WHAT WAS CLAIMED
A graph depicts the rise in global temperatures associated with higher concentrations of atmospheric CO2.
OUR VERDICT
False. The graph understates CO2's warming effect by five to six times.
AAP FACTCHECK - Carbon emissions are still causing temperatures to rise, despite social media posts that include a graph that massively understates CO2's warming effect.
Experts say the graph underestimates CO2's warming effect by five to six times and is based on discredited research.
The claim appears in a Facebook post that features a bar graph showing the effect of CO2 levels on global temperature.
"Carbon dioxide does not cause temperatures to rise, as climate alarmism insists. This is deliberate distortion," the post reads.
"Once CO2 hits 20 parts per million (ppm) it has very little further effect on slowing reflected infra-red radiation. It's not exponential & can't cause runaway warming."
The graph appears to show that the warming effect from CO2 rapidly declines when concentrations exceed 20ppm and contributes very little additional warming at present concentrations of around 430ppm.
But climate scientists say this is wrong as carbon dioxide continues to warm the planet well beyond 20ppm.
The graph appears to have originated from a 2010 version of a book called the Skeptics Handbook.
According to a footnote, the graph assumes that CO2's climate sensitivity - its effect on temperatures - is 0.15C per watt per square metre.
This equates to a rise in global temperatures of around 0.5C every time CO2 levels double.
These figures are derived from a 2009 paper by Richard Lindzen and Yong-Sang Choi.
The 0.5C estimate was much lower than most other estimates at the time.
The researchers' methodology was criticised by other scientists in a February 2010 paper, who said it failed to avoid "pitfalls", and the analysis was called "erroneous" in a May 2010 paper, both published in the same journal.
Their paper was revised to provide a slightly higher estimate of CO2's climate sensitivity at 0.7C and published in a Korean journal in July 2010.
However, the revised paper was rejected for US publication in 2011, with peer reviewers unanimously deeming its conclusions unjustified.
Professor Lindzen admitted in 2012 to making "some stupid mistakes" in the original paper that were "embarrassing", according to The New York Times.
The latest Intergovernmental Panel on Climate Change report on climate science states the best estimate for the temperature effect of doubling of CO2 from pre-industrial levels is 3C - not 0.5C - with a likely range of between 2.5C to 4C (page 926).
It goes on to state that "all lines of evidence" help rule out the likelihood that doubling CO2 levels would cause less than 1.5C of heating.
Steven Sherwood, an expert in climate change physics at the University of New South Wales, said the graph and paper it's based on understated CO2's climate sensitivity by a factor of about six.
"Nobody accepts the method used in that old paper, and a large body of more relevant evidence of various kinds shows that the true number is around five or six times higher," Prof Sherwood told AAP FactCheck.
While the graph's values are greatly understated, he said, it accurately depicts CO2's warming effect as a logarithmic rather than a linear pattern.
"If you multiply all the numbers in the bar graph by five or six and add them together then I think you will get global warming numbers in line with mainstream predictions from the scientific community," Prof Sherwood said.
AAP FactCheck has previously reported that this logarithmic growth means that every time CO2 concentrations double, it is expected to result in roughly the same temperature increase of about 3C.
So a doubling of CO2 from 10ppm to 20ppm would have the same temperature effect, in degrees Celsius, as doubling from 200ppm to 400ppm.
David Romps, a climate physicist at the University of California, Berkeley, also said the graph was wrong and underestimated CO2's climate sensitivity.
"If the values on this plot were multiplied by a factor of 5-6, then it would more accurately reflect the science," Professor Romps told AAP FactCheck.
Martin Manning, a former IPCC director and climate change scientist at Victoria University of Wellington, also said the heating caused by CO2 does not stop at 20ppm as claimed.
"The Lindzen and Choi, 2009, paper does not say that the effect of CO2 stops at some concentration," he told AAP FactCheck.
However, Prof Manning also noted that several reviews showed that the paper's estimate of CO2's climate sensitivity was an outlier and was based on several methodological pitfalls.
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