Acid burn: how ocean acidity might make climate change worse

All opinions my own and do not necessarily reflect those of Novo Nordisk.

I was at the University District Farmer’s market this past weekend, thinking about making paella, and so I stopped by the shellfish booth and bought a pound of clams.  As I wandered around, looking for some tomatoes and onions, I swung my canvas bag, now heavy with thick-shelled Manila clams, and thought back to some time I spent in Spain during my first postdoc.  That’s when I learned to make paella.  That’s also where I studied calcification in the green alga Acetabularia acetabulum, which grows thick in the shallows of the waters of the Mediterranean.

Calcification is the process of depositing calcium carbonate along, within or around a biological structure.  It’s an important process in the oceans, contributing to ecological roles like protection via the shells of mollusks like the clams in my bag.  And it’s getting harder to do for ocean creatures to do because the oceans are becoming more acidic.

Calcium carbonate, the major mineral component laid down during calcification, is sensitive to pH.  The more acidic the environment, the more the chemical equilibrium shifts from solid calcium carbonate to its constituent,soluble ions, calcium and carbonate.   This means it’s harder for organisms to precipitate calcium carbonate out of seawater.

As the levels of carbon dioxide have increased in the atmosphere, a fair amount of that has been absorbed by the oceans, leading to an increase in ocean acidity because some of dissolved carbon dioxide converts into carbonic acid.  The problem with acidification for marine species like clams, corals and oysters has been documented in many places (including a few of my own, here and here).  A recent study from Nature Climate Change  (paywall, but synopsis here), however, suggests that the direct damage to calcification of ocean species is only one of the unfortunate consequences of ocean acidification.  An unanticipated additional effect is that it could in turn make climate change worse.

The research involved incorporating previous data derived from studying ocean mesocosms and building computer models of the effects of ocean acidity on atmopheric warming.  Mesocosms are large structures that are built to enclose, as tightly as possible and within a reasonable space, a specific environment.  In this case the mesocosms  enclosed atmosphere-seawater ecosystems in which the atmospheric and water conditions could be well controlled to allow testing of the effects of water acidity.  Think of really big baggies filled with seawater and floating in the ocean.  And capped, so that the atmosphere could be carefully measured over time.

The mesocosm work measured the release of biogenic dimethylsulfide (DMS)–basically, sulfur compounds generated by microscopic, photosynthesizing plankton.  The researcher found that as seawater pH decreased, so did the amount of DMS.  DMS is a major atmospheric trigger for cloud formation.  Less DMS = potentially fewer clouds = more sunlight reaching the earth’s surface instead of being reflected into space = warmer surface temperatures.  The carbon dioxide that is contributing to both climate change and ocean acidification may have its effect amplified even more due to this indirect effect on phytoplankton metabolism.

The authors of the current study took that data and incorporated it into models for climate change.  If the amount of DMS reduction is not offset by other factors (such as an increased growth rate of phytoplankton in a warmer overall ocean), then the models suggest the atmosphere could warm by an additional quarter to a half a degree centigrade, on average.

Whether this will actually happen is  unclear since there are factors which could contribute to DMS production and that have yet to be incorporated into these models or even discovered.  If we’re lucky those factors may cancel out and we’ll just be left with the climate change we currently expect.  Which, frankly, is bad enough as it is.

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