Warming climate will slow ocean circulation.

Later this century, rising concentrations of greenhouse gases in Earth's atmosphere will slow the ocean currents that bring warm waters to the North Atlantic, thereby affecting that region's climate, computer simulations suggest.

When the waters of the Gulf Stream and other warm currents of the North Atlantic reach an area just south of Greenland, they cool, become denser, and sink. That, in turn, pulls more surface water northward, says Thomas L. Delworth, a climate scientist at Princeton University. The rate of this so-called thermohaline circulation depends on the temperature and salinity of the surface waters. The warmer and fresher those North Atlantic surface waters are, compared with underlying layers, the more buoyant they are and the slower the circulation becomes.

Using a new computer model, Delworth and his colleague Keith W. Dixon simulated various scenarios for ocean circulation in the North Atlantic from now until 2100. They calibrated the model using weather and ocean-circulation data gathered since 1860.

Throughout the 20th century, rising concentrations of greenhouse gases such as carbon dioxide warmed the atmosphere and ocean surface, but not enough to slow the thermohaline circulation. That's because large amounts of air pollutants known as aerosols have scattered sunlight back into space and counteracted the greenhouse effect somewhat, says Delworth.

In the remaining years of the 21st century, however, growing concentrations of greenhouse gases will begin to overwhelm the cooling effect of aerosols, Delworth and Dixon suggest. By the year 2040, thermohaline circulation could carry only 80 percent as much warm water to the North Atlantic as it does now. The researchers report their findings in the Jan. 28 Geophysical Research Letters.--S.P.