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| Photo: A study published in the journal Nature reconstructed 2 million years of global average temperatures. (Antarctic Climate & Ecosystems CRC and Australian Antarctic Division) |
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The Evolution of Global Temperature over the Past Two Million Years paper reconstructed 2 million years of global average temperatures.
It found temperatures gradually cooled until about 1.2 million years ago and then stalled after that, and concluded that Earth's temperature could rise by between 3 and 7 degrees Celsius over the next thousand years.
But that prediction has come under fire from prominent climate scientists, including Dr Gavin Schmidt, a climate scientist and director of the NASA Goddard Institute for Space Studies.
He said he did not think the conclusion was correct.
"In fact, I'm pretty certain that is an incorrect calculation," he said."The ratio that gave that, which was the very high sensitivity that she calculates, comes from a correlation between temperature and the carbon dioxide and other greenhouse gases from the ice cores, but as we all know, correlation does not equal causation.
"And in this case, the causation is the orbital wobbles of the Earth's climate that are controlling both the temperature and the carbon dioxide at the same time and so that's giving you an exaggerated view of how carbon dioxide affects temperature directly."
However, Dr Schmidt welcomed the temperature history provided by the study, which analysed about 60 different sediment cores.
"The meat of this study is really a synthesis of deep sea ocean sediment cores that come from all around the world but put together in a way that allows you to say something about the global temperatures at every point between recently and 2 million years ago," he said.
"And so that's a really impressive synthesis."
But he said that would likely be outweighed by the temperature increase error.
"I think it's unfortunate both for the journal which is going to be accused of hyping sensational results without being scientific rigorous," he said.
"I think it's unfortunate for the author whose really good work is being overshadowed by this particular error, and it is unfortunate for the public because what you're seeing is a very confused message that people are going to take away all sorts of different messages from."
'A very confused message'
Professor Jeffrey Severinghaus, from the Scripps Institution of Oceanography at the University of San Diego, also found a problem with the study.
"She made a very, very basic logical error," he said."Climate sensitivity is essentially the change in temperature divided by the change in CO2.
"The important part about that is that if you want to infer that from an actual situation in the Earth, you know, what the Earth did in the past, you have to make sure that temperature change is only due to an increase in CO2, whereas the ice ages, we know very well the temperature change was due to a combination of increasing CO2 and changes in the Earth's orbit around the sun.
"In fact, it's probably something like two-thirds of the temperature change is due to the orbit and only one-third to the CO2.
"So that's probably why she got a factor of three larger."
The study's author Carolyn Snyder was not available for an interview.
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