Many scientists have proposed that Earth's oceans and forests might be able to take the edge off global warming by absorbing some of the excess carbon dioxide in the atmosphere. A computer experiment described in today's Nature has now confirmed part of the theory, but also warns that changes in ocean circulation could pump even more carbon dioxide into the atmosphere.
In 1995, the Intergovernmental Panel on Climate Change (IPCC) issued several possible scenarios for global warming; these provided a scientific basis--albeit a disputed one--for last year's Kyoto agreement on greenhouse gas emissions. But none of the IPCC scenarios took into account the way that global warming might change the ocean's "carbon cycle." This cycle is partially driven by upwellings and downwellings that transport carbon-rich water. On the biological side, tiny marine organisms (mostly plants) called phytoplankton absorb carbon dioxide from the atmosphere and are consumed by fish. Eventually the fish die and sink, transporting carbon toward the bottom of the ocean.
Jorge Sarmiento, a biogeochemist at Princeton University, and three colleagues coupled a sophisticated computer model of ocean currents to two simple biological models of proxies of phytoplankton and fish abundance. As the temperature rose in the simulation, more ocean water evaporated and more rain fell on the Southern Ocean. This put a freshwater "lid" on the ocean, decreasing the circulation between the surface and the depths. As a result, the deep ocean received less carbon dioxide. "If the ocean's ability to absorb carbon dioxide is, in fact, being compromised, then the future growth of atmospheric carbon dioxide may be higher than the [IPCC] projections," says Sarmiento. But the computer also showed that if the plankton got enough nutrients--which is still an open question--their growth could compensate completely for the weakening of the physical carbon cycle.
Skeptics of human-induced global warming point out that the ocean circulation results could take some of the blame for high atmospheric carbon dioxide off of human activities. "We had a substantial warming of the oceans from 1880 to 1940," says S. Fred Singer, an atmospheric physicist at the University of Virginia. "If [Sarmiento] is correct, it means that the warming reduced the ability of the ocean to act as a carbon sink." This would have led to an increase in atmospheric carbon dioxide, regardless of human activities. If so, the "anthropogenic" increase may not be as large as scientists have assumed.