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How Much of an Increase in Temperature Can We Tolerate?

An IPCC author assails the current two-degree target as scientifically bankrupt and geographically biased.
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(Photo: Tom Wang/Shutterstock)

(Photo: Tom Wang/Shutterstock)

One of the most bothersome phrases in environmental journalism is “the climate change debate.” The climate change debate was settled years ago—it’s real. Today, there are dozens of climate change debates: How much will the sea level rise? Which species will struggle or disappear? What will happen to agricultural productivity? These are more interesting and important than “the” climate change debate.

The hottest debate right now is how much temperature rise we can tolerate. For nearly two decades, the prevailing view has been that a global mean temperature increase of more than two degrees Celsius must be avoided in order to ward off the worst of global warming’s consequences. Throughout that period, however, many scientists and policymakers have dissented. The latest salvo came recently, as Petra Tschakert, a Penn State professor who is the coordinating lead author for the Intergovernmental Panel on Climate Change, assailed the two-degree limit as “utterly inadequate.”

The two-degree target has a strange history. In 1975, Yale economist William Nordhaus speculated that a global mean temperature increase of two or three degrees would be unprecedented in the “last several hundred thousand years.” Nordhaus was not proposing those figures as potential policy targets, nor did he suggest that they represented any kind of threshold for disaster.

As Archbishop Desmond Tutu argued, a two-degree global mean temperature rise might result in Africa’s temperature rising as much as 3.5 degrees—a potentially disastrous change.

In 1990, however, the predecessor to the IPCC published forecasts suggesting that, were the temperature to cross the two-degree Rubicon, “grave damage to ecosystems” and “nonlinear responses” would follow. Although many researchers questioned the use of a single number to indicate the moment of catastrophe, the notion became popular. Malcolm Gladwell’s hit book The Tipping Point, in particular, primed public interest in the idea of trend inflections. Policymakers, scientists, and many politicians soon coalesced around the two-degree target. It was part of the Copenhagen Accord and statements from the G8, and it has formed the basis of most global negotiations.

There are a few major objections to the two-degree target. First, it’s somewhat arbitrary. The climate is a complex system. To the extent that we accept the notion of tipping points, there are likely to be several rather than one. Two degrees is, in the view of many, a handy rallying cry rather than a scientific threshold. A widely cited paper on the history of the target compared it to a speed limit—it may not be a scientifically optimal number, but it serves as a useful focal point. The paper concludes: “The two-degree target emerged nearly by chance.... Policy makers have treated it as a scientific result, scientists as a political issue.” (Incidentally, the paper was part of an endorsement of the target. That’s how lukewarm its support is among climatologists.)

Others say the target is simply too high. That’s the point Tschakert made in Climate Change Responses. She cites evidence that most of the world’s coral reefs would perish and sea levels would rise more than three feet after a two-degree increase. Moreover, the temperature rises associated with anthropogenic climate change are not uniform across the globe. As Archbishop Desmond Tutu argued at the 2009 Copenhagen climate conference, a two-degree global mean temperature rise might result in Africa’s temperature rising as much as 3.5 degrees—a potentially disastrous change.

That bring us to another complaint: The perceived consensus around the target involves a geographic bias. Among African, Caribbean, and Pacific Island countries, Tschakert says the majority prefer a 1.5-degree limit. The two-degree target is the choice of wealthy nations, for the most part, because the increased heat and sea levels won’t affect them as much.

Arguing over whether the target should be 1.5 or two degrees is slightly absurd, however, since we’re not on track to achieve either. (The most likely business-as-usual scenario will result in a three-degree increase by the end of this century.) It’s a little like debating whether we’re better off sending astronauts to Mars or to Jupiter. Nevertheless, we need a goal that is both achievable and worthwhile, so the argument over degrees is one worth having.

The next time you’re at a party and someone asks what you think about climate change, just tell them where you stand on the two-degree target. Let’s make this debate the climate change debate—the conversation worth having.

This post originally appeared on Earthwire as “An Argument of Degree” and is republished here under a Creative Commons license.