WASHINGTON (AP) _ Sixteen senators, including the Energy Committee chairman, unveiled a comprehensive plan Thursday to combat the global overheating that scientists say threatens the planet in the 21st Century.

Their legislation, with proposals ranging from developing safer nuclear power to minimizing destruction of Third World forests, represents the broadest Capitol Hill response yet to warnings from scientists about the so- called greenhouse effect.

''The greenhouse effect is the most significant economic, political, environmental and human problem facing the 21st Century,'' said Sen. Tim Wirth, D-Colo., the principal author of package.

Joining him at a news conference was Bennett Johnston, D-La., the energy chairman who has over the years been identified more with energy development and production than with environmental protection.

Johnston, who recently held a hearing on the greenhouse effect, said that ''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 that Johnston has become convinced that global action is necessary to combat the production of carbon and other greenhouse gases that scientists say trap heat in the atmosphere, threatening major changes in climate and higher ocean levels.

Johnston, who said he would hold more hearings in coming weeks, stopped short of endorsing a key element of Wirth's proposal: requiring new car fleets to average 55 miles per gallon by 2010, or double the 1985 fuel efficiency standard.

The legislation 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 greenhouse effect. The United States is said to be responsible for 20 percent of the world's carbon dioxide emissions.

The proposal would require administration officials to draft a national energy plan emphasizing least-cost ways of reducing fossil fuel use and increasing use of renewables such as solar energy.

The package would authorize about $1 billion through 1992 for research and development of non-fossil energy sources and a new generation of safer and cheaper nuclear reactors. It also calls for greater reliance on cleaner- burning natural gas for heating and transportation.

On the international front, the secretary of state would be urged to hold an international meeting aimed at getting other nations to commit to a 50 percent cut in greenhouse gas production by 2015.

The legislation would authorize this country to spend more than $1.5 billion in the early 1990s to provide birth control information in an effort to slow the demand for fossil fuels in burgeoning nations.

It also calls for the United States to work through international lending and development agencies to slow the destruction of forests, which consume carbon dioxide when alive but which emit huge amounts when they are cut down.

Johnston said that to slow pollution in the Third World, ''we're going to have to pay for their non-development in some way, and it's going to be expensive.''

He said that in 1989, ''we'll have to begin the serious process of legislating. We'll act as fast as the public's willing to support us.''

Johnston said that economic interests like automakers and coal producers ''must understand that this is not a passing fad, that it's not going to go away. This will not change with administrations. It's not a Democratic-Rep ublican issue. It's not a liberal-conservative issue. And I'm not sure we can solve it.''

Joining as cosponsors were Max Baucus, D-Mont.; Jeff Bingaman, D-N.M.; Dale Bumpers, D-Ark.; John Chafee, R-R.I.; John Danforth, R-Mo.; Wyche Fowler, D- Ga.; Albert Gore, D-Tenn.; John Heinz, R-Pa.; Daniel Inouye, D-Hawaii; John Melcher, D-Mont.; Claiborne Pell, D-R.I.; Spark Matsunaga, D-Hawaii; Terry Sanford, D-N.C.; and Robert Stafford, R-Vt.

The package was introduced a day after Stafford, Baucus, Chaffee and David Durenberger, R-Minn., submitted legislation to end all U.S. production and use of chlorofluorocarbons by 1999. These chemicals contribute to global warming as well as destroying the ozone layer that shields the Earth from the sun's cancer-causing ultraviolet rays.

Their bill would also force power plants to reduce emissions of the pollutants that cause acid rain and require new homes to contain the most efficient heating and cooling systems available.