"This was an act of anger, desperation and indignation" is the way Gar Smith, editor of Earth Island Journal, described the terrorist attack on America two days after it happened. He offered an "environmental analysis" of the event, tracing "every terrorist attack against the United States . . . back to one common factor: Oil." Mr. Smith's solution to terrorism was "to transform our economy into one that operates on clean, renewable energy."

Meanwhile, the Green Party USA suggested that we respond to the attacks by letting "U.S. corporations, so busy using up Earth's resources and beggaring Earth's life forms, protect themselves." It proposed an end to the American "manufacture and sale of most pesticides and industrial toxic chemicals."

That's one way of looking at Sept. 11 -- as if Osama bin Laden will leave us alone if we stop making plastic and start using ethanol. What is so odd about this, coming from environmentalists, is that prosperity is the reason why so many environmental trends are positive today, and prosperity owes a great deal to energy production, technology and markets.

You don't believe that environmental trends are positive? You are not alone: The media treat the environment as a subject of ceaseless decline, hastened by the indifference of ruthless capitalists and their toady politicians. But The Skeptical Environmentalist (Cambridge, 515 pages, $27.95 paper), a superbly documented and readable book by a former member of Greenpeace, has a different story to tell.

The author, Bjorn Lomborg, is a professor of statistics at the University of Aarhus in Denmark and a self-described "man of the Left." A few years ago he read an article about the economist Julian Simon in which Simon claimed that the state of humanity and the natural environment were both improving. Mr. Lomborg didn't believe it. He directed his students to find the "real" data that would debunk this "right-wing" American. What they found stunned him and inspired this book.

Mr. Lomborg begins with what he calls The Litany. "We all know it," he dryly notes: "Our resources are running out. The population is ever growing, leaving less and less to eat. The air and water are becoming ever more polluted. . . . The world's ecosystem is breaking down." There is "just one problem," he continues. The Litany "does not seem to be backed up by the available evidence."

Using uncontroversial data, Mr. Lomborg shows that the environment is improving, and the state of humanity too. Why? "Only when we are sufficiently rich can we start to think about, worry about and deal with environmental problems." In other words, economic growth is the friend of the natural world, not its enemy.

We all remember the dire predictions, from Stanford University Prof. Paul Ehrlich and others, that global famines would kill billions in the 1970s and 1980s. Well, economic growth and technological progress got in the way. Mr. Lomborg shows that, since 1960, the average amount of food per person in developing countries has increased 38%, and the percentage of malnourished poor people has fallen to 18% from 35% and will likely drop by another six percentage points in 10 years or so. Meanwhile, the cost of food is a third of what it was in the 1960s.

But surely a burgeoning population will lead to famines in the future? Actually, no. Mr. Lomborg notes that food production in developing countries has plenty of scope for increase since farms there are still far below the yields of farms in developed countries. In short, no new miracle technologies are needed, only the application of techniques already in use.

What about air pollution? Concentrations of sulfur dioxide are down 80% in the U.S. since 1962; carbon monoxide down 75% since 1970; nitrogen oxides down 38% since 1975. These trends are evident in all developed countries. And poor countries will be able to cut their pollution levels earlier simply by using the technologies developed by the West.

What about the forests? Mr. Lomborg finds the claims of environmentalists to be exaggerated. An authoritative United Nations survey found that global forest cover has been reduced by a minuscule 0.44% since 1961. In fact, temperate forests in the U.S. and Europe are expanding as crop productivity reduces the amount of farmland needed for growing food.

As for the Brazilian rain forests: The data show that 86% of them remain uncut, and the rate of clearing is falling. As the Brazilian economy has grown, its government reversed ill-advised land-tenure policies that encouraged poor farmers to cut down trees. Meanwhile, the spread of plantation forests is reducing the demand for the logging of natural ones.

As for global warming, Mr. Lomborg shows that it is unlikely to be a catastrophic problem. Why? First, actual measured temperatures aren't increasing nearly as fast as the computer climate models say they should -- in fact, any increase is likely to be at the low end of the predictions and no one thinks that the low end spells disaster. Second, the emissions of warming gases will decline, inevitably, as industries and consumers switch to forms of energy that will be cheaper than fossil fuels by the middle of the century.

Even if temperatures increase substantially, Mr. Lomborg argues, a draconian cut in fossil-fuel use is not the answer. A Kyoto Protocol-like reduction would cost humanity between $107 trillion and $274 trillion over the next century. (The GDP of the U.S. is about $10 trillion.) Such costs would mean that people living in developing countries would lose more than 75% of their expected increases in income. That would be a human tragedy and an environmental one as well, since poor people have little time for improving the environment.

Mr. Lomborg's analysis, as good as it is, fails to identify the chief cause of many environmental problems: the open-access "commons" (e.g., fisheries, airsheds) where people don't own a resource and so have no incentive to protect and conserve it. Clearly regulation has worked to improve these commons areas: Our air and streams are cleaner than they were. But there is good evidence that assigning property rights and market mechanisms to such resources would have resulted in a faster and cheaper cleanup.

"Things are better now," writes Mr. Lomborg, "but they are still not good enough." He's right. Only economic growth will allow, for example, the 800 million people who are still malnourished to get the food they need.

But will they get it? Not if the anti-Westerners win out. As The Skeptical Environmentalist makes clear, those who hate modern industrialized societies -- whether they are Islamic radicals or radical environmentalists -- threaten the hopes of the poor and imperil the natural world as well.