[Coral-List] Coral bleaching and ocean acidification

[Thomas Goreau]

Ocean acidification has become the latest bandwagon fad to hit coral  
reefs, and is being claimed as a "shocking new revelation" that will  
kill reefs and dissolve the evidence.

In fact there is nothing new about knowledge of the problem, all  
carbonate chemists have always known that CO2 is the major acid in the  
atmosphere and so its concentration controls the equilibrium pH of the  
ocean, along with the equilibrium with solid limestone minerals. More  
than 30 years ago at Harvard we would make undergraduate geochemistry  
students routinely calculate the equilibrium decline in ocean pH for  
doubling of CO2 as a homework problem! It is long known that periods  
in the past with no coral reefs or limestone sediments were caused by  
acidification, either due to higher atmospheric CO2, or more often, to  
changes in ocean circulation that resulted in CO2 build up in deep  
waters from decomposition of organic matter in anoxic basins.

But the fact is that these changes take thousands of years to develop,  
because they depend on the circulation time of the ocean and reaction  
rates with deep sea sediments. The increase in direct surface  
temperature is a far more serious and immediate threat to reefs than  
acidification. Acidification will only dissolve the dead skeletons  
centuries to millenia after high temperatures kill the corals, so  
focusing on acidification amounts to a red herring and effectively  
ignores a far larger and more immediate problem.

Recently “Declining coral calcification on the Great Barrier  
Reef” (De'ath, Lough, Fabricius, 2009, SCIENCE, 2 January, p. 116)  
shows field data convincingly indicating a strong negative  
relationship between rising temperatures and coral growth rates, and  
attributes decreasing coral growth in the last two decades to  
declining ocean pH caused by rising atmospheric CO2. Major flaws with  
this hypothesis are not discussed.

1) Coral bleaching is never mentioned. Yet there have been many  
episodes of mass coral bleaching in the Great Barrier Reef in the last  
two decades, and these are accurately predicted from high sea surface  
temperatures (1, 2). Bleached corals stop skeletal growth (3) even if  
temperatures are not high enough to kill them. This is because coral  
calcification as a function of temperature has a temperature optimum  
that is only slightly below the bleaching and death thresholds (4).

2) Tropical surface waters are not in equilibrium with atmospheric CO2  
due to the strong inverse relationship of CO2 solubility with  
temperature. As a result tropical surface oceans have partial  
pressures of CO2 above equilibrium with the atmosphere, and are a  
SOURCE, NOT A SINK, of atmospheric CO2 (5). CO2 dissolves in cold  
polar waters, where it takes about a thousand years to upwell back to  
surface waters. As a result of this natural ocean CO2 cycle, tropical  
surface waters will be the LAST part of the oceans where limestone  
becomes undersaturated. Furthermore calcium carbonates are anomalous  
minerals that become less soluble at high temperatures, not more  
soluble like almost all other minerals (6). Therefore the alarm about  
acidification effects on coral reefs is based on fundamental  
misunderstanding of the CO2 cycle in tropical surface waters. This is  
not to say that it is not an important long-term problem, but only  
that it is trivial compared to bleaching as a source of coral  
mortality and growth decline.

It is therefore likely that the decline in coral calcification  
reported in the Science paper is due to repeated temperatures above  
bleaching thresholds, which has happened increasingly in the past two  
decades (2), and that impacts of ocean acidity dissolving limestone  
will only take place long after the corals are directly killed by high  
temperature.

There is no question that we need to stabilize CO2 at safe levels  
immediately because IPCC has seriously underestimated the sensitivity  
of temperature and sea level to CO2 as shown by the paleoclimatic  
record (7). But that is needed in order to take care of the immediate  
temperature problem, not the long term acidification, at least as far  
as coral reefs are concerned.

If we take care of the CO2 stabilization in time to solve the  
bleaching problem, we will not only save coral reefs from mass  
extinction, we will automatically solve the ocean acidification  
problem. If we focus on solving the acidification problem first, it  
will come far too late to save coral reefs.

1) T. J. Goreau, & R. L. Hayes, 1994, Coral bleaching and ocean "hot  
spots", Ambio, 23: 176-180

2) T.J. Goreau, & R.L. Hayes, 2005, Global coral reef bleaching and  
sea surface temperature trends from satellite-derived Hotspot  
analysis, World Resource Review, 17: 254-293

3) T. J. Goreau & A. H. Macfarlane, 1990, Reduced growth rate of  
Montastrea annularis following the 1987-1988 coral bleaching event,  
Coral Reefs, 8: 211-215

4) C. Clausen, 1971, p. 246-269 in Experimental Coelenterate Biology,  
H. M. Lenhoff and L. Muscatine (Eds.), University of Hawaii Press,  
Honolulu

5) T. Takahashi, S. C. Sutherland, C. Sweeney, A. Poisson, N. Metzl,  
B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J.  
Olafsson, & Y. Nojiri, 2002, Global sea–air CO2 flux based on  
climatological surface ocean pCO2, and seasonal biological and  
temperature effects, Deep Sea Research Part II: Topical Studies in  
Oceanography, 49: 1601-1622

6) R. M. Garrels & C. R. Christ, 1965, Solutions, minerals, and  
equilibria, 450 p., Harper & Row, New York.

7) T. J. Goreau, 1990, Balancing Atmospheric CO2, Ambio, 19: 230-236

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
617-864-4226
goreau at bestweb.net
http://www.globalcoral.org