FEWER CLOUDS FOUND IN TROPICS

NASA scientists discover new evidence of climate change

After examining 22 years of satellite measurements, NASA researchers find that more sunlight entered the tropics and more heat escaped to space in the 1990s than in the 1980s. Their findings indicate less cloud cover blocked incoming radiation and trapped outgoing heat.

"Since clouds were thought to be the weakest link in predicting future climate change from greenhouse gases, these new results are unsettling," said Dr. Bruce Wielicki of NASA Langley Research Center, Hampton, Va. Wielicki is the lead author of the first of two papers about this research appearing in the Feb. 1, issue of "Science."

"It suggests that current climate models may, in fact, be more uncertain than we had thought," Wielicki added. "Climate change might be either larger or smaller than the current range of predictions."

The observations capture changes in the radiation budget-the balance between Earth's incoming and outgoing energy-that controls the planet's temperature and climate.

The previously unknown changes in the radiation budget are two to four times larger than scientists had believed possible. The reason why and the degree to which it changed are surprising scientists and create a powerful new test for climate models.

Inspired by this puzzle, a research group at NASA Goddard Institute for Space Studies (GISS) developed a new method of comparing the satellite observed changes to other meteorological data.

"The new method is a conceptual breakthrough in how we analyze data," said Anthony Del Genio, a scientist at GISS and co-author of the companion paper.

"What it shows is remarkable," said Wielicki. "The rising and descending motions of air that cover the entire tropics, known as the Hadley and Walker circulation cells, appear to increase in strength from the 1980s to the 1990s. This suggests that the tropical heat engine increased its speed."

The faster circulation dried out the water vapor that is needed for cloud formation in the upper regions of the lower atmosphere over the most northern and southern tropical areas. Less cloudiness formed allowing more sunlight to enter and more heat to leave the tropics.

In response, several of the world's top climate modeling research groups agreed to take on the challenge of reproducing the tropical cloud changes. But the climate models failed the test, predicting smaller than observed variability by factors of two to four.

"It's as if the heat engine in the tropics has become less efficient using more fuel in the 90s than in the 80s," said Wielicki. "We tracked the changes to a decrease in tropical cloudiness that allowed more sunlight to reach the Earth's surface. But what we want to know is why the clouds would change."

The results also indicate the tropics are much more variable and dynamic than previously thought.

"The question is, if this fluctuation is due to global climate change or to natural variability," said Del Genio. "We think this is a natural fluctuation, but there is no way to tell yet."

While the current 22-year radiation budget record-the longest and most accurate ever compiled-is still too short to pinpoint a cause, the newly discovered change acts as a standard by which to measure future improvements in cloud modeling.

"A value of this research is it provides a documented change in climate and a target for climate models to simulate," said Del Genio.

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The major atmospheric circulation systems are shown. The Hadley cells are composed of rising air in the heart of the tropics and sinking air in the subtropics. The rising branch of the Hadley cell is characterized by thick deep convective clouds similar to our summer thunderstorms. The sinking branch is characterized by deserts over land, and over ocean by sheets of low level cloud near the surface with clear air above.

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Outgoing Longwave radiation is the heat that the Earth sheds into deep space. Very high altitude ice clouds in the tropics trap longwave radiation, and cause the low emission over India in this image from the CERES (Clouds and the Earth's Radiant Energy System) instrument on NASA's Terra satellite. These clouds are at temperatures equal to or colder than the Antarctic at the bottom of the globe, and as a result, allow very little heat to escape to space. Changes in tropical cloudiness therefore have a profound effect on heat loss to space by the atmosphere and surface. Image courtesy of NASA SVS.

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The graph shows the amount of heat emitted from the Earth (Longwave Anomaly in Watts per meter squared) since 1979. Results are shown from an integration of observations over the entire tropics (20N latitude to 20S latitude) from satellite instruments on seven different spacecraft, a task never before attempted. Anomalies are departures from the average value during the 1985 through 1989 baseline period. Image courtesy of NASA CERES (Clouds and the Earth Radiant Energy System) Science Team.

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Illustration of the Walker Cell. Rising convective motion over the warmer ocean water near Indonesia in the western Pacific Ocean, and sinking motions over the cooler Eastern Pacific ocean. There are three such major East/West cells in the tropics, the Walker Cell with convective clouds rising over Indonesia, and two other tropical cells with rising motion over S. America, Africa.

Video Clip
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The video shows the amount of thermal infrared radiation emitted to deep space by the Earth as seen by the CERES instrument on the NASA Terra spacecraft. This thermal radiation releases the heat that we capture from the sun. The amount of thermal radiation is proportional to the temperature, with higher temperatures emitting more heat. The time-lapse video covers 15 months starting in March 2000 and shows a "climate view" of the Earth by averaging 2 weeks of data for each image. Yellow and red regions show that the Earth's highest cooling to space occurs in the relatively clear and warm subtropics. White is the lowest cooling to space and is found over cold Antarctica. Blue is the next lowest emission and is found in both cold high latitudes and in the tropics where thick high convective clouds dominate. The tops of the these clouds are very cold and as a result they trap much of the tropical radiative emission trying to escape from the warmer lower levels of the atmosphere. The Hadley and Walker cells are formed by upward motion in these blue cloudy regions of the tropics, and downward motions in the red/yellow regions. CERES is the Clouds and the Earth's Radiant Energy System experiment and is part of NASAs Earth Observing System. The NASA Terra spacecraft has been providing global climate data since March, 2000.

(Click on image above to view larger image)	The major atmospheric circulation systems are shown. The Hadley cells are composed of rising air in the heart of the tropics and sinking air in the subtropics. The rising branch of the Hadley cell is characterized by thick deep convective clouds similar to our summer thunderstorms. The sinking branch is characterized by deserts over land, and over ocean by sheets of low level cloud near the surface with clear air above.
(Click on image above to view larger image)	Outgoing Longwave radiation is the heat that the Earth sheds into deep space. Very high altitude ice clouds in the tropics trap longwave radiation, and cause the low emission over India in this image from the CERES (Clouds and the Earth's Radiant Energy System) instrument on NASA's Terra satellite. These clouds are at temperatures equal to or colder than the Antarctic at the bottom of the globe, and as a result, allow very little heat to escape to space. Changes in tropical cloudiness therefore have a profound effect on heat loss to space by the atmosphere and surface. Image courtesy of NASA SVS.
(Click on image above to view larger image)	The graph shows the amount of heat emitted from the Earth (Longwave Anomaly in Watts per meter squared) since 1979. Results are shown from an integration of observations over the entire tropics (20N latitude to 20S latitude) from satellite instruments on seven different spacecraft, a task never before attempted. Anomalies are departures from the average value during the 1985 through 1989 baseline period. Image courtesy of NASA CERES (Clouds and the Earth Radiant Energy System) Science Team.
(Click on image above to view larger image)
Illustration of the Walker Cell. Rising convective motion over the warmer ocean water near Indonesia in the western Pacific Ocean, and sinking motions over the cooler Eastern Pacific ocean. There are three such major East/West cells in the tropics, the Walker Cell with convective clouds rising over Indonesia, and two other tropical cells with rising motion over S. America, Africa.