Friday, 21 August 2009

Pipes published

Finally, after two inexplicably long review-revision-resubmission cycles, our paper on the efficacy (or not) of ocean pipes is finally formally published. I say "inexplicably" because neither review cycle turned up much in the way of major problems with the paper. Reading between the lines, I suspect that difficulties getting reviewers, and getting them to submit reviews on time, has been the main source for our frustrations.

Anyway, what does the paper say? Essentially it explores what the consequences would be if you "plumbed-in" the whole of the ocean (well, wherever it's > 1000 m in depth) with pipes that transfer seawater from its interior to the surface (cf. the figure to the left). The idea behind such pipes is that they would channel high concentrations of deep nutrients to surface waters, where they can fuel the growth of phytoplankton, increase the uptake of CO2, and thus potentially decrease climate change.

In our work we did quite short simulations of the recent past (1995-2004) in which we explored what happened to primary production, export production and the air-sea flux of CO2 when pipes of three lengths (200, 500 and 1000 m) were switched on, relative to a control simulation for the same period.

What we found was that the pipes did the business in terms of primary and export production, with significant increases in both (although we did assume that the pipes were moving a comparable amount of water to that naturally translocated by ocean circulation). However, we also found that, by contrast, the air-sea flux of CO2 was only marginally increased by the pipes. In part, this was because we just ran the pipes everywhere without any consideration as to whether they would bring up more DIC than they'd be able to sequester, and the consequences of this can be seen in the figure below. The big blue areas in the lower row are places where, because of the deep carbon:nutrient ratios, the pipes channel more CO2 to the surface than can be drawn down by the transported nutrients.

However, even factoring out areas where the ocean was turned from a sink of CO2 into a source, we found that for every 100 moles of extra carbon fixed by phytoplankton, there'd be only 2-3 moles extra drawn from the atmosphere. And then, by doing a big back-of-the-envelope calculation, we estimated, based on our results, that to increase the ocean's uptake of CO2 by 50%, we'd need to deploy around 800 million pipes in the oligotrophic tropical ocean. Of course, there are a lot of caveats in our paper that cover what's wrong with our model, our simulation and our assumptions, but we tend to think that our sums are about right to first order (but then we would!).

In the paper, we completely avoided tackling what the "side effects" of the pipes could be (although one of my co-authors finds some really interesting ones in a separate study), just so we could focus on the basic: "do the pipes work?". Taking a somewhat ethical stance, one thing that they would do would be to replace the oligotrophic "desert" communities of the subtropical gyres into the more mesotrophic communities typical of upwelling areas. As a biologist, I don't like the idea of decreasing biodiversity just so that we can avoid the unwanted consequences of our actions, but at the same time I think it's good to know how efficient this sort of geoengineering scheme is just in case we do need a drastic solution to climate change down the line. Though hopefully we'll see sense before then ...

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