Global warming, which has raised Earth's average air temperature by less than 1 degree Fahrenheit over the past century, is turning out to be relatively benign, scientists say. So far.

First, the effect has been to boost nighttime lows rather than daytime highs. Also, there is evidence that in the Northern Hemisphere the warming is happening mainly in the winter and spring and somewhat in the fall. In summer, when heat stress is hardest on living things and when ice caps melt, temperatures are no warmer than they were in the 1860s and 1870s.

Moreover, according to one of several reports on global warming being released today in the June issue of the National Geographic Society's journal Research & Exploration, the atmospheric phenomenon thought to account for the warming at night -- increased cloud cover -- is probably caused by the warming itself and, thus, is likely to continue to moderate the effect as long as warming continues by keeping daytime temperatures lower.

Another article in the same journal, however, suggests the cloudiness is caused by fossil fuel pollution, which will gradually diminish.

Some scientists have warned that global warming could disrupt agriculture, damage forests and other plants, and trigger coastal flooding as melting polar ice caps raise sea levels.

"The popular vision of climate apocalypse is wrong," concluded Patrick J. Michaels, author of the most optimistic report. Michaels is Virginia's state climatologist and a professor of environmental sciences at the University of Virginia.

Michaels suggested that if the trend continues, one significant effect could be to lengthen the growing seasons in agricultural regions because the frost-free season would begin sooner in spring and end later in fall.

The evidence of warmer nights is not so much a new discovery as a new appreciation of data that have been published many times in tables of numerical data. "But nobody," Michaels said, "ever pulled it together. It was there, waiting to be synthesized from the literature."

Still, the varied views of global warming in the journal show that atmospheric scientists are still grappling with major uncertainties in their understanding of the "greenhouse effect."

This is the phenomenon named for the way the glass in a greenhouse allows sunlight in to warm the surface but blocks the resulting heat from radiating back into space. Certain gases in the air -- such as carbon dioxide -- have the same effect, trapping heat near the ground instead of letting it radiate into space.

The fact that this heat is held close to Earth's surface means it no longer reaches the upper levels of the atmosphere. As a result, those upper air layers become cooler than before. That, in turn, means the water vapor in the cooled air is more likely to condense into clouds.

Moreover, atmospheric scientists agree, a warming climate is likely to put more water vapor into the atmosphere simply by increasing the rate of evaporation from the ground.

According to one study, cloudiness over the United States increased 3.5 percent between 1950 and 1988. A German study in 1990 reported a decline in sunshine in that country and a 1988 report by the Department of Energy concluded from numerous shipboard observations that the skies over the oceans have grown cloudier.

Clouds make the days cooler (by blocking sunlight) and the nights warmer (by absorbing the ground's heat, which would otherwise radiate into space, and radiating it back toward the ground). As a result, clouds counteract daytime warming and enhance nighttime warming.

Michaels, who long has doubted that greenhouse warming would lead to catastrophe, might be expected to emphasize data that minimize the threat.

But scientists on the other side of the controversy are in accord on this point. James Hansen of the Goddard Institute for Space Studies in New York City, the most prominent scientist to sound the global warming alarm in 1988, agrees that the warming observed so far has been mainly at night.

"We see the nighttime warming both in observational data and in our computer simulations," Hansen said. "And there is some evidence for increasing cloud cover."

Hansen, author of another report in the same journal, differs from Michaels, however, because he thinks increased cloudiness results less from warming and more from particles of air pollution in the atmosphere -- mostly sulfates from the burning of fossil fuels. These have a cooling effect both by reflecting sunlight back into space before it can reach the ground and by acting as surfaces upon which water vapor condenses to form clouds.

If he is right, Hansen said, the increased cloudiness should diminish over time because the burning of fossil fuels is not increasing as fast as before and because pollution controls are limiting the sulfate output.

While this means the output of carbon dioxide will also decline, the amount already in the atmosphere will stay there for decades. Sulfates, on the other hand, fall out in days (as acid rain). As a result, the CO2 can keep contributing to a greenhouse effect long after the sulfate levels decline, along with their cooling effects.

In the long run, Hansen said, he expects the warming factors to overtake the cooling factors and to lead to significant global temperature rise in the daytime as well as at night. Hansen's forecast, however, is not as dire as it used to be.

Just a few years ago, for example, it was estimated that CO2 levels would double by 2030 and that the global average temperature then would be as much as 5 degrees Fahrenheit higher than now. As it happens, carbon dioxide output is not growing as fast as before and the doubling is not expected to occur until the second half of the next century, by which time it may plateau. Hansen now says the warming probably won't reach the 5-degree increase until then.

Michaels believes Hansen is wrong about both the amount of warming and the role of aerosol particles like sulfates. First, he notes that Hansen's forecast is based on computer models that overestimate greenhouse warming. Hansen acknowledges that when his computers are asked to estimate today's temperature, they say it should have risen twice as much as has been measured over the last century.

Second, Michaels believes the increased cloudiness is not a result of aerosol particles. These, he said, stay mostly in low altitude air -- the haze zone visible from airplanes -- whereas atmospheric cooling has been observed at cloud-prone altitudes above the aerosol layer.

Another sign that the warming is relatively benign has come from British scientists who studied global temperature variations over the last 140 years and published their findings in the latest issue of a journal called the Holocene. (The title refers to the most recent epoch of geological time.)

P. D. Jones and K. R. Briffa of the University of East Anglia, which maintains a major climate research center, have found that the 1 degree F. warming in the Northern Hemisphere occurred only in winter, spring and autumn. "Summers," they wrote, "are now no warmer than in the 1860s and 1870s." In the Southern Hemisphere the same amount of warming has happened throughout the year.

Jones and Briffa also found great unevenness in the temperature change from place to place. For example the Amazon Basin has cooled on a year-round basis. In the United States, the chief variations have been a warming in winter and a cooling in spring (counter to the general Northern Hemisphere trend of a warming in spring).

Commenting on another area of uncertainty, Hansen said the poles have not warmed as fast as computers predicted they would. As a result, there is debate over the validity of previous forecasts of a dramatic rise in sea level that would flood out millions of coastal dwellers. For this to happen, the polar ice caps would have to melt faster in summer than they grow in winter.

He noted that if the poles do warm, meteorologists would expect snowfall to increase because it is now often too cold to snow there. But if air and water temperatures rise, the melting rate could increase as well. The rival trends could cancel each other out.