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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Archived Rebuttal

This is the archived Advanced rebuttal to the climate myth "Corals survived during past periods of high CO2". Click here to view the latest rebuttal.

What the science says...

Only when atmospheric carbon dioxide rises sharply does seawater become 'corrosive' to coral. Ancient reef coral were either almost completely annihilated, or repeatedly went extinct, as a result of multiple ocean acidification events.

Although coral in some shape or form have existed for over 400 million years, their existence has not been a continuous one through time. The paleo record shows a number of 'reef gaps' where there is no trace of coral at all. These reef gaps exist because ancient coral were repeatedly devastated by changing global conditions such as ocean warming and ocean acidification, which were the result of large carbon dioxide pulses from intense volcanic activity and, at other times, the rapid release of methane hydrates from seafloor sediments.

Some of these acidification episodes resulted in the complete extinction of coral, and it took many millions of years for new, and genetically unrelated, coral reefs to evolve. At other times (reef crises) coral reefs have suffered massive die-back, but have managed to hold out in isolated refuges. Populations only bouncing back once ocean conditions were again hospitable.

The important distinction to note is that it is only when the concentration of atmospheric carbon dioxide rises very quickly, like modern-day conditions, that ocean acidification (a rapid increase in acidity) occurs. This is because Earth's natural 'ocean carbonate compensation system' cannot supply alkalinity (an ability to neutralize acidity) back to the ocean quickly enough, and the surface ocean becomes corrosive to the coral skeleton.

This largely explains why ancient coral can exist during intervals of sustained high carbon dioxide (such as the Paleocene), but are devastated when atmospheric CO2 rises in a geologically-abrupt manner (the Paleocene-Eocene Thermal Maximum).

On the subject of past coral extinction, Kiessling & Simpson (2011) examine the link between ocean acidification and ancient coral reef extinctions as suggested by Veron (2008), and find that there is strong evidence for ocean acidification being implicated in the demise of coral reefs in 4 extinction events, but also caution that ocean warming, which accompanied these episodes, is also just as likely for the mass die-off (see figure 1).

Figure 1 -

Vulnerability of modern-day coral to ocean acidification

The coral skeleton is made from two crucial building-blocks, calcium and carbonate (chalk), which are drawn from the surrounding seawater. Therefore the concentration of these two building-blocks in the ocean (the aragonite saturation state) has a direct influence on the rate of growth, and maintenance of the coral skeleton.

The concentration of calcium ions (electrically charged particles) in the ocean is rather uniform today, and only changes on extremely long time scales. This slow pace of change is dictated by the weathering process, where rainwater wears calcium ions away from rocks, and eventually washes them into the oceans (more on this later). Therefore the concentration of this calcium 'building block' is effectively constant in the modern ocean.

The concentration of carbonate ions, on the other hand, is determined by the ocean carbonate system and, as such, it declines as more CO2 is added to the atmosphere and ocean pH falls. See figure 2. This presents a problem for coral as carbonate ions are the other crucial building-block in the coral skeleton, and a decline in their concentration makes it ever more energetically expensive for coral to maintain their skeleton.

Figure 2 - A speciation diagram for the carbonic acid system in seawater as a function of pH. The y-axis gives the fraction of each species present. A vertical line drawn at any pH value gives the relative proportion of each species. This plot is simplified to illustrate the concept; in real seawater several other factors like salinity, temperature and pressure are important. Carbonate = CO₃^2- (blue-green line)

Numerous experiments over the last decade have repeatedly shown that coral are sensitive to ocean acidification, with growth rates declining as the concentration of carbonate ions (the aragonite saturation state) falls (link - Keyplas?). In severely aragonite undersaturated (corrosive) seawater the coral skeleton begins to dissolve. Outside of the lab, this decline in growth rates is now being observed in coral reefs around the world (De'ath [2009], Bak [2009], Tanzil [2009] and Bates [2010]), but this isn't only due to ocean acidification.

Why ancient seawater wasn't necessarily corrosive with high atmospheric CO2

That ancient coral existed and did just fine during prolonged periods of high CO2 might seem paradoxical, but this is simply down to the speed at which alkalinity (an ability to neutralise acidity) is supplied back to the ocean. When the increase takes place gradually on timescales of a hundred thousand years or more, the Geological Carbon Cycle is fully capable of countering acidity, and prevent the oceans from becoming corrosive to coral.

The carbonate and silicate weathering processes are the key mechanisms which supply alkalinity back to the ocean, and also prevent atmospheric CO2 from inexorably drifting up or down over long timescales. Weathering simply means the slow wearing away of carbonate and silicate rocks by weather. Carbon dioxide in the atmosphere forms a mild acid (carbonic acid) when dissolved in rainwater, and reacts with carbonate and silicate rocks to flush calcium, bicarbonate and carbonate, into rivers which eventually discharge into the sea. (See SkS post: Always take the weathering for more detail on the carbonate cycle).

Weathering pic?

So the weathering process absorbs CO2 from the atmosphere, recycling it on very long timescales, but it also supplies alkalinity back to the ocean. The whole system works to maintain more-or-less equitable global temperatures because as more CO2 accumulates in the atmosphere it raises global temperature and increases rainfall, and this works to draw down more CO2 from the atmosphere. The only catch being that despite the increased weathering rate we are still looking at very long intervals of time.

And what about the mixing rate, mate?

An additional factor to consider is the mixing rate of the ocean, the ocean currents and circulations which distribute seawater-entrained minerals and gases to the very deep ocean. The rate of transport to the deep ocean is in the order of up to a thousand years.

Once again, gradual change allows mixing of CO2, and all its attendant changes in ocean chemistry, to the very deep ocean, and mixing into a far larger volume dilutes pH changes in the surface ocean. But rapid increases in CO2 overload the 'transport system' and CO2 accumulates in the surface ocean, which has the effect of intensifying acidification. This 'pooling' of CO2 in the modern-day surface ocean is shown in figure (?) below.

NOAA PMEL pic?

Calcite Compensation Depth

Many marine life incorporate calcium carbonate into their shells or skeletons, and phytoplantkon (microscopic surface-dwelling marine plants) make up a substantial proportion of this. When calcified marine life die and fall to the seafloor, they export a great deal of calcium carbonate out of surface waters. The bodies build up into deep sediments over time

As a function of both lower temperature and increased pressure with depth, seawater becomes more acidified the deeper one goes into the ocean. Travel downwards into the oceans and one finds that pH drops and so does the corrosiveness of seawater. Eventually seawater that is slightly corrosive the lysocline, a threshold below which calcium carbonate forms dissolve. This is otherwise known as the calcite compensation depth (CCD), and is another way in which the 'ocean carbonate compensation system' seeks to maintain a balance.

When there is a uptick in atmospheric CO2, the surface ocean quickly absorbs the excess CO2 reducing the saturation state of calcium carbonate. This means that less calcium carbonate precipitates (a solid forming in solution) out of seawater in tropical areas where it is warm and seawater is strongly supersaturated. This reduces CO2 outgassing from the ocean, because the formation of calcium carbonate is a source of CO2 to the ocean.

Although the size of the system feedback depends on the scale and speed of the pulse of CO2, the outgassing reduction is typically smaller in size than the CO2 pulse, and isn't enough to immediately restore ocean pH. The upper ocean pH (acidity) therefore falls, and the calcite compensation depth shoals (rises toward the surface). The rising CCD then dissolves calcium carbonates sediments on the seafloor that were previously above the saturation horizon, and these calcium and carbonate ions (charged particles) mix into the upper ocean, thereby restoring alkalinity to the ocean.

Pic?

An ocean affinity for alkalinity

The weathering process, the mixing rate and the calcite compensation depth are slow processes that are able to accomodate likewise slow rises in atmospheric carbon dioxide, by supplying alkalinity back to the ocean, therefore keeping them supersaturated with respect to calcium carbonate (non-corrosive to coral). But because they are speed-limited they are overwhelmed by large rapid pulses of CO2 into the atmosphere. Nevertheless they continue to toil away in the background such that, over long intervals, low calcium carbonate saturation states (corrosive seawater) cannot be maintained. Eventually the system works to increase alkalinity and the oceans finally end up strongly supersaturated and therefore non-corrosive to coral and other shelled marine life.

Past vs present ocean chemistry -one of these things is not like the other

So far I just looked at the

Modern-day coral are incredibly accomplished reef-builders, and this is readily apparent in coral that have recovered from mass bleaching events. If conditions are right (which is unfortunately not common enough), they are able to rebuild their skeletons so quickly they can almost return to pre-bleaching health and numbers in about a decade.

But this modern day proficiency is misleading, coral weren't always the mega reef-builders we observe today, for prolonged periods they were only bit players, or rarely contributed to reef-building at all. An example of this is the Cretaceous period (145-65 million years ago), the time of the dinosaur. During the Cretaceous the equatorial regions were much warmer than today and the oceans very salty. The dominant reef-builders of this time were rudist bivalves, ginormous ancient counterparts to modern-day mussels and clams, although shaped and sized very differently. See figure 3 below.

Figure 3 - fossilized remains of Cretaceous rudist reef. Image from Ferrebeekeeper.wordpress.com

Although the outer shell of the rudists were made of calcite, which is less easily dissolved than the aragonite skeleton of modern-day coral, this wasn't enough to save them from extinction either. The rudists began to die out near the end of the Cretaceous, and the asteroid that hit the Yucatan Peninsula in Mexico, may have put the finishing touches on them, as well as the dinosaur.

A reconstruction which demonstrates the contribution of coral, and other reef-builders over time is provided in figure 4 below.

(pic)

The lesson here is that only have coral reefs not always been the dominant reef-builder, or reef-builder at all, but that the survival of all reef-builders that have ever existed, from microbes to overgrown clams, have ebbed and flowed throughout time. None have proven immune to rapid global change, not one.

Show us what you're made of - Aragonite vs Calcite skeletons

A Cretaceous Scleractinian Coral with a Calcitic Skeleton - Stolarski (2007)

Scleractinian Coral Species Survive and Recover from Decalcification - Fine (2007) - Lab experiments show that some coral at least, can survive the complete dissolution of their protective skeleton by highly acidified seawater, with the surviving coral resembling anemone (link to picture). But in a real world situation this is little reason for cheer, the coral skeleton exists for a reason - to protect the coral from predators who might choose to eat it. Without protection the coral becomes easy pickings. But perhaps the more relevant concern is that once the coral reef structure is no longer able to keep ahead of the forces of bioerosion (physical and biological factors such as wave action and coral grazing fish) then the reef will begin to break down and eventually be reduced to rubble. This is likely to happen long before seawater is acidic enough to completely dissolve coral skeletons (link to Ove's study)

Paradox lost

Summary with bullet points

One-Third of Reef-Building Corals Face Elevated Extinction Risk from Climate Change and Local Impacts - Carpenter (2008)

Extinction and recovery patterns of scleractinian corals at the Cretaceous-Tertiary boundary - Kiessling (2004)

Coral Reefs Threats and Conservation in an Era of Global Change - Riegl (2008)

Skeletons and ocean chemistry: the long view - Knoll & Fischer (2011)

The evolution of modern corals and their early history - Stanley Jr (2003)

A negative feedback mechanism for the long-term stabilzation of earth's surface temperature - Walker 1981

Calcite and aragonite seas and the de novo acquisition of carbonate skeletons - Porter (2010) - species unable to switch from aragonite to calcite-based skeleton building.

Repeated loss of coloniality and symbiosis in scleractinian corals - Barbeitos (2010)

Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites (higher ca+ in the Cretaceous) - Horita (2002)

Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms (ca+ in the Phanerozoic) - (2011)

The carbon perturbation is estimated to have been of the order of 0.3 to 0.5 Gt C yr_,(Kump et al. 2009) during the end Permian mass extinction.

Distal “Impact” Layers and Global Acidification of Ocean Water at the Cretaceous–Paleogene Boundary (KPB) 1 - Premovic (2011) - yup, that was probably OA-related too.

Schneiber & Speijer [2008]

Coastal Ocean Last Glacial Maximum to 2100 CO2-Carbonic Acid-Carbonate System: A Modeling Approach - Lerman (2011)

"This decadal rate of decline of the carbonate ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene. Hence, when viewed against the millennial to several millennial timescale of geologic change in the coastal ocean marine carbon system, one can easily appreciate why ocean acidification is the “other CO2 problem"

Updated on 2011-12-31 by Rob Painting.

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