Ninth International Conference on Permafrost ... - IARC Research

Accelerated Arctic Land Warming and Permafrost DegradationDuring Rapid Sea Ice LossDavid M. LawrenceNational Center for Atmospheric Research, Boulder, CO, USAAndrew G. SlaterCooperative Institute for Research in the Environmental Sciences, Boulder, CO, USAIntroductionRobert A. Tomas, Marika M. Holland, Clara DeserNational Center for Atmospheric Research, Boulder, CO, USAIn September 2007, the annual minimum sea ice extentshattered the previous observational-record low. CRUTEM3data indicate that 2007 August to October western Arctic landtemperatures were the warmest of the last 30 years (+2.3°Cwarmer than the 1978 to 2006 average). The striking seaice decline in 2007 raises the specter that a period of abruptsea ice loss, such as those simulated in Community ClimateSystem Model (CCSM3) 21 st century A1B simulations(Holland et al. 2006), is a distinct possibility. Rapid sea iceloss events (RILEs) in CCSM3 typically last between 5 and10 years and exhibit negative sea ice extent trends that areroughly 4 times larger than average simulated (or recentlyobserved) trends.Whether or not the 2007 sea ice record minimum isa precursor of a sustained period of rapid loss remains tobe seen, but it provides motivation to assess the potentialconsequence for adjacent land climate. Here, we evaluateArctic land temperature response to RILEs in CCSM3.We find that the secular 21 st century land-warming trend isaugmented by a factor of 3.5 during RILEs, which is likelyto have adverse impacts on permafrost. Through idealizedexperiments with the Community Land Model (CLM), weassess the impact of a RILE and its timing on permafrost.The results presented here are excerpted from Lawrence etal. (2008).Arctic Land Temperature Trends DuringRapid Sea Ice LossNine RILEs are identified across the eight-memberCCSM3 A1B 21 st century ensemble (Holland et al. 2006b).By computing a lagged composite of sea ice extentanomalies across the nine events, we form a picture of thetypical sea ice extent trajectory during abrupt loss periods(Fig. 1a). A corresponding composite for western ArcticOctober to December (OND) land T airshows an increasein warming during RILEs (Fig. 1a). Figure 1b shows thewestern Arctic linear T airtrend during and outside RILEs.Warming is accelerated during RILEs throughout most ofthe year with statistically significant increases in warmingrates apparent in the summer and early autumn, likely dueto increased open water area, as well as in late autumn andwinter, when the thinner ice pack less efficiently insulatesthe atmosphere from the comparatively warm ocean waterbelow. Accelerated warming spans most of the terrestrialFigure 1. (a) Composite lagged time series of September sea iceextent (solid line) and OND T air(dashed line) over Arctic land area(65°–80°N, 60°–300°E). Composites are centered around the midpointsof the nine rapid sea ice loss events seen in the CCSM3 A1Bsimulations. Results shown as anomalies from the average of years-10 to -5. (b) Average monthly Arctic land air temperature trendsduring rapid sea ice loss periods and outside sea ice loss periods.Trend is statistically significant at the 90% (*) and 95% ( **) levels.(c) Maps of air temperature trends for OND during and outsideabrupt sea ice loss periods.western Arctic juxtaposed to the area of sea ice contractionin CCSM3. It is strongest along the Arctic coast where itis as high as 5°C decade -1 in the autumn, but a signal ofenhanced warming can extend 1500 km inland (Fig. 1c).Annually averaged, the warming trend during RILEs is 3.5times greater than outside these periods (1.60°C decade -1versus 0.46°C decade -1 ).Impact of Accelerated Warming on PermafrostTo evaluate the impact of abrupt warming and its timingon permafrost, we construct four synthetic trend scenariosbased on the results shown in Figure 1. We then use thesescenarios to force a version of CLM (Oleson et al. 2004)that includes some improvements in permafrost dynamics,namely explicit representation of the thermal and hydrologicproperties of organic soil (Lawrence & Slater 2007) and a50 m soil column that represents thermal inertia provided bydeep ground (Lawrence et al. 2008).167