NASA researchers have found that more sunlight entered the tropics and more heat escaped into space in the 1990s than in the 1980s, meaning there was less cloud cover to block incoming radiation or to stop the heat from escaping into the atmosphere. They determined this after examining 22 years of satellite measurements.
?Since clouds were thought to be the weakest link in predicting future climate change from greenhouse gases, these new results are unsettling,? says Dr. Bruce Wielicki of NASA. ?It suggests that current climate models may, in fact, be more uncertain than we had thought. Climate change might be either larger or smaller than the current range of predictions.?
What?s known as the radiation budget, the balance between Earth?s incoming and outgoing energy, controls the planet?s temperature and climate. These changes in the radiation budget are two to four times larger than scientists had believed possible.
A research group at NASA Goddard Institute for Space Studies (GISS) has developed a new method of comparing the satellite observations to other meteorological data. ?The new method is a conceptual breakthrough in how we analyze data,? says Anthony Del Genio, a scientist at GISS.
?What it shows is remarkable,? says 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.?
Faster air circulation dried out the water vapor that is needed for cloud formation in the atmosphere over the most northern and southern tropical areas. Fewer clouds allowed more sunlight to enter and more heat to leave the tropics.
Several of the world?s top climate modeling research groups tried to reproduce the tropical cloud changes. But their climate models predicted smaller changes than were actually observed by the satellite. ?It?s as if the heat engine in the tropics has become less efficient using more fuel in the 90s than in the 80s,? says 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.?
?A value of this research is it provides a documented change in climate and a target for climate models to simulate,? says Del Genio. ?The question is, if this fluctuation is due to global climate change or to natural variability. We think this is a natural fluctuation, but there is no way to tell yet.?
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