DOI: 10.1126/science.1139426 , 1533 (2007); 315Science et al.Mark C. Serreze,CoverPerspectives on the Arctic's Shrinking Sea-Ice www.sciencemag.org (this information is current as of May 22, 2008 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/315/5818/1533version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services,found at: can berelated to this articleA list of selected additional articles on the Science Web sites http://www.sciencemag.org/cgi/content/full/315/5818/1533#related-content 18 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/315/5818/1533#otherarticles 1 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/atmosAtmospheric Science : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience2007 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on May 22, 2008 www.sciencemag.orgDownloaded fromREVIEWPerspectives on the Arctic’sShrinking Sea-Ice CoverMark C. Serreze,1* Marika M. Holland,2Julienne Stroeve1Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This iceloss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmospheresystem and a growing radiative forcing associated with rising concentrations of atmospheric greenhousegases, the latter supported by evidence of qualitative consistency between observed trends and thosesimulated by climate models over the same period. Although the large scatter between individual modelsimulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, thistransition to a new arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the icecover is expected to affect the Arctic’s freshwater system and surface energy budget and could bemanifested in middle latitudes as altered patternsofatmosphericcirculation and precipitation.The most defining feature of the Arctic Oceanis its floating sea -ice cover , which hastraditionally ranged from a maximum extentof about 16 × 106km2inMarchtoaminimumextent of 7 × 106km2at the end ofthe summer melt season in Sep-tember (Fig. 1). Consistent satellite-derived monthly time series ofsea-ice extent are provided by theNimbus-7 Scanning MultichannelMicrowave Radiometer (October1978 to August 1987) and theDefense Meteorological SatelliteProgram Special Sensor Microwave/Imager (1987 to present). Based onregression analysis of the combinedrecord over the period 1979 to 2006,ice extent has declined for everymonth (Fig. 2), most rapidly forSeptember, for which the trend is–8.6 ± 2.9% per decade or about100,000 km2per year . Ice extent isdefined as the area of the ocean witha fractional ice cover (i.e., an iceconcentration) of at least 15% (1–3).Every year since 2001 hasyielded pronounced Septemberminima, the most extreme of whichwasin2005(5.56×106km2). Whencompared to the mean ice extentover the period 1979 to 2000, thisrepresents a spatial reduction of21% (1.6 × 106km2), an area roughly the sizeof Alaska (Fig. 1). Comparisons with earlierrecords, which combine visible-band satellite im-agery and aircraft and ship reports, suggest thatthe September 2005 ice extent was the lowest inat least the past 50 years. Data for the past fewyears suggest an accelerating decline in wintersea-ice extent (4).Evidence for accompanying reductions in icethickness (5) is inconclusive. Upward-lookingsonar aboard submarines provides informationon ice draft—the component of the total thick-ness (about 90%) that projects below the watersurface. Comparisons between early sonar records(1958 to 1976) and those for 1993 to 1997 indicatereductions of 1.3 m in mean late summer ice draftover much of the central Arctic Ocean (6), butsparse sampling complicates interpretation. Fur-ther analysis of the submarine-acquired data inconjunction with model simulations points tothinning through 1996 but modest recoverythereafter (7). Results from an ice-tracking al-gorithm applied to satellite data from 1978 to2003 document decreasing coverage of old, thickice (8).Understanding the Observed Ice LossThe observed decline in ice extent reflects a con-flation of thermodynamic and dynamic processes.Thermodynamic processes involve changes insurface air temperature (SAT), radiative fluxes,and ocean conditions. Dynamic processes involvechanges in ice circulation in response to windsand ocean currents. These include changes in thestrength and location of the Beaufort Gyre (amean annual clockwise motion in the westernArctic Ocean) and characteristics of the Trans-polar Drift Stream (a motion of ice that progress esfrom the coast of Siberia, across the pole, and intothe North Atlantic via the Fram Strait). Nearly allof the ice export from the Arctic to the Atlanticoccurs through this na rrow s trait be tweennorthern Greenland and Svalbard (Fig. 1).Estimated rates of SAT changeover the Arctic Ocean for the pastseveral decades vary depending onthe time period and season, as wellas the data source being consid-ered. Although natural variabilityplays a large role in SAT variations,the overall pattern is one of recentwarming, which is in turn part of aglobal signal (9). Using a record thatcombined coastal station obser-vations with data from drifting buoys(from 1979 onward) and Russian“North Pole” stations (1950 to1991), Rigor et al.(10) foundpositive SAT trends from 1979 to1997 that were most pronouncedand widespread during spring. Al-though there are biases in the buoydata relative to the North Pole da ta ,especially for October throughApril (11), independent evi denc efor warming during spring, sum-mer , and autumn since 1981 isdocumented in clear-sky surfacetemperatures retriev ed from ad-vanced very-high-resolution radio-meter satellite imagery