A while ago, we wrote about Global Dimming – a reduction in downward solar radiation of about 4% or about 7W/m2 from 1961 to 1990 was found at stations worldwide. We said at the time that there were hints of a recovery underway post-1990; now research has been published showing this. From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth’s Surface by Martin Wild et al. (Science 6 May 2005; 308: 847-850; subscription required for link) uses surface measurements; Do Satellites Detect Trends in Surface Solar Radiation? by Pinker et al., Science 2005 308: 850-854 uses satellites; both find a recovery of surface downward radiation since about 1990.
Wild et al. use high-quality surface observations primarily from Europe, North America, China, Australia, Japan and Antarctica but also some from Africa and India. The densest network of observations is over Europe: binning the observations into equal-area cells, and considering the two periods 1950-1990 and 1985-2000, they find 24 showing decreases and 8 increases for the earlier period. For the latter, this reverses with 26 showing increase and 6 decrease (and none of those 6 are statistically significant). Around the world (although there are large gaps in the network) a similar pattern of recent increase is seen. Evidence for continued dimming seems to be restricted to India and Zimbabwe. The average increase, estimated from the 8 most accurate stations, was 0.66 W/m2/year.
Pinker et al. derive surface downward shortwave radiation from satellite measurements since 1983. There must be some slight cautions about the quality of the satellite data, and Pinker et al. devote considerable space to an analysis of why they think their data can be trusted. For their period, they find a (significant) global linear trend of 0.16 W/m2/year, which is about 0.1%/year. Fitting a second-order polynomial to the same data shows a small decreasing trend to about 1992, with increases since then. Since they have global data, they can split it into land and ocean, and do: finding an insignificant negative trend over land and a significant positive trend, 0.24 W/m2/year, over the oceans. The Wild et al. paper discussed first used land stations only, of course. The Wild trends are larger, but comparing 8 land points to global data is difficult.
So what does this all mean? The “dimming” may have lead to a slight negative radiative forcing, somewhat masking the global warming signal; the reversal, Wild et al. suggest, may have removed this masking effect and lead to the signal being more obvious in the 1990s. Aerosol emissions have decreased, particularly in Europe and the US over the 1990s, largely due to clean air legislation. Thus relative to the 1980s there was probably an additional positive forcing from the aerosol decrease. However, as before we cautioned against over-interpreting the importance of the dimming, we offer similar cautions for the brightening.