The amount of sunlight reaching Earth"s surface appears to be growing. The phenomenon, which some dub "global brightening," (1)_____ scientists with a puzzle. If the (2)_____ is real and global, how long will it last and what are the consequences for climate change, the planet"s water cycle, and other (3)_____ that draw energy from sunlight? (4)_____, the answer might seem obvious: More sunlight reaching the ground in a warming world means that temperatures will get warmer (5)_____ Not so fast, some researchers say. Additional warming would be certain (6)_____ nothing else in the climate system changes. And the climate system is (7)_____ static. Some combinations of changes could reinforce the heating; others could (8)_____ it. Unraveling these interactions and forecasting their course require an accurate accounting of the sunlight reaching the surface and the (9)_____ the surface sends skyward. Moreover, researchers say, measurements of the sun"s strength at Earth"s surface are potentially powerful tools for (10)_____ human influences on the climate. Earth"s radiation "budget" (11)_____ an "extremely important parameter that is (12)_____ known," says Robert Charlson, an atmospheric scientist at the University of Washington at Seattle. "It needs to be (13)_____ much better than it is." (14)_____ about the amount of sunlight reaching Earth"s surface were first raised in 1974.@Researchers from the United States and Israel recorded a 12% drop (15)_____ sunlight over 40 years at a (16)_____ station in the southern Sinai Peninsula. Since then, others have used a variety of techniques to try to track (17)_____ sunlight. Three years ago, for example, a (18)_____ led by Beate Liepert at Columbia University"s Lamont-Doherty Earth Observatory gathered data from ground (19)_____ around the world and found that solar radiation reaching the surface fell (20)_____ 4% from 1961 to 1990.