New evidence suggests that the greenhouse effect had little to do with last summer's drought, and a major new study should make it easier to determine how much of a threat the ballyhooed warming trend really presents.

Scientists at the University of Chicago said Friday they have determined for the first time how much clouds protect the Earth by cooling it.

"The largest uncertainty in understanding climate change, from the drought of last summer to an ice age lasting thousands of years, is the way that clouds interact with the sun's radiation," said Veerabhadran Ramanathan, leader of the U. of C. team."This work will finally allow us to verify our models of climate and weather. We'll be able to develop hypotheses about how clouds will change if the atmosphere is increasingly dominated by a greenhouse effect."

The cooling effect of clouds is "surprisingly large," Ramanathan said. "The planet would be significantly warmer without this net cooling."

It is crucial that scientists gain more understanding of the greenhouse effect because influential governmental and public interest groups are mobilizing to fundamentally transform the way the world uses energy. Proposals range from developing safer nuclear power to slowing down the destruction of Third World forests.

The greenhouse effect became a household term last summer largely because of the drought, which was viewed by many scientists and politicians as a "signal" of future global warming. But a recent scientific study instead blames the drought on developments in the Pacific Ocean.

Researchers with the National Center for Atmospheric Research in Boulder, Colo., said the freakish weather was due to the interaction of two massive currents in the tropical Pacific, known as La Nina and El Nino.

El Nino (Spanish for "the boy") is a huge strip of abnormally warm water that periodically appears off the west coast of South America and wreaks havoc on weather patterns throughout the world.

Every so often, El Nino alternates with its chilly counterpart, La Nina (or "the girl"), a mass of cold water that wells up from the ocean depths along the equator and drifts for thousands of miles.

Nino and Nina represent two extremes, the opposite ends of a gradual but continuous back-and-forth oscillation of eastern Pacific water temperatures. The total cycle takes between three and six years.

El Nino last appeared in 1986-87, and it was Nina's turn in 1988, according to climatologists Kevin Trenberth, Grant Branstator and Phillip Arkin.

Using computer modeling, the scientists suggest that a double whammy of Nina and Nino occurred in the northern spring of 1988. Cold water along the equator clashed with warmer than normal water southeast of Hawaii.

The result was both the U.S. drought and the devastating floods that swamped Bangladesh, the scientists said. Nina's cooler water disrupted tropical weather patterns and distorted the path of the jet stream across North America.

Then, the jetstream shoved rain-producing weather systems away from the interior of the U.S., resulting in drought.