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Observational Determination of Albedo Decrease Caused by Vanishing Arctic Sea Ice

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Observational Determination of Albedo Decrease Caused by Vanishing Arctic Sea Ice Observational determination of albedo decrease caused by vanishing Arctic sea ice Kristina Pistone, Ian Eisenman1, and V. Ramanathan Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0221 Edited by Gerald R. North, Texas A&M University, College Station, TX 77843, and accepted by the Editorial Board January 6, 2014 (received for review September 30, 2013) The decline of Arctic sea ice has been documented in over 30 y of we assess the magnitude of planetary darkening due to Arctic sea satellite passive microwave observations. The resulting darkening ice retreat, providing a direct observational estimate of this ef- of the Arctic and its amplification of global warming was hypoth- fect. We then compare our estimate with simulation results esized almost 50 y ago but has yet to be verified with direct from a state-of-the-art ocean–atmosphere global climate model observations. This study uses satellite radiation budget measure- (GCM) to determine the ability of current models to simulate ments along with satellite microwave sea ice data to document these complex processes. the Arctic-wide decrease in planetary albedo and its amplifying Comparing spatial patterns of the CERES clear-sky albedo effect on the warming. The analysis reveals a striking relationship with SSM/I sea ice concentration patterns, we find a striking between planetary albedo and sea ice cover, quantities inferred resemblance (Fig. 1), revealing that the spatial structure of from two independent satellite instruments. We find that the Arc- planetary albedo is dominated by sea ice cover both in terms of tic planetary albedo has decreased from 0.52 to 0.48 between 1979 the time average (Fig. 1 A and B) and the changes over time (Fig. ± 2 and 2011, corresponding to an additional 6.4 0.9 W/m of solar 1 C and D). Fig. 1 focuses on the month of September, when energy input into the Arctic Ocean region since 1979. Averaged the year-to-year sea ice decline has been most pronounced, over the globe, this albedo decrease corresponds to a forcing that although there is also agreement between sea ice cover and is 25% as large as that due to the change in CO2 during this period, planetary albedo during every other sunlit month of the year considerably larger than expectations from models and other less (Fig. S1). There is also a smaller effect associated with dimin- direct recent estimates. Changes in cloudiness appear to play ishing albedo in central Arctic regions which have nearly 100% a negligible role in observed Arctic darkening, thus reducing sea ice cover throughout the record (Fig. 1D and Fig. S1), as the possibility of Arctic cloud albedo feedbacks mitigating future would be expected from a warming ice pack experiencing more Arctic warming. surface melt (16). To further probe the relationship between planetary albedo he Arctic has warmed by nearly 2 °C since the 1970s, a tem- and sea ice cover, we split the Arctic into six regions (Fig. S2) Tperature change three times larger than the global mean (1). and examine changes in the sea ice cover and albedo averaged During this period, the Arctic sea ice cover has retreated sig- over each region. As anticipated from Fig. 1, monthly mean nificantly, with the summer minimum sea ice extent decreasing values of the clear-sky planetary albedo demonstrate a close by 40% (2). This retreat, if not compensated by other changes relationship with sea ice cover, which is illustrated in Fig. 2 for the – such as an increase in cloudiness (3 6), should lead to a decrease Eastern Pacific region (Figs. S3 and S4 illustrate other regions). in the Arctic planetary albedo (percent of incident solar radia- This relationship, however, is not linear: the slope of the curve tion reflected to space), because sea ice is much more reflective in Fig. 2 is steeper for months with more ice. Because the than open ocean. Such an amplified response of the Arctic to presence of snow gives ice-covered regions a higher albedo in global warming was hypothesized and modeled in the 1960s by – winter and spring, whereas melt ponds contribute to lower al- Budyko (7) and Sellers (8). As per the Budyko Sellers hypoth- bedos in summer and fall, we hypothesize that the nonlinearity in esis, an initial warming of the Arctic due to factors such as CO2 forcing will lead to decreased ice cover which exposes more of Significance the underlying darker ocean and amplifies the warming. In 1975, this phenomenon was simulated in a 3D climate model by Manabe and Wetherald (9), who showed that under conditions The Arctic sea ice retreat has been one of the most dramatic climate changes in recent decades. Nearly 50 y ago it was of a doubling of CO2, tropospheric warming in the polar regions was much larger than in the tropics, due in part to the albedo predicted that a darkening of the Arctic associated with dis- decrease from shrinking snow/ice area. Previous studies have appearing ice would be a consequence of global warming. addressed aspects of this question using a combination of radi- Using satellite measurements, this analysis directly quantifies ative transfer models and indirect albedo change estimates (10– how much the Arctic as viewed from space has darkened in 13), but this study uses satellite measurements of both the region’s response to the recent sea ice retreat. We find that this decline has caused 6.4 ± 0.9 W/m2 of radiative heating since 1979, radiation budget (14) and the sea ice fraction (15) to quantify the considerably larger than expectations from models and recent radiative effects of ice retreat. less direct estimates. Averaged globally, this albedo change is Empirical measurements determining the response of plane- equivalent to 25% of the direct forcing from CO during the tary albedo to sea ice decline are critical for assessing the radi- 2 past 30 y. ative impacts of Arctic changes and for evaluating climate models. ’ The Clouds and Earth s Radiant Energy System (CERES) satel- Author contributions: K.P., I.E., and V.R. designed research; K.P. and I.E. performed re- lite program (14), which includes global measurements of incident search; K.P., I.E., and V.R. analyzed data; and K.P., I.E., and V.R. wrote the paper. and reflected solar radiation, provides an ideal tool to address this The authors declare no conflict of interest. issue. In this study, we analyze CERES clear-sky and all-sky This article is a PNAS Direct Submission. G.R.N. is a guest editor invited by the Editorial planetary albedo data (0.2–4.5-μm wavelength range) as well as Board. sea ice observations inferred from passive microwave satellite 1To whom correspondence should be addressed. E-mail: [email protected]. instruments [the Special Sensor Microwave Imager (SSM/I) This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and its predecessor and successor (15)]. Using these datasets, 1073/pnas.1318201111/-/DCSupplemental. 3322–3326 | PNAS | March 4, 2014 | vol. 111 | no. 9 www.pnas.org/cgi/doi/10.1073/pnas.1318201111 Downloaded by guest on September 30, 2021 Sea ice: 2007-2011 Clear-sky albedo: 2007-2011 AB Sea ice: 2007-2011 minus 2000-2004 Clear-sky albedo: 2007-2011 minus 2000-2004 CD Fig. 1. (A) Sea ice concentration and (B) CERES clear-sky albedo averaged over each September during the last 5 y of the CERES record (2007–2011) and the change in (C) sea ice and (D) clear-sky albedo between the mean of the last five Septembers (2007–2011) and the mean of the first five Septembers (2000– 2004) of the CERES record. Results for other months are included in Fig. S1. Fig. 2 arises due to the seasonal evolution of the ice albedo itself. and then we average over the regions and months to derive the To test this interpretation, we generate an estimate of the sea- Arctic annual albedo (Methods). Note that the linear relation- sonal cycle in the albedo of ice-covered regions by linearly ex- ships effectively function as a monthly and regionally varying trapolating the CERES clear-sky planetary albedo to 100% ice empirical “kernel,” in analogy with model-based kernels typically cover (Methods). This isolates what seasonal changes in ice al- used in feedback analyses (19). bedo would be required to produce the nonlinearity in Fig. 2. We The results are indicated by the dashed lines in Fig. 4A. For then use an empirical relationship (17) to estimate the sea ice the clear-sky case, we observe a decrease in albedo from 0.39 to surface albedo from the clear-sky planetary albedo associated 0.33 during the 33-y period from 1979 to 2011. The change in all- with ice-covered regions (Methods). The CERES-inferred sur- sky albedo, which is less pronounced due to clouds masking some face albedo is consistent with in situ measurements (18) of the of the decrease (12), declines by 0.04 ± 0.006, from 0.52 to 0.48, seasonal cycle of sea ice surface albedo (Fig. 3), suggesting that during this period. This change in all-sky albedo corresponds to the nonlinearity in Fig. 2 is indeed the result of the seasonal cycle an increase of 6.4 ± 0.9 W/m2 over the Arctic Ocean during this in sea ice surface albedo. The same basic structure of the re- period. Here the error bar is based on uncertainty in the linear lationship between CERES albedo and SSM/I ice cover (Fig. S3) relationships used to estimate the albedo during 1979–1999 and between the observations and model (Fig.
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