A Cozy Layer of Filthy Air

When dust from Earth's surface is swept up into the atmosphere or belched there by volcanoes, it blocks sunlight, leading to cooler global temperatures. But a paper in this week's issue of Nature attempts to turn this conventional wisdom on its head: It presents evidence that an increase in atmospheric dust levels tends to warm ice-covered regions and may even have caused armadas of icebergs to break away from northern ice sheets during ice ages.

Dust tends to reflect light, which increases the Earth's overall albedo, or reflectivity. The higher the albedo, the less efficient sunlight is at raising global temperatures. Therefore, the notion that dust might in some regions actually promote warming "strikes me as being counterintuitive," says Scott Lehman, a climatologist at the University of Colorado.

But Jonathan Overpeck, head of the National Oceanic and Atmospheric Administration's paleoclimatology program, and his colleagues performed a computer simulation of global temperatures based on ice-core samples. The simulation found that during ice ages, dust tended to increase the temperature of icy regions--such as Greenland and northern Asia--by 5 degrees Celsius or more. The reason, says Overpeck, is that while dust has a higher albedo than most land and ocean features, it is less reflective than ice sheets. So while extra atmospheric dust may increase Earth's overall albedo, the reflectivity of icy regions decreases. "Too much or too little of something can produce the opposite effect of what you would expect," says Paul Mayewski, a climatologist at the University of New Hampshire.

Overpeck argues that this counterintuitive warm-up may be responsible for Heinrich events, warming periods during ice ages that cause herds of icebergs to calve from ice sheets. "The trigger [of the warming periods] is elusive," says Overpeck. "We provide another clue regarding what can generate these events."

Some experts are unconvinced. "I'm not ready to accept dust as a trigger," says Lehman. "Nevertheless," he says, "the results are interesting."