The fate of corals and the impending mass extinction

J.E.N. Veron, formerly chief scientist of the Australian Institute of Marine Science, has spent decades studying coral reefs around the world. And he thinks they are in dire trouble.

Brain coral (Diploria labyrinthiformis). Photo source: Wikipedia

Corals are extremely primitive animals, which form the sister group to jellyfish in the Class Cnidaria (pronounced nye-DAR-ee-ah). The Scleractinia, or stony corals, include both coral that are solitary and live in all marine environments and others that are colony builders and live mostly in clear, shallow tropical waters.

In colonies the reef-building coral polyps secret calcium carbonate which form exoskeletons for support and protection.  The structures that result from these secretions form the center of important marine ecosystems. The reefs provide protection from predators and currents, which allow animals of all sorts to live, breed and grow. It’s been estimated that reefs provide the home for 25% of all marine species with species from 32 of the 33 animal phyla on the planet. J.C. Sylvan, “How to Protect a Coral Reef: The Public Trust Doctrine and the Law of the Sea,” 7 Sustainable Develop Law & Pol’y 32 (2006).

We have a couple of times looked at the five mass extinctions that have taken place during the history of life on earth. What should be noticed about these events is that they were all associated with the extinction of corals that built reefs. Because the reefs themselves leave good evidence in the fossil record we can determine when there has been a “reef gap.” And following each of the five major extinction events there has been a significant reef gap showing that it took millions of years for reef systems to recover. (In some cases the coral that built reefs became entirely extinct; it took the appearance of other types of corals to begin building reefs again.)

The 5 mass extinction events and the coral reef gaps (the black rectangles) from Veron 2008 (cited in text). Click to enlarge image.

In “Mass extinctions and ocean acidification: biological constraints on geological dilemmas,” Coral Reefs 27:459-472 (2008) (abstract; paper behind paywall) Veron examined each of the five extinction events and the proposed causes for the destruction of reefs. The extensive time of reef gaps (at least 4 million years) eliminated most of those unrelated to the carbon cycle. In depth examination showed that in each case causes associated with events in the carbon cycle (acid rain, hydrogen sulphide, oxygen and anoxia, methane, carbon dioxide, changes in ocean chemistry and pH) were  most likely the operative cause, and especially the acidification of the ocean.

This of course bodes ill for us because we are substantially lowering the pH of the oceans by our unremitting, and unprecedented, increase in carbon dioxide in the atmosphere (see, for example this post and this one).  The destruction of the reef-based ecosystems will not only affect marine life but also human (and other life forms) that depend on it. In 2003 the World Wildlife Federation concluded that the net economic benefit from coral reefs amounted to $30 billion annually. Herman Cesar, Lauretta Burke and Lida Pet-Soede, “Economics of Worldwide Coral REef Degradation” (pdf file). Of course that pales in comparison to the amount that the GOP is planning to rebate to the wealthy, and it doesn’t involve their clients so that argument would have no persuasive power to our new rulers. They will also undoubtedly brush off all concern about the health of the planet, because that would be based on “unproven speculation,” as is all science to them.  But the prospect is quite disturbing. If a quarter of the marine species are at substantial risk of extinction (and in each of the previous mass extinctions a considerably large portion became extinct) it is hard to conceive how this would not have a significant domino effect on terrestrial life, particularly the terrestrial life we are most interested in–large vertebrates. The mechanics of the domino effect are not spelled out in the fossil record, but it is clear that in each of the cases where there were tetrapods, the large animals suffered perhaps even more severely.

Veron has once again written another warning. This time as part of Yale’s Environoment 360 series: “Is the End in Sight for the World’s Coral Reefs?” (December 6, 2010). The entire piece of course is worth detailed consideration. He explains the effect of ocean acidification:

Ocean physics dictates that we will observe the effects of acidification in colder and deeper waters before it spreads to shallower tropical climes. The early stages of acidification have now been detected in the Southern Ocean and, surprisingly perhaps, in tropical corals. On our current trajectory of increasing atmospheric CO2, we can expect that by 2030 to 2050 the acidification process will be affecting all the oceans of the world to some degree. At that point, the relatively cool, deep-water tropical regions that have offered refuges to corals from temperature stress will be those most affected by acidification.

No doubt different species of coral, coralline algae, plankton, and mollusks will show different tolerances, and their capacity to calcify will decline at different rates. But as acidification progresses, they will all suffer from some form of coralline osteoporosis. The result will be that corals will no longer be able to build reefs or maintain them against the forces of erosion. What were once thriving coral gardens that supported the greatest biodiversity of the marine realm will become red-black bacterial slime, and they will stay that way.

But what is particularly important to note is that we have already “committed” to substantial increase in CO2 concentration in the atmosphere, which in turn will reduce the pH of the oceans. This commitment is the amount that will take place even if we completely ended greenhouse emissions:

The atmospheric levels of CO2 we are already committed to reach, no matter what mitigation is now implemented, have no equal over the entire longevity of the Great Barrier Reef, perhaps 25 million years. And most significantly, the rate of CO2 increase we are now experiencing has no precedent in all known geological history.

NOAA Photo Library

We are on the verge of monumental effects not seen in the history of life on earth. And yet the conference in Cancun this week promises a more “modest” set of agreements compared to the expectation of Copenhagen last year (which itself ended in farce). This year the President has stayed home to deal with the more pressing concern: $700 billion in tax cuts to millionaires.

We are like Nero fiddling while Rome burns. And like Nero, we are the cause.

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    • katesisco
    • January 3rd, 2011

    Interesting. Only lately have we learned that sponges serve themselves (everting) as food for the reefs. Sponges can be seen as the foundation of reef life. But am not convinced CO2 is going to do us in. Seems like Sol has personal ax to grind in this dog fight. 26 million year extinction events no matter what–sounds like sun as nothing else fits. Silly to postulate non-existent planet to account for a sun cycle of which we know nothing. More likely something up close and personal, like a plasma phase change.

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