circulation would lead to a more rapid rate of rise of global sea level, reduce upwelling of nutrients, and exert a chilling influence on the North Atlantic region as Gulf Stream heat transport is reduced. It would also decrease the rate at which CO2 is transported to the deep ocean. Finally, temperature increases over permafrost areas could possibly lead to the release of additional methane (CH4) into the atmosphere; if seabed temperatures rise by a few degrees, hydrated CH4 trapped in solid form could also escape into the atmosphere.
Although it is possible to draw many conclusions about past arctic climate change, it is evident that further research is still needed.The complex processes of the atmosphere, sea-ice, ocean, and terrestrial systems should be further explored in order to improve projections of future climate and to assist in interpreting past climate. Reconstructions of the past have been limited by available information, both proxy and instrumental records.The Arctic is a region of large natural variability and regional differences and it is important that more uniform coverage be obtained to clarify past changes. In order for the quantitative detection of change to be more specific in the future, it is essential that steps be taken now to fill in observational gaps across the Arctic, including the oceans, land, ice, and atmosphere.
Chapter 2: Arctic Climate - Past and Present
2.2 Arctic atmosphere
2.3 Marine Arctic
2.4 Terrestrial Water Balance in the Arctic
2.5 Influence of the Arctic on global climate
2.6 Arctic climate variability in the twentieth century
2.7 Arctic climate variability prior to 100 years BP
2.8 Summary and key findings of ACIA on Arctic Climate - Past and Present