Surface insolation trends from satellite and ground measurements: Comparisons and challenges

Global "dimming'' and "brightening,'' the decrease and subsequent increase in solar downwelling flux reaching the surface observed in many locations over the past several decades, and related issues are examined using satellite data from the NASA/Global Energy and Water Cycle Experiment (GEWEX) Surface Radiation Budget (SRB) product, version 2.8. A 2.51 W m(-2) decade(-1) dimming is found between 1983 and 1991, followed by 3.17 W m(-2) decade(-1) brightening from 1991 to 1999, returning to 5.26 W m(-2) decade(-1) dimming over 1999-2004 in the SRB global mean. This results in an insignificant overall trend for the entire satellite period. However, patterns of variability for smaller regions (continents, land, and ocean) are found to differ significantly from the global signal. The significance of the computed linear trends is assessed using a statistical technique that accommodates the autocorrelation typically found in surface insolation time series. Satellite fluxes are compared to measurements from surface radiation stations on both a site-by-site and ensemble basis. Comparison of an ensemble of the most continuous Global Energy Balance Archive (GEBA) sites to SRB data yields a root-mean-square difference and correlation of 2.6 W m(-2) and 0.822, respectively. However, the GEBA time series does not correspond well to the SRB global mean owing to its extremely limited distribution of sites. Simulations of the Baseline Surface Radiometer Network using SRB data suggest that the network is becoming more representative of the globe as it expands, but that the Southern Hemisphere and oceans remain seriously underrepresented in the surface networks. This study indicates that it is inappropriate to describe the variability of global surface insolation in the current satellite record using a single linear fit because major changes in slope have been observed over the last 20 years. Further efforts to improve the quality of satellite flux records and the spatial distribution of surface measurement sites are recommended, along with more rigorous analysis of the origins of observed insolation variations, in order to improve our understanding of both long- and short-term variability in the downwelling solar flux at the Earth's surface.