Wednesday, March 19, 2008

Reef relief

There are 284,300 km2 of coral reef in the world. They are basically made of calcium carbonate and if porosity accounts of one third of their volume, their density would be about 1.87 gm/cm3. This then is a carbon density of 0.22 gm/cm3. Coral is very stressed by silt and nitrogen runoff currently and may soon be attacked by ocean acidification, but is one of the ways that atmospheric carbon dioxide is converted into limestone. Coral seeks a certain depth below the surface of the ocean to have the right amount of light for its growth. We may well see 30 cm of sea level rise in the next 50 years since sea level rise appears to be accelerating. How much carbon would be sequestered by coral if we were to ensure that it can grow 30 cm where it is already established?
The total volume would be 8.5x1016 cm3 so the total carbon mass would be 19 Gigatonnes. This covers about 2 years of current carbon emissions or 4 years of carbon that is not already currently being absorbed from the atmosphere. To cover emissions since 1950 when emissions were 5 times lower, we'd need to increase the areas of coral reefs by about a factor of 5. Since corals grow in a range of conditions, establishing reefs in ways that anticipate both water temperature and sea level changes while doing what is needed to control silt and nitrogen runoff can apparently remove the extra carbon dioxide we have introduced into the atmosphere. Growth rates of corals are lower than what is needed (10 gm/m^2/day) by about a factor of three, so increasing the coral surface area by a factor of 15 would make some sense. Once sea level begins to recede, further coral growth would be limited, providing protection against removing too much carbon from the atmosphere.

Doing what needs to be done to protect coral so that it can grow would also likely revive mollusk populations which also sequester carbon as calcium carbonate so that the amount of new reef needed may be less. One way to control nitrogen runoff is to use biochar as a buffer and this also had carbon sequestration potential itself. The changes we need to make to unmake our carbon dioxide waste look as though they also improve our ability to get food through restored land and ocean productivity.


green with a gun said...

Difficult to do.

Coral grows pretty slowly, and with global warming already happening... the warmer water with more CO2 in it - well, hotter and more acidic water kills coral, it doesn't encourage its growth.

Really, coral growth is more a result of a stabilised climate than something which can contribute to it.

Chris Dudley said...

I compensated for the growth rate by expanding the area and for the changing water temperature by anticipating how temperature will change. The present rate of sea level rise is just about ideal for encouraging coral growth but it will likely get ahead of the growth rate before too long. I don't think we kill off coral owing to acidity yet and this is something I'd like to try to avoid with the coral's help. Coral reduces the acidity by removing the disolved cabon dioxide.

This was a quick post in response to a draft paper I read which I'll link later when it is submitted. I mainly want to point out the potential for large scale biomineralization of carbon here. We do eventually need to tap into the geological carbon cycle on the sink side since we have gone so far on the source side.