Politics and the Planet1
Instead, Hansen argues that we should look to Earth's past for clearer answers. We know from basic physics that, all else being equal, an increase in greenhouse gases will warm the planet. But by how much? From studying a wealth of prehistoric data—the movement of ice sheets, samples of atmosphere trapped in ice cores, changes in the sun's brightness—we can assemble snapshots of the Earth's "energy balance" over millions of years, and pinpoint the factors that altered it. The past is full of huge temperature changes: fifty million years ago, for instance, Alaska had tropical vegetation and crocodiles. And as it turns out, the biggest factor affecting prehistoric temperature shifts has been fluctuating levels of carbon dioxide in the air. (Previous changes in carbon dioxide levels were due to natural factors, such as rock weathering, and occurred very sluggishly, over many millenia.) Historically, a doubling of carbon dioxide has raised global temperatures by about 3°C. There is every reason to believe that, by burning fossil fuels, humans are charting a similar course today—only this time in fast motion.
One consequence of these past temperature swings has been dramatic changes in sea levels, and this is where Hansen gets unnerved by his data. For the past seven thousand years, sea levels have stayed remarkably stable, a happy event that allowed human civilization to develop and prosper. But at plenty of other points in the geological record, sea levels have been extremely volatile. The ice sheets at the poles can disintegrate rapidly once they start melting, and sea-level rises of a few meters per century are not unheard of. Hansen's research suggests that, in the past, it has not taken much to set off this process. Sea levels have been a couple meters higher when the world was only 1°C or 2°C warmer than it is today.
Now, at many global climate conferences, a 2°C rise is considered a best-case scenario. Hansen is saying that even our most ambitious targets are flirting with catastrophe. To preserve a livable climate, we need to rapidly bring carbon-dioxide levels in the air back down below 350 parts per million (we are currently at about 387 ppm and rising fast). And, Hansen contends, the only way to do that is for the world to phase out coal use quickly. Oil doesn't get the same harsh treatment because Hansen thinks the idea of weaning ourselves off of oil in such short order is unfeasible. What’s more, there is probably not enough conventional oil left to tip the climate over the edge. ("Unconventional" oil from shale or tar sands is a different matter.) There is, however, more than enough coal left in the ground to send temperatures soaring if we keep burning it. So, he says, we should put a moratorium on any new coal-fired plants unless they can capture and bury their emissions.