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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 repeatedly went extinct as a result of ocean acidification.

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 conditions such as ocean acidification. 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 been ravaged, 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 buffering systems cannot keep up with abrupt increases in CO2, and the surface ocean becomes corrosive to the coral skeleton. Gradual changes in atmospheric CO2 do not invoke a similar response, the 'ocean carbonate system' is able to keep pace, preventing the oceans from becoming corrosive.   

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 warn that the rapid warming which accompanied these events is also just as likely for the mass die-off (see figure 1).

Figure 1 -

Vulnerability of modern-day coral to ocean acidification

Present-day coral growth depends heavily on the concentration of carbonate ions in the surrounding seawater, because these are one of the crucial building-blocks in the coral skeleton. Although the coral skeleton is built of calcium carbonate (chalk), the concentration of calcium in the ocean is quite uniform today, and only changes on extremely long timescales, as rainwater leaches it from rocks, before it is washed by rivers into the oceans (more on this later). Therefore this calcium 'building block' is not a limiting factor in the modern ocean.

The concentration of carbonate ions, on the other hand, is part of the ocean carbonate system and as such, it declines as more CO2 is added to the atmosphere and ocean pH falls. See figure 2.

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. 

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 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 (link from calcification rebuttal).

Weathering & the Geological Carbon Cycle 

The speed or rate of change in atmospheric CO2 So how does a steady state climate differ, and why doesn't it seem to affect coral? This has to do with the long-term, or geological carbon cycle, in which slowly changing ancient climates counter-balanced the rise in atmospheric CO2 by dissolving more carbonates into the oceans, and therefore replenished the supply of carbonate 'building blocks'. This process is known as silicate & carbonate weathering, and is part of a negative feedback system which operates on timeframes of at least tens to hundreds of thousands of years.

Simply described, carbon dioxide forms a mild acid (carbonic acid) when dissolved in rainwater. This reacts with carbonate and silicate rocks to form various salts, including bicarbonate and carbonate, which get washed into rivers and end up in the sea. So these reactions not only raise carbonate levels in the ocean, but also lower atmospheric CO2, which is consumed in the chemical reactions. The beauty of the cycle is that increasing levels of CO2 raise global temperature, and this accelerates the rate of weathering, because the rate of chemical reactions speeds up with increasing temperature. The cycle is later completed by CO2 being released by calcium carbonate (chalk) formation, and outgassing from the ocean to the atmosphere.  

As I described earlier, this 'negative feedback' weathering process, is a tortuously slow one. It works fine over 100,000-year-plus timeframes to supply carbonates back to the ocean, raising seawater alkalinity and limiting the build-up of atmospheric CO2 from periods of moderate volcanic activity, but cannot keep pace with (geologically) rapid change. Two examples of this from Earth's history are the massive volcanic outgassing of CO2 that accompanied the formation of the Siberian Traps in modern-day Russia during the Permian-Triassic Extinction (otherwise known as the Great Dying) around 250 million yearsa ago, and the rapid warming and ocean acidification of the Paleocene-Eocene Thermal Maximum (around 56 million years ago).    

Today we are observing this acidification accumulate in the surface ocean, because the geological carbon cycle (silicate/carbonate weathering) is unable to operate fast enough to cycle carbonate ions back to the ocean, and because the ocean circulation is too sluggish to mix the dissolved man-made CO2 into deeper layers of the ocean.

Pic of CO2 accumulating in surface ocean (NOAA?)

One of these things is not like the other 

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

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.


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)       

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 2011-12-25 by Rob Painting.

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