|The rate of warming increased by a factor of 3.8 from 1992 to 2002 corresponding to the period of "global brightening," and was followed by global cooling.|
*CO2 forcing 1992 - 2002 calculated using the IPCC formula: 5.35*ln(373/356) = .25 Wm-2
Journal of Climate 2012 ; e-View
Kaicun Wang,1 Robert E. Dickinson,2 Qian Ma,1 John A. Augustine,3 and Martin Wild4
1 State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, 100875, China
Surface incident solar radiation (G) determines our climate and environment. G 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”), i.e., at a rate of -3.5 W m−2 per decade (or -2% per decade) from 1960 to 1990. Since the early 1990s, the diffuse and direct components of G have been measured independently and a more accurate G was calculated by summing these two measurements. Data from this summation method have suggested that G has increased at a rate of 6.6 W m−2 per decade (3.6% per decade) 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, we used 17 years (1995-2011) parallel measurements by the two methods from nearly 50 stations to test whether these two measurement methods of G provide similar long-term trends. Our 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 per decade 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, i.e., the dimming might have been less if measured with current summation methods.