WHAT WAS CLAIMED
If atmospheric CO2 levels nearly doubled, global temperatures would only increase by 0.35C.
OUR VERDICT
False. Scientists estimate that doubling atmospheric CO2 levels will increase the average global surface temperature by about 3C.
AAP FACTCHECK - A doubling of CO2 levels is expected to increase global temperatures by about 3C, not 0.35C as some social media users have claimed.
The false contention was made in a Facebook post with the caption: "CO2 only has minimal effect on climate."
"Increase from current 410ppm [parts per million] to about 800ppm is only expected to result in +0,35C increased temperature," the post reads.
"And the extra CO2 would improve plant growth greatly and reduce their water needs. Hardly a problem really!"
The post includes a graph that shows the heating effect of atmospheric CO2 tapering off as concentrations increase.
It's unclear where the graph originated. It has previously been shared by a person associated with a US-based climate sceptics group.
However, experts estimate a doubling in atmospheric CO2 levels would increase temperatures by 3C, with a likely range of between 2.5C and 4C.
Current CO2 levels are around 430ppm, according to NASA, up from around 280ppm prior to industrialisation.
The post is correct that CO2's impact on global temperatures decreases as concentrations increase, however it greatly overstates the significance of this effect.
The warming effect of atmospheric CO2 increases in a roughly logarithmic manner, which means growth slows the more it occurs - the opposite of exponential growth, where growth occurs at an increasing rate.
Because of this, each time the atmospheric concentration of CO2 doubles, global temperatures increase by about the same amount - in this case about 3C - as opposed to doubling each time.
This is because of the way CO2 warms the planet. It traps thermal radiation, preventing heat from escaping the atmosphere - known as the greenhouse effect - and results in the earth warming up.
However, atmospheric CO2 traps and absorbs thermal radiation at particular wavelengths.
According to a 2013 paper published in the Royal Meteorological Society journal Weather, as more CO2 is added to the atmosphere, it starts to absorb all the heat radiation available at its most effective wavelengths.
This means there is less radiation available for additional CO2 molecules to absorb, reducing the warming effect of rising CO2 levels.
However, the paper notes that increasing levels of CO2 continue to heat the atmosphere even at very high concentrations, because it still absorbs radiation at a lesser rate at wavelengths that do not become saturated.
Robin Wordsworth, a climate researcher at Harvard University, wrote a paper on the warming effect caused by CO2 at a quantum level, which involves the study of very small particles like atoms.
He told AAP FactCheck the claim in the Facebook post wasn't supported by evidence.
"The most probable global average temperature increase following atmospheric CO2 doubling is 2C to 5C," Professor Wordsworth said.
Dave Frame, a physics professor and climate researcher at New Zealand's University of Canterbury and a lead author of multiple IPCC reports, also told AAP FactCheck the projections in the Facebook post were "way too low".
He recommended figures in the latest IPCC report on the physical science of climate change. A UK Met Office climate scientist, Timothy Andrews, referred AAP FactCheck to the same IPCC report.
Based on multiple lines of evidence, the report states, the best estimate of the long-term warming caused by doubling CO2 is 3C, with a "likely range" of 2.5C to 4C, and a "very likely range" of 2C to 5C (page 926).
It goes on to state that "all lines of evidence" help rule out the likelihood that doubling CO2 levels will cause heating below 1.5C.
The IPCC report also estimates the immediate impact of doubling CO2 levels within 70 years, without accounting for the long-term climate feedbacks (the "transient climate response" value, or TCR).
The best estimate is that this will lead to 1.8C of warming, with a likely range of 1.4C to 2.2C and a very likely range of between 1.2C and 2.4C (p927).
These projections were based on CO2 concentrations doubling from pre-industrial levels of around 280ppm.
Experts AAP FactCheck spoke to agreed that the doubling of CO2 levels from 400ppm to 800ppm, the figures cited in the post, would result in a temperature increase of around 3C, not the 0.35C claimed.
Dr Andrews said this was due to the logarithmic nature of CO2's warming effect.
"It's why the effect of doubling atmospheric CO2 concentration will approximately cause the same amount of warming whatever the starting point," Dr Andrews said.
This logarithmic relationship has been well established and is outlined in IPCC guidelines for climate change scenario modelling.
Prof Frame agreed the doubling CO2 levels from 400 to 800ppm "should have a pretty similar warming effect" to those in the IPCC projections because of the logarithmic phenomenon.
David Romps, a climate physics expert at the University of California Berkeley, who has written about the logarithmic nature of CO2's warming, agreed.
"You should expect about the same increase in temperature going from 400 to 800ppm as you would from 280 to 560ppm," Professor Romps told AAP FactCheck.
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