Industrial countries warmed most at night.

A study of temperature records in the United States, China and the former Soviet Union indicates that these regions have warmed over the last 40 years, but that most of the warming has occurred at night, with daytime temperatures staying about the same through the decades.

"This could be rather important in terms of understanding the impact of a [greenhouse] warming," says Thomas R. Karl of the National Climatic Data Center in Asheville, N.C., who worked with U.S., Soviet and Chinese researchers on the project. The team presents its results in the December GEOPHYSICAL RESEARCH LETTERS.

Karl and others noted in 1984 that nighttime temperatures in North America had risen over the previous four decades. But, he says, "we were really quite surprised to find an even stronger effect in China and the Soviet Union."

The researchers analyzed daily records from 497 stations in the United States, 57 in China and 190 in the former USSR. They adjusted the temperature data for the so-called "heat-island effect," which plagues weather stations in regions with a growing urban population. The analysis revealed statistically significant increases in the annual averages of each country's daily minimum temperatures.

The group also found an increase in extreme minimum temperatures in U.S. and Soviet records -- meaning that the coldest nights had grown less frigid over four decades. But certain areas, including the eastern United States, showed the opposite trend: progressively lower minimum temperatures during winter over the last four decades.

The researchers say they do not know why the nights are warming more than the days, though they suspect that sulfur-based aerosol pollutants from fossil-fuel burning play a role. These microscopic droplets of sulfuric acid can stimulate the growth of clouds, which trap heat during the night. However, the rise in nighttime temperatures may result from other factors, including natural ones.

The day/night discrepancy suggests that the task of predicting long-term climate change is even tougher than scientists thought, Karl says.

If sulfur aerosols account for the "extra" nighttime warming, then less industrialized regions should show greater uniformity between their daytime and nighttime warming trends. Karl and his co-workers are now examining records from other parts of the world. Preliminary analysis of data from the Australian interior seems to show a more balanced night and day warming, he says.

Climate experts are becoming increasingly interested in sulfur aerosols because they can block sunlight, cooling Earth's surface. Some scientists suggest that sulfur aerosols have so far offset a significant portion of the greenhouse warming over industrialized regions of the Northern Hemisphere.

The newly reported increase in nighttime temperatures supports that idea, says James Hansen of NASA's Goddard Institute for Space Studies in New York City. But he warns that society cannot view aerosol pollution as a solution to the greenhouse problem, because the warming from greenhouse gases will dominate in the long run.