Coral reefs as CO2 sinks?

An interesting article that has appeared in the news recently suggests that countries in the Asia Pacific region have proposed to utilise their coral reefs as carbon sinks under the new climate change protocol being developed in Bali.

Indonesian Fisheries and Marine Affairs Minister, Freddy Numberi, who opened the six countries’ senior official meeting to discuss the action plan to conserve the coral area, said that the area, which is dubbed as the Amazon of the Seas because it contains 53 percent of the world’s coral reef and over 3,000 fish species, was the earth’s epicenter of marine life and diversity.

“We have made efforts to conserve it during the past five years, so we want the world to appreciate it. One of the ways is to include it into the Kyoto Protocol framework so that it can be turned into a carbon sink, and later trade it for carbon credit,” Freddy told reporters. (Read more)

On the topic of coral reef sequestration of carbon, Dr Thomas Goreau from the Global Coral Reef Allience (a delegate at the United Nations conference in Bali) has the following to say:

Not only is the entire claim that coral reefs are a CO2 sink completely incorrect, they are in fact a source of CO2 to the atmosphere even while they remove carbon from the ocean. This has been understood by carbonate chemists for a very long time but we keep having to deal with this popular error over
and over again.

Fundamentally, because the ocean is a pH buffered system in which electrical charge is conserved, for every atom of bicarbonate in seawater that is converted to carbonate and deposited as limestone one molecule of bicarbonate is converted to carbonic acid and then to CO2 to balance the charge. So in effect for each atom of carbon removed from the ocean into limestone, one atom is released as CO2 to the atmosphere.

On a geological time scale limestone deposition and volcanic emissions are the two major sources of atmosheric CO2 (since photosynthesis and respiration plus decomposition balance),and are balanced by the fact that CO2 dissolves in fresh water, where it is the major acid once it ionizes, and is then neutralized by chemical weathering of limestone on land and of igneous and metamorphic rocks, being converted into bicarbonate which washes into the sea, resuming the cycle.
Half of all the limestone buried in the sea is buried in coral reefs (since most oceanic production dissolves in the deep sea), but to put it into perspective, this natural source of CO2 is 50 TIMES SMALLER THAN FOSSIL FUEL INPUTS, showing how seriously we have perturbed the natural carbon cycles.

The only way that reefs could be a CO2 sink would be if they were autotrophic ecosystems that buried most of the algae carbon before it could decompose. But in fact reef sediments have very low buried organic carbon content, because the organic carbon is almost entirely decomposed. In fact, reefs are not autotrophic at all, they are heterotrophic systems that rely on external organic carbon input from land and oceanic zooplankton.

Whenever I have measured oxygen in a reef it has always been below saturation, except directly over dense shallow seagrass beds in full sunlight. Overall the reef organic carbon cycle is consuming oxygen and producing CO2, as well as the CO2 produced by limestone deposition. Coral reefs are the first and worst victims of global warming, but they do not contribute to removing CO2 form the atmosphere at all. We must save them for their biodiversity, fisheries, shore protection, and tourism services, not because of false and misqguided claims that they are carbon sinks.

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