New paper finds 2007 Arctic sea ice anomaly was due to decreased clouds

A paper published today in the Journal of Geophysical Research - Atmospheres finds the Arctic summer sea ice anomaly of 2007 was primarily due to a decrease in cloud cover that increased solar [shortwave] radiation at the surface. According to the authors, "Summer 2007 had the largest persistent cloud, radiation, and sea ice anomalies in the climatology. During that summer, positive net shortwave radiation anomalies exceeded 20 Wm-2 over much of Arctic Ocean. This enhanced shortwave absorption resulted primarily from cloud reductions during early summer, and sea ice loss during late summer." By way of comparison, the alleged longwave forcing from CO2 increase since the beginning of the satellite record in 1979 is only about 0.8 Wm-2*, or 25 times less than the forcing from these cloud reductions noted during 2007. Thus, there is no evidence of a man-made influence on the 2007 sea ice anomaly; rather, it was due to a natural decrease in cloudiness.

*5.35*ln(394/337) = 0.8 Wm-2

Observational constraints on Arctic Ocean clouds and radiative fluxes during the early 21st century

Jennifer E. Kay, Tristan L'Ecuyer

Abstract:  Arctic Ocean observations are combined to create a cloud and radiation climatology for the early 21st century (March 2000 - February 2011). Data sources include: observed top-of-atmosphere (TOA) radiative fluxes (CERES-EBAF), active (CloudSat, CALIPSO) and passive (MODIS) satellite cloud fraction observations, and observationally constrained radiative flux and cloud forcing calculations (CERES-EBAF, 2B-FLXHR-LIDAR).Uncertainty in flux calculations is dominated by cloud uncertainty, not surface albedo uncertainty. The climatology exposes large geographic, seasonal, and inter-annual variability in the influence of clouds on radiative fluxes but, on average, Arctic Ocean clouds warm the surface (+10 Wm-2, 2B-FLXHR-LIDAR) and cool the TOA (−12 Wm-2, CERES-EBAF, 2B-FLXHR-LIDAR).Shortwave TOA cloud cooling and longwave TOA cloud warming are stronger in 2B-FLXHR-LIDAR than in CERES-EBAF, but these two differences compensate each other yielding similar net TOA values. During the early 21st century, summer TOA albedo decreases are consistent with sea ice loss, but are unrelated to summer cloud trends that are statistically insignificant. In contrast, both sea ice variability and cloud variability contribute to inter-annual variability in summer shortwave radiative fluxes. Summer 2007 had the largest persistent cloud, radiation, and sea ice anomalies in the climatology. During that summer, positive net shortwave radiation anomalies exceeded 20 Wm-2 over much of Arctic Ocean. This enhanced shortwave absorption resulted primarily from cloud reductions during early summer, and sea ice loss during late summer. In summary, the observations show that while cloud variability influences absorbed shortwave radiation variability, there is no summer cloud trend affecting summer absorbed shortwave radiation.