Update: Ocean Productivity Holding Steady?

by Bill Chameides | May 2nd, 2011
posted by Erica Rowell (Editor)

Permalink | 2 comments

The ocean’s little green guys are doing fine, according to new research.

Last summer TheGreenGrok covered a paper published in the journal Nature by Daniel Boyce of Dalhousie University in Nova Scotia and colleagues, which reported evidence of a decades-long decline in global ocean productivity (i.e., the amount of photosynthesis carried out by those one-celled algae we call phytoplankton). The decline was staggeringly large: about one percent per year, amounting to an overall productivity decrease of about 50 percent since 1899. The authors argued their detected decrease was “unequivocal” and speculated that it could likely be attributed to global warming.

While I thought the connection between global warming and a decline in ocean productivity advanced by Boyce et al was reasonable, I had a hard time swallowing the size of the decline they reported. A factor-of-two decrease in ocean productivity is a huge change whose impact on the carbon cycle and global fisheries would be hard to miss. In my post from last August, I wrote that I found the size of the decline ”surprising” and inserted my own speculation that “in all likelihood the overall decrease was not quite so large.” I left off the discussion with the fact the Boyce et al paper left me with “some questions unanswered” that would “have to wait for additional research.”

Additional Research Arrives

That has now arrived in the form of three new critical comments published this month in Nature, which take the questioning of the Boyce et al’s findings a major step farther than my own. Collectively, these criticisms do not just conclude that the decline is overstated but that in all likelihood there has been no significant decline at all.

Two of these critiques — one by David Mackas of the Institute of Ocean Sciences in Sidney, British Columbia, and the other by Ryan Rykaczewski and John Dunne of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey (a part of the National Oceanic and Atmospheric Administration) — argue that the trend detected by Boyce et al is an artifact of the methodology they used. This methodology combined two datasets: an older one based on an analysis of seawater samples and a modern one based on ocean color measurements from satellites. The new communications argue that most of the decline Boyce et al reported could be attributed to having mixed two incompatible datasets as opposed to an actual decline in productivity.

The third criticism, written by Abigail McQuatters-Gollop of the Sir Alister Hardy Foundation for Ocean Science in the United Kingdom and co-authors, reported on an analysis of the long-term trend in phytoplankton abundance using a different, independent dataset derived from the 80-year Continuous Plankton Recorder survey. Rather than a decline, this dataset suggests that ocean productivity, especially in most recent decades, is on the rise.

And a Rebuttal Follows

As is the custom in scientific circles, these criticisms were rebutted by Boyce et al in the same issue of Nature. The rebuttal to both the Mackas and the Rykaczewski and Dunne critiques gets fairly technical but basically claims that the critiques fail to take into account the methods Boyce et al used to combine the two datasets to make them compatible for a trend analysis. In response to the other criticism, Boyce et al point out that the dataset McQuatters-Gollop et al used was designed to monitor subsets of the total phytoplankton population in part in areas outside of areas of interest to Boyce et al and thus is not really a comparable record of ocean productivity.

A Model for Public Discourse?

Given the hyperbole and personal attacks that so often masquerade for debate today, it’s worth noting the tenor of this debate. First of all, such exchanges are quite common in the literature, where critical comments on a paper are published along with a rebuttal by the original authors. And as a rule, these exchanges are made in a civil manner and in the spirit of advancing scientific understanding. This exchange on ocean productivity is no exception. For example, in their concluding paragraph, Rykaczewski and Dunne commend Boyce et al for addressing “this critical issue,” and refer to the exchange as a “dialogue.” Similarly, in their rebuttal, Boyce et al characterize the three critiques as “thoughtful” and they “welcome the critical suggestions.” That this topic is part and parcel to the highly contentious issue of global warming did not prevent these scientists from acting as colleagues.

So, what’s the bottom line from the exchange? For one, it would appear that the report of the decline of ocean phytoplankton may have been premature, or maybe not; we’ll have to see. And two: reports of the decline of scientific discourse aimed at advancing understanding are plain wrong.

filed under: climate change, faculty, global warming, nature, oceans, science
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  1. MattN
    May 3, 2011

    I give you points for posting a correction to what was obviously poorly done science.

    • Bill Chameides
      May 17, 2011

      MattN: Not at all clear Boyce et al’s results were based on “poorly done science.” Jury still out.

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