My cousin David, successful lawyer, businessman, and champion bridge player, has a sharp, analytical mind. He enjoys intellectual combat and loves to argue about global warming. How to convince him that Al Gore’s movie is sheer fantasy and that the “Stern Report,” just released in Britain, spells economic ruin? I plan to meet the challenge by appealing to logic.

I pose three fundamental questions to him:

(1) Is there evidence for or against an appreciable human contribution to current climate warming?

(2) Would a warmer climate be better or worse than the present one?

(3) Realistically speaking, is it possible to influence the climate by policy actions in an effective way?

First, the climate is always changing — either warming or cooling — on timescales ranging from decades to millions of years. Nearly 20 ice ages have come and gone in the past 2 million years, controlled by predictable changes in Earth’s orbit and in the tilt of the axis. Our present interglacial warm period is 12,000 years old and may soon end. Geological evidence also has uncovered a 1,500-year climate cycle, likely caused by the sun — and also unstoppable. On top of all this, we have irregular, unpredictable short-term fluctuations. Since 1979, weather satellites show a slight warming trend that is well within historical experience. How can we tell whether this warming is due to human influences, such as the rise in atmospheric greenhouse gases (mainly carbon dioxide from the burning of fuels), or whether it is simply another natural fluctuation?

The melting of glaciers and ice sheets, the rise in sea level, severe storms, floods, droughts — all of these are interesting, to be sure, but really irrelevant to our question. They may well be connected to a warmer climate — or maybe not — but they cannot tell us what causes the warming. Nor can a vote among scientists settle this issue; science is not democratic. History shows that the majority is quite often wrong.

Climate models, run on giant computers, give scary results, but these are simply theoretical exercises, not to be confused with real evidence. Nor can we argue that the rough correlation of current warming with the rising level of greenhouse gases proves a cause-and-effect relationship. World climate cooled between 1940 and 1975, while energy use and carbon dioxide levels rose sharply. Correlation is not causation — a truth that often is forgotten.

So what’s left? All working scientists agree that one must compare the observed patterns of warming with the patterns calculated from greenhouse models — and look for agreement of distinctive geographic and altitude distributions of the temperature trends. Directed by the National Oceanic and Atmospheric Administration, a May 2006 interagency government report, “Temperature Trends in the Lower Atmosphere,” using the best available temperature data and climate models, conclusively demonstrates: The patterns do not agree. (See figure 5.4G in climatescience.gov/Library/sap/sap1-1/finalreport/default.htm.)

Logically, even an agreement could not prove that the warming is due to human causes; it would only make it plausible. But when we find a significant disagreement between observations and models, then we can argue that the influence of human effects is minor compared to natural climactic fluctuations. This discrepancy also shows us that existing models cannot reliably be used to make predictions about future climate warming and its consequences.

The second question is clearly in the realm of economics. The ongoing debate assumes, generally without much analysis, that a warmer climate presents a “danger” or a “threat,” implying serious consequences for the economy, human health, ecology, and so forth. To the contrary, in “The Impact of Climate Change on the United States Economy” (Cambridge University Press, 1999), edited by Robert Mendelsohn and James Neumann, a prestigious group of resource economists showed positive consequences: The GDP of our country would increase for assumed modest temperature increases.

One can also look at historical evidence. We know from voluminous records that human existence was good and more comfortable during the Medieval Warm Period (circa 1100 common era), when the Vikings settled Greenland, than during the following Little Ice Age, when crops failed, people starved, and disease was rampant. Life then was nasty, brutish, and short.

Another way of tackling this question is to ask if things were better in 1976, when it was colder than today. It soon becomes obvious that climactic effects are minor compared to everything else that happened in the last 30 years. But that is exactly the point. Technological progress and the mobilization of capital in a market economy far outweigh any climactic factor in promoting prosperity — here and throughout the world. Would a slightly warmer climate, of, say, one degree Celsius in 100 years, have serious effects on the way we live, on the economy, on human health, or on anything else? Indeed, since most agree that a colder climate would damage the economy, one might ask, “What is the probability that the present climate just happens to be the Panglossian optimum?”

The third topic is perhaps easiest to deal with. People often talk about “stabilizing the climate.” What they really mean is stabilizing the concentration of greenhouse gases in the atmosphere. But even that is a daunting task. We all know what it would take to stabilize the level of carbon dioxide in the atmosphere: reducing emissions worldwide by between 60% and 80%. In practical terms, this calls for reducing energy use by comparable amounts. All also agree that the Kyoto Protocol is a puny effort. Its effect on climate would be miniscule — a calculated temperature reduction of only one-twentieth of a degree, unmeasurable by ordinary thermometers. Nevertheless, America and Australia have faced heavy criticism from other industrialized nations for not joining them in ratifying Kyoto.

Since it is unlikely that the current warming has much of a human component and also unlikely that something substantial could be done to reduce carbon dioxide growth, what is the point of having a vast and costly program for mitigation, when most likely, a warmer climate would have positive rather than negative effects? This is particularly true when tangible costs must be incurred today to avoid hypothetical damages in the far future.

I’ll try these arguments on cousin David next time I am in New York and see if he agrees.

Mr. Singer is professor emeritus of environmental sciences at the University of Virginia and author of “Unstoppable Global Warming — Every 1500 Years.”