Published online 24 November 2010 | Nature | doi:10.1038/news.2010.631


Peer-reviewed 'oil budget' appeases scientists

Government agencies release revised account of fate of oil from Deepwater Horizon spill.

oil spillUS government agencies estimate that a quarter of oil from the Deepwater Horizon spill has dispersed - but this is based on very few data.Stephen Lehmann/U.S. Coast Guard

Scientists have welcomed a long-awaited government report on the short-term fate of the oil spilled in the Deepwater Horizon disaster last summer. But they warn that little is yet known about the ultimate ecological consequences of the oil that remains in the environment.

The new report replaces a previous 'oil budget calculator' created to guide the response to the spill. In use since June, this budget was harshly criticized when released publicly in August because it was not peer reviewed, and provided little information on how the estimates it contained for dispersion rates and other factors. That left critics concerned that the report was motivated by a political desire to downplay the risks from the spill.

Both reports were produced by the Federal Interagency Solutions Group, a consortium that includes the National Oceanic and Atmospheric Administration (NOAA), the US Geological Survey and the National Institute of Standards and Technology. The new version is peer reviewed, and contains clearer information about how the various calculations were carried out and the uncertainties associated with each estimate.

Follow the oil

This report, released on 23 November, reaches broadly similar conclusions to the original. Roughly a quarter of the oil was either recovered at the well, burned or collected using skimmers. A quarter evaporated or dissolved. A quarter broke up and dispersed — either naturally or with the help of chemical dispersants. The final quarter is assumed to have washed onto beaches or to remain in the water, for example in small, sticky blobs called tarballs.

“When you look at the underlying evidence, it simply isn't there to support the estimates presented.”

Jeffery Short
Conservation advocacy group Oceana

The most significant changes between the reports are in the oil thought to be dispersed. The estimate for natural dispersion has been reduced by 3%, and the amount of oil thought to have been broken up by chemical dispersants has been doubled to 16% of the total — changes that NOAA chief Jane Lubchenco attributes to information that has become available since the original budget was produced.

But those numbers are still far from certain. "So little data was collected during this particular spill event," says Ira Leifer, an oil expert from the University of California, Santa Barbara, and one of several non-governmental authors on the report. He believes that the true chemical dispersion rate is probably substantially lower than the estimate used, although within the large stated levels of uncertainty.

Jeffrey Short, a retired NOAA environmental chemist based in Juneau, Alaska, who works with the conservation-advocacy group Oceana and was a leader of the group assessing damage caused by the 1989 Exxon Valdez oil spill, also points out the paucity of solid data on which to base dispersion estimates. From the information that is available, he feels that the report has ended up on the optimistic side. "I'm concerned that this will be used to justify unwarranted dispersant use on future spills," he says. "When you look at the underlying evidence, it simply isn't there to support the estimates presented."

Too optimistic

Criticisms of the earlier report mainly attacked its reporting procedures and presentation. Many felt that its release was handled too optimistically, adding to the growing public distrust of the way that the US government presented research on the spill.

At a press conference on 4 August accompanying the original oil budget's release, Lubchenco and Carol Browner, director of the White House Office of Energy and Climate Change, focused on the idea that the oil was degrading rapidly and was effectively gone from the environment. The same day, Browner said on NBC's Today show that "the vast majority of the oil is gone".

When Lubchenco spoke about the new report on 23 November, she was careful to clarify its limitations, saying that questions such as where the remaining oil is now located, and its ecological effects, must be answered through further research.

"I think if they had used the approach they did today two months ago, the problems of reliability would never have surfaced," says Leifer. Perhaps more important, he says, the overly optimistic presentation of the first oil budget might have made legislators complacent about the need for further research to assess the threat from the oil.


Scientists were also upset because Lubchenco and Browner claimed at the August press conference that the oil budget had been peer reviewed. NOAA also told Nature at the time that about half of the 11 independent scientists listed on the report acted only as reviewers. Such claims proved to be false.

On 23 November, Lubchenco said: "I was in error in the press conference when I said the report had been peer reviewed." The University of New Hampshire has coordinated peer review of the new report, and researchers, including Leifer, say that the process was handled well.

But there is still much work to do. "Even if the numbers are dead on it's still not very comforting," says Jim Cowan, an oceanographer at Louisiana State University in Baton Rouge. "I don't think we know the consequences of having that much oil still in the environment." 

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