How does rising atmospheric CO2 affect marine organisms?

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Atmospheric CO2 Enrichment: Boon or Bane of the Biosphere?
Volume 2, Number 8: 15 April 1999

In the 2 April issue of Science, Kleypas et al. (1999) describe theoretical calculations that suggest that the rising CO2 content of earth's atmosphere could lower the saturation state of the carbonate mineral aragonite in the surface waters of the world's oceans. They then state that this phenomenon could result in reduced calcification rates in coral reefs, which could in turn lead to weaker coral skeletons, reduced coral extension rates, and increased coral susceptibility to erosion, noting that these primary effects could lead to "a host of secondary changes in community structure, reproduction, and overall community functions." They further note that aragonite calcite precipitation in the tropics should have already decreased by 6 to 11% since 1880, as a result of the increase in atmospheric CO2 we have experienced to date, and that these reductions could reach 17 to 35% by 2100, as a result of expected increases in the air's CO2 content over the next century. And in the wake of the media attention that has followed the publication of their paper, we are treated to headlines that trumpet "CO2 Could Kill Coral" and "Great Barrier Reef Faces Death Knell."

At the oppostie end of the optimism spectrum, in a brief research synopsis published in the March issue of Natural History, Laurance (1999) discusses the recent discovery that tropical forests throughout the Amazon and Central America are growing amazingly better than they did three decades ago. "Not only are these forests producing more trees per acre," Laurance notes, "the existing trees are growing larger faster." Indeed, the total mass of living trees on each acre of studied land within the rainforests has risen by an average of 17 metric tons (37,000 pounds) since the beginning of observations thirty years ago. And why are today's forests "producing more and bigger trees per acre?" Laurance's answer is that the accelerated growth is likely "a direct response to high levels of carbon dioxide in the atmosphere."

Boon or bane? Which is it? Will the rising CO2 content of earth's atmosphere devastate the planet's coral reefs while it simultaneously fertilizes its forests? Will this ongoing global environmental change be both good and bad at the same time? Or is there a problem with one of these scenarios?

In comparing the two reports, we see that the coral reef scare is indeed a scenario, a description of the way in which something could come to pass. It is based upon certain chemical and thermodynamic equations that are perfectly fine in the abstract, but which may not tell the whole story in the real world, where the "all else being equal" part of their sphere of application is rarely valid. Second, the scenario assumes a number of interrelated cascading effects, each of which is dependent upon its predecessor, to finally arrive at the end result. We proceed, for example, from (1) the basic equations describing seawater's aragonite saturation state to (2) an inference about coral reef calcification rates to (3) several conjectured "primary effects," which are really tertiary effects if the counting is done correctly, to (4) even more ethereal "secondary changes," which are really fourth-order phenomena and beyond. It is a tortuous road of tentative linkages reminiscent of "the best laid schemes of mice and men."

The rainforest report, on the other hand, is not a scenario. It is an actual description of what has already happened, derived from repeated measurements "at fifty sites scattered throughout the Amazon and Central America ? of more than a hundred thousand trees over the past thirty years." It too was originally published in Science (Phillips et al., 1998; see our Journal Review CO2 Sequestration by Tropical Forests), wherein it was noted that this phenomenal increase in forest growth rate is large enough to account for approximately 40% of the missing carbon sink of the entire globe. In fact, Laurance (1999) states that "in the Amazon basin alone, intact rainforests could be absorbing over one billion tons of carbon dioxide each year." And another data-driven study, published in the same issue of Science as the Phillips et al. report, demonstrates that North American broadleaf forests located between 15 and 51N latitude are yearly removing from the atmosphere a quantity of carbon dioxide equal to all of the CO2 annually released to the air by all fossil fuel combustion in both the United States and Canada (Fan et al., 1998; see our Journal Review CO2 Sequestration in North America).

With respect to the coral reef scenario, it should also be noted that reefs don't materialize out of thin air, or water, as it were. They are laid down by biological activity; and the work performed by reef-building organisms in accomplishing this task may be able to overcome what in the absence of directed effort might otherwise be a serious impediment to reef development. Interestingly, Kleypas et al. recognize this fact, noting that it may be possible for reef-building organisms to adapt to gradual changes in ocean-water carbonate chemistry via "mitigative physiological effects, such as CO2 fertilization of calcareous algae or the symbiotic algae within coral tissues." Indeed, in reviewing the results of hundreds of experiments that were conducted under growth-limiting conditions, Idso and Idso (1994) found that atmospheric CO2 enrichment is particularly effective in helping terrestrial plants overcome various environmental stresses and resource deficiencies (see CO2-Induced Amelioration of Environmental Stresses); and there is no reason to expect that it would not act similarly to help aquatic plants do likewise.

It is also unsettling that the CO2-induced declining-coral reef scenario is built upon such a tenuous foundation. Kleypas et al. note, for example, that the key "calcification versus saturation state data are scarce," and that they are derived from experiments that "were conducted over days to weeks." To extrapolate so few results obtained over such a short time period to months or years is risky, and to do so for all of earth's reef systems for an entire century is very risky. The world is a complex place, as evidenced by the several papers in the special Complex Systems section of the issue of Science in which the Kleypas et al. paper appears (pages 79-109); and given enough time -- like a century -- that complexity is sure to alter the perceptions one may have derived from experiments lasting no more than a month.

So where does this all leave us? Like A Tale of Two Cities, this "Tale of Two Papers" gives us the best of times and the worst of times all at the same time. One or the other is therefore probably (but not necessarily) wrong. Is it the paper based on "scarce data" accumulated "over days to weeks"? Or is it the paper based on "studies of more than a hundred thousand trees over the past thirty years"? Or are they both right or wrong?

Rarely are simply phrased questions about complex living systems ever answered satisfactorily without years of painstaking research; and Kleypas et al. are well aware of this fact. As they say in their concluding paragraph, "the possibly dire consequences of reduced reef calcification warrant a much closer look at the biogeochemistry of shallow water carbonate secretors." Clearly, their paper makes a reasonable case for more research in this area; and it therefore recognizes its own main contribution as being to raise a warning flag about a potential environmental threat that ultimately may or may not be found to be valid. Whatever the outcome, the final word on the subject of CO2 and reefs will likely be significantly different from the simplistic and apocalyptic headlines the Kleypas et al. paper has generated in the popular press.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P. 1998. A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science 282: 442-446.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J-P., Langdon, C., and Opdyke, B.N. 1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284: 118-120.

Laurance, W.F. 1999. Gaia's lungs: Are rainforests inhaling earth's excess carbon dioxide? Natural History (April), p. 96.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical forests: Evidence from long-term plots. Science 282: 439-442.