A University of Virginia professor cautioned Tuesday against taking drastic action to combat the atmosphere's greenhouse effect until scientists learn whether it causes droughts, heat waves and other climatic changes.

"Our policy should be commensurate with the state of our scientific knowledge," Dr. Patrick J. Michaels told a House Energy and Commerce subcommittee studying the global warming problem.Dr. Stephen Schneider, a scientist for the National Center for Atmospheric Research, agreed that better information is needed, but he said action was needed now as "insurance against catastrophic change."

The greenhouse effect "gives you an additional reason" to reduce burning of fossil fuels and take other action to benefit the environment, the climate specialist said.

The subcommittee was told that it may take two decades to learn whether the greenhouse effect is responsible for temperature changes like the searing heat waves of last summer.

Schneider and Michaels agreed that scientists are unable to attribute last summer's heat to the buildup of greenhouses gases, and Schneider showed the committee slides of news articles that he said prematurely blamed last summer's temperatures on the building up greenhouse gases.

Schneider said "another decade, or possibly two, will be required to be sure that the warming of the 1980s (the warmest decade recorded on a global basis) will . . . continue into the 1990s and beyond."

Michaels, a professor of environmental sciences, said climate experts have a "clouded vision" of future global warming, and told lawmakers that "basing sweeping environmental policy" on such uncertainties "is especially risky, even if the policy is otherwise rational."