Surface Radiation Measurements

The Global Energy Balance Archive (GEBA) contains measurements of downwelling solar radiation at the surface that were made at various locations around the world during the past several decades, primarily in North America and Eurasia and only over land (Gilgen and Ohmura 1999). Not all stations are trustworthy; many report unrealistically large interannual and decadal variations in solar radiation flux, particularly in the United States and in developing countries. Furthermore, sensor degradation may lead to a spurious reduction in observed solar radiation (M. Wild, pers. comm.). Nonetheless, several studies have made use of high-quality stations and documented a geographically widespread decrease in surface solar radiation from the 1960s until the mid-1980s, followed by an increase from the 1990s onward (Wild et al. 2005). These trends are popularly known as "global dimming" and "global brightening," even though they do not necessarily occur around the entire world. The reported trends probably represent regional rather than global mean changes in downwelling solar radiation, and may suffer from an urban bias (Alpert et al. 2005).

Several explanations for the observed "global dimming" and "global brightening" have been offered, including trends in cloud cover and cloud optical thickness. The most plausible general cause, however, is a rise followed by a decline in anthropogenic aerosol burden. According to this scenario, haze from fossil fuel and biomass burning has increased since the middle of the 20th century and has, as a result, enhanced atmospheric reflection and absorption of solar radiation ("global dimming"). Air pollution control laws enacted during the 1980s and later in certain countries reduced this haze, thus allowing more solar radiation to reach the surface ("global brightening"). Another likely factor contributing to "brightening" in Europe was the shutdown of fossil fuel combustion sources following the collapse of communism in Eastern Europe. In certain developing countries such as India, "global dimming" continues and there has been no reversal to "brightening" (Wild et al. 2005).

It has been difficult to distinguish impacts of cloud changes from aerosol changes using GEBA data because the values typically exist only as monthly averages of the direct plus diffuse flux. Thus, cloudy intervals cannot be separated from cloudless intervals in the time series so as to distinguish cloud radiative effects and potential aerosol impacts on clear-sky solar radiation. This shortcoming is alleviated in the Baseline Surface Radiation Network (BSRN) dataset, which provides data on downwelling SW and LW radiation every minute at a small number of stations as of1990 (Ohmura et al. 1998). For the larger and lengthier GEBA record, the radiative effects of cloud cover variations on surface solar radiation can be reliably estimated from surface synoptic cloud reports. The recent study of Norris and Wild (2007) demonstrated that cloud cover variability, associated with weather and atmospheric circulation regimes, was the dominant cause of interannual anomalies in surface solar radiation over Europe but not of long-term trends. Removal of the cloud cover contribution from the solar radiation time series revealed more distinct "dimming" and "brightening" trends than were apparent in previous studies.

Retrievals of the components of the surface radiation budget are available from several satellite programs (e.g., Gupta et al. 1999, Zhang et al. 2004); however, care must be taken in using them because a signifi cant amount of modeling is used to create such products. A score or so of well-instrumented surface sites (e.g., BSRN) provide a reference both for satellite retrievals and for the much larger number of surface sites with limited instrumentation. It must also be remembered that the signals of an enhanced greenhouse effect are much larger at the surface than at the top of the atmosphere, so direct surface measurements should play an important part in the detection and attribution of global warming.

One intriguing, new potential method of monitoring changes in the Earth's global albedo is that of "earthshine" observations (Pallé et al. 2004). This approach would use a network of surface telescopes to measure (a) variations in the brightness of the non-sunlit part of the Moon and thus (b) how much solar radiation is reflected by the Earth to the Moon and back again.

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