6 May 2005
Global Brightening Creates Warming Worries
by Kate Melville
Earth's surface has been getting brighter for more than a decade, a reversal from a dimming trend that may accelerate warming at the surface and unmask the full effect of greenhouse warming, according to an exhaustive new study of the solar energy that reaches land.
Since the 1980s, scientists have been aware of a dimming trend that began in 1960 that has seen a 4 to 6 percent decline in sunlight reaching the Earth's surface. Since then, atmospheric scientists have been working on theories to explain this trend and how it would relate to the greenhouse effect, the warming caused by the buildup of carbon dioxide and other gasses. But a group of researchers, led by Martin Wild at the Swiss Federal Institute of Technology in Zurich, has been checking figures from the international Baseline Surface Radiation Network (BSRN) archive and come to the surprising conclusion that the dimming trend ended a decade ago and that there are now increasing amounts of sunlight reaching the Earth. The report, appearing in the journal Science, suggests that the planet's surface has brightened by about 4 percent in the past decade. The brightening trend is corroborated by other data, including satellite analyses that are the subject of another paper in the same issue of Science.
Co-researcher Charles N. Long, at the Pacific Northwest National Laboratory, said the researchers were expecting to see the dimming trend continuing. "When we looked at the more recent data, lo and behold, the trend went the other way," said Long.
Sunlight that isn't absorbed or reflected by clouds as it plunges earthward will heat the surface. Because the atmosphere includes greenhouse gasses, solar warming and greenhouse warming are related. "The atmosphere is heated from the bottom up, and more solar energy at the surface means we might finally see the increases in temperature that we expected to see with global greenhouse warming," Long said. He further suggests that we may have already been seeing those effects in our most temperature-sensitive climates, with the melting of polar ice and high altitude glaciers.
The report's authors stopped short of attributing a cause to the cycle of surface dimming and brightening, but listed such suspects as changes in the number and composition of aerosols - liquid and solid particles suspended in air - and how aerosols affect the character of clouds. Long also sounded a cautionary note, saying that 70 percent of the planet's surface is ocean, for which there are no long-term surface brightening or dimming measurements, making conclusions difficult to establish.