Prime Minister Gro Harlem Brundtland of Norway Monday accused rich nations of playing lethal games with the Earth's life-support system and made a proposal to combat the greenhouse effect and protect the atmosphere.

In a keynote address to the Canadian-sponsored World Conference on the Changing Atmosphere, Brundtland, a noted environmentalist, told more than 300 scientists and politicians from 40 countries:"The impact of climatic change may be greater and more drastic than any other challenges that mankind has faced with the exception of the threat of nuclear war."

She called for a "new global ethic" in which protection against air and water pollution, ozone depletion and acid rain is a prerequisite for economic development.

"We have been playing lethal games with vital life-support systems," said Brundtland.

Her five-point plan included development of renewable energy sources in the next century; the transfer of modern, low-polluting technologies to Third World countries; more research on climatic change; and consideration of a global convention on protecting the atmosphere.

Canadian Prime Minister Brian Mulroney opened the four-day conference by offering to host a U.N.-run World Conference on Sustainable Development in 1992.

By that time, he urged participants, an "international law of the atmosphere" should be negotiated to limit all types of air pollution.

Those attending include the heads of U.N. environmental agencies; cabinet ministers from Indonesia, the Netherlands and Senegal; scientists from Harvard University and the Soviet Academy of Sciences; and Sen. Timothy E. Wirth, D-Colo., and Rep. George E. Brown Jr., D-Calif.

Scientists will discuss ozone depletion, acid rain, water management and food prospects, but much interest centers on the recent warming of the Earth's surface due to the greenhouse effect, in which gases humans have introduced into the atmosphere prevent heat from escaping.

Although experts are not yet ready to ascribe the 1988 drought to the greenhouse effect, the Earth has been warmer in the first five months of this year than in any comparable period since measurements began 130 years ago, a NASA climatologist, James Hansen, told a congressional hearing in Washington last week.

Five of the warmest years on record have occurred in the 1980s, and scientists at Environment Canada predict global temperatures could rise as much as 8 degrees by the year 2050, partially melting the polar ice caps and raising sea levels three feet.

"In the last 1,000 years, the climate fluctuated about a half of a degree. Now we can expect it to go up at 10 times that rate, a phenomenal change unprecedented in human history," Henry Hengeveld, a climate expert for Environment Canada, told a news conference.

The problem is manmade. The burning of oil, gas, wood and coal in factories, homes and cars sends the greenhouse gases of carbon dioxide,nitrous oxide, methane, ozone and chlorofluorocarbons into the atmosphere.

For much of the world, there are no alternatives to fossil fuel. In developed countries, public opinion has turned against the cleaner option of nuclear power because of the risk of catastrophic accidents.

Another major atmospheric problem is the gradual destruction of the thin ozone layer, which protects the Earth from the sun's harmful ultraviolet rays. Chlorofluorocarbons used in spray cans, insulation, refrigerators and industrial cleaning tear holes in the ozone, and the result has been a marked increase in skin cancer.

Twenty-four nations signed the Montreal Protocol on ozone last year, pledging to reduce the use of chlorofluorocarbons by 50 percent by 1999.

Canada's first atmospheric priority is acid rain. The government blames power plant emissions from the U.S. Midwest for at least half the acid rain falling in eastern Canada, which has led to the destruction of fish life in 14,000 lakes and extensive forest damage.