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Earth and Sky
Higher Ocean Temperatures Linked to Cooling in Midwest
While Earth as a whole has warmed during the last half-century, much of the continental United States has grown slightly colder. This trend toward cooler temperatures in the central and eastern United States is due to warmer ocean temperatures, an LAS researcher says.
"Although portions of the U.S. have failed to get warmer, they have gotten cloudier," says Walter Robinson, professor of atmospheric sciences. "Our models suggest there is a strong correlation between this increased cloudiness and higher sea-surface temperatures in the tropical Pacific Ocean."
Warmer sea-surface temperatures in the tropical Pacific Ocean make the tropical atmosphere warmer and wetter. Additional moisture is swept from the Gulf of Mexico into North America, where it increases cloud cover over the central and eastern United States. "By reflecting more solar radiation back into space, this additional cloud cover is directly responsible for the cooling," says Robinson.
Conventional wisdom has associated the lack of warming in the continental United States with the influence of anthropogenic aerosols, such as sulfate particles, Robinson says. Unlike greenhouse gases—which contribute to global warming—sulfate particles have a cooling effect. Coal-burning power plants in the Midwest are a major source of such particles. The general atomospheric circulation models used by Robinson and his colleagues—James Hansen at the NASA Goddard Institute for Space Studies and Reto Reudy at Science Systems and Applications, Inc.—showed no such influence.
The deeper question that Robinson will now explore is whether this effect is a manifestation of global warming or a result of natural variability in the climate system. "If the tropics are getting warmer because of global warming, then we can expect the cooling trend in the U.S. to continue," Robinson says.
"But if the effect is due to natural variability—and historical records indicate that recent temperature variations in the tropical Pacific are not unusual—then it's only a matter of time before our temperatures catch up with the rest of the world."
Summer 2001