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Scientists: How Matt Ridley misinterpreted new climate sensitivity paper

  • 22 May 2013, 16:30
  • Roz Pidcock

A new paper suggesting temperatures might not rise as much as some models predict in the near future has been interpreted in some corners - notably by   Matt Ridley in the Times - as a sign that climate change no longer poses a problem. But the authors have spoken out against Ridley's arguments, highlighting why pinpointing earth's sensitivity to greenhouse gases relies on more than one estimate.

The story stems from a  letter in Nature Geoscience on Sunday with a new estimate of what scientists call equilibrium climate sensitivity. That's the total warming we can expect from a doubling of carbon dioxide in the atmosphere, compared with pre-industrial levels.

Climate sensitivity is important because the higher it is, the more warming there will be. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) estimated a likely range of between  two and 4.5 degrees Celsius, with a best estimate of three degrees.

More than one way to estimate climate sensitivity

There are different ways to go about calculating climate sensitivity - which is part of the reason the IPCC's range is so large. The new study used what's called an energy budget model, which used land, atmosphere, ice and ocean temperatures between 1970 to 2009 to see how warming in all parts of the climate system has changed in that time. We wrote more about the new paper  here.

Scientists can also use natural recorders of temperature - called  climate proxies - to look at how earth's temperature has changed in response to greenhouse gases throughout its distant past. The third way is using complex climate models to simulate how processes affecting temperature rise are likely to evolve in the future.

Uncertainties about some of these processes - particularly how clouds affect the rate of warming - means climate model estimates can vary quite widely.

New estimate

Most of the media coverage surrounding the new paper focusses on earth's Transient Climate Response (TCR). This is a simplified measure of climate sensitivity that estimates the amount of surface warming we can expect at the point of a doubling of carbon dioxide.

Importantly, TCR doesn't take into account heat that's going into the deep oceans rather than staying in the atmosphere. Scientists think natural cycles are driving the deep oceans to take up more heat at the moment, causing surface temperatures to rise slower recently than in previous decades.

The new paper suggests a best estimate of TCR based on data from the last decade of 1.3 degrees, but because of uncertainty around the measurements it could be as low as 0.9 degrees or as high as two degrees.

The authors suggest that unlike their estimate of equilibrium climate sensitivity - which is largely consistent with model projections - their range of TCR is lower than some models predict. This is the point some parts of the media have picked up on.

Media misinterpretation 

In a piece for the Times, Conservative peer and member of skeptic thinktank the Global Warming Policy Foundation, Matt Ridley, compared the new paper's estimate of TCR with that of the Met Office's HadGem2 model, which suggests a value of 2.5 degrees.

In a blog for yesterday's Guardian, co author on the new paper Myles Allen disagrees with the point Ridley's trying to make with his comparison. Allen says:

"[Ridley] is worried that government policies are misguided because they place their faith in climate models, like one of the Met Office models that puts the warming instead at 2.5 degrees Celsius, almost twice our estimate."

But, as Allen explains, scientists don't place all their confidence in just one model. He says:

"Climate scientists are all well aware the Met's model (HadGEM2) is at the top end of the current range. The Met Office's advice to government is based on the range of results from current climate models, not just their own."

Allen is making the point that scientists use a number of climate models to make robust temperature projections - and if a comparison is to be made, it should be with the average of all the models, not just HadGem2. He says:

"The relevant comparison is not with the 2.5 degrees Celsius response of one model, but with the average of climate models used by the UN's climate science panel [the IPCC] in its upcoming major report, which is 1.8 degrees Celsius. Now, 1.3 degrees is 30 per cent less than 1.8 degrees Celsius, but this is hardly a game changer."

Allen's explains that at 1.8 degrees, the average value of climate sensitivity from all the models is within the 0.9 to two degree range of uncertainty that accompanies the new paper's estimate. In other words, the two estimates aren't inconsistent with each other.

As part of a longer discussion on the paper's conclusions, lead author on the paper Alexander Otto makes a similar point in an article for the Met Office.

The fact that the uncertainty ranges of different methods of estimating climate sensitivity are still quite large is another reason the IPCC's range is unlikely to be refined just yet.

Damage done

Ridley goes on to suggest that given the new lower estimate of sensitivity, the negative effects of climate change we're likely to experience before the end of the century are negligible. He says:

"There is little doubt that the damage being done by climate-change policies currently exceeds the damage being done by climate change, and will for several decades yet … At this rate, it will be the last decades of this century before global warming does net harm."

Part of Ridley's argument is that rolling out renewables "dwarf[s] any possible [economic] effects of worse weather, for which there is still no actual evidence anyway: recent droughts, floods and storms are within historic variability."

Although it's certainly difficult to attribute particular events to climate change, scientists suggest rising temperatures are stacking the decks towards more extreme weather. As Allen puts it:

"Try explaining to a casino bouncer that it doesn't matter you are using loaded dice because a triple-six is within historic variability."

Warming in the pipeline

While TCR gives a useful indication of the surface warming we can expect in the next few decades, scientists turn to equilibrium climate sensitivity for the full picture of the total warming - at earth's surface and in the oceans - we can expect in the long term. We wrote a bit more about the difference between the two measures of sensitivity here.

The new paper's estimated range of equilibrium climate sensitivity is consistent with that of current climate models. Contrary to Ridley's suggestion that scientists are "backing away from rapid warming", this means projections of how much warming we can expect in the future remain unchanged - at least for now.

While the precise value of climate sensitivity poses an interesting scientific question, many argue tackling the policy response is by far the bigger problem. As Allen puts it:

"If this means we can move on from a sterile debate about the global response to much more interesting questions about regional impacts, the rights of different generations, and, most interesting of all, what to do about it, that's great. Ridley, welcome to the real climate debate."

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