IT IS SUMMERTIME and the skies are angry, filled with smoke from forest fires, the greenhouse gases building up almost before your eyes, the fragile ozone layer barely protecting humanity from the sun's destructive rays.

Soon, perhaps, a melting ice cap will put your favorite beach resort under water. Then, you will turn on your tap and no water will come out. The reservoirs will be dry.

Since the environmental movement first gained strength 30 years ago, predictions of the demise of the world as we know it have been a staple. Mankind loves an imminent apocalypse. End-of-the-world movements have probably had adherents since the beginning of the world. The top fiction hardcover book on The New York Times best-seller list is The Remnant: Armageddon is Near, the latest installment of the immensely popular Left Behind series, a dramatization of Christian end-of-the-world prophecies.

The problem with such prophets is that the world, darn it, doesn't end on their schedule. Adherents lose patience and faith. And that means such apocalyptic tendencies can hurt the environmental movement. Since various predicted cataclysms have not happened, then it all must be a bunch of hogwash.

One of the most famous examples of this was a wager between Paul Ehrlich and eco-skeptic Julian Simon in 1980. Ehrlich -- whose 1968 book The Population Bomb used that Cold War terminology to warn of the coming crisis of world population -- accepted a challenge from Simon. Ehrlich bet that the price of five natural resources -- copper, chrome, nickel, tin, and tungsten -- would go up in the next decade because of growing scarcity. Instead their prices went down. Ehrlich paid up, and Simon became a hero to those who would debunk the environmental movement.

Many of Ehrlich's dire predictions have not come true, including the size of catastrophic famines that would ravage the world. No one took up another bet Ehrlich offered in 1969 -- even money that England would cease to exist by 2000. And there were myriad prophecies during the gas-line days of the 1970s that the world would run out of oil in a few years that also failed to come to pass.

These issues were used against the environmental movement in a recent book, The Skeptical Environmentalist by the Danish scientist Bjorn Lomborg. He attempts to show that despite the warnings, the environment is improving. Yet the dire predictions continue, he says, to help in fund-raising efforts.

Steve Fetter, associate director of the Joint Global Change Research Institute at the University of Maryland, College Park, says that most environmental scientists are overly cautious, but that is not always how it comes out in the popular media.

"Folks trying to catalyze political action need to have some visible symptom to get peoples' attention," says Fetter, who has studied global warming. "So if you have a really hot summer, forest fires, hurricanes, the temptation is to use these things to get the political support you need. It's not really scientific, but it's understandable."

Katherine McComas, who teaches at the university's journalism school, studied media coverage of the first wave of global warming warnings a decade ago. She found that early stories responded to apocalyptic visions of rising seas and other disasters, but that then the pendulum shifted to coverage of economic arguments and disputes among scientists.

"Some scientists seemed to be really scaring people, so what they set up is the possibility that in the future, people would then tend to be less willing to pay attention to those issues," she says. "It's the 'boy-who-cries-wolf' scenario."

Andrew Miller, an associate professor in the department of geography and environmental systems at the University of Maryland, Baltimore County, says that environmentalists are often driven to issue dire warnings because more studied statements are dismissed by those making money off the status quo.

"Personally, I don't walk around telling students to expect the apocalypse, but as a skeptic, I am also skeptical of those who say that nothing bad will happen because it hasn't happened yet," Miller says. "There is almost a total consensus among scientists that we are now experiencing a total climatic shift. How large that will be, what the consequences will be, it is hard to say. The fact is we are running a huge uncontrolled experiment in a very complex system. Arguing that we should continue to run that experiment because it has not resulted in catastrophe so far is self-serving for those who make that argument."

The apocalyptic language is understandable in the global- warming debate because, as Robert Park, head of the Washington office of the American Physical Society, says, the two sides of the global warming argument "came at this thing from two religious points of view."

But he says the result was good science. "Both sides were out there working [hard] ... doing really good research. They played by the rules and gave an honest accounting of what they saw. It worked great. We learned more about the climate in a few years than we had in decades."

And he says the bottom line is a consensus that man's use of fossil fuels is warming up Earth. Now the argument is if this will mean a catastrophe or just a wardrobe change?

Park says the results could be cataclysmic: "If you look far enough down the line, the apocalypse will happen, but it will happen pretty slow."

Says Miller: "I think a lot of things are seriously at risk. That does not mean that human ingenuity cannot come up with a way to deal with this."

Climate-shift scientists do have a couple of doomsday scenarios. In one, global warming affects the ocean in a way that messes up the long-standing flow of currents. The Gulf Stream ceases delivering warm water to Europe. Ironically, Earth's warming brings an ice age to that continent.

"This has happened in the past in a matter of years, definitely less than 10 years," says Fetter. "It would be a catastrophe for European agriculture."

The other scenario is a collapse of the west Antarctic ice sheet, a huge chunk of ice that is attached to land. "If that shelf were to break off and fall into the ocean, it would be like plopping a big ice cube in a glass of ice tea," Fetter says. "That could raise sea levels by as much as 17 feet, putting a lot of the world's ports under water."

Fetter says the odds are that these things won't happen, but that it would be irresponsible for scientists who see these possibilities not to point them out.

The problem with these and other predictions of catastrophe is that they assume a linear progression in the future -- that we keep doing what we've been doing. But we don't. We change our behavior, in part because of what those predictions tell us.

So Ehrlich's famines did not come about because the world was able to produce more food more efficiently. But one reason people figured out how to do that was because predictions of famine gave them the incentive.

Fetter points to the atmosphere's ozone depletion problem as the system working properly. A scientist in the 1970s said that certain chemicals were damaging the ozone layer which could lead to a marked increase in skin cancers.

This caused some action -- the replacement of propellants in aerosols -- and made people start paying attention. Several years later, they found a hole in ozone layer over Antarctica. It was caused by a different chemical mechanism than the one originally outlined, but because the alarm had been raised, the world was ready to take action. The damaging chemicals were banned and the ozone regenerated.

So the doomsday scenario did not materialize, lending support to those who say technology can take care of these problems. But if the doomsday scenario had not been raised, then technology might not have been ready to prevent it.

"In part, you make these predictions to get people's attention," says M. Gordon "Reds" Wolman, professor of geography and environmental engineering at the Johns Hopkins University. "Lomborg's argument [in The Skeptical Environmentalist] is that the Cassandra style is beginning to be counterproductive, but the counterargument is that progress would not take place without the equivalent of a bunch of Cassandras."

Miller says it is necessary to be dramatic.

"I think that society responds stronger to something that impacts them directly. How many societies look multiple decades ahead and invest in trying to prevent problems that do not have a direct impact yet?" he says. "Politics doesn't work that way. You have to get peoples' attention."

Other than climate change, water -- Wolman's specialty -- is the main item on the agenda of serious environmentalists. Again, Lomborg and his allies point to improving water quality in a variety of once-polluted areas as proof that modern society can handle this. Wolman says that water is better in cities in North America and Europe, but the same cannot be said about Asia and those parts of Africa where water quality is measured.

"It is hard to argue that we are doing immensely better everywhere," he says. "It can be demonstrated that we are doing awfully well in some places, once people put their mind to it, and their money. But that doesn't happen without a tremendous amount of push."

And that push doesn't happen, quite often, until doomsayers get the attention of people and their political leaders.

"Some people say just relax, but you can't relax," he says. "The world we live in doesn't get better by itself."