Twenty years ago, Hansen kicked off this issue by testifying before Congress that the planet was warming and that people were the cause. At the time, we could only guess how much warming it would take to put us in real danger. Since the pre-Industrial Revolution concentration of carbon in the atmosphere was roughly 275 parts per million, scientists and policymakers focused on what would happen if that number doubled — 550 was a crude and mythical red line, but politicians and economists set about trying to see if we could stop short of that point. The answer was: not easily, but it could be done.
In the past five years, though, scientists began to worry that the planet was reacting more quickly than they had expected to the relatively small temperature increases we’ve already seen…
The rapid melt of most glacial systems, for instance, convinced many that 450 parts per million was a more prudent target.
But the data just keep getting worse. The news this fall that Arctic sea ice was melting at an off-the-charts pace and data from Greenland suggesting that its giant ice sheet was starting to slide into the ocean make even 450 look too high. Consider: We’re already at 383 parts per million, and it’s knocking the planet off kilter in substantial ways. So, what does that mean?
It means, Hansen says, that we’ve gone too far. “The evidence indicates we’ve aimed too high — that the safe upper limit for atmospheric CO2 is no more than 350 ppm,” he said after his presentation. Hansen has reams of paleo-climatic data to support his statements (as do other scientists who presented papers at the American Geophysical Union conference in San Francisco this month). The last time the Earth warmed two or three degrees Celsius — which is what 450 parts per million implies — sea levels rose by tens of meters, something that would shake the foundations of the human enterprise should it happen again.