Dim view: darkening skies a regional phenomenon.

The decline in the solar radiation reaching Earth's surface in the latter half of the 20th century--a trend observed at many locations worldwide for several decades--turns out to have been primarily a regional phenomenon, new research suggests.

On average, about 342 watts of solar radiation strike each square meter at the top of Earth's atmosphere. As much as one-third of that radiation immediately bounces back into space. A small amount gets absorbed within the atmosphere. The rest--about two-thirds of the total--arrives at the planet's surface.

Radiation reaching the ground at many locations declined significantly from the 1950s to the 1980s, a trend that some scientists have dubbed global dimming. At some sites, the solar radiation dropped as much as 2.7 percent per decade, says Yoram J. Kaufman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

Now, analyses by Kaufman and his colleagues indicate that dimming was much stronger in some regions than in others. The researchers found that solar radiation reaching ground level at 318 sites worldwide declined, on average, about 0.27 watt per square meter (W/[m.sup.2]) each year between 1964 and 1989. However, the 144 of these sites that are located near cities with more than 100,000 residents experienced an even stronger average dimming--about 0.41 W/[m.sup.2] annually, says Kaufman.

The largest dimming was chronicled in and near densely populated sites between latitudes of 10[degrees]N and 40[degrees]N, where most of the world's industrial activity occurs. At those sites, solar radiation dropped about 1.25 W/[m.sup.2] each year. These findings, which dispel the notion that dimming is uniform globally, also hint that the phenomenon has a human cause, says Kaufman. The researchers present their findings in the Sept. 16 Geophysical Research Letters.

Because most of the world is sparsely populated, dimming is essentially a regional effect, Kaufman's team argues. However, even the 0.16 W/[m.sup.2] average annual decline seen at sparsely populated sites adds up over a quarter century to a decrease of 4 W/[m.sup.2], notes Beate G. Liepert, a climatologist at Lamont-Doherty Earth Observatory in Palisades, N.Y.

"That's still a lot," she says.

Kaufman and his colleagues showed that at some tropical sites, the skies got brighter, not dimmer. At the 21 sparsely populated sites between 15[degrees]N and 15[degrees]S, solar radiation reaching the ground rose by 0.58 W/[m.sup.2] per year. That may be a result of decreasing cloudiness in those locales, Kaufman notes.

On average, about 60 percent of the dimming effect comes from increased cloud coverage, Liepert adds.

Understanding the size and distribution of the dimming ether observed in the past, as well as determining how much of it resulted from pollutants and how much from clouds, will enable scientists to refine climate models, says Rachel T. Pinker of the University of Maryland in College Park.

Dimming trends since the early 1990s have been mixed. When communism--and the economies--collapsed in many eastern European countries in the early 1990s, industrial emissions declined and the skies became clearer. More recently, however, skyrocketing industrialization in China, India, and many areas of southern Asia has spawned the Asian brown cloud, a plume of pollution that causes acid rain (SN: 6/16/01, p. 381) and blocks sunlight more effectively than some natural clouds do (SN: 1/6/01, p. 15).