Darker at noon: less sunlight reaching Earth

In the second half of the 20th century, Earth became, quite literally, a darker place.

Defying expectation and easy explanation, hundreds of instruments around the world recorded a drop in sunshine reaching the surface of Earth, as much as 10 percent from the late 1950s to the early '90s, or 2 to 3 percent per decade. In some areas like Asia, the United States and Europe, the drop was even steeper. Hong Kong saw a 37 percent decrease in its sunlight.

No one is predicting that it may soon be night all day, and some scientists believe the skies have brightened in the past decade as the suspected cause of global dimming -- air pollution -- clears up in around the world.

Still, the dimming trend -- noticed by a handful of scientists two decades ago but dismissed as unbelievable -- is now attracting wide attention. Satellite measurements show that the sun remains as bright as ever, but less and less sunlight has been making it through the atmosphere to the ground.

Pollution dims sunlight in two ways, scientists believe. Some light bounces off soot particles in the air and back into outer space. Pollution also causes more water droplets to condense out of air, leading to thicker, darker clouds that also block more light.

But the dynamics and effects of global dimming are not completely understood. Antarctica, which would be expected to have clean air, has also dimmed.

"In general, we don't really understand this thing that's going on," said Dr. Shabtai Cohen, a scientist in the Israeli Agriculture Ministry who has studied dimming for a decade. "And we don't have the whole story."

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