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Climate sensitivity in the media: A case of mistranslation?

  • 09 Apr 2013, 18:30
  • Roz Pidcock

Climate sensitivity is a measure of how sensitive earth is to greenhouse gases - and is key to forecasting future temperature rise. But as the media discusses climate sensitivity, some of the scientific detail is being lost. Carbon Brief looks at what scientists really mean when they talk about climate sensitivity.

Estimating climate sensitivity

Understanding how sensitive the climate is to greenhouse gases isn't just about atmospheric emissions. A warming atmosphere has knock on effects that can amplify or dampen warming, such as the loss of sea ice which reflects sunlights and helps keep global temperatures down. These are known as  feedback processes - and they make climate sensitivity inherently difficult to quantify.

In 2007, the Intergovernmental Panel on Climate Change (IPCC)  estimated a doubling of carbon dioxide above pre-industrial levels - 280 parts per million - was likely to raise global temperature by between two and 4.5 degrees Celsius. But it didn't rule out higher values of up to six or seven degrees.

The amount of temperature rise following a doubling of carbon dioxide levels is called "climate sensitivity". If climate sensitivity is higher, climate change will happen faster. And if it's lower, the planet will warm more slowly as greenhouse gases are put into the atmosphere.

There are  three main ways to estimate climate sensitivity. One method is to look at how earth responded to natural greenhouse gas changes in its geological past. Another matches global atmospheric temperatures with greenhouse gas concentrations over the last century or so. The third uses climate models to predict the theoretical effect of a doubling of carbon dioxide.

As the graphic below shows, the different methods produce generally comparable results.

Climate Sensitivity Estimates

Source: Skeptical Science adapted from Knutti and Hegerl (2008)

But there is still some important uncertainty in climate models - mainly to do with how they simulate clouds, which can have a warming or cooling effect. A few models project higher values, which means the right hand side of the yellow curve in the graph above extends beyond 4.5 degrees. Scientists have called the small chance that these high values could be right the " long tail" of climate sensitivity.

The "long tail"

The long tail idea has been around for a while. But now, scientists are discussing two possibilities. The first is whether new research means this long tail of climate sensitivity can now be ruled out.

As climate scientist James Annan explains on his blog:

"It's increasingly difficult to reconcile a high climate sensitivity (say over four degrees Celsius) with the observational evidence for the planetary energy balance over the industrial era."

Essentially, Annan believes that looking at how the climate changed over the past few centuries gives enough information to rule out the long tail of climate sensitivity.

Some skeptic bloggers have suggested Annan's perspective is controversial among climate scientists. But climate science blog Skeptical Science explains, "there are few if any mainstream climate scientists who would disagree with [Annan's suggested upper limit]". Andy Revkin's Dot Earth blog also has a useful discussion of what scientists think the likely range is.

Model-data mismatch

The second strand of scientific discussion is whether a subset of climate models projecting higher temperature rise than has occurred recently could be overestimating climate sensitivity.

The last decade or so has seen a fairly muted rise in global atmospheric temperature, meaning temperatures are now approaching the lower boundary of where IPCC models predict they should be.

                   Hawkins

Source: Ed Hawkins, Climate Lab Book

Scientists are speculating about reasons for the mismatch. Oxford University climate scientist Myles Allen told the Guardian recently:

"[R]ecent observations [are] indeed suggesting that very high values of the so-called "climate sensitivity" (the long-term warming we should expect on doubling carbon dioxide), values greater than five Celsius or so, [are] looking less likely."

So this might be another reason to rule out the very high 'long tail' of climate sensitivity - values above five degrees, in this case. But it's important to flag that it's only one possible explanation. We wrote more about this here after a Mail on Sunday piece suggested the recent mismatch meant climate models "were wrong all along" - which is clearly not the case.

To summarise, both scientific discussions point to the possibility of ruling out the higher values of climate sensitivity - above 4.5 degrees - not about changing the likely range of climate sensitivity according to the IPCC. That's one mistake recent articles in both the Economist and the Telegraph appear to have made.

Media misinterpretation

Some high-profile papers have emerged recently supporting values at the lower end of the IPCC range, which both articles picked up on to support their conclusions. One is an un-peer reviewed and unpublished study by Norwegian researchers suggesting a value of 1.9 degrees. We reported on the study here. The Economist also mentions - among others - a paper by Julia Hargreaves, suggesting a mean estimate of 2.5 degrees.

But as this Skeptical Science blog explains, the lower estimates are based on atmospheric temperature change and don't include the role of the oceans. This means they could  underestimate recent warming. A recent paper suggests the rate of ocean warming has accelerated during the current slowdown in atmospheric temperature rise.

Solid foundations

The bottom line is that the IPCC likely range can't be narrowed down any further just yet. A number of studies still point towards both ends of the IPCC range. As a recent blog post from the Committee on Climate Change says:

"While it is true that recent studies based on temperature observations question the very highest model projections of warming, even if confirmed, these would not justify a wholesale downward revision of the [IPCC] range..."

In any case, many argue the policy response should be the same regardless of the climate sensitivity. Atmospheric carbon dioxide concentration is already approaching 400 ppm and the IPCC's high emissions scenario (A1F1) suggests we could see a doubling of carbon dioxide above pre-industrial levels by 2050.

Even with a climate sensitivity at the lower end of the IPCC range, by mid century the world would be committed to two degrees of warming above pre-industrial levels. That's the target set by the United Nations Framework Convention on Climate Change (UNFCCC) to avoid serious climate change. As Piers Forster, climatologist at the university of Leeds, explained recently:

"Even with a suggested [climate sensitivity] of around 2.5 Celsius or so we can end up with a very significant climate change by 2100 if we don't do something."

And if climate sensitivity is at the upper end of the IPCC's likely range - even excluding the higher 'long tail' - reports suggesting a four degree temperature rise above pre-industrial levels by the end of the century look optimistic given current emission trajectories.

A scientific question

Scientists have been grappling with what the likely value of climate sensitivity is for a long time now - and it's proving a tricky one to pin down. But the question surrounding the true value of climate sensitivity is ultimately a scientific one not a political one - if emissions continue unabated we're looking as serious climate change this century whatever happens.

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