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 ate“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.

[NOTE: Images and a movie supporting this release are available at]