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Environment

43: Man-Made Particles Dim Sun

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It may have escaped your notice, but the skies are significantly darker these days than they were when John F. Kennedy was president. There is simply more stuff in our atmosphere, scientists say, and much of it is man-made.

In May climatologists Michael Roderick and Graham Farquhar of the Australian National University released data suggesting that a little-known trend, commonly called solar dimming, is a global phenomenon. Their study involves the rate of water evaporation from metal pans placed at various land sites around the world. Because Earth has been on a steady warming trend, it would stand to reason that water would evaporate more quickly now than in, say, the 1960s. But observations show otherwise. “Pan evaporation has been, on average, declining,” says Roderick.

Many scientists think the reason is due to aerosols like smoke that are man-made, particulates from volcanic eruptions, and increased cloudiness. Tiny chemical particles thrown into the atmosphere reflect sunlight back into space. The particles may also contribute to cloud formation. Although the net observed effect appears unnoticeable, agricultural scientists are concerned that dimming will adversely affect crop productivity.

At least two groups of climatologists, including Helen Power at the University of South Carolina and Martin Wild of the Swiss Federal Institute of Technology, caution that solar dimming may have slowed or even reversed in recent years, but they say it will take some time for them to sort through recent dimming data to be certain.

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