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Date: May 21, 1998
Computer Models Suggest Global Warming Disturbs Ocean Circulation
Oceans May Absorb Less Carbon Dioxide Than Believed, Princeton Researchers Find
Princeton. N.J. -- In this week's Nature, scientists from Princeton University and from the Geophysical Fluid Dynamics Laboratory report that changes induced by global warming in Southern Ocean circulation may be reducing its ability to absorb excess atmospheric carbon. If other computer simulations support their results, the researchers suggest, an international effort to collect data on how the great Southern Ocean responds to climate change should be undertaken immediately.
The research was led by Professor of Geosciences Jorge Sarmiento and carried out jointly with with scientists from GFDL, which is part of the National Oceanic and Atmospheric Administration and located on Princeton's Forrestal campus.
The world's oceans act like a CO2 sponge: that is, atmospheric CO2 dissolves in the water and gets carried into the deep ocean by the circulation. Previous estimates of how steeply we must decrease carbon dioxide emissions in order to stabilize CO2 concentrations in the atmosphere and to slow global warming have assumed the oceans remain constant over time. That is, the computer models on which the estimates are based assume that ocean circulation patterns and ocean biota don't change.
This work examines what would happen if you treated the oceans and the atmosphere as part of a connected system. The simulation used CO2 estimates from the years 1765 to 1990, and projections from 1990 to 2065.
The results suggest that the oceans will not remain unchanged, but may respond to global warming with changes in salinity, vertical circulation patterns, and biotic activity. According to this model, global warming triggers increased rainfall over vast stretches of the great Southern Ocean. This causes freshening of the water, which in turn creates increased stratification: that is, the water at the surface -- which is fresher and thus lighter than the saltier water below -- floats on top. This floating layer of water interferes with the usual ocean cycle, in which deep water wells up and the surface water sinks, a process that helps absorb CO2 as well as heat from the air. The fresh water, in effect, acts like a lid, slowing down CO2 uptake.
If the biota, particularly the tiny organisms that take up the dissolved carbon, remain constant or increase despite this stratification, they may continue to absorb CO2 and counteract the reduction that results from this freshwater "lid." However, the authors say, we don't know enough about the biota to predict what it will do. Therefore, if other simulations, which also use coupled models but vary the simulation parameters, indicate these circulation changes are likely, we ought to start collecting real data -- salinity and temperature, as well as nutrients, oxygen, and other signals of biological life -- in the Southern Ocean as soon as possible.
"Our simulations are particularly relevant to international efforts to control future atmospheric carbon dioxide," says Sarmiento. "If the ocean's ability to absorb CO2 is, in fact, being compromised, then the future growth of atmospheric CO2 may be higher than the projections that were used to craft agreements such as the Kyoto Protocols."