The most important number on Earth is almost certainly 450. And just as certainly, it’s not a number that means much to most policymakers. Not yet, anyway.

Everyone without a severe ideological kink knows by now that global warming is a looming problem. Even in the United States, two decades of energy industry disinformation is finally wearing off: Hurricane Katrina and Hurricane Gore have finally blown most doubt away. But many fewer people realize either the real magnitude of the problem or the speed with which it may be bearing down on us.

Here’s the short course. Before the Industrial Revolution, the atmospheric concentration of carbon dioxide was roughly 280 parts per million. CO2, by virtue of its molecular structure, regulates how much of the sun’s energy stays trapped in our narrow envelope of atmosphere—Mars, which has very little, is cold; Venus, with a lot, is hellish. We were in a sweet spot, where human civilization developed and thrived. But as we burned coal, gas, and oil, the extra carbon dioxide that combustion produced began to accumulate in the atmosphere. By the late 1950s, when people first started to measure it, atmospheric concentrations were already above 315 parts per million. Now, that number has reached 380 parts per million, and its rise has accelerated: In recent years, we’ve been adding about 2 parts per million annually to the atmosphere. And, predictably, the temperature has begun to rise.

Twenty years ago, when global warming first came to public consciousness, no one knew precisely how much carbon dioxide was too much. The early computer climate models made a number of predictions about what would happen if we doubled the amount of CO2 in the atmosphere to 550 parts per million. But, in recent years, as the science has gotten more robust, scientists have tended to put the red line right...