High-Resolution Image

New analyses by Long et al., just submitted to the Journal of Geophysical Research, show that, over the continental US, sunlight reaching the surface has increased dramatically over the past decade (so-called “global brightening”). Most of this brightening is due to changes in clouds, not to reductions in aerosol optical depth. This effect is not captured well, or at all, by climate models since their errors in cloud and aerosol amount are at the order-zero level and brightening is caused by order-one changes in cloud and/or aerosol. Nevertheless, the amounts of radiation involved, several Watts/m^2 per decade, are order-zero for climate change and in particular for those changes in precipitation that are related to surface fluxes.

The top panel of the figure shows the brightening for 1996 through 2007 for the best radiation sites in the continental US: the ARM Oklahoma site and the six NOAA SURFRAD sites located in rural areas of Pennsylvania, Mississippi, Illinois, Montana, Colorado, and Nevada. Brightening has occurred at every site, averaging about 8 Wm^-2/decade, which is substantially greater than the 2 Wm^-2/decade reported by Wild et al. [2008] over most of the Earth’s land surface from 1986 to 2000, and which is more than twice the magnitude of the 2-3 Wm^-2/decade increase in downward surface longwave radiation during the same time period. While changes in longwave radiation at the tropopause are the prime mover of global warming, at the surface things are quite different: changes in solar radiation are of equal or greater importance (to the net radiation budget).

Brightening is commonly attributed to decreasing aerosol optical depth. However, these new results show that reductions in dry aerosols and/or direct aerosol effects alone cannot explain even half of the brightening. Changes in cloudiness play the dominant role. The lower panel of the figure shows the strong correlation between cloud fraction changes and brightening on seasonal time scales. A similar figure (not shown) for aerosol shows only a weak correlation between aerosol optical depth and brightening.

A conclusion of this work is that cloud and especially aerosol effects on brightening require local and regional explanations. Yet the current brightening is occurring globally except in areas like India and China which are injecting massive amounts of pollution aerosol. Thus, while the variety and balance of individual causes is great, they work to create a brightening almost everywhere. This would indicate some overall principle at work, whose details play out differently in different regimes.

The network of US and indeed worldwide officially-certified radiation sites is still far smaller than needed, considering the essential role radiation plays in climate. It has languished while networks like AmeriFlux have grown from zero to over a hundred carbon-flux towers just in the US alone. A considerably expanded radiation network is needed to address the issue of global brightening with the precision needed for climate work.

(submitted by Warren Wiscombe (NASA) and Chuck Long, Pacific Northwest National Laboratory)