Send in the clouds

During the summer of 1976, a dozen years before climate change burst on the scene as a hot-button issue, climate scientist Bruce Wielicki saw the future, and it was in clouds.

A grad student at the time, Dr. Wielicki was summering at the Woods Hole Oceanographic Institution on Cape Cod, taking part in a 12-week brainstorming session on climate.

At one point, he recalls, he teamed up with a climate modeler studying how the Earth and atmosphere balance the energy they receive from the sun. Wielicki had been working on a simple computer model to simulate the effect changing sea-surface temperatures have on climate, including cloudiness. The two decided to see what would happen if they linked their models.

"I added clouds to his model, and of course, they blew his results right up," Wielicki says, laughing. "He was sure I'd messed up something in my calculations. So we spent the summer figuring out that, no, my calculations were right and clouds were that powerful."

"That was 25 years ago," he says, "and we still haven't figured clouds out."

Indeed, Joni Mitchell's '60s-era lyrics still resonate with climate scientists today as they probe the mysteries of clouds and their impact on Earth's climate system.

"We've made a lot of progress" in understanding and modeling clouds and their impact on climate, acknowledges Jeffrey Kiehl, who heads the climate modeling section at the National Center for Atmospheric Research in Boulder. Colo. "But in the last five years, progress has flattened out in trying to sort out cloud processes." Clouds are still a weak point, he says, in reducing the uncertainties in climate forecasts.

Small clouds hard to gauge

Some of the challenges, he notes, lie in the nature of modeling itself. Many of the key processes that take place in clouds occur on scales so small that large climate models can't see them. As knowledge about these small-scale processes grows, the potential to include them in models exists, researchers say, but to do so would require an enormous increase in computing power. As an alternative, researchers are using smaller-scale models that can accurately capture features, important to events such as thunderhead formation, and then using those results as reality checks for how larger-scale models treat such phenomena.

Challenges also lie in understanding basic cloud processes, such as factors that influence where a cloud's base forms or what determines how high clouds get, Dr. Kiehl says. "These are critical to understanding whether clouds present a positive or negative feedback to the system" - essentially whether they act to heat or cool the planet.

A thick layer of stratus clouds, for example, which can range from fog at ground level to cloud masses a few hundred feet above the surface, typically moderates daytime temperatures by reflecting most of the sunlight striking them back into space.

High-flying cirrus clouds, on the other hand, can be thin enough to let sunlight through while trapping heat that rises from Earth's surface. Researchers note that when the sun is low on the horizon, sunlight can strike the underside of high-latitude cirrus layers, which reflect that radiation back toward Earth.

By some estimates, a 50 percent increase in cirrus-cloud cover could warm the climate much more than a 50 percent increase in atmospheric carbon dioxide.

In other cases, the challenge lies in more subtle interactions between clouds and other constituents of the atmosphere.

Enough progress has been made on basic cloud processes, that to continue to focus on them "may be looking under the streetlight for the lost keys," says James Hansen, a climate researcher at the National Aeronautics and Space Administration's Goddard Institute for Space Science in New York. He holds that the "real uncertainty lies in the effect aerosols have on clouds."

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