Arctic sea ice (CLIM 010) - Assessment published Sep 2008
Climate change (Primary topic)
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
- CLIM 010
Key policy question: .
- The extent of the sea ice in the Arctic has declined at an accelerating rate, especially in summer. The record low ice cover in September 2007 was roughly half the size of the normal minimum extent in the 1950s.
- The summer ice is projected to continue to shrink and may even disappear at the height of the summer melt season in the coming decades. There will still be substantial ice in winter.
- Reduced polar ice will speed up global warming and is expected to affect ocean circulation and weather patterns. Species specialised for life in the ice are threatened.
- Less ice will ease access to the Arctic's resources. Oil and gas exploration, shipping, tourism and fisheries will offer new economic opportunities, but also increase pressures and risks to the Arctic environment.
Area of multi-year Arctic sea ice in March 1957-2007
Note: Note: The area of thick, multi-year sea ice in the Arctic Ocean is decreasing
Nghiem, S. V.; Rigor, I. G.; Perovich, D. K; Clemente- Colon, P.; Weatherly, J. W. and Neumann, G., 2007. Rapid reduction of Arctic perennial sea ice. Geophysical Research Letters 34, L19504
Observed and projected Arctic September sea-ice extent 1900-2100
Note: Note: The retreat of the sea ice has been faster than predicted: Arctic September sea-ice extent from observations (thick orange line) together with the mean value (solid grey line) from 13 IPCC AR4 climate models and the variance (dotted black line) of models runs.
Stroeve, J.; Holland, M.; Meier, W.; Scambos, T. and Serreze, M., 2007. Arctic sea ice decline: Faster than forecast. Geophysical Research Letters 34, L09501, DOI:10.1029/2007GL029703
The 2007 minimum sea-ice extent
Note: Note: The extent of the summer sea ice in September 2007 reached a historical minimum, 39 % below the climatic average for the first two decades of satellite observations (red line)
- Arctic Sea Ice News Fall 2007 provided by National Snow and Ice Data Center (NSIDC)
- Daily Updated AMSR-E Sea Ice Maps provided by University of Bremen
The extent of the minimum ice cover at the end of the melt season in September 2007 broke all previously observed records. If older ship and aircraft observations are taken into account, sea ice coverage may have halved since the 1950s (NSIDC, 2007; Meier, 2007). Since the more reliable satellite observations started in 1979, summer ice has shrunk by 10.2 % per decade (Comiso et al., 2008; NSIDC, 2007). This strong negative trend was further reinforced when the second lowest minimum extent was recorded summer 2008. The reduction in maximum winter extent is smaller, with a decrease of 2.9 % per decade (Figure 1) (Stroeve et al., 2007). Both summer and winter declines have accelerated (Comiso et al., 2008).
The Arctic sea ice is also getting thinner and younger since less ice survives the summer to grow into thicker multi-year floes. There has been a remarkable shift in its composition towards less multi-year ice and larger areas covered with first-year ice (Figure 2). The first-year ice is weaker and melts easier in summer.
Observations of thickness are more scattered, and it is hard to calculate trends for the whole ice cover. Submarine data have been considered to be most representative and have demonstrated a decrease of 40 % from an average of 3.1 m in 1956-1978 to 1.8 m in the 1990s (UNEP 2007). British submarine data from 2007 show continued thinning (Wadham, pers. com.). German observations from the area around the North Pole and towards northeast Greenland indicate that ice thickness there decreased by 44 % from 2001 to 2007. This was due mainly to a fundamental regime shift from multiyear ice to first-year ice. But there has also been a general thinning of the ice (Nghiem et al., 2007; Haas et al., 2008). These results are in stark contrast with observations between Ellesmere Island and 86oN, where ice thickness was still above 4 m in 2006 (Haas et al., 2006).
Arctic sea ice reacts very sensitively to changes in air and ocean temperatures as well as winds, waves and ocean currents (both thermodynamic and dynamic forcing). There are strong imprints of natural variability in the observed changes, e.g. due to regular shifts in the circulation patterns of the polar atmosphere. However, the changes that can be attributed to increases in greenhouse gases seem to be increasing over time (Stroeve et al., 2007).
The summer ice is very likely to continue to shrink in extent and thickness, leaving larger areas of open water for an extended period. It is also very likely that conditions for freezing in winter will persist so that winter sea ice will still cover large areas.
The speed of change is however uncertain. Several international assessments until recently concluded that mostly ice-free late-summers may occur by the end of this century (ACIA, 2004; IPCC, 2007a; UNEP, 2007). But the actual melting has been faster than the average trends simulated by the climate models used for these assessments (Figure 3). New model studies suggest that ice-free summers may occur in a much nearer future. (Winton, 2006; Holland et al., 2006; Stroeve et al., 2007). Exactly when is impossible to predict with confidence, due both to the limited understanding of the processes involved and the large variability of the system.
Most studies emphasise that it is very likely that thinner and more vulnerable ice will break up more easily so more heat from the sun will be absorbed in the open water. This can lead to abrupt melting and a high susceptibility to dynamic stress like strong winds when weather conditions are favourable, as in the summer of 2007. An increased influx of warm Atlantic water can also be an important mechanism for further weakening of the sea ice. Unless followed by consecutive years of cold winters, such events will produce a thinner, younger and even more vulnerable ice cover that can melt more easily the next summer, and be more easily transported out of the Polar Ocean.
Extent of Arctic perennial sea ice
provided by Jet Propulsion Laboratory
Arctic sea ice extent at the end of the melt season in September
provided by University of Colorado (CIRES)
Sea ice index
provided by National Snow and Ice Data Center (NSIDC)
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