Global warming. The words themselves conjure up beads of sweat. Is Mother Nature getting a fever? Can she survive it? Can we afford the cure?
Leaders from 160 nations are flying to Rio de Janeiro, Brazil, this week to take the planet's temperature at the United Nations Conference on the Environment and Development, better known as the Earth Summit.
Top on the agenda of the 12-day session is a treaty that would pledge world leaders to avert a global warm-up by curbing emissions of so-called "greenhouse gases," chiefly carbon dioxide.
Many scientists worry that the accumulation of carbon dioxide and other heat-trapping gases will turn up the thermostat on the Earth's natural greenhouse, raising global temperatures. The result could be nightmarish problems, such as coastal flooding, drought, famine and extinction of plants and animals.
In Maryland, for instance, the projected sea-level rise could aggravate persistent shoreline erosion in the Chesapeake Bay, inundating historic waterfront communities like Smith Island and St. Michaels. Soft clams might vanish from the bay, and the recently restored striped bass, or rockfish, could be stressed trying to live in water even slightly warmer than it is now.
In dry, hot regions of the world, where food production is already difficult, rising temperatures could worsen droughts, according to Cynthia Rosenzweig, an agronomist at Columbia University. As a result, up to 120 million more people might go hungry each year, said Dr. Rosenzweig, co-author of a study on the effect of climate change on world food supply.
Skeptics counter that global warming, if it occurs, will be mild and mainly benign. They foresee lower heating bills and longer growing seasons for farmers in temperate regions like the United States.
These arguments among scientists mirror the bickering among politicians that has begun even before the opening gavel in Brazil. Environmentalists have accused President Bush of courting global disaster by getting the proposed climate treaty stripped of targets and timetables.
"History gave him a mandate to lead at a critical juncture in the world's history, and he has absolutely refused to do so," Sen. Al Gore, D-Tenn., said last week of Mr. Bush.
"I think it's disgraceful," added Senator Gore, an outspoken environmental advocate who is leading a Senate delegation to Rio.
Some business groups and conservatives, meanwhile, have complained that, despite its lack of teeth, the treaty remains a folly, an economic straitjacket being peddled by doomsayers, who were warning just 15 years ago that another Ice Age was coming.
"Why should the United States want to take the lead in pushing such hokum?" asked Murray L. Weidenbaum, a former chief economist in the Reagan administration.
Global warming first captured headlines and the public's attention in the searing summer of 1988 when Dr. James Hansen of the National Aeronautics and Space Administration's Goddard Institute of Space Studies in New York told a congressional committee that he was "99 percent confident" that "the greenhouse effect has been detected and it is changing our climate now."
The theory behind Dr. Hansen's assertion is nearly a century old. It was posed in 1896 by a Swedish chemist, Svante Arrehnius, who predicted that carbon dioxide emitted by coal-burning factories of his era could eventually warm the planet.
Put simply, the greenhouse gases in the Earth's atmosphere act as an invisible blanket. They capture the sun's warming infrared rays after they are reflected off the planet's surface. Otherwise, much of the sun's energy would bounce off the Earth into space, leaving it as cold and apparently lifeless as Mars.
Naturally occurring water vapor makes up the largest part of the greenhouse gases. But scientists say there is little that can be done to influence the amount of moisture in the planet's atmosphere.
The chief threat of excess warming comes from carbon dioxide, mainly from burning fossil fuels, and a few other gases, most of which occur naturally but which also are byproducts of human activity.
Carbon dioxide, which accounts for more than half the man-made greenhouse gases, has risen 25 percent since pre-industrial times. Another gas, methane, released by cattle, plant decay, rice paddies and coal mines, has doubled.
The evidence of global warming so far has been less than conclusive.
Temperature records dating back to the 1850s indicate that the planet has warmed about 1 degree Fahrenheit in the past century.
Everyone has felt the heat in the past decade. The 1980s had six of the 10 hottest years ever measured.
But skeptics like Dr. Richard Lindzen, a meteorologist at the Massachusetts Institute of Technology, argue that the rise in temperature so far can be chalked up to the natural vagaries of climate -- a string of hot summers and mild winters, but nothing to get alarmed about.
"The politics have not only outrun the science but redefined it," Dr. Lindzen said.