Atmospheric impacts on climatic variability of surface incident solar radiation

Abstract. The Earth's climate is driven by surface incident solar radiation (R_s). Direct measurements have shown that R_s has undergone significant decadal variations. However, a large fraction of the global land surface is not covered by these observations. Satellite-derived R_s has a good global coverage but is of low accuracy in its depiction of decadal variability. This paper shows that daily to decadal variations of R_s, from both aerosols and cloud properties, can be accurately estimated using globally available measurements of Sunshine Duration (SunDu). In particular, SunDu shows that since the late 1980's R_s has brightened over Europe due to decreases in aerosols but dimmed over China due to their increases. We found that variation of cloud cover determines R_s at a monthly scale but that aerosols determine the variability of R_s at a decadal time scale, in particular, over Europe and China. Because of its global availability and long-term history, SunDu can provide an accurate and continuous proxy record of R_s, filling in values for the blank areas that are not covered by direct measurements. Compared to its direct measurement, R_s from SunDu appears to be less sensitive to instrument replacement and calibration, and shows that the widely reported sharp increase in R_s during the early 1990s in China was a result of instrument replacement. By merging direct measurements collected by Global Energy Budget Archive with those derived from SunDu, we obtained a good coverage of R_s over the Northern Hemisphere. From this data, the average increase of R_s from 1982 to 2008 is estimated to be 0.87 W m⁻² per decade.