Earth Lightens Up
Long, C. N., Pacific Northwest National Laboratory
Radiation Measurements
Radiative Processes
Wild, M., H. Gilgen, A. Roesch, A. Ohmura, C. N. Long, E. G. Dutton, B. Forgan, A. Kallis, V. Russak, and A. Tsvetkov, (2005): From dimming to brightening: Decadal changes in solar radiation at the Earth's surface, Science, 308, Issue 5723, 847-850, [DOI:10.1126/science.1103215]
Global distribution of surface observation sites used in this study. Sites measuring an increase in surface solar radiation after 1990 are marked in yellow; sites measuring a decrease are shown in brown. High-quality observation sites fulfilling the Baseline Surface Radiation Network standards are shown as triangles; other sites from the updated Global Energy Balance Archive are shown as crosses. Information from 300 sites across Europe and 45 sites across Japan are displayed as aggregated regional means. The majority of the sites show an increase in surface solar radiation after 1990.
"Dimming" and "brightening" are terms scientists are currently using to describe trends that have an impact on the amount of solar energy reaching the Earth's surface. Past studies have shown a decreasing trend in downwelling solar energy reaching the Earth's surface occurring since the 1950s through the 1980s, an apparent "dimming" trend. In the May 6 issue of Science magazine, a paper co-authored by a researcher from DOE's Atmospheric Radiation Measurement (ARM) Program shows a reversal in the trend starting in the mid to late 80s, resulting in a widespread "brightening" trend. The article, titled "From dimming to brightening: Decadal changes in solar radiation at the Earth's surface," highlights more recently available and expansive data, such as long-term cloudless sky estimates from algorithms developed through the ARM Program. Because a decrease in solar energy input to the surface would decrease the amount of warming of the surface air, the "dimming" trend may have acted to somewhat mask the projected global greenhouse warming trend. With the current "brightening" trend, the projected greenhouse warming signal might become more apparent.
Presently, scientists can only speculate as to what phenomena might be responsible for solar dimming and brightening trends. Possibilities include changes in atmospheric aerosol amounts or types, and aerosol/cloud interactions. As discussed in the paper, at some sites the brightening trend is about the same for cloudless skies as it is for cloudy skies, suggesting that clouds play little, if any, role in the phenomenon. However, it must be noted that "cloud" is a nebulously defined term in the sense of being distinguished from hazy conditions. Haze is included in the "cloudless sky" category, but includes the same types of aerosol interactions as clouds. Thus, in these "same trend" cases, the similarity in clear/cloudy trend magnitude does not necessarily mean a difference in cause. Other sites in the study exhibit distinct differences in trend magnitude between the cloudless and cloudy results, adding complexity to the puzzle.
Though the possible causes of these subtle dimming and brightening trends were beyond the scope of the article, it is only through long-term detailed radiation, cloud, and aerosol measurements that the causes might be determined. The DOE ARM Climate Research Facility—with sophisticated surface remote sensing sites in several locations around the world to sample tropical, mid-latitude, and polar climates—is currently the only source of the needed data. In addition, ARM scientists are among the world leaders in the development of techniques for retrieving cloud microphysical properties from these measurements, which are essential for studying any possible cloud/aerosol interactions as the cause.
