The environmental left has received some severe blows lately. One is the declining cost of oil, which environmental nannies fear will lead Americans to forget that they have a moral duty to consume less fossil fuel. The other is a decision by the World Health Organization to lift its ban on the use of the insecticide DDT for combating malaria in the Third World.

The latter strikes at the heart of the modern environmental movement, which was spawned in part by Rachel Carson’s famous 1962 polemic, “Silent Spring.” In lyrical — some might say hysterical — terms, she wrote of the dangers of chemicals like DDT that supposedly threaten to upset the natural balance. This led to a ban on the manufacture and export of DDT, resulting in millions of unneeded deaths in the malarial regions of the world.

The enviros still insist that malaria can be stopped by the widespread use of bed-nets and less harmful chemical substances. In the real world, DDT is still the cheapest, most effective, and easiest to use anti-malaria agent, a critical consideration in impoverished places like Africa, which accounts for about 95% of the one million deaths a year from malaria. And if used responsibly, according to a 2005 study in the British medical journal Lancet there is no evidence that it poses a threat to human beings.

Researchers haven’t even been able to show conclusively that DDT is the cause of widely-cited declines in populations of eagles and other animals.There appeared to be a strong correlation, but the type of DDT use being recommended by WHO — indoor spraying to reduce the risk of mosquito bites to sleeping humans — is no threat to nature. All this was known more than three decades ago, but the environmental lobby had become so powerful that rational decision-making was all but impossible.

There is an important lesson here. Policy decisions that aren’t based on a cold, hard appreciation of costs and benefits, as well as solid science, cost lives.That’s as true of oil policy as it is of DDT policy.

The drumbeat these days is about global warming.T he science is “settled,” we are told. Failure to ban or sharply reduce use of fossil fuels will lead to a global silent spring. The fate of the earth, Al Gore informs us, depends on acting without delay.

That, of course, is exactly what Rachel Carson told us. Just as with the near-extinction of some bird species, natural phenomena are occurring that seem to be consistent with the theory that human activity is artificially heating up the planet. But there could be other explanations as well. There are still wide variations in predictions of just how much the earth is likely to warm, strongly suggesting that the science is anything but settled. And there are some scientists who have expressed doubts about the underlying theory of global warming itself.

Yet the environmental left, no doubt animated by visions of a giant leap forward for Big Government, is eager to clamp controls on energy use. They have allies on the right as well. The new Treasury secretary, Henry Paulson, recently delivered a speech expressing about “China’s severe environmental hazards,” in particular its fast-growing consumption of fossil fuel.

Mr. Paulson’s real target may have been no less than President Bush, who has called for an end to America’s own “oil addiction” but — fortunately — hasn’t done much about it. Could the administration be preparing to cave in to those who see the answer to global warming as government controls on energy use?

Citing the so-called “precautionary principle,” advocates of such controls argue that we must act even before all the facts are in. To delay is to risk catastrophe down the road, they assert. But insofar as the precautionary principle is valid, it might offer a strong reason not to act too quickly. The one thing we know about government, after all, is that it often gets things wrong — as it did with DDT.

The Kyoto Protocol calling for reductions in CO2 emissions to pre-1990 levels would cost hundreds of billions — without actually slowing global warming very much, even its supporters agree. Just imagine what an effort to roll back energy use even more radically would cost. Over time, the loss of income and jobs will mean poorer, meaner, deadlier societies — and a less healthy environment. It would be the DDT tragedy writ large.

Mr. Bray is a columnist based in the Detroit area.