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.
Even warming theory believers acknowledge that, so far, the average temperature increase is not big enough to be blamed on human activity.
Scientists have looked back in time for clues, poring over the logs of ancient Viking explorers for references to sea ice. Others have studied glaciers and tree rings.
BTC But the bulk of the case for global warming relies largely on computer projections. Using complex mathematical models that simulate the Earth's climate, scientists predict that if the current buildup of man-made greenhouse gases continues, the planet will warm by 3 to 8 degrees Fahrenheit by the end of the next century.
That may not seem much, but the average temperature was no more than 3 degrees colder during the "little Ice Age" in the 14th century, when the Baltic Sea froze over twice. And for the past 10,000 years, Earth was at no point more than 4 degrees warmer than it is now, according to Irving Mintzer, a senior research scholar at the University of Maryland's Center for Global Change.
A U.N.-sponsored panel of climate experts, looking at all the uncertainties, this year reaffirmed its earlier finding that global temperatures are likely to rise if current emission trends continue.
Recently, the Bush administration has concurred that the signs point to warming.
But given the limitations of computer models, scientists caution that they cannot predict with certainty how far or how fast the mercury will rise, or what that means to inhabitants of different parts of the world.
One widely expected impact, however, is a rise in sea level as glaciers and northern ice melt. Scientists forecast that the waters will rise by from 1 foot to more than 3 feet over the next century.
Half the world's population lives in coastal areas, and flood-prone countries like Bangladesh and Indonesia are most at risk. In this country, experts say that low-lying areas along the East and Gulf coasts are vulnerable to flooding, increased shore erosion and storm damage.
Maryland's Atlantic coast and bay shoreline could be especially at risk. Tide gauge records show that the sea level has been rising 3 to 4 millimeters a year. But part of that apparent rise is because the land itself is sinking for unknown reasons, said Dr. J. Court Stevenson, a wetlands ecologist at the University of Maryland.
Tidal marshes, vital sources of food and shelter for waterfowl and fish, are already rapidly disappearing under water, Dr. Stevenson said, and those losses will increase if the sea level rises as predicted.
St. Michaels in Talbot County could suffer severe flooding in even a moderate storm if the sea level rises 3 feet by the end of the next century, Dr. Stevenson said, and all that would be left above water on Smith Island would be the fishing community's dump and graveyard.
In the past century, the sea level has risen about 1 foot along the bay, working through shore erosion to wash away Sharp's Island near the mouth of the Choptank River and more than a dozen other islands, many of them once inhabited, said Dr. Stephen Leatherman, a geographer at the University of Maryland.
Shore erosion already is severe at some points around the bay, where the land recedes 6 to 8 feet a year. Dr. Stevenson and Dr. Michael S. Kearney, a University of Maryland geographer, predict that waterfront property worth from $90 million to $120 million could wash away every year if erosion doubles, as expected.
The potential impact on farming is more optimistic, at least in this part of the world. Since plants need carbon dioxide, doubling its concentration in the air can lead to a 30 percent to 50 percent improvement in crop yield.
"With crops, I think the evidence is overwhelming that they will respond positively," said Dr. Bert Drake, a staff scientist at the Smithsonian Institution's environmental research center in Edgewater.
But that may not be so in warmer countries like Bangladesh, India and Indonesia, where rice is the staple. Rising temperatures can reduce rice fertility and even shorten the growing season, said Dr. Alan Teramura, a University of Maryland botanist.
Natural ecosystems could be upset. Up to one-fifth of the 15,000 plant species growing in North America may be unable to survive if average temperatures rise 8 degrees, according to preliminary results from a utility industry-funded study by the Nature Conservancy.
The more common plants and trees will survive and may be able to "migrate," as they have during past climate changes. But as many as half the plants now considered endangered could be wiped out because their numbers and their range already are extremely limited, according to Dr. David Maddox, a research botanist for the conservancy.
Given the uncertainties and the potential for calamity, some have argued that world leaders need to begin curbing greenhouse gas emissions now. But the Bush administration balked at setting any binding limits, fearing that they could hamper America's economic recovery.
One study by CONSAD Research Corp. warned that the United States could lose 600,000 jobs by the end of the decade if the country went along with one warming remedy proposed by environmentalists -- a "carbon tax" on the burning of coal, oil and other fossil fuels.
But environmentalists and some businessmen contend that curbing greenhouse gases need not bankrupt the country. In the absence of conclusive proof of global warming, they recommend taking steps that make sense for other reasons, such as reducing pollution or oil imports.
They advocate promoting energy efficiency and conservation, development of "renewable" energy like solar and wind power, and reforestation, and they note that Europe and Japan are aggressively seeking to develop new products and industries in these fields.
"It's not good enough to wait until there's a catastrophe, and then decide to act," said Senator Gore. "If we act now, we're going to be better off, since the range of possible outcomes include catastrophes that are almost biblical in scale."
TOMORROW: Sizing up the summit.