Scientists behind a major study that claimed the Earth's oceans are warming faster than previously thought now say their work contained inadvertent errors that made their conclusions seem more certain than they actually are.
Weeks after the high-profile study was published in the journal Nature, its authors have submitted corrections to the publication.
The Scripps Institution of Oceanography, home to several of the researchers involved, also noted the problems in the scientists' work and corrected a news release on its website, which previously had asserted that the study detailed how the Earth's oceans "have absorbed 60 per cent more heat than previously thought".
"Unfortunately, we made mistakes here," said Ralph Keeling, a climate scientist at Scripps, who was a co-author of the study.
"I think the main lesson is that you work as fast as you can to fix mistakes when you find them."
The central problem, according to Keeling, came in how the researchers dealt with the uncertainty in their measurements. As a result, the findings suffer from too much doubt to definitively support the paper's conclusion about how much heat the oceans have absorbed over time.
The central conclusion of the study — that oceans are retaining ever more energy as more heat is being trapped within Earth's climate system each year — is in line with other studies that have drawn similar conclusions.
And it hasn't changed much despite the errors.
But Keeling said the authors' miscalculations mean there is a much larger margin of error in the findings, which means researchers can weigh in with less certainty than they thought.
"I accept responsibility for what happened because it's my role to make sure that those kind of details got conveyed," Keeling said. (He has published a more detailed explanation of what happened here.)
The study's lead author was Laure Resplandy of Princeton University. Other researchers were with institutions in China, Paris, Germany and the US National Centre for Atmospheric Research and Geophysical Fluid Dynamics Laboratory.
"Maintaining the accuracy of the scientific record is of primary importance to us as publishers and we recognise our responsibility to correct errors in papers that we have published," Nature said in a statement to the Washington Post.
"Issues relating to this paper have been brought to Nature's attention and we are looking into them carefully. We take all concerns related to papers we have published very seriously and will issue an update once further information is available."
The original study, which appeared on October 31, derived a new method for measuring how much heat is being absorbed by the oceans.
Essentially, the authors measured the volume of gases, specifically oxygen and carbon dioxide, that have escaped the ocean in recent decades and headed into the atmosphere as it heats up.
They found that the warming "is at the high end of previous estimates" and suggested that as a result, the rate of global warming itself could be more accelerated.
The results, wrote the authors, may suggest there is less time than previously thought to curb greenhouse gas emissions. The study drew considerable media attention.
However, not long after publication, an independent Britain-based researcher named Nicholas Lewis published a lengthy blog post saying he had found a "major problem" with the research.
"So far as I can see, their method vastly underestimates the uncertainty," Lewis said in an interview, "as well as biasing up significantly, nearly 30 per cent, the central estimate."
Lewis added that he tends "to read a large number of papers, and, having a mathematics as well as a physics background, I tend to look at them quite carefully, and see if they make sense. And where they don't make sense — with this one, it's fairly obvious it didn't make sense — I look into them more deeply."
Lewis has argued in past studies and commentaries that climate scientists are predicting too much warming because of their reliance on computer simulations, and that current data from the planet itself suggests global warming will be less severe than feared.
It isn't clear whether the authors agree with all of Lewis's criticisms, but Keeling said "we agree there were problems along the lines he identified".
Paul Durack, a research scientist at the Lawrence Livermore National Laboratory in California, said that promptly acknowledging the errors in the study "is the right approach in the interests of transparency".
But he added in an email: "This study, although there are additional questions that are arising now, confirms the long known result that the oceans have been warming over the observed record, and the rate of warming has been increasing".
Gavin Schmidt, head of the Nasa Goddard Institute for Space Studies, followed the growing debate over the study closely on Twitter and said that measurements about the uptake of heat in the oceans have been bedeviled with data problems for some time — and that debuting new research in this area is hard.
"Obviously you rely on your co-authors and the reviewers to catch most problems, but things still sometimes slip through," Schmidt wrote in an email.
Schmidt and Keeling agreed that other studies also support a higher level of ocean heat content than the Intergovernmental Panel on Climate Change, or IPCC, saw in a landmark 2013 report.
Overall, Schmidt said, the episode can be seen as a positive one.
"The key is not whether mistakes are made, but how they are dealt with — and the response from Laure and Ralph here is exemplary. No panic, but a careful re-examination of their working — despite a somewhat hostile environment," he wrote.
"So, plus one for some post-publication review, and plus one to the authors for re-examining the whole calculation in a constructive way. We will all end up wiser."