South stays cooler in greenhouse models.

South stays cooler in greenhouse models

A surprise has surface in simulations of the future climate. A new computer model, coupling the ocean with the atmosphere, suggests large portions of the Southern Hemisphere will heat up much more slowly from greenhouse warming than the rest of the globe, report Kirk Bryan, Syukuro Manabe and Ron J. Stouffer from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

The simulations tested the climate effects of an accumulation of greenhouse gases in Earth's atmosphere from 1958 to 2058. Most strikingly, it showed that the regions in the Southern Hemisphere south of 50[degrees]S lag far behind their counterparts in the north. The Antartic continent warms by one 2[degrees]C, and the circumpolar ocean warms even less. North of the equator, the temperature change is stronges at the pole, reaching 7[degrees]C by 2058, according to the model.

The imbalance appears to arise from strong currents in the circumpolar ocean that mix surface water with deep water. In other areas of the world, vertival mixing is much weaker and the ocean's surface layers warm as the atmosphere warms. But in the well-mixed circumpolar southern ocean, currents carry heat downward, preventing the surface layers from warming quickly, Bryan says.

To test whether their model accurately simulates ocean mixing, the researchers are checking it against actual measurements of how quickly human-made chemicals such as chlorofluorocarbons have prenetrated the deep ocean. Other climate models do not show such a drastic imbalance between hemispheres, and more work is needed to explain therese differences, Manabe says. The researchers also describe their results in the Dec. 7 NATURE.