Study finds thicker storm clouds over warmer tropical waters, affect climate

September 19, 2002

Over warmer ocean waters, tropical storm clouds become thicker, more extensive and reflect more sunlight back into space than they do over cooler waters, NASA researchers report.

Using data from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite, Anthony Del Genio, a physical scientist at NASA's Goddard Institute for Space Studies, N.Y., and lead author of the study, and his co-author, Columbia University's William Kovari, were able to isolate raining cloud systems and compare the rain they produce with the water that stays in the clouds and reflects sunlight. The researchers found that storm clouds over warmer waters are denser and cover wider areas of the tropics than those over cooler waters.

"We now have a better understanding of tropical storm clouds." said Del Genio. "Such clouds play a key role in global climate change, and it is essential to understand this role thoroughly if we are ever to comprehend humanity's effect on the Earth's climate."

The study provides a clearer picture of the behavior of storm clouds than was previously available. It also runs counter to a recent theory which claims that storm cloud cover lessens as tropical ocean temperatures rise, thus making climate more strongly resistant to the warming that would occur with an influx of greenhouse gases.

A key observation was that while warmer storm clouds do release more of their moisture as rain, as a theory known as "Adaptive Iris" states, such clouds also have more moisture to begin with and thus also form bigger clouds. These NASA researchers believe the theory is inaccurate because it does not take into account all the factors that come into play when storm clouds form over warmer tropical regions.

If the Iris theory were correct, there would be less cloud cover and less humidity and mostly greater heat loss to space, strongly countering the warming effects of an influx of greenhouse gasses. The TRMM satellite data, however, indicate that the climate is more sensitive to warming caused by increases in greenhouse gases.

The Adaptive Iris model claims that tropical clouds cool the Earth, but to a much greater extent than Del Genio's research indicates. The Iris theory predicts that clouds in the tropics grow thinner and less extensive as temperatures rise, thus trapping progressively less heat.

According to the hypothesis, increased warmth in the tropics would create more turbulent storm conditions and cause clouds to quickly drop their moisture as rain, thus leaving less water in the clouds and making them both thinner and less extensive. If this were the case, changes in tropical clouds could potentially cool the atmosphere as fast as greenhouse gas accumulation would heat it, making them a natural damper on global warming. However, such a claim is not supported by these satellite observations.

Scientists agree that the atmospheric concentration of greenhouse gases in the atmosphere will double compared to pre-Industrial Revolution levels within the next few decades, but there is still widespread debate over how great an effect this influx of gases will have on the world's climate. Although doubling the concentration of greenhouse gases like carbon dioxide would by itself raise Earth's temperature, the change in atmospheric composition also may trigger other effects that could amplify or diminish this temperature change.

"It's not just the heat the greenhouse gases themselves will trap," said Del Genio. "We have other factors to worry about. Clouds are one of the most influential climate factors, and one of the most difficult to understand."

All clouds both trap heat and reflect solar energy. They hold heat in like a blanket, preventing it from escaping the Earth, and their white upper surfaces also reflect sunlight back into space before it can warm the atmosphere. The net effect can either heat or cool the planet, depending on how thick, wide and high in altitude the clouds are.

Del Genio emphasizes that although their work should not be used to directly predict climate change, it will allow scientists to develop more accurate climate models that can be used with greater confidence.
The research appears in the Sept. 15th issue of the American Meteorological Society's Journal of Climate.

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NASA/Goddard Space Flight Center

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