杏吧原创

Headlines this聽week proclaimed the聽worst-case scenarios for climate change were 鈥渄ebunked鈥� and 鈥渘ot credible鈥�. As you might expect, things aren鈥檛 that simple.

The stories were sparked by a study聽by at the University of聽Exeter, UK, and his colleagues, who聽attempted to work out how much聽warming will result from a given聽increase in carbon dioxide levels. Specifically, if we doubled CO₂ levels in the atmosphere and waited for the temperature to stabilise, how much would the world warm?

This is known as the equilibrium climate sensitivity, and climate scientists have been trying to work it聽out for decades. Hundreds of studies聽have produced a wide range of results, which means there is a lot of uncertainty, but the consensus says 1.5 to 4.5掳C is most likely.

Cox鈥檚 study narrows this to between 2.2 and 3.4掳C. That is excellent news if it is right, but it isn鈥檛 a definitive answer. 鈥淭his study does not set the final boundaries,鈥� says of Duke University, who studies climate sensitivity. Cox agrees. 鈥淲e聽don鈥檛 know for sure,鈥� he says.

Heated debate

Yet other recent studies say sensitivity is higher. One last month put it between 3 and 4.2掳C. Because studies often use different methods, it聽isn鈥檛 obvious why the results vary or which should be given more weight.

While there is uncertainty about the聽upper range of climate sensitivity, nearly all major recent studies agree that low values can be ruled out. Some climate deniers claim that sensitivity is聽so low we don鈥檛 need to worry about聽climate change 鈥� we are now very sure they are wrong.

In fact, even the high-end estimates are likely to underestimate the eventual rise in global temperatures. That鈥檚 because these studies only look at the warming seen a few decades after a doubling of CO₂, whereas in reality it would take thousands of years for temperatures to stabilise.

The issue here is that how much the world warms depends on lots of feedback effects. Some are very fast, such as changes in clouds. Others are slow, like changes in vegetation or the melting of the great ice sheets.

By convention, climate scientists only include the rapid feedbacks when聽calculating equilibrium climate sensitivity. The argument for this is that the response over the next few decades is most relevant to policy. But聽in the long run there will be more warming 鈥� the 鈥渢rue鈥� climate sensitivity could be 6掳C or more .

We also need to be cautious about studies that are based on how the climate has behaved over the past century or so, . There could be surprises in store as the world warms further.

And keep in mind, the worst-case scenario is the business-as-usual scenario, in which we continue to pump out C0₂ as we are today. The consensus is that could warm the world by 4掳C by 2100, give or take 1掳C. (This is a projection of the actual warming, not the sensitivity, which is a more abstract measure of the climate). If Cox is correct, that estimate is still in the right ballpark.

The good news is that the world isn鈥檛 expected to stay on this high-emissions pathway. Even so, CO₂ levels are rising faster than ever, so there is no room for complacency. We aren鈥檛 doing nearly enough to limit warming to 2掳C, even assuming Cox is right.