Senators are using the closing weeks of the 100th Congress to put the issue of global warming from the "greenhouse effect" at the top of their environmental agenda for next year.

They got the ball rolling this week with the introduction of legislation prompted by a series of hearings over the last two years at which scientists warned that the greenhouse effect threatens the planet with higher temperatures, climate changes and rising oceans in the next century.Sixteen senators, including Energy Committee Chairman Bennett Johnston, D-La., unveiled a comprehensive plan on Thursday that calls for a variety of steps, from developing safer nuclear power to slowing the destruction of forests in the Third World.

A day earlier, a group headed by Robert Stafford, R-Vt., submitted a package to end U.S. use of chlorofluorocarbons, the chemicals that contribute to global warming and destroy the Earth's vital ozone layer.

Johnston, who has been identified more with energy development and production than environmental protection, said he would hold a series of hearings in coming weeks to set the stage for 1989.

Next year, he said at a news conference, "we'll have to begin the serious process of legislating. We'll act as fast as the public's willing to support us."

Johnston is a cosponsor of most elements in a package developed by Sen. Tim Wirth, D-Colo., that represents the broadest Capitol Hill response yet to the greenhouse warnings.

"The greenhouse effect is the most significant economic, political, environmental and human problem facing the 21st Century," Wirth said.

Johnston, who recently held a hearing on the greenhouse effect, said: "The evidence of global warming is virtually undeniable. It is so much more comprehensive, dangerous and life-changing than anything we've ever seen."

Aides said Johnston has become convinced that global action is necessary to combat the production of carbon and other gases that scientists say trap heat in the atmosphere.

Johnston stopped short of endorsing a key element of Wirth's proposal: reducing carbon monoxide emissions by requiring new car fleets to average 55 miles per gallon by 2010, a doubling of the 1985 fuel efficiency standard.

The Wirth package would force this country to reduce carbon dioxide emissions by 20 percent by 2000. Scientists estimate that carbon dioxide, coming mostly from fossil fuel combustion, produces about half the world's greenhouse gases. The United States is said to be responsible for 20 percent of the world's carbon dioxide emissions.