Recent Septembers have seen the extent of Arctic sea ice cover fall to about 60% of levels in the early 1970s. The past two years, 2007 and 2008, saw the lowest and second-lowest ice extent ever recorded. Scientists have long expected that a shrinking Arctic sea ice cover will lead to strong warming of the overlying atmosphere. This anticipated warming, known as Arctic or polar amplification because it is large in comparison to the temperature rise in lower latitudes, may further accelerate climate warming well beyond the Arctic. Mark Serreze, with NSIDC colleagues Julienne Stroeve, Andrew Barrett, David Kindig, and Andrew Slater, analyzed observations and model simulations to look for the signals of Arctic amplification. Their conclusion: it has arrived. But its effects have only just begun, raising new questions about how once-stable climate patterns will begin to alter.
As the Northern Hemisphere's refrigerator, the Arctic influences climate patterns well beyond the boundaries of the Arctic itself. The loss of the ice cover changes the refrigerator, and climate patterns in turn. How does Arctic amplification work? Sea ice helps to keep the Arctic atmosphere cold. Its whiteness reflects much of the Sun's energy back to space, and it physically insulates the Arctic atmosphere from the underlying Arctic Ocean. With less sea ice, the refrigerator door is left open: more dark open water is exposed, which readily absorbs the Sun's energy in summer, heating the ocean and leading to even more melt. With less sea ice there is also less insulation, so that heat from the ocean escapes to warm the atmosphere in the autumn and winter.
A closer look
Figure 1: In this chart of temperature and ice cover anomalies, strong atmospheric warming in the area of recent ice loss is obvious. The chart shows a vertical cross section of autumn (September to November) temperature anomalies, relative to 1979 to 2007 means. The chart also shows September anomalies in the number of days with ice cover (ice concentration greater than 55%). Reds and pinks indicate positive temperature anomalies, and blues indicate negative ice cover anomalies. Both temperature and ice cover anomalies are along a transect (left to right) from 50° N to the North Pole along the Date Line, and from the pole southward to 50° N along the Prime Meridian. Temperature anomalies are from the NCEP Atmospheric Reanalysis for the years 2003 to 2007. The light blue boxes along the zero anomaly line indicate land. (Adapted from Serreze et al., 2009)
Atmospheric data shows clear evidence of Arctic warming linked to declining ice extent (Figure 1). To see if the loss of Arctic sea ice had really begun to speed up the effects of climate warming, the team looked at output extending through 2007 from the NCEP/NCAR Reanalysis, a retrospective form of numerical weather prediction. They also examined the satellite-derived sea ice record, and compared results with simulated surface air temperature variability based on the Community Climate System Model Version 3. The results consistently pointed at Arctic amplification starting in the last decade. Their analysis also indicates that as the sea ice cover continues to shrink, Arctic amplification will grow.
The implications of Arctic amplification are only beginning to be understood. Scientists at NSIDC are working with groups around the world to understand the likely impacts of Arctic amplification and whether they are already emerging. Wind patterns are expected to spread the warming over high-latitude land areas, warming the tundra and its underlying permafrost. There is growing recognition that this could thaw permafrost and release the carbon stored in these soils back to the atmosphere, further accelerating climate warming. Some numerical simulations indicate that loss of the sea ice cover may lead to changes storm tracks and rainfall patterns over Europe or the American West. Still other studies find that the effects of Arctic amplification on atmospheric circulation will be largely limited to the Arctic itself. While there is tantalizing evidence that the atmospheric circulation is already responding to the loss of ice, the answers are not yet in.
Serreze, M.C., A.P. Barrett, J.C. Stroeve, D.N. Kindig, and M.M. Holland. 2009. The emergence of surface-based Arctic amplification. The Cryosphere