The Earth's accidental sun shade -Studies show days have been growing dimmer for years Gerald Stanhill was reviewing solar radiation records in the late 1980s when he stumbled onto something that amazed him: Days were growing dimmer. "It was difficult to believe," said Stanhill, an Israeli government scientist. "So I started looking around for other continuous, long series of solar radiation measurements to make sure it wasn't an error." It wasn't. He found that scientific papers since 1973 have documented downward trends in surface solar radiation in Japan, the Soviet Union and Germany, among other places. Stanhill, in about a dozen papers beginning in 1992, went on to detail declines in Israel, Australia, Hong Kong, Ireland and even the Arctic and Antarctic. In 1998, researchers at the Swiss Federal Institute of Technology published a 30-year analysis that found a 10 percent average drop at sites concentrated in the Northern Hemisphere. Three years ago, Stanhill, emeritus biologist of Israel's Agricultural Research Organization, and agency colleague Shabtai Cohen published a study of all available evidence. They found surface solar radiation had dropped an average 3 percent per decade since the late 1950s. In all, sunlight had plummeted more than 10 percent worldwide between 1958 and 1992. Stanhill called the phenomenon "global dimming." A dramatic drop in sunshine could have implications for everything from solar energy to plant growth, but its main impact may be on our understanding of man- made climate change, or global warming. Indeed, many climate scientists who read the 2001 paper either ignored it or insisted -- given that global dimming seems to run counter to the overall warming trend -- that the measurements had to be wrong. "It's very difficult to explain how there can be a substantial reduction in solar radiation at the Earth's surface and at the same time a substantial increase in the temperature," Stanhill said. As a result, said Veerabhadran Ramanathan, an atmospheric scientist with Scripps Institute of Oceanography in La Jolla, Calif., "many took the easy route to just disbelieve it." Then came the work of Graham Farquhar and Michael Roderick at Australian National University in Canberra, studying evaporation. For years, researchers had been puzzled by a steady, worldwide decrease in the amount of water that evaporates from open pans, despite the Earth's rising temperatures. Farquhar, a plant physiologist, and Roderick, an ecologist, came up with an explanation. They compared 50 years of evaporation measurements with Stanhill's solar radiation measurements and got a match. Less sunlight was hitting the water's surface, they reasoned, resulting in less surface heat and thus less evaporation. For solar power generation, the consequences of global dimming are obvious: Less sunshine falling on solar cells generates less electricity. For plant growth, the consequences are murkier. Even though less sunlight is reaching plants, a greater fraction of that light is scattered, which can be beneficial. "I think it's a bit of an open question what (global dimming) does for photosynthesis," Farquhar said. Farquhar and Roderick published their results in the journal Science in 2002, giving global dimming a toehold in the wider scientific world. The topic has gained enough currency to merit a special session at the joint meeting of the American and Canadian geophysical unions in Montreal this May. What causes global dimming? Scientists think smoky pollution plays a big part. Tiny particles suspended in the atmosphere can reflect or absorb sunlight before it reaches the ground. This promotes formation of bigger, more persistent and more reflective clouds that less readily condense into rain, said Beate G. Liepert, an atmospheric physicist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. Industrialized regions have dimmed the most. For example, Israel's coastal plain now gets 30 percent less solar radiation than 50 years ago, Stanhill said. Ramanathan found that vast brown hazes generated in Asia and hanging over the Indian Ocean and the Himalayas can cut surface sunlight by as much as 30 percent. A worrisome possibility, Liepert said, is that at least some of the decrease in sunshine is connected to the warming caused by greenhouse gases. Unlike particulates, these gases don't wash out of the air in a matter of days once cars and factories stop spewing them out. "Hot air can hold more water," she said -- and under hotter conditions, you get more and thicker clouds. Add up the effect of particulates, greenhouse gases and reduced evaporation and what global dimming might be telling us is that we're in for an even hotter, drier future than anticipated from global warming, both Liepert and Ramanathan said. With only 35 years of data, some researchers aren't ready to go that far. "It's an interesting phenomenon, but maybe it's a bit premature to call it global," said Martin Wild, a climatologist at the Swiss Federal Technical Institute in Zurich. "We have no idea what's going on over the oceans, which cover 70 percent of the Earth." Nor do scientists know how long global dimming has been going on or whether it will last. Researchers are still analyzing records from the last decade. There are early indications that the phenomenon may have eased as pollution controls were taking effect.