A well-known climate sceptic has said scientists have failed to prove that human emissions of carbon dioxide are driving global warming – despite him spending two decades probing for an answer.
Professor Ian Plimer made the claim in a written piece (archived) summing up his recent speech at the Sydney CPAC event.
His wording differs slightly in his written piece, in which he states: “I don’t have opinions. I have facts. They are repeatable and validated. Fact #1: No one has ever proven that human emissions of carbon dioxide drive global warming. For more than two decades I have been asking scientists for this proof.”
But the claim is misleading. Proof does not exist in science and scientists do not set out to prove anything. Instead they offer theories backed up by – often overwhelming amounts of – evidence.
Experts told AAP FactCheck that Prof Plimer’s claim is not only meaningless but misleading. They said no scientist would ever be able to answer his question and said he is playing on the difference between a scientist’s meaning of the word proof and that of the general public.
“Proof only occurs in pure mathematics,” Professor Steven Sherwood, a research team leader at UNSW’s Climate Change Research Centre, said.
“We do not ‘prove’ anything in science, we cannot even prove the sun will come up tomorrow.
“We also do not ‘prove’ things in court – we only demand that guilt be demonstrated beyond a reasonable doubt. We know beyond any reasonable doubt that CO2 is driving the warming we are seeing.”
Dr John Cook, a researcher at the Centre for Climate Change Communication at George Mason University in the US, agreed.
“Empirical science doesn’t ‘prove’ anything – it collects lines of evidence and as more independent evidence builds up, our scientific confidence increases,” he told AAP FactCheck in an email.
“When it comes to human-caused climate change, there are multiple lines of evidence that human activity is driving current climate change.”
Professor Michael Biercuk, director of the Quantum Control Laboratory at the University of Sydney, made a similar point in a piece he wrote for The Conversation.
Prof Biercuk said that as long as the evidence is consistent with the scientific theory, the theory is considered validated – but it will never be proven.
He added in the 2011 article that this concept is often played on by sceptics.
“A critic or sceptic may view a scientist’s hedging on the issue of ‘proof’ as a sign of weakness – really it’s just a sign the scientist’s meaning of the word is different to the general public’s,” he explained.
Dr Pep Canadell, chief research scientist at the CSIRO Climate Science Centre, said Prof Plimer’s claim was misleading the public given the overwhelming evidence backing the theory.
“Putting all our understanding and data of how the earth and climate works into global climate models (models are the only tools that can provide information on the many interactions in the earth system), we cannot explain the observed rise of temperature in any way except when we include the observed rise of greenhouse gases, particularly CO2,” he said in an email.
Dr Cook added that “anyone claiming there’s no evidence for human-caused global warming is not looking very hard – or more likely, engaging in an extreme form of wilful ignorance.”
Prof Sherwood said “no reasonable analyst would assign any doubt” to the theory given the overwhelming evidence (see here, here, here, here, here and here).
He added: “We know beyond any reasonable doubt that CO2 is driving the warming we are seeing. We know this because (1) we can measure the impact of CO2 on the planetary energy balance, (2) we know from the laws of physics that this leads to warming, and (3) we observe the expected warming.”
David Karoly, honorary professor in the School of Geography, Earth and Atmospheric Sciences at the University of Melbourne, pointed to IPCC reports (here and here) supported by multiple peer-reviewed studies as evidence supporting the theory.
“The increase in observed global average temperature is only possible with observed increase in CO2 concentrations due to human activity,” he told AAP FactCheck via email.
Prof Plimer is a geologist and long-time climate change sceptic who has been the subject of fact-checks previously (see here and here).
The Verdict
The claim that scientists have never been able to prove that human emissions of carbon dioxide drive global warming is misleading. Science does not deal in absolute proof and all scientists do is provide evidence to support a theory.
Experts told AAP FactCheck Mr Plimer’s claim is therefore meaningless and he misleads by playing on the general understanding of the concept of proof. They say there is overwhelming evidence and data that puts the theory beyond any reasonable doubt.
Misleading – The claim is accurate in parts but information has also been presented incorrectly, out of context or omitted.
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