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22 January 2015 17:15

Factcheck: Scientists hit back at claims global warming projections are “greatly exaggerated”

Roz Pidcock

Roz Pidcock

NASA's Earth Observatory
Roz Pidcock

Roz Pidcock

22.01.2015 | 5:15pm
FactchecksFactcheck: Scientists hit back at claims global warming projections are “greatly exaggerated”

The MailOnline today reports on a study claiming scientists’ projections of climate change are overstated. Using an alternative “simple” model, there is “little evidence for alarm” about the scale of future warming, say the authors.

Today’s news report is picking up on a study published earlier this month in the Chinese journal Science Bulletin, lead-authored by climate skeptic politician Viscount Christopher Monckton. The MailOnline headline reads:  “Is climate change really that dangerous? Predictions are ‘very greatly exaggerated’, claims study”.

But climate scientists have been quick to point out serious flaws with the new research, calling its approach “cherry-picked”, “meaningless” and “simply physically implausible”.

                          Screenshot 2015-01-22 14.31.05

Source: MailOnline, 22nd January 2015

A simple model

The new paper uses a simple model to project future warming. The authors look at the relationship between greenhouse gases and surface temperature in the past, and extend it forwards.

They conclude the United Nations official climate body, the Intergovernmental Panel on Climate Change (IPCC), “very greatly exaggerates” the amount of warming we can expect in the next few decades. The press release claims the new research “exposes serious errors in complex computer models and reveals that Man’s influence on the climate is negligible”.

The new paper’s approach isn’t new – it’s been around for decades. Simple models like these are frequently used in IPCC reports, with the caveat that they could result in unrealistically low warming as they don’t take into account the extent to which ‘feedbacks’, such as ice loss and water vapour, can enhance warming.

Advanced climate models take a more complicated approach, using mathematical equations to represent processes known to affect the climate, including interactions between the land, oceans, atmosphere and plants.

The new paper claims this extra complexity leads to “major errors and exaggerations”, and, ultimately, to climate models overestimating future warming. By contrast, the new paper’s model is so simple that a “high-school student with a pocket scientific calculator” can use it, according to co-author  Dr Willie Soon, a solar physicist at the Harvard-Smithsonian Centre for Astrophysics, in the MailOnline article.

Other co-authors on the paper are David Legates, a professor of hydroclimatology at the University of Delaware and William Briggs, who describes himself as “statistician to the stars”.

A flawed approach

One of the authors’ criticisms of complex climate models, according to the MailOnline, is that they “incorrectly assume ‘temperature feedbacks’ would double or triple direct manmade greenhouse warming”. Instead, the new simple model suggests feedbacks could reduce warming overall, say the authors.

But Richard Allan, a professor of climate science in the meteorology department at the University of Reading, tells Carbon Brief there’s no clear scientific basis for this claim:

“The basic physics relating to water vapour is robust and observations confirm that water vapour strongly amplifies climate change â?¦ Even stronger counteracting feedbacks required to explain the simple model result have not been observed.”

The authors say that, using their simple model, the amount of warming we can expect per doubling of carbon dioxide is around one degree Celsius or less, or a third of the 3.3 degrees Celsius that climate models predict.

It’s important to note that these numbers refer to the warming we’d get each time the level of carbon dioxide in the atmosphere doubles. This is known as the climate sensitivity. This is not the same as – and is much less than – the total warming we’ll see if we continue on current emissions trends.

Cause for concern

Prof Reto Knutti, an atmospheric climate scientist at the Swiss Federal Institute of Technology, says the model makes assumptions that aren’t relevant today. He tells Carbon Brief:

“I’m particularly concerned about three aspects at least in this study. First, they use temperature variations over the last 800,000 years as a constraint, but of course the feedbacks in a world with whole continents covered by snow and ice are very different from a world today.”

A number of the statements the new paper makes are “simply physically implausible”, Knutti tells us. Another concernsis that the new paper’s approach overlooks recent warming in the oceans, he adds:

“[T]he use of surface temperature alone is problematic as 90 per cent of the additional energy that goes in the climate system is taken up by the ocean. That is a critical piece of information that is not used here.”

Myles Allen, professor of geosystem science at the University of Oxford, echoes this point, telling Carbon Brief:

“[The new] model asserts that there has been no warming below the surface layers of the oceans over the past few decades. It would be nice if that were true, because then â?¦ we would be looking at future warming near the low end of the IPCC’s uncertainty range. Unfortunately, the observations indicate that the oceans have warmed substantially since 1970, and that energy must have come from somewhere.”

Piers Forster, professor of physical climate change at the University of Leeds, seems to broadly underline scientists’ concerns about the new study, telling us:

“There are many plausible simple models one can use to approximate the complexities of the Earth’s climate [but they] are only ever as good as the assumptions and approximations that go into them. [These] authors cherry pick numbers and incorrectly infer ranges of parameters from past IPCC reports to build their model … I cannot see any robust estimates of climate change emerging from their study.”

For example, a line in the MailOnline piece claims “modellers are said to have failed to cut their estimate of global warming in line with a new, lower feedback estimate from the IPCC”. But Forster says the IPCC does not identify or quantify significant changes in feedback estimates between past and present reports.

Monckton has been responding to comments on a blog post by Dr Jan P Perlwitz, a Columbia University climate scientist who works at NASA GISS. You can see an extended set of comments Dr Perlwitz sent to Carbon Brief, here. A personal blog by Richard Telford, an associate professor of palaeoecology at the University of Bergen, has more [UPDATED].

Comparison with observations

The paper goes on to compare the IPCC’s long -erm outlook with “real world” warming over the past 25 years. This is represented in the graph below by the Met Office’s global temperature dataset, HadCRUT4 (top green line).

Screenshot 2015-01-22 11.08.19

The paper argues warming in the past 25 years has been less than 1.4 degrees per century, which it suggests is “half the IPCC’s central prediction in 1990”.

Incidentally, Monckton appears to contradict this statement later in the MailOnline article, claiming there has been no significant warming in almost the last two decades. He says:

“[E]ven if we stop emitting greenhouse gases, the simple model – confirmed by almost two decades without any significant global warming – shows there is no committed but unrealised man-made warming still to come”

Allan tells us it is not appropriate to compare long-term projections of future changes in surface temperature made in 1990 with recent observed temperature change, calling it a comparison between “apples and oranges”.

In another part of the paper, the authors compare the IPCC’s projections with satellite measurements of temperature in the lower part of the atmosphere. But the temperature there will be different to that at the surface, Allan tells us:

“[T]he satellite data shown for the last 20 years … measures heat emitted from a broad layer of the atmosphere, and so is not simply related to surface temperature.”

Open access

If you want to peruse the new paper yourself, you can. It’s freely available after the  Heartland Institute, a controversial think-tank which regularly promotes climate skeptics, paid to make the article open access.

The paper is published in the journal Science Bulletin, formerly the Chinese Science Bulletin, which is described in the press release as “equivalent to Science or Nature”. Journals are rated according to an ‘impact factor’, which is a measure of how many times the papers in that journal are referenced by other papers. The impact factor of Nature is 42, for Science it’s 31, and for the Science Bulletin it’s 1.4

The Chinese Academy of Science, which co-publishes the journal, forcefully  distanced itself from the Heartland Institute in 2013 after the think-tank used the fact that the Academy had translated one of its reports to suggest broad support for its climate-skeptic views.

Main image: US eastern seaboard at night from the ISS.

UPDATED 11:20am 23rd January 2015: This article has been updated to include a link to comments we received from Dr Jan Perlwitz. You can find the comments in full, here. We have also ammended the text to clarify Prof Richard Allan's title and Dr Perlwitz's affiliation.

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