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Scientists take the Mail on Sunday to task over claim that warming is half what IPCC said last time

  • 16 Sep 2013, 15:40
  • Roz Pidcock

The Mail on Sunday yesterday claimed the international climate community is conceding the world hasn't warmed as much since the middle of last century as previously thought. We ask climate scientists what they make of the story.

A major new report on the state of the climate is due imminently from the Intergovernmental Panel on Climate Change (IPCC). An involved process of review by experts and governments worldwide is coming to a close, and the first part of the report is set for release at the end of next week.

Last week, a draft summary of the document was leaked to journalists. In yesterday's Mail on Sunday, climate skeptic journalist David Rose claims to have seen information in the summary which "reveals scientific forecasts of imminent doom were drastically wrong".

The Mail on Sunday piece is entitled 'World's top climate scientists confess: Global warming is just HALF what we said'. It follows a series of other articles Rose has written for the Mail on Sunday recently which, taken together, suggest that the fundamentals of climate science are being thrown into doubt.

The Telegraph quickly repurposed the Mail on Sunday article under the headline, 'Top climate scientists admit global warming forecasts were wrong'. Meanwhile, The Australian went a step further with a story headlined 'We got it wrong on warming, says IPCC'.

Rate of warming not halved

The Mail on Sunday gives the story a generous double page spread, and repeats many of the same arguments Rose has made in previous pieces. So let's focus on what's new - the central claim of the article that scientists have cut their assessment of warming since the middle of last century by half.

The Mail on Sunday says:

"Back [in 2007], [the IPCC] said that the planet was warming at a rate of 0.2 degrees Celsius every decade … But the new report says the true figure since 1951 has been only 0.12 Celsius per decade - a rate far below even the lowest computer prediction"

Dr Richard Allan, a climate scientist at the University of Reading, tells us this statement is quite simply wrong. He says Rose has mixed up the numbers in the last IPCC report.

"The main claim by David Rose in the Mail on Sunday is that rate of global warming since 1951 has been halved since the last IPCC report. This is completely incorrect."

In 2007, the IPCC said the rate of warming since 1951 had been not 0.2 but 0.13 degrees Celsius per decade. If the new report says 0.12 degrees Celsius, as the Mail on Sunday suggests, this is a very minor revision of 0.01 degrees.

Professor Myles Allen from Oxford University, who is quoted pretty heavily in the piece, posted a comment below the article pointing out Rose's error. He said:

"Neither the IPCC in 2007 nor the current crop of climate models ever suggested that the world has been, or should have been, warming at 0.2 degrees per decade since 1951. So the headline should have been "Global warming is just 92 percent of what we said it was", on an apples-for-apples comparison."

And Dr Ed Hawkins, a climate scientist at Reading University, tells us:

"The trend over the past 50 years [the Mail on Sunday] says is in [the new IPCC report] is almost identical to the [last report in 2007] so the article's headline and premise that global warming is half of what was said ... is incorrect."

So where does the 0.2 degree per decade figure come from? Richard Allan tells us it does appear in the last IPCC report, but refers to a 15-year period in the run up to the report's release, not the warming per decade since 1951. He says:

"The 0.2 degrees Celsius per decade figure relates to an observed warming over the period 1990-2005 which clearly cannot be compared with the period since 1951".

So the two figures Rose compares are not measuring the same thing. As the Met Office's Richard Betts tweeted yesterday: "Rose created a headline by misrepresenting [the 2007 IPCC report]."

Confusion abounds

The Mail on Sunday's error has led one or two other journalists astray, and resulted in some fairly confused media coverage.

The Telegraph appears to have repeated the Mail on Sunday story wholesale. But the paper was forced to make a swift correction yesterday after suggesting the 2007 IPCC report claimed warming since 1951 had been 1.3 degrees per decade - ten times the actual figure.

This was quickly changed online to the correct figure of 0.13 degrees per decade. The change is likely to leave readers confused by what the article is actually claiming, as it now reads:

"The "summary for policymakers" of the report, seen by the Mail on Sunday, states that the world is warming at a rate of 0.12 degrees Celsius per decade since 1951, compared to a prediction of 0.13 degrees Celsius per decade in their last assessment published in 2007."

As Richard Allan tells Carbon Brief:

"That's 0.01 degrees Celsius per decade difference, which makes the title [Top climate scientists admit global warming forecasts were wrong] seem completely ridiculous and is an embarrassment to the serious reporting of climate change elsewhere."

Climate sensitivity

The Mail on Sunday also suggests climate models overestimate how much warming we will get in the future and by extension, the need to bring emissions down.

In 2007, the IPCC estimated a doubling of carbon dioxide above pre-industrial levels was likely to raise global temperature by between two and 4.5 degrees Celsius, with a best estimate of three degrees. This value is called climate sensitivity.

The Mail on Sunday says the new IPCC report "admits it is 'likely' [climate sensitivity] may be as little as 1.5 degrees Celsius - so giving the world many more decades to work out how to reduce carbon emissions before temperatures rise to dangerous levels."

It remains to be seen exactly what the new report says about climate sensitivity. The New York Times reported recently that the lower end of the range of likely values has come down from two to 1.5 degrees since the last report. We will have to wait for the report's publication to see how the IPCC has tackled the issue.

But Myles Allen tells us Rose's suggestion that a revision of the lower bound estimate on climate sensitivity could take the pressure off efforts to reduce emissions is "patently absurd". He says:

"I have explained [this reasoning] to David several times, and he does understand this, so he also understands that any revision in the lower bound on climate sensitivity does not affect the urgency of mitigation."

Richard Allan gives a good explaination of this point on his University of Reading blog, saying:

"[A]rguing over a few tenths of a degree in climate sensitivity at the bottom of the range masks the real issue which is the expected substantial climate change in response to the continued emissions of greenhouse gases, which are at present following the  worst case emissions scenarios."

The future of the IPCC

The piece also quotes Myles Allen, an IPCC lead author, in such a way as to suggest Allen thinks the IPCC design and process is flawed. Rose writes:

"One of the report's own authors, Professor Myles Allen …  last night said this should be the last IPCC assessment - accusing its cumbersome production process of 'misrepresenting how science works'.

For the record, Allen tells us he supports the continued work of the IPCC to assess climate science:

"I did not say this should be the last IPCC report, I said that in my view producing a massive volume once every six years has become counterproductive … I would favour much shorter annual update reports, plus special reports on specific issues."

The IPCC won't comment on new material in the report before it is published, cautioning against drawing conclusions from the draft as the findings are still subject to change. With the official version of the summary for policymakers out next Friday, at least there's not long left to wait now.

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