Saved by the storm?
Clouds formed by thunderstorms may help brake global warming. They're already challenging climate forecasts.
Summer in south Florida seems to bring out as many towering thunderheads as it does bottles of sunscreen. For tourists, the storms can quickly douse beach plans. But for scientists, they generate a type of cloud that lies at the heart of one of the biggest mysteries in climate forecasting. These tropical cirrus clouds do double duty: They can trap heat like a blanket or reflect sunlight back into space like a mirror. No one has yet figured out which effect dominates and whether cirrus clouds represent a natural brake on global warming.Skip to next paragraph
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Now, two years after they wrapped up the largest airborne assault ever on these clouds, scientists are closing in on an answer. Although more tests will be needed beyond Florida, the data already are yielding insights that promise a major overhaul in the way climate forecasters treat cirrus formations.
"We've got some amazing results that no one anticipated," says Anthony Del Genio, a climate modeler with NASA's Goddard Institute for Space Studies at Columbia University in New York. "It's humbling to find out how often you're wrong."
Among the surprises: the possible presence of natural "antifreeze" at 50,000 feet, the key role of distant pollution, and smaller than expected ice crystals, which dramatically bolster both the mirror and blanket effects.
At first glance, spending up to $20 million on flying six aircraft, enlisting three satellites, deploying three sensor-laden ground stations, and tapping 450 scientists might seem like overkill for a research target as tenuous as tropical cirrus clouds.
Researchers beg to differ.
"When you look down on the earth from space, especially over the tropics, you see these gigantic cirrus anvils" topping thunderheads there, notes Owen Toon, a climate scientist at the University of Colorado at Boulder. These cirrus clouds play a significant role in determining the amount of sunlight the planet reflects back into space. Without the clouds, the light would reach the surface to be absorbed and reradiated as heat. "So you want to know how bright the cirrus are and what controls that brightness," he says. The brightness tells scientists how much light is being reflected back into space. The size of the cloud's ice crystals plays a key role in setting its brightness.
More important is the story the cirrus clouds tell about water vapor - the atmosphere's most abundant greenhouse gas - high in the atmosphere.
When humans add carbon dioxide to the atmosphere, only about 25 percent of the atmosphere's warming comes from the CO2, Dr. Toon says. The rest comes from related effects. The most important of these effects is the addition of water vapor to the atmosphere as temperatures warm and seawater evaporates and builds convective thunderheads in response.
Water vapor is "a huge amplifier of the greenhouse effect," he adds. And the portions of the atmosphere the research team studied - the upper troposphere - is "where the lever is."
If warming placed increasing amounts of water vapor in the upper troposphere, warming would be expected to continue.
On the other hand, "if something up there worked [in] the other direction and stabilized things, that would keep the greenhouse effect from growing," he says. If thunderheads grow more vigorously as the climate warms, for example, this could accelerate the downdrafts surrounding the thunderheads. The souped-up downdrafts might pull increasing amounts of moisture out of the upper troposphere, offsetting warming.
The month-long field project, known as CRYSTAL-FACE, highlights the interplay of real-world measurements and computer simulations in climate research.
One of the most fundamental questions surrounds the size of the ice crystals that make up tropical cirrus clouds. A team led by Timothy Garrett at the University of Utah found that the ice crystals in anvil cirrus over south Florida are smaller and reflect light more effectively than most models assume. The results suggest that when the clouds are thick as they first form over the top of a thunderhead, they reflect substantially more light back into space than models currently show.