Venting Unexpectedly |
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Wielicki says that he and his team collected and merged data over a 22-year period from a number of satellites that measure both solar and thermal radiation reflected by and emitted from the atmosphere over the tropics. They brought together older data from Nimbus 7, which was one of the earliest satellites to measure radiation over the Earth, and the Earth Radiation Budget Satellite (ERBS), which has kept a continuous record of the Earth’s radiation since 1985. New data sets were also used from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) instruments that fly aboard the Tropical Rainfall Measuring Mission (TRMM) as well as the newer Terra satellite (Wielicki et al. 2002). “What we found was a 4-watt-per-square-meter change within the climate system that the climate models did not predict,” says Wielicki. Over the last 15 years, without anyone’s knowledge, the amount of thermal, long-wave radiation escaping the atmosphere above the tropics increased by 4 watts per square meter. At the same time the amount of reflected sunlight, which is mostly in the form of short-wave visible and near-visible light, decreased by 4 watts per square meter. The change appears to have occurred gradually over the past decade and a half and as such was likely completely independent from El Niño (Wielicki et al. 2002). Though 4 watts of energy is only a fraction of the 342 watts per square meter of solar energy that hits the Earth’s outer atmosphere, the Earth’s energy budget is usually extremely stable over the long term, and changes of more than a couple of watts are significant. |
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At first, Wielicki and the research community did not quite know how to interpret the results. They suspected that cloud cover must have been changing over large regions of the tropics (Wielicki et al. 2002). Fewer high, thin clouds, for instance, would likely result in more thermal energy leaving the Earth, and fewer low-lying, thick clouds would result in more solar energy reaching the oceans. In order to verify their suspicions, the scientists would need proof. “But observing the obvious cloud patterns spatially across the tropics couldn’t give us these long-term changes in long-wave (thermal) radiation,” says Wielicki. |
Satellite-borne instruments have measured energy fluxes at the top of the atmosphere for more than 15 years over the tropics (from 20° North to 20° South). The top two graphs show monthly changes in the amount of heat emitted and sunlight reflected back to space, from 1985 through 1999, relative to the long-term average (gray bar). The graphs reveal that increasing amounts of heat energy escaped to space in the 1990s as the Earth was absorbing more sunlight. The bottom graph shows changes in the Earth’s total net radiation budget during this same period—notice the erratic up and down pattern in the 1990s as compared to the relative stability of the net energy fluxes through the 1980s. (Graphs by Robert Simmon, based on data from the CERES Science Team, NASA LRC) | |
He explains that the relatively short-term El Niño and La Niña cycles that occur every three to seven years dominate cloud patterns across the tropics. During a typical El Niño year, the trade winds that travel from east to west across the Pacific slow down or even reverse during the winter months. Without these winds pushing surface waters west, warm water from the western Pacific moves east. Since the warm water generates rising air and thus clouds, the clouds shift eastward en masse with the water. During a La Niña, the opposite phenomenon occurs. Trade winds speed up and the warm waters and clouds bunch up in the western Pacific and Indian oceans during the winter. These massive swings in water temperature and air pressure in the Pacific impact the tropical latitudes around the globe, causing cloud cover to shift in over the Atlantic Ocean, the Indian Ocean, and the continents as well. Though this short-term reshuffling of cloud cover does not result in a net difference in the amount of clouds or outgoing thermal radiation over the tropics as a whole, it obscures any subtle, long-term fluctuation. Attempting to locate a 4-watt-per-square-meter difference in cloud cover over 15 years across the tropics would be akin to listening for a gradual build up at a violin concerto as a foghorn intermittently blasts away. |
Clouds and the Earth’s Radiant Energy System (CERES) measurements show the reflected solar radiation (left) and emitted heat radiation (right) for January 1, 2002. In both images, the lightest areas represent thick clouds, which both reflect radiation from the Sun and block heat rising from the Earth’s surface. Notice the clouds above the western Pacific Ocean, where there is strong upwelling of air, and the relative lack of clouds north and south of the equator. The animations, created from daily data, show how rapidly these measurements change. (Image and animation by Robert Simmon and Reto Stöckli, based on data from the CERES Science Team, NASA LaRC) animations:
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