There's no denying the Earth is getting hotter. Now we just have to draw a line in the sand.
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 around 450 parts per million. That's where NASA's James Hansen, America's foremost climatologist, has said we need to stop if we want to avoid a temperature rise greater than two degrees Celsius. Why would two degrees be a magic number? Because as best we can tell, it's where the melting of the Antarctic and Greenland ice sheets would become rapid and irrevocable. The ice above Greenland alone contains about 23 feet of sea-level rise, which is more than enough to alter the Earth almost beyond recognition.
So far, the diplomatic effort to do something powerful about climate change has been blocked by a couple of factors. One is the complete intransigence of the United States, where 5 percent of us produce a quarter of the world's carbon dioxide. But assuming that the next president finally gets us on some kind of new course, the international negotiations that could then resume in earnest will still be hampered by the lack of a real and understandable goal. The Kyoto treaty was as much about process as outcome -- it began to build the plumbing for an international system of carbon controls. But the time was not yet ripe to set a real, urgent, ultimate target for that work.
That time has now come. Instead of vague promises about taking global warming seriously, we need numbers. It will be incredibly difficult to stop at 450 parts per million -- it will require large-scale technological and social change, with the investments of financial and political capital that such shifts imply. Even if we muster that will, we won't solve the problem: The Earth will continue to heat, with fairly dire, if not catastrophic, implications. (There are, it should be added, some scientists who think we've already breached the red lines -- that the Earth's feedback systems are already producing a kind of runaway greenhouse effect.) But without a goal, one that we can track as easily as the Dow Jones average or the size of the gross domestic product, the chances for focused and concentrated progress are almost nil. You’ll know the real statesmen and women of the future -- they'll be the ones with the little gold "450" pin on their lapels. In a very real sense, it may be the only number that counts.