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
