Arctic climate change: the past 100 years
The Arctic is a region particularly sensitive to climate change, since temperatures are, on average, near the freezing point of water. Slight shifts in the average temperature can greatly change the amount of ice and snow cover in the region, due to feedback processes. For example, as sea ice melts in response to rising temperatures, more of the dark ocean is exposed, allowing it to absorb more of the sun's energy. This further increases air temperatures, ocean temperatures, and ice melt in a process know as the "ice-albedo feedback" (albedo means how much sunlight a surface reflects). The 20% loss in Arctic sea ice in summer since 1979 has given rise to concerns that this "ice-albedo feedback" has taken hold and will amplify until the Arctic Ocean is entirely ice-free later this century. Should we be concerned? Has the Arctic been this warm in the past and the sea ice survived? The answers are yes, and yes.
Figure 1. Annual average change in near surface air temperature from stations on land relative to the average for 1961-1990, for the region from 60 to 90° north. Image credit: The Arctic Climate Impacts Assessment (ACIA).
The past 100 years
The Arctic Climate Impact Assessment (ACIA), published in November 2004, was a uniquely detailed study of Arctic climate compiled by 300 scientists over three years. The study found that while temperatures in the Arctic have increased significantly since 1980 (Figure 1), there was also a period in the 1930s and 1940s when temperatures were almost as warm. If one defines the Arctic as lying poleward of 62.5° north latitude (Polyakov, 2003), the 1930s and 1940s show up being warmest period in the past 100 years. Looking at Figure 1, one cannot dismiss the possibility that temperatures in the Arctic oscillate in a 50-year period, and we are due for a cooling trend that will take temperatures below normal by 2030.
However, the period since 1980 was a time when the entire globe (except the bulk of Antarctica) warmed, and the 1930s and 1940s were not. Thus, the 1930s and 1940s warming in the Arctic is thought to be fundamentally different. Furthermore, the past 20 consecutive years have all been above normal in temperature, whereas during the 1930s and 1940s there were a few cooler than average years interspersed with the very warm years. A detailed breakdown by month and region of the 100-year history of Arctic temperatures was performed by Overland et al. (2004). They found no evidence of a 50-year cycle in Arctic temperatures, and concluded that the warming since 1980 was unique. However, they stopped short of blaming the recent warming on human-emitted greenhouse gases (anthropogenic forcing). The ACIA, though, concluded that humans were likely to blame for the recent Arctic warming, but not definitely:
It is suggested strongly that whereas the earlier warming was natural internal climate-system variability, the recent surface air temperature changes are a response to anthropogenic forcing. There is still need for further study before it can be firmly concluded that the increase in Arctic temperatures over the past century and/or past few decades is due to anthropogenic forcing."
This is the first in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday over the next two weeks. Next blog: The skeptics attack the ACIA report--and how the position of the pole star is indicative of Arctic climate change.
Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.
Overland, J.E, M.C. Spillane, D.B. Percival, M. Wang, H.O. Mofjeld (2004), "Seasonal and Regional Variation of Pan-Arctic Surface Air Temperature over the Instrumental Record", Journal of Climate, 17:17, pp3263-3282, September 2004.
Polyakov, V., et al. (2003), "Variability and Trends of Air Temperature and Pressure in the Maritime Arctic, 1875-2000", Journal of Climate, 16, 2067-2077.