Climate crunch: A burden beyond bearing

The climate situation may be even worse than you think.

In 2007, environmental writer Bill McKibben approached climate scientist James Hansen and asked him what atmospheric concentration of carbon dioxide could be considered safe. Hansen’s reaction: “I don’t know, but I’ll get back to you.”

After he had mulled it over, Hansen started to suspect that he and many other scientists had underestimated the long-term effects of greenhouse warming. Atmospheric concentration of CO2 at the time was rising past 382 parts per million (p.p.m.), a full 100 ticks above its pre-industrial level. Most researchers, including Hansen, had been focusing on 450 p.p.m. as a target that would avoid, in the resonant and legally binding formulation of the United Nations Framework Convention on Climate Change, “dangerous climate change”. McKibben was aware of this: he was thinking of forming an organization called 450.org to call attention to the number, and his question to Hansen was by way of due diligence.

As he thought about McKibben’s question, Hansen, who runs NASA’s Goddard Institute for Space Studies in New York, began to wonder if 450 p.p.m. was too high. Having spent his career working on climate models, he was aware that in some respects the real world was outstripping them. Arctic sea ice was reaching record lows; many of Greenland’s glaciers were retreating; the tropics were expanding. “What was clear was that climate models are our weakest tool, in that you can’t trust their sensitivity in any of these key areas,” he says. Those warning signs