Holman W. Jenkins Jr., in the Wall Street Journal, notes perfectly accurately that they believe in it because it is apparent that a significant majority of the elite believes in it.

It would surprise the public, and even the Supreme Court, to know how utterly the science of global warming offers no evidence whatsoever on the central proposition. What fills Mr. Gore’s film, books, speeches and congressional testimony are scientific observations and quasi-scientific observations, all right. They concern polar bears, mosquitoes, hurricanes, ice packs and everything but whether humans cause global warming.

Some of this evidence may suggest, weakly or strongly, the existence of warming trends in particular parts of the world (such local trends, both cooling and warming, have been observed in many places and many times). More dubiously, some may indicate a generalized warming. But none offers any evidence that carbon dioxide is causing warming. Mr. Gore’s method is the equivalent of trying to prove that Jack killed Jane by going on and on about how awful it was that Jane was killed.

Polemicists in favor of human-caused global warming liken skeptics to tobacco lobbyists who denied the link between smoking and lung cancer. In fact, it makes a useful analogy.

Suppose the world consisted of exactly one smoker who could be observed only from a distance to test the theory that smoking causes lung cancer. If he died of cancer, it wouldn’t prove smoking causes cancer. If he failed to die of cancer, it wouldn’t prove smoking doesn’t cause cancer.

The link between smoking and cancer is made by observing millions of smokers and nonsmokers. Indeed, what led scientists to seek systematic evidence of a link in the first place was anecdotal evidence that smokers, of whom there have been millions, appeared to die in unusual numbers from lung cancer.

Nothing remotely similar has been involved in developing the hypothesis that carbon dioxide creates warming. The relevant observations are a mess: Measured global temperature has both risen and fallen for considerable periods during the past century, even as CO2 has risen steadily. The geologic record suggests the world was much cooler in the past despite CO2 concentrations higher than today’s. Unlike smoking and cancer, there’s no anecdotal observation for the hypothesis that CO2 causes planetary warming. It may or may not be true, but to believe it is a “scientific truth” is to make a leap of faith, not science.

The consensus that human activities are causing global warming is purely a social invention — there’s no way of showing it to be so, and no self-evident reason for preferring to believe it’s so. The “consensus” is, in truth, a product of itself.

Now we are prepared to get the joke. It came during last fall’s Supreme Court oral argument about global warming, when the learned Justices, allowing the word “consensus” to serve as evidence of manmade warming, devoted themselves instead to a solemn discussion of how many inches of sea-level rise, and thus how many square miles of coastal inundation, the EPA is guilty of failing to prevent by refusing to regulate U.S. tailpipe emissions (which account for just 8% of human CO2 output).

Sen. James Inhofe is notorious for saying the theory of manmade global warming is a “hoax.” Obviously we need a better theory than Mr. Inhofe’s of when head-counting is a useful way of estimating the validity of a factual proposition and when it isn’t. Until then, it’s perhaps sufficient to say that many people believe in manmade global warming because many people believe in manmade global warming; Al Gore believes in it because many people believe in it; many people believe in it because Al Gore believes in it; and so on, right up to the highest court in the land.