How hot will it get?

Lately, there’s been much discussion about this thing called climate sensitivity. But what the devil is it? Climate sensitivity refers to the amount by which the temperature is expected to increase with each doubling of atmospheric carbon dioxide (CO2) from pre-industrial levels. It’s important because if it turns out that the climate is very sensitive, then the temperature increase we can expect will be higher rather than lower. Pre-industrial CO2 levels were 280 ppm (parts per million). They have almost reached 400 ppm today.

Current climate sensitivity estimates range from 1°C – 5°C. As we are getting close to the 1°C mark already, I think this estimate is a little on the low side.

What I find missing from many of the discussions in the blogosphere and mainstream media is evidence from the fossil record. A little while ago I wrote about A world of giant snakes, which described a period in Earth’s history when there was no ice at either pole and where crocodiles lived in the Arctic along with frost-intolerant subtropical plants. This is very strong evidence for a climate sensitivity at the high end of estimates. A climate sensitivity of only 1°C or 2°C will not provide an equable habitat for crocodiles in the Arctic.

If Earth’s climate had remained fairly stable over millions of years, then this would suggest it was less sensitive to changes in atmospheric CO2. But the climate has shifted dramatically in the past, from ice age to hot house and back again.

When models are used to generate conditions during the hot house period of the Eocene (~50 million years ago), the results are not hot enough to reproduce the conditions that the fossil record suggests for that time1. This problem is known as the equable climate problem. While the models can match conditions at the tropics reasonably well, they do not accurately represent the climate at the higher latitudes and poles, which, according to the fossil record, were significantly warmer than the models suggest.

If we’re placing bets on how much the temperature will rise with each doubling of atmospheric CO2, I’m putting my money on 3-4°C.

Let’s say humans decide to continue with business as usual and make no attempt to wean ourselves off fossils fuels.  Then in one hundred years there will be as much CO2 in our atmosphere as there was 50 million years ago2, when crocodiles lived in Greenland and giant snakes nested in the tropics. It is worth noting that humans did not exist at this time.


1. Progress in greenhouse climate modeling Mathhew Huber
2. The early Eocene equable climate problem revisited M. Huber, R. Caballero