Why numbers matter...

Last Updated 20 May 2013, 14:28 IST

Finding the climate sensitivity number is the holy grail of climate science. Several recent papers have offered best estimates for climate sensitivity that are below 4 degrees F, rather than the previous best estimate of just above 5 degrees. But, total emissions may still wind up being excessive. The number could indeed turn into a ray of hope, but only if it is then followed by a broad new push to get the combustion of fossil fuels under control, writes Justin Gillis

Since 1896, scientists have been trying to answer a deceptively simple question: What will happen to the temperature of the earth if the amount of carbon dioxide in the atmosphere doubles?

Some recent scientific papers have made a splash by claiming that the answer might not be as bad as previously feared. This work — if it holds up — offers the tantalising possibility that climate change might be slow and limited enough that human society could adapt to it without major trauma.

Several scientists say they see reasons to doubt that these lowball estimates will in fact stand up to critical scrutiny, and a wave of papers offering counter arguments is already in the works. “The story is not over,” said Chris E Forest, a climate expert at Pennsylvania State University.

Still, the recent body of evidence — and the political use that climate contrarians are making of it to claim that everything is fine — sheds some light on where we are in our scientific and public understanding of the risks of climate change.

The topic under discussion is a number called “climate sensitivity.” Finding this number is the holy grail of climate science, because the stakes are so high. The fate of the earth hangs in the balance.

Arriving at a number

The first to take a serious stab at it was a Swede named Svante Arrhenius, in the late 19th century. After laborious calculations, he declared that if humans doubled the carbon dioxide in the air by burning fossil fuels, the average temperature of the earth would rise by something like 9 degrees Fahrenheit, a whopping figure.

He was on the high side, as it turned out. In 1979, after two decades of meticulous measurements had made it clear that the carbon dioxide level was indeed rising, scientists used computers and a much deeper understanding of the climate to calculate a likely range of warming. They found that the response to a doubling of carbon dioxide would not be much below 3 degrees F, nor was it likely to exceed 8 degrees.

In the years since, scientists have been pushing and pulling within that range, trying to settle on a most likely value. Most of those who are experts in climatology subscribe to a best-estimate figure of just over 5 degrees F.

That may not sound like a particularly scary number to many people — after all, we experience temperature variations of 20 or 30 degrees in a single day. But as an average for the entire planet, 5 degrees is a huge number.

The ocean, covering 70 per cent of the surface, helps bring down the average, but the warming is expected to be higher over land, causing weather extremes like heat waves and torrential rains. And the poles will warm even more, so that the increase in the Arctic could exceed 10 or 15 degrees F. That could cause substantial melting of the polar ice sheets, ultimately flooding the world’s major coastal cities.

What’s new is that several recent papers have offered best estimates for climate sensitivity that are below 4 degrees F, rather than the previous best estimate of just above 5 degrees, and they have also suggested that the highest estimates are pretty implausible.

Notice that these recent calculations fall well within the long-accepted range — just on the lower end of it. But the papers have caused considerable excitement among climate-change contrarians.

It is not that they actually agree with the new numbers, mind you. They have long pushed implausibly low estimates of climate sensitivity, below 2 degrees F in some cases. But they appear to be calculating that any paper with a lowball number is a step in their direction.

James Annan, a mainstream climate scientist working at a Japanese institute, offers a best estimate of 4.5 degrees F. When he wrote recently that he thought some of the highest temperature projections could be rejected, skeptics could not contain their enthusiasm.

“That is what we call a landmark change of course — by one of climatology’s most renowned warmist scientists,” declared a blogger named Pierre L Gosselin. “If even Annan can see it, then the writing is truly emblazoned on the wall.”

But does this sort of claim — that we can all breathe a sigh of relief about climate change — really hold up?

Warming threat

Annan said in an email that the Intergovernmental Panel on Climate Change, a mainstream body that periodically summarises climate science, should be bolder about ruling out extreme temperature scenarios, but he still believes global warming is a sufficient threat to warrant changes in human behaviour. He noted that climate skeptics “are desperate to claim that the IPCC is being unreasonably alarmist, but on the other hand they don’t really want to agree with me either, because my views are close enough to the mainstream as to be unacceptable to them.” He added that he finds it “amusing to watch their gyrations as they try to square the circle.”

It will certainly be good news if these recent papers stand up to critical scrutiny, something that will take at least a year or two to figure out. But the need for additional scientific vetting before we accept the lower numbers is not the biggest flaw in the contrarian argument.

Remember, the climate sensitivity number, whatever it turns out to be, applies to a doubling of carbon dioxide. Given how weak the political response to climate change has been, there is no reason to think that human society is going to stop there. Some experts think the level of the heat-trapping gas could triple or even quadruple before emissions are reined in. Only recently, the level of carbon dioxide passed a milestone of 400 parts per million at the flagship monitoring station atop Mauna Loa, in Hawaii, evidence that efforts to control emissions are failing.

Even if climate sensitivity turns out to be on the low end of the range, total emissions may wind up being so excessive as to drive the earth toward dangerous temperature increases. So if the recent science stands up to critical examination, it could indeed turn into a ray of hope, but only if it is then followed by a broad new push to get the combustion of fossil fuels under control.

(Published 20 May 2013, 14:28 IST)

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