A senior NASA official has taken the extraordinary step of personally rejecting the claims of One Nation senator Malcolm Roberts that the agency had falsified key data to exaggerate warming in the Arctic.
Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, told Senator Roberts he was "mistaken" in his assertion that the US agency had "removed" Arctic data to mask warming in the 1940s.
"You appear to hold a number of misconceptions which I am happy to clarify at this time," Dr Schmidt told Senator Roberts in letters and emails obtained by Fairfax Media. "The claim that GISS has 'removed the 1940s warmth' in the Arctic is not correct."
"We are certainly gratified by the attention Australia pays to our analysis, but in case you have remaining questions, I urge you to perform your own analyses."
The claim that NASA tampered with decades-old Arctic data is a favourite conspiracy theory among global warming sceptics who argue the current run-up in temperatures – especially at the North Pole – is nothing exceptional, and so action to address climate change is unnecessary.
In his letter to NASA dated November 14, Senator Roberts explained his interest in the agency's temperature calculations, saying they had "influenced" the Intergovernmental Panel on Climate Change's warnings on global warming that in turn had informed Australian government policy.
Senator Roberts, a former coalmining manager, rejects the idea climate change is related to carbon-dioxide levels and last week attacked the CSIRO's climate science.
Fairfax Media has sought comment from Senator Roberts.
On Twitter, he said he "had not had a chance to reply as the email only arrived Saturday".
Iceland weighs in
"In Australia, we have considerable concern about temperature adjustments made by NASA over many years," Senator Roberts wrote, including charts from Icelandic stations at Vestmannaeyjar and Teigarhorn.
"In dropping the temperatures for the early period, the [Arctic] warmth for the 1930s and 1940s appears to have been removed," he said. "What is your specific reason for doing this?"
In an email, Truasti Jonsoon, senior meteorologist with a specialty in historical climatology at the Icelandic Meteorological Office, told Senator Roberts that the temperature "adjustments" are "quite sound".
The original data was not from NASA but the Smithsonian publication "World weather records", he said.
"During this early period there was a large daytime bias in the temperature data from Iceland as presented in this publication," which accounted for much of the "discrepancy" at Teigarhorn and less so at Vestmannaeyjar, Mr Jonsoon said.
For the latter station, it was relocated in October 1921 to a higher elevation. "Comparative measurements at both sites have shown that the later location is about 0.7 degrees Celsius colder than the former – this relocation has to be 'adjusted' for," he said.
"I assure you that these adjustments are absolutely necessary and well founded although the finer details of the resulting series shown in your letter differ slightly from my own version," he told Senator Roberts.
Dr Schmidt said the Arctic was "not so much" the target of data critics.
"But the insistence that the data must've been inappropriately adjusted [is what the sceptics say] all the time," he told Fairfax Media. "[It's] pretty much the definition of denial."
Dr Schmidt said he was not surprised that the query about Arctic temperatures was coming from Australia.
"I'm aware of who Malcolm Roberts is, and the only surprise is that he is in fact a senator," he said.