Why 2 Degrees?

By Michael D. Lemonick

Thanks to a tidy mathematical coincidence, it doesn’t matter whether you’re talking Fahrenheit or Celsius when you talk about trying to limit global warming to 2 degrees. That’s the proposed target announced last summer by the G-8 countries, and it’s the number scientists talk about as well — in Celsius if you’re counting from the average temperature of pre-industrial times, and in Fahrenheit, if you’re counting from today.

It’s easy to get the impression that 2 degrees represents a tipping point (especially since people actually use that term) — a threshold beyond which the climate will go out of control, wreaking terrible destruction.

And it might be — but nobody has any actual evidence to that effect. What we do know is that the more the temperature goes up, the greater the impact will be on people, property and the natural world. It’s already risen about .7 C over pre-industrial levels, and we’re already seeing rising seas, changing weather patterns, more frequent droughts and melting glaciers, to name just a few effects. When we get to 2C, it’s unlikely that something magical will happen — although climate scientists generally believe that the risk of serious disruption to the climate system begins to rise more sharply around that level. 2C is a tipping point once removed: a point at which tipping points become more likely.

So it’s probably more realistic to think of global warming the way you think of cholesterol (or the way your doctor wants you to think, anyway). If your total cholesterol goes over 200, you’re supposed to be worried. But that doesn’t mean a level of 190 is just fine, or that 210 means you’re about to die. If your cholesterol is 400, your doctor will be thrilled to see it go down to 200. But if it’s 200, the doctor will still want you to get it down.

It’s the same with 2C. Scientists and political leaders realize that this is about the best limit we can manage with strenuous effort, given how tough it is to curb emissions. But it’s not a magic number, and if we could limit the rise to only 1C, that would make it more likely that we’d avoid serious impacts.

Given that we probably can’t, 2C is a more reasonably attainable goal.