For a long time now -- indeed, since the first Earth Day in 1970 -- self-styled "environmentalists" have been warning the rest of us that our planet is spinning its way toward ecological Armageddon.

It's a depressing litany: Melting glaciers, rising temperatures, violent weather, crop failures -- and nearly all of it, we're told, the fault of human beings engaged in such unforgivable activities as creating businesses, driving cars and ... well, breathing.

"We humans are about as subtle as the asteroid that wiped out the dinosaurs," New Scientist magazine says. "The damage we do is increasing ... We are heading for cataclysm." The Washington-based Worldwatch Institute finds "the key environmental indicators are increasingly negative." And Greenpeace predicts that "half the Earth's species are likely to disappear in the next 75 years."

It sounds pretty frightening -- until you look beyond the headlines. Then you discover that such claims rest mostly on hype, rather than on science.

Take forests. They're shrinking, right? That's what the Worldwatch Institute says -- a "fact" dutifully parroted in classrooms and newsrooms nationwide. But as Danish professor Bjorn Lomborg points out in his book "The Skeptical Environmentalist," Worldwatch makes this sweeping claim without sources.

Data available from the United Nations show that "forest cover has remained remarkably stable over the second half of the 20th century," Lomborg says, and actually appears to have increased slightly.
(Lomborg, by the way, is a former Greenpeace member who originally set out to prove that Julian Simon, the late economist who had spent years debunking environmental doomsayers, was wrong. But, time after time, he found the facts supported Simon.)

How about air pollution? We're told that's on the rise. And it is -- in the developing world. In industrialized countries such as the United States, where the total number of car miles traveled has more than doubled over the past 30 years, emissions have decreased by a third and the amount of pollutants such as lead by 80 percent and more. Why? Because, Lomborg says, only nations with growing economies can afford clean-air technology.

Then there's global warming. The conventional wisdom is that climate change can be explained as simple cause-and-effect: As greenhouse gases (such as carbon dioxide) rise, so do average temperatures. Industrial activities belch these gases into the air, trigger warming and invite environmental calamities.

But is it really that simple? The fact is, many scientists admit that we can't be sure how much of an impact human activity has on global temperatures.

One study, for example, conducted by NASA's Goddard Institute for Space Studies, suggests carbon dioxide may not be the biggest contributor to greenhouse gases. Even a report from the National Academy of Sciences (a global-warming advocate) says there is "considerable uncertainty in current understanding of how the climate system varies naturally and reacts to emissions of greenhouse gases."

It says warnings about the "magnitude of future warming should be regarded as tentative and subject to future adjustments (either upward or downward)."

But why be surprised? As Dr. Kenneth Green of the Reason Public Policy Institute notes, we've been taking temperature readings for a relatively short portion of the Earth's total life-span (about the last 150 years). As technology improves, we're gaining a better understanding of other variables that affect climate, from cloud changes and "carbon sinks" (forests that soak up carbon dioxide) to solar radiation and volcanic aerosols.

I'm not suggesting that all environmental warnings are groundless -- only that we shouldn't swallow every doomsday scenario whole. Factory smokestacks aren't the only source of hot air.

Edwin Feulner is president of The Heritage Foundation, a Washington-based public policy research institute.