As atmospheric temperatures have risen over the past century, a naturally occurring air-ocean circulation system in the tropical Pacific has sped up -- redistributing heat to regions where it dissipates more easily, the scientists reported in the Feb. 14 issue of Science. "The global consequence is a reduction in warming," they said.
The Columbia scientists said that the phenomenon -- which is closely related to El Niño -- has been underestimated or overlooked in computer models that simulate Earth's climate and that attempt to forecast how the climate will change as heat-trapping industrial gases continue to build up in the atmosphere. The Pacific's cooling effect may explain why global temperatures in the 20th century have risen only half as much as the models said they should have, the scientists said.
The new research was conducted by Mark Cane, Amy Clement, Alexey Kaplan, Yochanan Kushnir, Dmitri Pozdnyakov, Richard Seager and Stephen Zebiak, all scientists at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y., and by Ragu Murtugudde of NASA's Laboratory for Hydrospheric Processes in Greenbelt, Md.
The researchers say that the ocean and atmosphere in the tropical Pacific region interact in a series of causes and effects that work this way:
Across the vast equatorial Pacific, prevailing trade winds push warm surface waters westward from Ecuador toward Indonesia. Deep, cold waters off the coast of South America rise, creating an east-west temperature contrast. That, in turn, lowers air pressure in the west, which draws in winds from the east.
When the global atmosphere warms, the pattern intensifies, the scientists say. Some of the warming in the east is offset by the upwelling of cold waters from below.
As a consequence, east-west temperature and pressure contrasts increase, trade winds strengthen, and even more cold water rises in the east and spreads throughout the tropical Pacific, cooling the region. This change in the tropical Pacific ocean circulation patterns moves heat north and south, where it dissipates more easily to the atmosphere and ultimately to space.
The scientists tested their theory using models of ocean circulation and models of the ocean-air interactions in the Pacific. The models indicated that tropical Pacific ocean-atmosphere dynamics had an impact on regulating tropical ocean temperatures. The scientists suggest this would affect the Earth's global mean temperature. In contrast, state-of-the-art computer models of the Earth's climate, which underestimate the Pacific Ocean's effect, show that Earth's temperature by now should have risen twice as much as it has, given the increased amount of greenhouse gases in the atmosphere.
Puzzled by the discrepancy, scientists have searched for other processes or factors that may affect Earth's climate. According to one theory, the models have not adequately accounted for the effect of sulfate aerosol particles from industrial gases, which may block out sunlight and could help reduce global warming. The new research, however, points to the tropical Pacific as another overlooked factor that may be playing an important role in counteracting the greenhouse effect.
Analyzing ocean surface temperatures, the Lamont-Doherty scientists showed evidence that surface waters in the eastern equatorial Pacific may have cooled over the past century, even as Earth's surface temperature has increased by between 0.6° and 1° F. That agrees with their theory and indicates that Earth's climate system is changing in response to increases in greenhouse gases.
"It is notable that the eastern equatorial Pacific shows a cooling trend despite the strong and frequent El Niño events in the period after 1975," the scientists said. El Niño is created by the same tropical Pacific ocean-atmosphere dynamics. During an El Niño, trade winds slacken and the pool of warm waters in the western Pacific begins to migrate back eastward, accompanied by a center of tropical rainfall. The shifting conditions in the tropical Pacific dramatically rearrange global wind and rainfall pattern, often causing droughts, floods and other destructive weather around the world. The irregularly occurring phenomenon is second only to the seasons themselves in driving worldwide weather patterns and has impacts on climate from the Far East, Australia and southern Africa to South America and the United States.
While tropical Pacific ocean-atmosphere dynamics may delay global warming, the scientists warn that the changing pattern of tropical Pacific temperatures could produce long-term ocean-atmospheric changes. These, in turn, could cause "changes in regional climate and climate variability over much of the Earth that would be likely to have substantial social and economic consequences," they said.