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Second, the science is extraordinarily complex. The impact of any single human activity—let’s pretend we tripled the number of airplane flights, for instance—depends on many different factors: the gases emitted, yes, but also how the planes affect things like convection and cloud formation.

To predict global surface temperatures, one must take into account these and many other factors, including evaporation, rainfall, and, yes, animal emissions. But even the most sophisticated climate models don’t do a very good job of representing such variables, and that obviously makes predicting the climatic future very difficult. By comparison, the risk models used by modern financial institutions seem quite reliable—but, as recent banking meltdowns have shown, that isn’t always the case.

The imprecision inherent in climate science means we don’t know with any certainty whether our current path will lead temperatures to rise two degrees or ten degrees. Nor do we really know if even a steep rise means an inconvenience or the end of civilization as we know it.

It is this specter of catastrophe, no matter how remote, that has propelled global warming to the forefront of public policy. If we were certain that warming would impose large and defined costs, the economics of the problem would come down to a simple cost-benefit analysis. Do the future benefits from cutting emissions outweigh the costs of doing so? Or are we better off waiting to cut emissions later—or even, perhaps, polluting at will and just learning to live in a hotter world?

The economist Martin Weitzman analyzed the best available climate models and concluded the future holds a 5 percent chance of a terrible-case scenario—a rise of more than 10 degrees Celsius.

There is of course great uncertainty even in this estimate of uncertainty. So how should we place a value on this relatively small chance of worldwide catastrophe?

The economist Nicholas Stern, who prepared an encyclopedic report on global warming for the British government, suggested we spend 1.5 percent of global gross domestic product each year—that would be a $1.2 trillion bill as of today—to attack the problem.

But as most economists know, people are generally unwilling to spend a lot of money to avert a future problem, especially when its likelihood is so uncertain. One good reason for waiting is that we might have options in the future to avert the problem that cost far less than today’s options.

Although economists are trained to be cold-blooded enough to sit around and calmly discuss the trade-offs involved in global catastrophe, the rest of us are a bit more excitable. And most people respond to uncertainty with more emotion—fear, blame, paralysis—than might be advisable. Uncertainty also has a nasty way of making us conjure up the very worst possibilities. (Think about the last time you heard a bump in the night outside your bedroom door.) With global warming, the worst possibilities are downright biblical: rising seas, hellish temperatures, plague upon plague, a planet in chaos.

It is understandable, therefore, that the movement to stop global warming has taken on the feel of a religion. The core belief is that humankind inherited a pristine Eden, has sinned greatly by polluting it, and must now suffer lest we all perish in a fiery apocalypse. James Lovelock, who might be considered a high priest of this religion, writes in a confessional language that would feel at home in any liturgy: “[W]e misused energy and overpopulated the Earth…[I]t is much too late for sustainable development; what we need is a sustainable retreat.”

A “sustainable retreat” sounds a bit like wearing a sackcloth. To citizens of the developed world in particular, this would mean consuming less, using less, driving less—and, though it’s uncouth to say it aloud, learning to live with a gradual depopulation of the earth.

If the modern conservation movement has a patron saint, it is surely Al Gore, the former vice president and recent Nobel laureate. His documentary film An Inconvenient