How sensitive is the earth to carbon dioxide? It’s a question that’s at the heart of climate science.

It’s also complicated, and scientists have been grappling with pinning down an exact amount for a while now.

But while the exact value of climate sensitivity presents a fascinating and important scientific question, it has little relevance for climate policy while greenhouse emissions stay as high as they are.

Nevertheless, each time a new research paper comes out suggesting climate sensitivity might be low it’s misused by parts of the media to argue cutting emissions isn’t so urgent after all.

The latest example comes in an article in today’s Times, which claims a new low climate sensitivity estimate means “Climate change could be slower than forecast”.

So what is climate sensitivity? What does and doesn’t it tell us about future warming?

               

Source: The Times, 26th September 2014

What is climate sensitivity?

Climate sensitivity is the warming we can expect when the concentration of carbon dioxide in the atmosphere reaches double what it was in preindustrial times.

Preindustrial levels were about 280 parts per million (ppm), we’re at nearly 400 ppm now and at current emission rates, we’re due to hit 560 ppm soon after 2050.

Scientists estimate the value of climate sensitivity lies between 1.5 and 4.5 celsius. This doesn’t mean we’ll get that temperature rise immediately. Parts of the climate system respond slowly. Heat entering the oceans takes time to cause atmospheric warming, for example.

That means the full warming effect of the greenhouse gases we release into the atmosphere might not materialise until decades or centuries later.

That’s why scientists sometimes talk about a simpler measure of climate sensitivity known as the Transient Climate Response (TCR).

TCR is easier to estimate because it ignores the slower changing parts of the climate system. It’s a useful way to look at the short term rather than centuries from now.

But the same rule applies to both definitions. The higher the climate sensitivity, the more warming we’ll see.

How do scientists calculate climate sensitivity?

Working out climate sensitivity is complicated, and there are three main ways to do it.

One method looks at how earth responded to natural greenhouse gas changes in its geological past. Another matches global surface temperatures with greenhouse gas concentrations and other forcings over the last century or so, to try and work out sensitivity from how the planet is responding – what’s known as the ‘energy budget model’ approach. The third uses climate models to predict the theoretical effect of a doubling of carbon dioxide based on scientists’ understanding of how different elements of the climate system interact.

Any estimate of climate sensitivity comes with a range – a lower and upper limit within which the actual value could reasonably lie.

In its 2013 report, the Intergovernmental Panel on Climate Change (IPCC) gave a likely range for climate sensitivity of between 1.5 and 4.5 degrees Celsius. The likely range for TCR was between 1.0 and 2.5 degrees.

Are estimates of climate sensitivity getting lower?

No.

Some high-profile papers based on the energy budget model approach have emerged recently, suggesting the values of climate sensitivity lies at the bottom of the IPCC’s likely range.

This week a paper by Judith Curry and Nic Lewis gave a value for TCR of 1.05 to 1.8 degrees Celsius, with a mid value of 1.33 degrees. This is similar to a paper last year by  Otto et al. Both estimates are about 25 per cent lower than the IPCC’s middle estimate of 1.8 degrees.

Though this flurry of recent papers has attracted some media attention and plaudits from climate skeptics, their publication doesn’t mean scientists are “backing off” higher climate sensitivity, as some have suggested.

Each way to estimate climate sensitivity comes with pros and cons. Critcisms  of the energy budget model approach are that it is sensitive to uncertainties in observations and doesn’t account for slow feedbacks between the atmosphere, deep oceans and ice sheets.

Climate sensitivity estimates using climate models support the higher end of the IPCC’s likely range. Such estimates are rooted in the behaviour of known physical processes but contain large uncertainties, such as the interaction of aerosols with clouds.

The IPCC considers all the different ways of calculating climate sensitivity, without making a value judgment about which is best. So as long as there is a body of literature supporting both ends of the likely range, it won’t be revising it any time soon.

It’s important not to oversell the significance of a single paper, or a collection of papers that use one of several available methods.

Pinning down climate sensitivity is an area of ongoing scientific debate.  The hope is that new research can help narrow the range of uncertainty.

When a new study is published, on any scientific topic, it doesn’t override all that came before it. It simply adds to the body of literature.

Does low climate sensitivity mean emissions cuts can happen more slowly?

No. Some parts of the media have interpreted low climate sensitivity as evidence that emissions cuts aren’t urgent after all. For example, an article by journalist Ben Webster in today’s Times said about Judith Curry and Nic Lewis’s paper:

“[I]f the new paper is correct, the emissions cuts needed to prevent a dangerous rise in temperature could take place more slowly than governments have proposed.”

This is definitely not the case. We’re emitting carbon dioxide so fast that the difference between a low and a high value of climate sensitivity is largely irrelevant in climate policy terms.

As Myles Allen, professor at Oxford University and IPCC author, told us recently:

“A 25 per cent reduction in TCR would mean the changes we expect between now and 2050 might take until the early 2060s instead … So, even if correct, it is hardly a game-changer.”

It’s also important to remember that climate sensitivity is not the same as total warming.

Instead, it’s what we’ll get every time the carbon dioxide concentration doubles above pre-industrial levels.

If emissions stay as high as they are, that means even a low value of climate sensitivity would see a significant amount of warming by the end of the century. The risks are large either way, Myles Allen explains:

“[A]ny revision in the lower bound on climate sensitivity does not affect the urgency of mitigation.”

On the other hand, if climate sensitivity is at the upper end of the IPCC’s likely range, reports suggesting a four degree temperature rise above pre-industrial levels by the end of the century are likely to be optimistic.

So while the precise value of climate sensitivity poses an interesting scientific question, tackling the policy response is by far the bigger problem.  Without swift emissions cuts, we can expect a serious level of warming – whatever climate sensitivity ends up being.

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30th September – Update