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