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Roz Pidcock

Roz Pidcock

06.03.2014 | 2:30pm
Climate sensitivityScientists challenge climate skeptic claims that UN panel overestimates warming
CLIMATE SENSITIVITY | March 6. 2014. 14:30
Scientists challenge climate skeptic claims that UN panel overestimates warming

Lord Lawson’s skeptic lobby group, the Global Warming Policy Foundation (GWPF), released a report today criticising scientists’ estimate of how sensitive earth’s climate is to carbon dioxide.

In what may be a sign of growing confidence in the scientific community about engaging online, climate scientists have been quick to respond, highlighting what they label the report’s “cherry picking” approach.

They have also pointed out Lawson’s lobby group appears to have unwittingly come out in support of the mainstream scientific view – that we can expect a serious level of warming if emissions aren’t brought down swiftly.

The GWPF report, entitled ‘Oversensitive: How the IPCC hid good the news on global warming”, argues the UN’s official climate body glossed over the possibility of modest future warming in its latest assessment, in favour of evidence that the risks could be much higher.

Authored by former financier Nic Lewis, who describes himself as an “independent climate scientist”, and freelance science writer Marcel Crok, the report claims to provide a “technically sound” and “independent” assessment of the IPCC’s conclusions.

But climate scientists strongly disagree, today pointing out issues with the analysis.

A matter of sensitivity

The new GWPF report centres on something called climate sensitivity – the warming we can expect when carbon dioxide concentration reaches double what it was in preindustrial times.

In its most recent report, the IPCC estimated we’re likely to see between one and 2.5 degrees Celsius at the point of doubling. This is what’s known as the Transient Climate Sensitivity ( TCR).

The crux of the GWPF’s argument is the IPCC’s estimate is “biased high”, because it attaches too much importance to climate models. Based on what it calls “observational” estimates, the GWPF report argues the true value of TCR is more like 1.35 degrees Celsius – about 25 per cent lower than average value from climate models.

Warming won’t stop when carbon dioxide doubles

Let’s suppose for a minute that the GWPF analysis is right.

It might sound from their arguments like their projections mean we’re on course for only a modest amount of warming. But if emissions stay high, Myles Allen, professor at Oxford University and IPCC author, tells us:

“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.”

Piers Forster, professor at Leeds University and another IPCC author, wrote on fellow climate scientist Ed Hawkins’s Climate Lab Book blog earlier today:

“[E]mploying the best estimates suggested by Lewis and Crok, further and significant warming is still expected out to 2100, to around 3°C above pre-industrial climate, if we continue along a business-as-usual emissions scenario (RCP 8.5), with continued warming thereafter.”

Hawkins adds:

“It is great to see the GWPF accepting that business-as-usual means significant further warming is expected. Now we can move the debate to what to do about it.”

AR5_temp _projections

The IPCC’s projections of future warming for a high emissions scenario (red) and one with “substantial and sustained” emissions cuts to limit warming to two degrees above preindustrial levels. Source: IPCC AR5 Summary for Policymakers ( SPM).

Cherry-picking

Even so, it’s worth digging deeper into where the GWPF’s numbers come from. As Forster explains, the GWPF report is placing all its eggs on one basket when it comes to estimating climate sensitivity. He says:

“Lewis & Crok perform their own evaluation of climate sensitivity, placing more weight on studies using “observational data” than estimates of climate sensitivity based on climate model analysis.”

Climate scientist at the University of New South Wales, Steve Sherwood, says this is exactly the sort of selective approach the GWPF is accusing the IPCC of. He tells Graham Readfearn’s Guardian blog:

“The [GWPF] report is standard cherry-picking. It offers no new evidence not already considered by the IPCC, relying very heavily on a few strands of evidence that seem to point toward lower sensitivity while ignoring all the evidence pointing to higher sensitivity.”

Many lines of evidence

The IPCC’s estimate of climate sensitivity takes into account all lines of evidence, including recent observations, records of temperature in earth’s distant past and climate models. As Forster explains:

“[T]he IPCC did not make such a value judgment about the different methods of evaluating climate sensitivity.”

What’s more, Forster says the method the GWPF opts for might not give a reliable result. Forster himself developed the original method Lewis and Crok base their argument on, and points out that it has its limitations. He says:

“[The studies the GWPF draws on], which employ techniques developed by us over a number of years â?¦ are limited by their own set of assumptions and data issues, making them not necessarily more trustworthy than other techniques.”

Forster adds:

“Particularly relevant, is our analysis that confirms that the Gregory and Forster method employed in the Lewis & Crok report to make projections â?¦ leads to systematic underestimates of future temperature change”

Touch sensitive

Working out climate sensitivity is complicated. Because of the many uncertainties involved, any estimate of climate sensitivity comes with a range, a lower and upper limit within which the real value could reasonably lie.

The crux of the GWPF’s argument is that its best estimate is lower than the one from the IPCC’s climate models.

But Allen tells us that once the uncertainty range of the estimate is taken into account, it is actually extremely similar to that of the IPCC models. He says:

“[I]t turns out Lewis and Crok’s [uncertainty] range (not in the GWPF report, but kindly provided by Nic Lewis) is 0.9 to 2.5 degrees Celsius, which is almost identical to the range of the [IPCC’s] models (1.1-2.6 degrees Celsius).”

So considering the uncertainty range that accompanies Lewis and Crok’s estimate – something that climate skeptics are normally keen to highlight – the GWPF’s estimate is very close to the IPCC’s.

Over-confidence

It would seem the GWPF is far more confident in its conclusions about climate sensitivity than the scientists on whose work its estimates are derived. It would also seem that climate sceptic arguments are moving closer to the scientific mainstream.

That may be welcome. But arguments over the precise value of climate sensitivity duck the wider point, which is that even if we’re lucky and climate sensitivity is on the low side of scientists’ estimates, we’re still heading for a substantial level of warming by the end of the century if greenhouse gas emissions aren’t addressed, as the IPCC has highlighted.

In a few weeks the IPCC will publish the second and third volumes of its Fifth Assessment Report, covering the impacts of climate change, and options to mitigate it.


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