By Mark Serreze, Ph.D.
Director, National Snow and Ice Data Center
Sea ice is any form of ice found at sea that originated from the freezing of sea water. It is the most visible feature of the Arctic Ocean, with its extent waxing and waning with the seasons. Ice thickness is highly variable, ranging from a thin veneer to tens of meters. While the existence of sea ice reflects the cold conditions inherent to high latitudes, sea ice also strongly modulates the energy budget and climate of the Arctic and beyond, particularly because it is white, and hence reflects much of the sun's energy back to space (it has a high albedo) and also through acting as a lid, insulating the underlying ocean from a generally much colder atmosphere.
Historically, at its maximum extent in March, Arctic sea ice covered an area more than 15 million square kilometers, somewhat less than twice the size of the contiguous United States. The minimum extent, occurring in September, the end of the melt season, was typically around 7.0 x106 km2. However, as assessed over the modern satellite record spanning 1979 to the present, Arctic sea ice extent exhibits downward linear trends for all months, weakest in winter and strongest for September. The downward September trend appears to have accelerated over the past decade. Through 2001, the September trend stood at -7.0% per decade. Through 2012, it was more than twice as large at -14.3% per decade. The six lowest September extents in the satellite record have all occurred in the past six years, with September of 2012 setting a new low mark. Decreased summer ice extent has been accompanied by large reductions in winter ice thicknesses that are primarily explained by changes in the ocean's coverage of thick multiyear ice (MYI). MYI is ice that has survived at least one summer melt season. In the mid-1980s, MYI accounted for 70% of total winter ice extent, whereas by the end of 2012 it had dropped to less than 20%. At the same time the proportion of ice older than 5 years declined from 50% of the MYI pack to less than 8%.
The emerging view is that the Arctic will lose essentially all of its summer sea ice cover by the end of this century, perhaps as early as 2030-2040. The environmental and societal consequences of current and future sea ice loss are only beginning to be fully appreciated. Ice loss is already contributing to increased coastal erosion in the Arctic; part of the reason is that with less ice, winds have a longer fetch over open water, so ocean waves become bigger. Ice loss is also contributing to strong rises in Arctic air temperature during autumn and winter, not just at the surface, but extending through a considerable depth of the atmosphere. As discussed, sea ice acts as a lid, insulating the underlying ocean from a generally much colder atmosphere. With less ice, the insulating effect is weaker, so heat can readily be transferred from the ocean to the atmosphere above. This strong warming, termed Arctic amplification, is starting to extend beyond areas of ice loss to influence Arctic land areas. Continued loss of the ice cover is in turn likely to impact on patterns of atmospheric circulation and precipitation not just within the Arctic, but into middle latitudes; there is evidence that this is already occurring. The basic reason for this is that the outsized warming of the Arctic changes the atmospheric stability and temperature differences between the Arctic and lower latitudes. Finally, as the ice cover retreats, the Arctic is becoming more accessible for marine shipping as well as oil and natural gas exploration, increasing the economic and strategic importance of the region.
Arctic Sea ice extent for September of 2012. Ice extent is defined as the region with an ice concentration of at least 15%. Arctic ice extent for September 2012 was the lowest over the period of satellite observations. The pink line shows the median ice extent for March and September, over a base period of 1979-2000 (from the NSIDC Sea Ice Index).