Computing the connection between carbon and temperature rise

New reports suggest global temperatures may be slightly less sensitive to rising CO2 levels than we thought. But the jury is still out and scientists say it is no excuse to get complacent about carbon emissions. James Mitchell Crow reports.

Climate scientists can’t agree exactly how sensitive our climate is to carbon dioxide, but one thing’s for certain: if carbon emissions don’t start falling soon, all summers will soon be scorchers. – Science Photo Library / Getty Images

If CO2 levels in the atmosphere double, how much warmer will the planet get? This relationship is called climate sensitivity. Getting the right answer is important because at current rates of carbon emission we’ll see CO2 levels double by mid-century. But lately there’s been vehement disagreement over what exactly that number is.

Most climate scientists agree we need to do our best to avoid raising temperatures above 2°C – seen as a tipping point. Beyond that we may have events like the large-scale release of methane from the Arctic tundra, and a runaway increase in the rate of global warming.

Researchers disagree as to how much CO2 we can pump into the atmosphere before we hit the 2°C threshold. And it’s unlikely we can nail this exact relationship any time soon.

Back in 2007, when the Intergovernmental Panel on Climate Change (IPCC) last weighed all the scientific evidence, the climate sensitivity question appeared simple. Doubling CO2 would likely cause between 2.0°C and 4.5°C of warming, the organisation reported. Their best guess was 3.0°C.

Last year the IPCC updated its report. Despite the extra years of research, they didn’t narrow the likely temperature response to a doubling of CO2. Instead they broadened it, to 1.5°C– 4.5°C, and didn’t even give a best guess. What went wrong?

Essentially there are two ways to estimate climate sensitivity, and the IPCC considers both. One way is to use computational climate models. The other is try to calculate it based on observed, measured data on the way the world has warmed so far as CO2 levels rise. Studies of the second type tend to estimate climate sensitivities at the very low end.

The latest was published in September by Judith Curry from Georgia Institute of Technology and Nicholas Lewis (a former financial sector worker who has no academic affiliation) in the journal Climate Dynamics. They say that doubling CO2 levels will lead to between 1.25°C to 2.45°C of global warming. Their best guess is 1.64°C, almost half the estimate of the IPCC. Their paper was lauded by climate sceptics who emphasised that rather than using climate models, the calculation was based on real world observations.

“It’s definitely a valid approach,” says Joeri Rogelj, a climate researcher at the International Institute for Applied Systems Analysis in Vienna. “But every approach has limitations.”

Climate change will play out differently in different parts of the world.

As Rogelj points out, painting this approach as using only cold, hard data rather than models is simply wrong. They still rely on models to calculate how human activity is perturbing aspects of the climate system. On the other hand, climate models use real world temperature data to test their assumptions.

Calculating climate sensitivity from observed warming data is also fraught with pitfalls. The climate is naturally in constant flux, explains Tom Wigley, a climate scientist now based at the University of Adelaide who has been working on the problem for decades. “In order to calculate climate sensitivity from the observations, we need to have a long data record”. Shorter records are meaningless because of “noise in the system”. Some years are naturally hotter than others, some decades are hotter; some centuries are hotter. That natural variability adds a huge element of uncertainty to our attempts to calculate climate sensitivity, explains Wigley.

Then there’s the added complication that Curry and Lewis made their estimates by using average global temperature. But climate change will play out differently in different parts of the world, says Rogelj. The dirty skies over large cities, for example, are cooled by atmospheric pollutants such as sulfur particles that reflect incoming sunlight.

So it’s by no means certain that observational studies give a better estimate of climate sensitivity than climate models. “There are other studies pointing towards the higher end that also have very good grounds for being possible,” says Rogelj.

Wigley agrees. He’s not about to declare that we’ve been overestimating climate sensitivity. “If you look at the observations from about 1940 onwards, and drive a climate model…using three degrees as the sensitivity, the models and observations agree fantastically well,” he says.

However this is not to say that climate models aren’t also under scrutiny. Lewis argues that climate models over-estimate both climate sensitivity and the cooling effect of pollutant aerosols.

So it’s a stalemate. “This is probably an uncertainty that will not go away very quickly,” says Rogelj. So we should consider both lines of evidence as equally important, he says. “If you want findings as robust as possible, I think it is useful to take as many different approaches and look at the outcomes.”

Nevertheless the calculations by Lewis and Curry are hardly cause for complacency. If they are right, then by the end of the century the climate will have warmed by 1.6°C, enough to put many low-lying Pacific island states at risk, according to a paper published by Rogelj at the start of the 2014.

“If the lower climate sensitivity estimates are true, there is no reason to be lenient or to think we have much more time in terms of emissions reductions,” says Malte Meinshausen at the University of Melbourne, a co-author on Rogelj’s paper.

Should we ditch the warming goal and make carbon the benchmark?

In 2009 at the Copenhagen conference on climate change , the international community agreed on the goal of keeping global warming to within 2°C of pre-industrial global temperatures, a decision ratified by the United Nations Framework Convention on Climate Change.

This October, a provocative opinion piece published in Nature by David Victor from the University of California, San Diego, together with Charles Kennel, also a former NASA associate administrator, argued the world should “ditch the 2°C warming goal”.

In part their view was based on this being an increasingly unreachable goal. Meeting it would require CO2 emissions to decline. But as a paper in Nature Geoscience last September showed, the planet’s CO2 emissions are still rising at 2.5% per year over the past decade, tracking the most extreme climate warming scenarios.

Victor and Kennel argue that the “unachievable” two degree goal allows politicians to “pretend that they are organising for action when, in fact, most have done little”.

Rather than tilting for an unrealistic goal, Victor and Kennel are in favour of taking the “vital signs” of the planet such as the actual concentration of CO2 and other greenhouse gases in the atmosphere, and heat content in the ocean. “Scientifically there are better ways to measure the stress that humans are placing on the climate system than the growth of average global surface temperature,” they write.

The scientific community has been divided in its response. Some have agreed it is high time to call out the implausibility of reaching the 2°C limit and find more concrete measures that politicians must adhere to. Others are dismayed that the high profile letter will merely act as an excuse to weaken resolve on lowering emissions.

Tom Burke, director of the NGO E3g, a UK government adviser and a former Rio Tinto executive told The Guardian, “There are lots of people who disagree with the idea it’s unachievable. It depends what effort you put in. Could we get to 2°C? Would we wreck economies to do it? Certainly not. Could we get the political will to do that? That’s the question. But if you say we shouldn’t even try then we definitely won’t get there.”

And as many scientists have subsequently pointed out, the IPCC already provides governments exhaustive lists of different climate metrics – including those argued for by Victor and Kennel.

For formulating climate policy, the 2°C target remains an essential yardstick, says Malte Meinshausen from the University of Melbourne. “The two degree target is first and foremost an indication of where the international community thinks that climate impacts become intolerable and so what we would like to avoid.”

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