The United States must take major steps to cut energy consumption - including more efficient automobiles and home heating systems - if expected global warming problems are to be eased, the Environmental Protection Agency said Friday.

In a report to a Senate environmental subcommitee, the EPA outlined actions that are needed by the end of this century if the movement toward a dangerously warmer earth is to be slowed.While the report said the United States should take the lead in meeting the global warming, or greenhouse, problem head-on, EPA Administrator William K. Reilly emphasized the problem is international in scope. "The response to this problem cannot be a unilateral one," Reilly told the Senate subcommittee on environmental protection, adding that if there is not international cooperation, U.S. actions could be "canceled out by other countries."

But Reilly said the United States "is going to have to lead by example" and that President Bush has made clear he intends to take a leadership role.

The EPA report, concluding two years of study, acknoweldged that there remains considerable uncertainty about the magnitude of global warming. But it said there is a "growing consensus" in the scientific community that if nothing is done to reduce the discharge of pollutants into the atmosphere, the Earth can be expected to become 4 to 11 Farenheit degrees warmer over the next century.

"There's no doubt that there is a consensus that the `greenhouse' phenomenon is very real," said Sen. Max Baucus, D-Mont., the subcommittee chairman.

The scientific consensus has been that pollution is causing a "greenhouse effect" by trapping the sun's infrared radiation instead of allowing it to go back into space. The major contributors to the warming process are carbon dioxide, chlorofluorocarbons, methane and nitrous oxides.

The EPA report said that "no single technology or policy can have significant impact in mitigating the accumulation of greenhouse gases" because of their wide use. Instead, it said, significant reductions in emissions can be achieved only by dealing with the problem broadly.

One focus, the agency said, must be aimed at reducing the use of fossil fuels that produce carbon dioxide, by far the largest single contributor to global warming.

To curtail energy use, the report proposed:

- Increasing average automobile fuel economy levels to 40 miles per gallon by the year 2000. This is in sharp contrast to the Reagan administration, which in recent years urged elimination of federal fuel economy standards and accommodated auto industry requests not to set fleetwide standards above 26.5 mpg.

- Taking measures to make new single-family homes more energy efficient so they will use an average 50 percent less fuel heat in the year 2000 than the average in 1980.

- Setting fees on carbon emissions ranging from 7 to 20 percent on natural gas, oil and coal to encourage less use of those fossil fuels.