Is there evidence for an aerosol indirect effect during the recent aerosol optical depth decline in Europe?

Abstract

1] Aerosol indirect effects are some of the largest uncertainties of anthropogenic climate change. To estimate the first aerosol indirect radiative effect (or cloud albedo effect), we analyzed global solar irradiance measurements under completely overcast skies during the recent period of aerosol optical depth decline in Europe. Although measurements from 15 Swiss and 8 northern German sites show clear evidence for an aerosol direct radiative effect under cloud-free skies, trends of transmitted solar irradiance (SW tran ) under overcast skies are ambiguous. Time series from 1981 to 2005 of SW tran for all overcast conditions show slightly negative, but nonsignificant trends. SW tran under overcast conditions with ''thick'' clouds (SW tran smaller than the long-term mean) exhibit on average an increasing trend of +0.29 [+0.01 to +0.57] W m À2 / decade. The increase of SW tran under ''thick'' overcast skies, however, is about nine times smaller than the increase under cloud-free skies. Since cirrus clouds are generally excluded from and low-level stratiform clouds are more frequently represented by ''thick'' overcast skies, the slight increase in SW tran may possibly result from a weak aerosol indirect effect. Alternatively, the increase in SW tran may be due to a decreasing trend in low-level stratiform cloud amount under overcast conditions observed for these sites. We further find that solar irradiance changes caused by decreasing aerosol direct effect and increasing sunshine duration can account for most of the observed increasing all-sky solar radiation trend. This suggests that the first aerosol indirect effect makes little contribution to surface solar radiation changes over Europe.Citation: Ruckstuhl, C., J. R. Norris, and R. Philipona (2010), Is there evidence for an aerosol indirect effect during the recent aerosol optical depth decline in Europe?,

Understanding the rapid summer warming and changes in temperature extremes since the mid-1990s over Western Europe

“…12b, g), indicating the dominant role of aerosol-radiation interaction in the surface solar radiation change over Western Europe. This is in line with Ruckstuhl et al (2010) who found a small contribution of cloud impact to the recent solar brightening over Europe. The increase in surface net SW and the decrease in upward latent heat flux (Fig.…”

“…12b minus 12g), highlighting the dominant role of aerosol-radiation interactions for the summer mean warming and changes in temperature extremes over Western Europe. Similar conclusions were reached based on observations for the role of aerosols on recent solar brightening over Europe (e.g., Ruckstuhl et al 2008Ruckstuhl et al , 2010.…”

Atmospheric impacts on climatic variability of surface incident solar radiation

“…9a shows increasing cloud cover but decreasing AOD. Direct concurrent measurements of R s and AOD (Norris and Wild, 2009;Ruckstuhl et al, 2010) have confirmed that the AOD impact of R s , i.e., the solar irradiance increase caused by the direct effect of decreasing aerosol, accounts for most of the observed increase of all-sky solar radiation over Europe from 1981 to 2005.…”

Aerosol emissions and dimming/brightening in Europe: Sensitivity studies with ECHAM5-HAM

“…[60] Concerning the relevance of indirect aerosol effects for European SSR changes, studies based on observational data suggest that there is little, if any, contribution from such effects [Norris and Wild, 2007;Ruckstuhl et al, 2008Ruckstuhl et al, , 2010. What can we learn in this respect from the comparison of modeled clear-sky and all-sky SSR trends?…”