SPOTLIGHT: Peer-reviewed studies, published six months apart, produce wildly different estimates.

BIG PICTURE: In June 2014, the US Proceedings of the National Academy of Sciences (PNAS) published a paper by a team of Spanish researchers. The PNAS website tells us it “strives to publish only the highest quality scientific research,” and that papers “undergo rigorous peer review” beforehand.

Titled “Plastic debris in the open ocean,” the paper acknowledges that the magnitude of this problem is “still unknown.” Its authors synthesized “data collected across the world to provide a global map and a first-order approximation.”

After performing numerous calculations involving 3,000 ocean samples, they concluded that the “global load of plastic on the open ocean surface” is “far less than expected.” In their estimation, it amounts to between “7,000 and 35,000 tons.”

Six months later, a second research paper was published in another peer-reviewed journal, PLOS ONE (which stands for the Public Library of Science). Written by individuals affiliated with reputable organizations in Australia, Chile, France, the US, and elsewhere, its version of reality is dramatically different.

According to this team, at least 268,940 tons of plastic is “afloat at sea.” These people explain their math and their models at some length. Their paper looks impressive and convincing to a layperson.

But let us be honest. There’s a huge difference between a maximum estimate of 35 thousand tons and a minimum estimate of 269 thousand tons.

TOP TAKEAWAY: When research papers published within months of each other reach starkly different conclusions, we need to tread carefully.

LINKS:

• Plastic debris in the open ocean, Andrés Cózar et al., Proceedings of the National Academy of Sciences, June 2014
• Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea, Marcus Eriksen et al., PLOS ONE, Dec. 2014
• The authors of the second paper appear to misunderstand the first one. The Spanish researchers tell us “mainly microplastics” (small bits) were found in their samples. But their study provides a big-picture estimate of all ocean plastic. The second team addresses the Spanish results this way: “A recent study on the global distribution of microplastic suggests that the total floating microplastic load ranges between 7,000 and 35,000 metric tons, and ours is 35,500 metric tons…The similarities between our results and those of this study gives us further confidence in our estimates…”

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