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Anthropogenic carbon and ocean pH

Ken Caldeira and Michael E. Wickett · Nature 425, 365 · 2003

Key finding. Oceanic absorption of fossil fuel CO2 may produce larger changes in surface ocean pH over the next several centuries than any inferred from the geological record of the past 300 million years, with the possible exception of rare extreme events such as bolide impacts or catastrophic methane hydrate degassing.

Panel a stacks three plots against year from 1750 to 3000: carbon emissions in gigatonnes of carbon per year, atmospheric partial pressure of carbon dioxide, and a filled contour section of ocean pH change with depth to 5 kilometres, showing a change reaching about minus 0.7 at the surface. Panel b plots the time interval over which a change occurs, from 100 to 10 million years, against atmospheric carbon dioxide from 100 to 10,000 parts per million, with contours of pH change and shaded regions marking historical and geological analogues.
Panel b is the comparison that made the point. The pH change from fossil fuel carbon falls outside the region occupied by the past 300 million years of ocean chemistry — not because the amount of carbon is unprecedented, but because the time interval is so short. Figure 1 from Caldeira and Wickett (2003), Nature 425, 365. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Most carbon dioxide released by burning fossil fuels ends up in the ocean, where it lowers pH. Whether that matters depends on how the change compares with what marine organisms have encountered before.

This paper quantified the pH change that continued CO2 release would produce and set it against pH changes reconstructed from the geological and historical records.

What did we find?

The comparison is against 300 million years of reconstructed ocean chemistry, not against the instrumental record — which is what makes the claim about unprecedentedness meaningful.

The exceptions named are catastrophes rather than trends: an asteroid impact, or a sudden release of methane hydrate. Nothing in the ordinary variability of the past 300 million years is comparable.

The largest change occurs at the surface, where most marine biota live.

Organisms building skeletons or shells from calcium carbonate — coral reefs, calcareous plankton and others — are identified as particularly exposed, while deep-ocean biota may be more sensitive to a given change even though the change there is smaller.

The paper is explicit that understanding of the biological consequences was limited by a paucity of relevant observations, which is a statement about what was not yet known rather than a claim of safety.

Why does it matter?

This is the paper that put ocean acidification on the map as a distinct consequence of carbon dioxide emissions, separate from warming and not addressed by anything that only manages temperature.

Framing the change against 300 million years rather than against a baseline is what gives it force. Marine organisms have had no evolutionary experience of a shift this fast.

It also set the agenda it identified as missing. The observational programmes and reef experiments that followed exist because this paper showed the magnitude and named the unknown.

Citation

Ken Caldeira and Michael E. Wickett (2003). Anthropogenic carbon and ocean pH. Nature 425, 365.

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