People are naturally prone to worry about dangers that are invisible: radioactivity, for one spectacular example. The media know this, and are forever trumpeting the discovery of new perils to scare us with. Hardly a week passes without someone announcing that some familiar food or other useful substance has just been discovered to cause cancer (though usually only when administered in huge doses to mice). Dangers associated with weather are special favorites because they are usually so difficult to cope with. In recent decades, we have been treated to alarmist reports about impending disasters to be caused by nuclear winter, acid rain and the ozone hole. But the Big Daddy of all such scare stories is “global warming.”

Al Gore, who had a dangerous brush with the presidency in 2000, has long been associated with this particular fright syndrome, and I have no doubt that he is perfectly sincere in believing that global warming is a real danger. But recently he has stepped forth with a brand new campaign to sell the American people on the peril. It is spearheaded by a documentary film entitled “An Inconvenient Truth,” in which Gore himself presents what he chooses to regard as overwhelming evidence of the reality of the danger. Given his political history, it is perfectly fair to wonder if this maneuver isn’t simply, or primarily, a device to promote his own candidacy for the presidency in 2008. But, whether it is or not, it is also a powerful blast in the propaganda war over the issue of global warming — and must be treated as such.

Gore begins by insisting that the scientific argument over the truth of the matter is over; climate scientists, he asserts, are virtually unanimous in endorsing it. Among the thousands of predictions on the subject, moreover, he invariably opts for the worst-case scenarios. The increase in the quantity of carbon dioxide in the atmosphere is caused, to an important extent, by human “pollution,” and this is the cause of a dangerous increase in the planet’s surface temperature. That, in turn, is causing glaciers, and the great ice caps of Greenland and Antarctica, to melt. This will inevitably result in the disastrous flooding of coastal areas all over the globe, and all sorts of ecological upsets (e.g. the extinction of the polar bear).

The trouble is that all of the statements in the last paragraph above are subject to challenge, and in several cases, are almost certainly false. Among the many systematic attacks being waged against the spurious case for global warming, one of the deadliest and most effective is a weekly report available on the Internet, called “The Week That Was” (TWTW). The author is the formidable S. Fred Singer, professor emeritus of Environmental Sciences at the University of Virginia and former director of the U.S. Weather Satellite Service. Each week, Singer summarizes or reprints the most recent studies debunking global warming, with generous references to still more information. In TWTW for June 17, he demolishes Gore’s contentions, one by one.

The basic flaw in the argument for global warming is its assumption that the Earth’s surface temperature is a constant, and that, if it threatens to vary in some inconvenient direction, mankind’s puny efforts are capable of maintaining it within a degree or two of its present level.

The truth is that the Earth’s temperature is always changing to some extent, up or down. Within historic memory, the canals of Venice froze solid during the medieval Little Ice Age, and Greenland was verdant enough, during a warm spell, to earn its (currently) wildly inappropriate name. Over longer geological periods, the Arctic has sported palm trees (no polar bears then!) and the latitude of Connecticut was under a mile-thick layer of ice. Just now, according to Dr. Singer, we are seeing a warming trend of about one-tenth of a degree centigrade per decade, or roughly a degree per century.

There is nothing we can do about this, and no reason why we should try — let alone spend hundreds of billions of dollars trifling with titanic forces we can’t even comprehend.

(William Rusher is a Distinguished Fellow of the Claremont Institute for the Study of Statesmanship and Political Philosophy.)