Skepticism about corals and rising CO2

The recent spate of "skeptical" climate change reporting and posting - which I personally feel is largely weather and opportunity-based - have included some serious misinterpretations and misrepresentations of coral reef science. For example, Climate Shifts tells the story of an amusing report from the "Center for the Study of Carbon Dioxide and Climate Change".

The latest screed comes from Watts up with that goes after the threat of rising CO2:

This does indeed sound alarming, until you consider that corals became common in the oceans during the Ordovician Era - nearly 500 million years ago - when atmospheric CO2 levels were about 10X greater than they are today.

What follows is a graph of temperature and CO2 over the past several hundred million years which I'll let the geologist argue over.

Now like the author of that post, I'm not one of the world's experts on reef development. So, instead, let me defer to one. This is a quote from the beginning of Charlie Veron's Corals of the World (page 33-34), the three volume tome found on the shelf of pretty much every reef scientist out there:

By the Middle Ordovician, complex algae and invertebrate reef communities had become widespread and reef biota had diversified... Reef development reached a peak in the Devonian Period and even after all this time, what remains today of those reefs are sometimes of awesome size... Corals were seldom the dominant organisms of Devonian reefs although rugose corals are often abundant in them and have a wide variety of growth-forms. Tabulate corals, which were a less varied group, mostly occupied protected or inter-reef environments.

There were reefs during those earlier era. Read on:

Unlike the Scleractinia (ed - the order of stony corals found today), both these groups of corals made excellent fossils becauase their skeletons were made of calcite, a far more stable form of calcium carbonate that the aragonite skeltons of Scleractinia.

Yes, corals evolved over time. Today's coral species date back in the tens not the many hundreds of millions of years. The type of calcium carbonate (aragonite) today's corals secrete is more sensitive to changes in the CO2 or carbonate concentration, no doubt in part because those corals have persisted through a period in which CO2 was lower than today.

Any reef experts wish to comment?

Update: A reader reminded me of this recent article by Veron in the journal Coral Reefs. The author hypothesizes that high CO2 /acidification may have caused some of the major marine extinction events in the geological past. It also addresses the very question of whether corals could have persisted in a high CO2 world in the past:

Two possibilities present themselves: (1) Reefs may not have proliferated at all
during CO2 highs; they may just appear to have survived because they were able to resume growth when levels fell. (2) The high apparent CO2 levels of ancient times may be
an artefact of a lack of data and measuring method.

Veron also again addresses the fact that today's corals are different:

Reef proliferation in the distant past during periods when atmospheric CO2 may have been high could mean that the reef builders and consolidators were better adapted to these conditions and could exploit the enhanced calcification and photosynthesis promoted by warmer sea surface temperatures without adverse effects. Many organisms in the ancient oceans would have been more tolerant to acidification through their dependence on calcitic skeletons rather than aragonite or high-magnesium calcite. There may have been other aspects of coral biology that allowed ancient corals to tolerate water chemistries that are lethal to today’s Scleractinia. If so, it would be more than interesting to know what those physiological mechanisms were.