Kaicun Wang¹, Robert E. Dickinson², Qian Ma³, John A. Augustine⁴, Martin Wild⁵

¹ State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, China;

² Department of Geological Sciences, The University of Texas at Austin, Austin, Texas;

³ College of Global Change and Earth System Science, Beijing Normal University, Beijing, China;

⁴ NOAA/Earth System Research Laboratory, Boulder, Colorado;

⁵ Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland.

 

Abstract: Surface incident solar radiation G determines our climate and environment, and has been widely observed with a single pyranometer since the late 1950s. Such observations have suggested a widespread decrease between the 1950s and 1980s (global dimming), that is, at a rate of -3.5 W m^-2 decade^-1 (or -2% decade^-1) from 1960 to 1990. Since the early 1990s, the diffuse and direct components of G have been measured independently, and a more accurate G has been calculated by summing these two measurements. Data from this summation method suggest that G increased at a rate of 6.6 W m^-2 decade^-1 (3.6% decade^-1) from 1992 to 2002 (brightening) at selected sites. The brightening rates from these studies were also higher than those from a single pyranometer. In this paper, the authors used 17 years (1995–2011) of parallel measurements by the two methods from nearly 50 stations to test whether these two measurement methods of G provide similar long-term trends. The results show that although measurements of G by the two methods agree very well on a monthly time scale, the long-term trend from 1995 to 2011 determined by the single pyranometer is 2–4 W m^-2 decade^-1 less than that from the summation method. This difference of trends in the observed G is statistically significant. The dependence of trends of G on measurement methods uncovered here has an important implication for the widely reported global dimming and brightening based on datasets collected by different measurement methods; that is, the dimming might have been less if measured with current summation methods.