Watching the World Rev its Heat Engine

Absorption of solar energy heats up our planet's surface and the atmosphere
and makes life for us possible. But the energy cannot stay bound up in the
Earth's environment forever. If it did then the Earth would be as hot as the Sun.
Instead, as the surface and the atmosphere warm, they emit thermal longwave radiation,
some of which escapes into space and allows the Earth to cool. This false-color image
of the Earth was produced on September 30, 2001, by the Clouds and the Earth°s Radiant
Energy System (CERES) instrument flying aboard NASA°s Terra spacecraft. The image
shows where more or less heat, in the form of longwave radiation, is emanating from
the top of Earth's atmosphere.

As one can see in the image, the thermal radiation leaving the oceans is fairly uniform.
The blue swaths across the central Pacific represent thick clouds, the tops of which are
so high they are among the coldest places on Earth. In the American Southwest, which can
be seen in the upper righthand corner of the globe, there is often little cloud cover to
block outgoing radiation and relatively little water to absorb solar energy. Consequently,
the amount of outgoing radiation in the American Southwest exceeds that of the oceans.
Also, that region was experiencing an extreme heatwave when these data were acquired.

Recently, NASA researchers discovered that incoming solar radiation and outgoing thermal
radiation increased in the tropics from the 1980s to the 1990s. They believe that the
reason for the unexpected increase has to do with an apparent change in circulation patterns
around the globe, which effectively reduced the amount of water vapor and cloud cover in the
upper reaches of the atmosphere. Without the clouds, more sunlight was allowed to enter the
tropical zones and more thermal energy was allowed to leave. The findings may have big
implications for climate change and future global warming.

"This suggests that the tropical heat engine increased its speed," observes Dr. Bruce Wielicki,
of NASA Langley Research Center. "It's as if the heat engine in the tropics has become less
efficient, using more fuel in the 1990s than in the 1980s."