How does rising atmospheric CO2 affect marine organisms?

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Coral Reefs: Doomed by Carbon Dioxide?
Volume 2, Number 1: 1 January 1999

While surfing last week - on the web, that is - we came across some material decrying the state of the world's coral reefs.  In one report, posted on 19 October 1998 (Hocking Voice Global Warming Report 9), corals were said to be expiring in record numbers because of warmer-than-normal temperatures.  "These corals are dying from heatstroke," it quoted Thomas Goreau of the Global Coral Reef Alliance as saying, noting that he and another scientist claim that "reefs will rebound only through dramatic reduction of fuel consumption."

Coincidentally, a frequent visitor to our website sent us a copy of the 19 December 1998 "Good Weekend" edition of The Sydney Morning Herald, wherein (on pages 28-33) Environment Editor Murray Hogarth suggests that anthropogenic CO2 emissions pose a serious threat to Australia's Great Barrier Reef, due to their purported central role in causing global warming.  In fact, Hogarth goes so far as to describe plans to mine oil shale deposits on Queensland's reef coastline as "bizarre and perverse," as he puts it, because of the CO2 that would ultimately be released to the air when the recovered oil is burned.

Citing the oft-repeated claim of Goreau that "if it keeps getting hotter due to human-induced global warming, ? then reefs are doomed," Hogarth joins him in equating the death of reefs with human enterprises that release CO2 to the atmosphere and presumably lead to escalating temperatures.  There are, however, a number of problems with this line of reasoning.

First, it has yet to be proven that the rise in the atmosphere's CO2 concentration that began with the Industrial Revolution has anything to do with the concomitant warming of the globe.  It is a well known fact, for example, that earth's near-surface air temperature oscillates on millennial time scales throughout glacial and interglacial periods alike, independent of any forcing from carbon dioxide.  Furthermore, for significant periods of time during the present interglacial, when there was much less CO2 in the air than there is currently, it was actually warmer than it is now; and earth's reefs did not succumb to the dreaded bleaching that Goreau and Hogarth attribute to global warming.

A good case in point is illustrated by the recent work of a large research team composed of M.K. Gagan, L.K. Ayliffe, J.A. Cali, G.E. Mortimer and M.T. McCulloch of the Australian National University's Research School of Earth Sciences, as well as J. Chappell and M.J. Head of ANU's Research School of Pacific and Asian Studies and D. Hopley of James Cook University's Sir George Fisher Center.  Together, these scientists studied corals from the Great Barrier Reef and determined that some 5,350 years ago the tropical ocean surface was 1.2°C warmer than it is presently.  Moreover, as they noted in the report of their research published in the 13 February 1998 issue of Science, "terrestrial pollen and tree-line elevation records elsewhere in the tropical southwest Pacific indicate that the climate was generally warmer from 7,000 to 4,000 [years before present]."  As for CO2, our Fact Sheet referenced above indicates that this was a period of time when the air's CO2 content was fully 100 ppm less than it is today!  And, of course, the corals survived.

Second, Hogarth's recitation of global warming's "growing list of perils for reefs" lacks rigorous scientific backing.  There is, for example, simply no empirical evidence that the global warming of the recent past has resulted in "more powerful cyclones to pound them [i.e., the reefs]."  Neither is there any evidence that such storms are becoming more frequent.  In fact, intense tropical cyclone activity has actually decreased in the North Atlantic over the last few decades; and hurricane financial damage in the United States, when adjusted for inflation, wealth statistics and population trends, has also dropped over the last half century.

Hogarth's claim that CO2-induced global warming will result in "more frequent and more intense El Niņos (and the counter-cycle La Niņas)" also lacks merit.  As Richard Grove of the Australian National University's Institute of Advanced Studies reported in the 28 May 1998 issue of Nature, "several ENSO [El Niņo-Southern Oscillation] events that occurred before 1880 had effects at least as intense and wide-ranging as those associated with the current event."  In fact, he notes that the one of 1789-93 - when the atmospheric concentration of CO2 was nearly 90 ppm less than it is today and earth was still in the frigid grip of the Little Ice Age -- was "one of the most severe on record."

But what about Hogarth's other "extremes of weather including more floods"?  Even the report prepared by the group that Hogarth refers to as "a panel of 2,500 top climatologists" has bluntly stated that "there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century," when, of course, we all know that it has warmed a bit.  Hence, it is abundantly clear that global warming, whatever its cause, just does not produce the extreme weather events that are so cavalierly claimed by so many pundits.

So what else could be damaging earth's reefs?  Here Hogarth provides a valuable service; for he names several offending phenomena, many of which have particular application to Australia's Great Barrier Reef: (1) rising nutrient levels caused by runoff from agricultural activity on land, (2) outbreaks of the coral-devouring crown-of-thorns starfish, (3) the barbed hooks and scything nets used in fishing, (4) tourists and the developers who build resorts and marinas for them, (5) increased sediment levels, (6) the nets of prawn trawlers stirring up the growing load of sediments, (7) the 6-10 tons of "bycatch" for each ton of prawns netted that are caught and die, which dramatically changes the composition of reef life, (8) sea life depleted to the point of exhaustion by over-fishing, (9) huge catamarans and dive boats that take thousands of visitors to the Barrier Reef each day and dump their sewage in the sea on the way home, (10) the live reef-fish trade, (11) fishermen using dynamite and cyanide, (12) coral diseases, and (13) pollution.  Moreover, a news item in the 25 July 1997 issue of Science reports that a group of marine scientists convened to consider the prospects of the world's coral reefs concluded that such immediate reef threats were by far the most serious, and that "the gradual warming expected in coming years was the least of their worries."  These observations also pull the rug out from under Hogarth's "canary in a coal mine" analogy; for with so many real and pressing problems with which to contend, it is unlikely that earth's coral reefs would ever be able to serve as an early warning system for a phenomenon so esoteric (CO2-induced global warming) that its very existence is still in question.

Nevertheless, vigilance in any worthwhile endeavor is a virtue; and reef conservation is no exception.  As Hogarth truly notes, surprises can happen, and we should try to anticipate them.  However, his invocation of the possibility of a collapse of oceanic deep-water formation has been bandied about for many years; and as even he admits, it is still but a theory that cannot be supposed with any certainty to be soon transformed into reality by the current slow warming of the globe, much less, we would add, by the ongoing rise in the air's CO2 content, which cannot be positively linked to the ongoing rise in mean global air temperature.  As for the newer concept of increasing atmospheric CO2 concentrations reducing calcification rates in tropical waters, this idea is just now beginning to make its way into the peer-reviewed scientific literature and is at least a decade or more behind the still-uncertain concept of the near-future sudden collapse of oceanic deep water formation in terms of its critical scrutiny by the world's scientific community.

Nevertheless, as we would emphasize once again, vigilance is always warranted; and continued study of many reef-related issues is definitely appropriate and should, in fact, be encouraged; for there is much that remains unknown about these amazing ecosystems.  Nevertheless, it seems illogical that our current ignorance should stand as a roadblock to the cautious and thoughtful - as opposed to Hogarth's "bizarre and perverse" -- development of valuable natural resources that could serve many useful purposes and positively impact the world economy and the living standards of its citizens.  And in this regard, the idea that anthropogenic CO2 emissions will harm earth's coral reefs through a string of numerous unproven linkages is, in our minds, just too remote to use as justification for impeding progress on such fronts.  We definitely do not believe that "it will all come to naught, unless we get a firm grip on the global warming problem," as the Hocking Voice Global Warming Report quotes U.S. Interior Secretary Bruce Babbitt as saying.  The disposition of any number of more immediate problems will have decided the fate of earth's coral reefs long before that.

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

Gagan, M.K., Ayliffe, L.K., Hopley, D., Cali, J.A., Mortimer, G.E., Chappell, J., McCulloch, M.T. and Head, M.J.  1998.  Temperature and surface-ocean water balance of the Mid-Holocene tropical western Pacific.  Science 279: 1014-1018.

Grove, R.H.  1998.  Global impact of the 1789-93 El Niņo.  Nature 393: 318-319.

Pennisi, E.  1997.  Brighter prospects for the world's coral reefs?  Science 277: 491-493.