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Planetary Watch: Ocean Acidification Faster Sooner

by Bill Chameides | December 11th, 2008
posted by Erica Rowell (Editor)

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Calcifiers, like the sea scallop, use oceanic carbonate and calcium to form their shells. The more acidic the ocean becomes, the less available these salts and metals are, putting an important part of the marine food web at risk.

Regardless of your stance on global warming, you should be worried about a related problem: ocean acidification. You don’t need a climate model to know this is serious, especially if you like — or depend on — seafood. Two new papers explore just how serious.

Well over a billion people the globe over depend upon the ocean as their primary source of protein. The fish we eat, in turn, rely on a complex food web that begins with phytoplankton (those one-celled green plants that float in the surface ocean). Key links in that chain are provided by calcifiers – organisms with hard shells (exoskeletons) made of the calcium carbonate minerals calcite and aragonite:

  • Calcifying phytoplankton, known as coccolithophores, sit at the bottom of the food chain.
  • Mollusks such as clams and mussels recycle carbon and nutrients by filtering organisms from the water they take in.
  • Coral reefs provide essential breeding grounds to support the fisheries we depend on.

It would be best if we did not mess with these and other calcifiers. But unfortunately we are.

To grow and develop, calcifiers must be able to take dissolved carbonate and calcium from ocean waters and solidify them into calcium carbonate for their shells. Today, that is not too hard to do because much of the surface ocean is supersaturated with calcium and carbonate. But that supersaturation depends upon the acidity of the ocean. (The term pH describes acidity – the more acid the lower the pH.) As ocean acidity increases (or pH decreases), calcium and carbonate become less saturated, making it harder for calcifiers to do their thing.

Here’s the rub: as we add carbon dioxide (CO2) to the atmosphere (by burning fossil fuels and cutting down tropical rain forests), more of that CO2 dissolves in the ocean, increasing its acidity. At some point, ocean acidity could get so high that calcifiers begin to decline or even die out. Not good news if you appreciate the beauty of coral reefs — and possibly disastrous news if you depend upon fish as a dinner staple.

New Paper: Acidification Rate Spells Trouble for Seafood

While scientists have known about the dangers of ocean acidification from CO2 for some time, the predictions of how fast it would occur and what its impacts will be have been largely limited to model calculations and lab experiments. There are very few empirical oceanic datasets to back these models up. Now, two papers, in the Proceedings of the National Academy of Sciences, present real-world data, and the results are, for the most part, not encouraging.

Timothy Wootton and colleagues from the University of Chicago report on an 8-year dataset of pH and related measurements collected off the coast of Washington state. The data show the acidity is far more variable than previous thought – responding for example to changes in sunlight, temperature, and time of day. But over the study’s 8-year span, the site’s average pH declined steadily as atmospheric CO2 increased. The decline is not surprising. What is surprising is its rate: the pH declined by a factor of 10 faster than what the models have predicted. In other words, ocean acidification is happening faster, at least in the waters studied by the Wootton team.

On the encouraging side, Wootton et al. found a more complex and somewhat unexpected ecological response to increased acidity. While dominant calcareous species such as the California and blue mussels declined with increased acidity, others like the acorn barnacle actually fared better.

The Wootton team’s findings will no doubt catalyze a lot of new research to better understand the mechanisms at play and their applicability to the oceans as a whole.

New Paper: Higher CO2 Levels Threaten Another Primary Food Source

Meanwhile, Ben McNeil of the University of New South Wales and Richard Matear of the Antarctic Climate & Ecosystems Cooperative Research Centre provide a new seasonal reconstruction (based on observations) of the Southern Ocean’s pH and carbonate ion saturation. This work has important implications for how acidification will play out in the Southern Ocean.

Their seasonal reconstruction showed that ocean pH tends to reach an annual minimum in winter. This means the onset of undersaturated conditions can occur earlier than if just annual average pH levels are considered. The researchers conclude that once atmospheric CO2 reaches 450 ppm, wintertime aragonite undersaturation will begin threatening the viability of pteropods – a type of zooplankton that is a main source of food in the region. This is a lower CO2 concentration than previously thought.

There are two notable aspects to McNeil’s work:

  1. Coincidentally, 450 ppm is also thought to be threshold beyond which the complete melting of the Greenland icesheet becomes inevitable; and
  2. Without a global agreement to curb greenhouse gas emission, we will likely cross the 450 ppm threshold sometime around 2030 – just a little more than 20 years from now.

Food for thought the next you sit down to a seafood dinner.

filed under: carbon dioxide emissions, faculty, global warming, oceans, Planetary Watch, science
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