Man-made gases that trap solar heat, resulting in the so-called "greenhouse effect," have left the Earth warmer today than ever before and increase the likelihood of the type of drought now parching U.S. farmland, a NASA scientist told a congressional hearing yesterday.

James E. Hansen, chief of NASA's Goddard Institute for Space Studies, said it is impossible to blame a specific heat wave or drought on the "greenhouse effect." But he said the record-breaking, global warming of the 1980s, hitting hardest in the American Southeast and Midwest, is the first firm evidence that gases emitted by automobiles and coal-burning factories have accumulated in the atmosphere, shrouding the Earth and trapping its heat like a greenhouse.

"The greenhouse effect has been detected and it is changing our climate now," Hansen testified at a hearing of the Senate Committee on Energy and Natural Resources. "We already reached the point where the greenhouse effect is important."

Although scientists have previously theorized about the greenhouse effect and issued vague projections of rising temperatures sometime in the next century, Hansen is the first to concretely link warming and greenhouse gases and to authoritatively proclaim the phenomenon's arrival.

He warned, moreover, of an "increasing tendency" for droughts and heat waves, especially in the Southeast and Midwest, during the balance of this decade and the 1990s.

None of the scientists joining Hansen at the hearing went as far in suggesting a link between current weather patterns and greenhouse gases. But Michael Oppenheimer of the Environmental Defense Fund appeared to express the consensus that the drought "is a warning. Whether or not it's related to global changes, it provides a small taste of the dislocations society will face with increasing frequency if we fail to act."

Syukuro Manabe, a scientist with the National Oceanic and Atmospheric Administration, lent theoretical support to Hansen's position. He said computer models show that with a doubling of carbon dioxide -- the most common greenhouse gas -- soil becomes drier over "very extensive," mid-continental regions of North America, including the American Great Plains, Southern Europe and Siberia.

Higher temperatures caused by carbon dioxide melt winter snows earlier than normal, increasing the land's absorption of solar energy that evaporates soil moisture, he explained. Moreover, the warmer, moisture-rich air moves north, reducing rainfall.

Manabe said the greenhouse effect probably aggravated the current drought but has not warmed the Earth enough to account solely for the dry conditions.

According to Hansen, a climate specialist at NASA, global temperature in the first five months of 1988 is "substantially warmer" than any previous period since measurements were first taken 100 years ago. This year is expected to be the warmest on record barring a "remarkable, improbable cooling" in later months, he said.

The Earth has warmed at a record pace in the past two decades, Hansen said, noting that the four warmest years of the last 100 years have been in the 1980s. The warming has been matched by increasing emissions of carbon dioxide, mostly from power plants.

Compared to the 30-year mean temperature, the recent warming pattern is three times larger than the standard deviation, giving NASA "99 percent confidence" that current temperatures represent a "real warming trend" rather than chance fluctuation, according to Hansen.

Although global climate models are not sufficiently reliable to predict the effects of greenhouse warming on regional climates, he said, studies projected into the late 1980s and 1990s indicate greater than average warming in the Southeast and Midwest, where high temperatures will be accompanied by less rainfall.

"It is not possible to blame a specific heat wave/drought on the greenhouse effect," he said. "However, there is evidence that the greenhouse effect increases the likelihood of such events."

Projections by as authoritative a figure as Hansen are expected to stimulate debate on the impact of greenhouse gases and fuel efforts in Congress to impose controls on sources of carbon dioxide, nitrous oxides, chlorofluorocarbons (CFCs), methane and other gases that do not easily break down in the atmosphere.

"We have only one planet," said Sen. J. Bennett Johnston (D-La.), committee chairman. "If we screw it up, we have no place to go. The greenhouse effect has ripened beyond theory."

Sen. Dale L. Bumpers (D-Ark.) said Hansen's report "ought to be cause for headlines in every newspaper of the country. What you have is all the economic interests pitted against our very survival."

Oppenheimer listed a number of measures designed to reverse the global warming trends, including a halt to deforestation because trees absorb carbon dioxide. He also recommended energy conservation, strengthening international controls on CFCs, development of alternative fuels and tapping solid waste landfills for methane.

Much of the opposition to such measures is expected to come from coal-producing states. Sen. Wendell L. Ford (D-Ky.) pointedly asked witnesses whether they were ready to endorse nuclear power as an alternative to coal-fired utilities.

1. Pollutants such as carbon dioxide and industrial gases are released into the atmosphere from automobiles, factories, etc..,

2. Instead of dissipating, the chemicals remain trapped in the atmosphere where they surround the Earth like a blanket.

3. Sunlight passes through the blanket and warms the planet, but the blanket prevents heat from escaping back into space.

4. Scientists believe that the blanket acts like the Earth's greenhouse. They predict that the warming associated with the "greenhouse effect" will cause major changes in global climate patterns.

Compiled by James Schwartz