Is Earth Getting Darker?

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The sun is not growing weaker, yet its light appears to be dimming. Between 1960 and 1990, some scientists believe, the amount of solar radiation reaching Earth's surface may have declined as much as 10% — and in some places, Hong Kong, for example, more than 35%.

What was going on? Well, it appears that increased air pollution during those 30 years — over Asia, in particular — with the help, perhaps, of some increased cloudiness, may have exerted a cooling influence on the surface of the planet even as carbon dioxide and other greenhouse gases were encouraging the atmosphere to warm. The impacts of that tug-of-war on the climate system could be devilishly difficult to untangle.

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At the same time, no task could be more urgent. For if global pollution has helped keep global warming in check, says Veerabhadran Ramanathan, an atmospheric scientist at the University of California at San Diego, then the full impact of the buildup of greenhouse gases has yet to be felt. This week, at the American Geophysical Union meeting in Montreal, Ramanathan and others will be presenting the latest data on the solar-dimming problem and pondering its implications for the climate system as a whole.

Many scenarios for global warming, for example, invoke a speedup in the hydrological cycle by which water evaporates and then comes down as rain. The cooling produced by solar dimming, however, may slow the rate of evaporation, while higher up in the atmosphere the pollutants responsible for absorbing and reflecting sunlight are likely to interfere with the process that produces rain.

Why? These pollutants, which take the form of tiny, airborne particles called aerosols, act as nuclei around which cloud droplets form. The problem is, there are too many aerosols in the atmosphere competing for water molecules, so the cloud droplets that form are too small and never become weighty enough to fall to the ground. As a result, says Beate Liepert, an atmospheric physicist at Columbia University's Lamont-Doherty Earth Observatory, the atmosphere could be filled with moisture while Earth's surface thirsts for rain.

Many questions remain, including the true extent of the dimming. One analysis pegs the average worldwide darkening to be about 4% over three decades, while another computes it to be more than twice that much. There are also questions about the reliability of the devices that measure the sunlight reaching Earth's surface. Known as radiometers, these instruments are nothing more than flat, black solar collectors capped with glass. They are sometimes finicky; a smudge of dirt or a speck of dust can cause bogus readings and change the calculated results.

Solar dimming, in other words, is a problem still in the process of being defined, and as its dimensions become clearer, so will the nature of the challenge the world faces. Although scientists have done a lot of thinking about global warming, they are just beginning to grapple with the problem of how global warming and solar dimming interact. As Ramanathan puts it, "It's like we have a new gorilla sitting down at the table"--and it could turn out to be a very big gorilla indeed.