Global warming lurks principally at night.

Cool evenings are getting rarer these days. Measurements at 5,400 weather stations around the globe reveal that global warming in the last half century has raised nighttime temperatures far more than daytime ones, according to a team of international meteorologists.

Between 1950 and 1993, globally averaged minimum temperatures rose at a rate of 0.186 [degrees] C per decade, whereas maximum temperatures increased at only 0.088 [degrees] C per decade, according to David R. Easterling of the National Climatic Data Center in Asheville, N.C., and his colleagues in the United States, England, Russia, New Zealand, and Australia. The scientists report their findings in the July 18 Science.

The nighttime warming has shrunk the gap between maximum and minimum temperatures. In a few spots, such as the southern United States and eastern Canada, daytime maximum temperatures have also dropped, further reducing the daily temperature range there.

The new results back up a finding 6 years ago by Easterling's colleagues, who studied just the United States, China, and the former Soviet Union. For the current study, Easterling's group expanded its data set to cover much more of the Southern Hemisphere, including stations in South America, New Zealand, Indonesia, and Australia.

Critics of the earlier study wondered whether urban growth around meteorological stations had caused the enhanced nighttime warming. Cities generally remain warmer at night than nearby rural areas do because concrete and asphalt store heat during the day and radiate it at night. When Easterling's group removed from its data the 1,300 stations near large urban centers, the results changed by only about 10 percent. "This shows it's not just due to urban warming. It's a real effect," says Easterling.

Meteorologists cannot say whether greenhouse gas pollution has caused the enhanced nighttime warming, although the results match some greenhouse computer simulations, says Alan Robock of the University of Maryland at College Park. He and his colleagues found that when greenhouse gases warm the climate, the atmosphere holds more water vapor. That extra water vapor blocks some incoming sunlight and thus limits the magnitude of greenhouse warming during the day. It does not weaken greenhouse warming at night.

Increasing cloudiness could also explain the trend. Some clouds reduce daytime temperatures and raise nighttime temperatures.

The higher nighttime temperatures, exaggerated in winter, could benefit society by trimming heating costs and the number of plant-damaging frosts. Scientists who study climate and agriculture, however, point out that warmer nights can stimulate the growth of harmful insects and weeds. They can also reduce yields by spurring plants to burn energy faster at night. Moreover, such climate change further limits the places where farmers can plant winter wheat, which requires cold temperatures, says Cynthia Rosenzweig of NASA's Goddard Institute for Space Studies in New York.