PNAS - Albedo Change Caused By Arctic Sea Ice Loss Equal To 25% Of All Carbon Forcing Since 1988
The Arctic sea ice retreat has been one of the most dramatic climate changes in recent decades. Nearly 50 y ago it was predicted that a darkening of the Arctic associated with disappearing ice would be a consequence of global warming. Using satellite measurements, this analysis directly quantifies how much the Arctic as viewed from space has darkened in 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, considerably larger than expectations from models and recent less direct estimates. Averaged globally, this albedo change is equivalent to 25% of the direct forcing from CO2 during the past 30 y.
The decline of Arctic sea ice has been documented in over 30 y of satellite passive microwave observations. The resulting darkening of the Arctic and its amplification of global warming was hypothesized almost 50 y ago but has yet to be verified with direct observations. This study uses satellite radiation budget measurements along with satellite microwave sea ice data to document the Arctic-wide decrease in planetary albedo and its amplifying effect on the warming. The analysis reveals a striking relationship between planetary albedo and sea ice cover, quantities inferred from two independent satellite instruments. We find that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 ± 0.9 W/m2 of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates. Changes in cloudiness appear to play a negligible role in observed Arctic darkening, thus reducing the possibility of Arctic cloud albedo feedbacks mitigating future Arctic warming.
The Arctic has warmed by nearly 2 °C since the 1970s, a temperature change three times larger than the global mean (1). During this period, the Arctic sea ice cover has retreated significantly, with the summer minimum sea ice extent decreasing by 40% (2). This retreat, if not compensated by other changes such as an increase in cloudiness (3??–6), should lead to a decrease in the Arctic planetary albedo (percent of incident solar radiation reflected to space), because sea ice is much more reflective than open ocean. Such an amplified response of the Arctic to global warming was hypothesized and modeled in the 1960s by Budyko (7) and Sellers (8). As per the Budyko–Sellers hypothesis, an initial warming of the Arctic due to factors such as CO2 forcing will lead to decreased ice cover which exposes more of 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 of a doubling of CO2, tropospheric warming in the polar regions was much larger than in the tropics, due in part to the albedo decrease from shrinking snow/ice area. Previous studies have addressed aspects of this question using a combination of radiative transfer models and indirect albedo change estimates (10??–13), but this study uses satellite measurements of both the region’s radiation budget (14) and the sea ice fraction (15) to quantify the radiative effects of ice retreat.
Empirical measurements determining the response of planetary albedo to sea ice decline are critical for assessing the radiative impacts of Arctic changes and for evaluating climate models. The Clouds and Earth’s Radiant Energy System (CERES) satellite program (14), which includes global measurements of incident and reflected solar radiation, provides an ideal tool to address this issue. In this study, we analyze CERES clear-sky and all-sky planetary albedo data (0.2–4.5-?m wavelength range) as well as sea ice observations inferred from passive microwave satellite instruments [the Special Sensor Microwave Imager (SSM/I) and its predecessor and successor (15)]. Using these datasets, we assess the magnitude of planetary darkening due to Arctic sea ice retreat, providing a direct observational estimate of this effect. We then compare our estimate with simulation results from a state-of-the-art ocean–atmosphere global climate model (GCM) to determine the ability of current models to simulate these complex processes.
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http://www.pnas.org/content/111/9/3322
