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Ocean acidification: Global warming's evil twin

Posted on 7 April 2010 by John Cook

While there's much focus on the impacts from warming temperatures, there's another more direct effect from the burning of fossil fuels and deforestation. More than 30% of the carbon dioxide emitted by humans is dissolved into the oceans, gradually turning ocean water more acidic. Coral reef researcher Ove Hoegh-Guldberg explains the threat of ocean acidification: "Evidence gathered by scientists around the world over the last few years suggests that ocean acidification could represent an equal – or perhaps even greater threat – to the biology of our planet than global warming". Thus a new paper Paleo-perspectives on ocean acidification (Pelejero et al 2010) labels ocean acidification the 'evil twin' of global warming.

As CO2 dissolves in the oceans, it leads to a drop in pH. This change in seawater chemistry affects marine organisms and ecosystems in several ways, especially organisms like corals and shellfish whose shells or skeletons are made from calcium carbonate. Today, the surface waters of the oceans have already acidified by an average of 0.1 pH units from pre-industrial levels and we're seeing signs of its impact even in the deep oceans.

The past gives us an insight into future effects of ocean acidification, as we continue to emit more CO2 and acidify the ocean even further. Ice cores give us accurate data on the evolution of CO2 in the atmosphere over the last 800,000 years. These reconstructions, together with data derived from foraminifera, find that the pH of ocean surface water was lower during interglacials (high levels of atmospheric CO2). Seawater pH was also higher during glacial periods  when atmospheric CO2 was low. Correspondingly, foraminifera seem to have grown thicker or thinner shells over glacial–interglacial timescales in time with changing CO2 levels.

Figure 1: Glacial–interglacial variability in surface water pH (filled blue symbols, note the reversed axis), superimposed on atmospheric CO2 concentration during the last 800,000 years (magenta curve) (Pelejero 2010).

Current atmospheric CO2 are at greater levels than seen over the last 800,000 years. Similarly, pH levels are already more extreme than those experienced by the oceans over this same period. By the end of the 21st century, the projected decline in seawater pH is expected to be three times larger than any change in pH observed as the Earth’s climate has oscillated between glacial and interglacial periods. The times when seawater pH changed fastest was during glacial terminations when the Earth came out of an ice age. The change in seawater pH over the 21st Century is projected to be around 100 times faster than this rate.

What will be the effect of seawater pH falling to such levels? Let's look further back at periods when pH fell to the levels projected for the end of the 21st Century. There have been several periods where pulses of CO2 have been injected into the atmosphere, from volcanic activity or melting of methane hydrates. One well known example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred around 55 million years ago. During this event, global temperatures increased by over 5°C over a time frame less than 10,000 years. This coincided with a massive release of carbon dioxide into the atmosphere, which led to ocean acidification. This change caused a series of biological responses, including the mass extinction of benthic foraminifera.

Looking further back, there are other examples of mass-extinctions coinciding with global warming and increases in atmospheric carbon dioxide.  Examination of the mass extinction that occured 251 million years ago during the end-Permian find that the patterns of mortality are consistent with the physiological effects of elevated CO2 concentrations (along with the effects of global warming). 205 million years ago at the Triassic–Jurassic boundary, a sudden rise in the levels of atmospheric CO2 coincided with a major suppression of carbonate sedimentation, very likely related to ocean acidification. A similar situation occurred 65 million years ago during the Cretaceous–Tertiary extinction event. Most of the planktonic calcifying species became rare or disappeared.

Future acidification depends on how much CO2 humans emit over the 21st century. By the year 2100, various projections indicate that the oceans will have acidified by a further 0.3 to 0.4 pH units, more than many organisms like corals can stand. This will create conditions not seen on Earth for at least 40 million years.

A highly recommended website is Climate Shifts, run by one of the coauthors of Pelejero 2010. with a strong emphasis on coral reefs. For more peer-reviewed research on ocean acidification, check out AGW Observer's Papers on Ocean Acidification.

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Comments 51 to 66 out of 66:

  1. To those wondering about historic pH levels, the WIKIPEDIA link provided by Jimbo leads to the following : The Royal Society Based upon current measurements of ocean pH, analysis of CO2 concentration in ice cores, our understanding of the rate of CO2 absorption and retention in the surface oceans, and knowledge of the CaCO3 buffer (Section 2.2.2), it is possible to calculate that the pH of the surface oceans was 0.1 units higher in pre-industrial times (Caldeira & Wickett 2003; Key et al 2004). Link here Which leads to : Caldeira & Wickett and Key et al More : Orr et al Orr et al Supplemental
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  2. Berényi Péter, it looks a bit weird to claim that the Galapos coral are "well and alive". Alive they are, not so well though. The appearance of new species and the discovering of a specie thought to be extinct say that they are recovering from a deep crisis. They suffered a 97% loss in 1982-83 and a further 99% losses in 1997-98; luckly they're managing to recover. So far so good. But for sure, stressing them more won't be of any help next time.
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  3. #19 doug_bostrom at 07:56 AM on 8 April, 2010 where we may read about naturally occurring pools of liquid C02 in the deep ocean? Here: Submarine venting of liquid carbon dioxide on a Mariana Arc volcano Lupton & al. G3 Volume 7, Number 8 10 August 2006 Q08007, doi:10.1029/2005GC001152 ISSN: 1525-2027
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  4. #52 Riccardo at 00:40 AM on 10 April, 2010 for sure, stressing them more won't be of any help next time You miss the point. These creatures are designed to survive El Nino events. When the problem is not too much dissolved CO2 in water, but lack of nutrients, including carbon dioxide. Solubility of CO2 drops with increasing temperature (due to El Nino). On the other hand, they are quite happy with upwelling oversaturated La Nina water, regardless of extremely low pH.
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  5. Berényi Péter, how come that "These creatures are designed to survive El Nino events" but they almost got completely extinct in the last two large El Nino events?
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  6. Doug Bostrom wrote: where we may read about naturally occurring pools of liquid C02 in the deep ocean? To which Berényi Péter replied: Here: Submarine venting of liquid carbon dioxide on a Mariana Arc volcano Lupton & al. [...] That's a really fascinating paper and I'm very glad you posted the link to it. Thank you. However, in the interest of accuracy, I'm compelled to point out that there's nothing in there about "pools of liquid CO2 in the deep ocean". They found droplets of liquid CO2 venting from a submarine hydrothermal field. They inferred that there was liquid CO2 beneath the seafloor, capped by a layer of clathrates. The authors note that liquid CO2 is less dense than water at this depth, so the droplets would rise buoyantly for a couple of hundred meters and then disperse. Were a "pool of liquid CO2" to somehow appear on the seafloor there, it would float upward and eventually dissolve. So, not really relevant to the claims about CO2 lakes, but fascinating nonetheless.
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  7. BP thanks for that! You are nothing if not a remarkably efficient ferret of fascinating papers. No pools of C02 sloshing about on the ocean floor but imagine that, C02 clathrates. I've seen some sequestration stuff related to manufacturing C02 clathrates but nothing about naturally occurring samples. What I'd like to know is what sort of creatures live in that environment, beneath the seafloor -in- the clathrates. I'd be surprised to learn it was devoid of life.
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  8. Doug, I certainly won't dismiss the possibility, but I'd be skeptical of life in a CO2 clathrate because there's no energy for life to extract. (CO2 and H2O being highly stable, it takes photosynthesis or some other energy source to use them.) Of course if there was also other material present, life could work with that.
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  9. BP, I'd like to take issue with your comment @50. Unfortunately I don't have time to do a good job of it, so I'll limit myself to two points. First, this form of argument -- "there's low pH at location or time X and species Y is doing okay there/then" -- is interesting but not convincing. Biochemically harsh environments like tide pools and thermal vents have species that do well in them, too, and there are shellfish in fresh water (pH below 7). One cannot conclude that critters in other environments will be unaffected by changes to pH that are within the range of "location X". For example, the shellfish doing well in fresh water (and low pH) should give us very little comfort that shellfish in the ocean are going to do fine as pH declines. Further, your suggestion that low pH is good because bleaching occurs during El Nino ignores interrelated factors that are important (eg. temperature!). The full suite of environmental parameters and ecological context (including species composition) matters. That brings me to my second point, and I repeat to some extent Riccardo's comment earlier @18. The reduction of pH is a problem because it increases the solubility of carbonate such that it forms bicarbonate which isn't very available to creatures to make their shells. My understanding is that the aragonite (a more highly soluble form of carbonate used in many invertebrate shells) saturation horizon will become shallower in the high latitudes before it changes much in the tropics (despite pH being generally lower in the tropics, in absolute terms). By 2100, the aragonite saturation horizon is projected to go from 120m (current) to 0m in the high North Pacific or Bering Sea, from 730m (current) to 0m in parts of the Southern Ocean, and 2600m (current) to 115m in the North Atlantic. For these kinds of contextual reasons, Alaska Department of Fish and Game (for example) is worried about their production of pink salmon (who feed on pteropods, who make shells of aragonite). What happens in specific coral reef locations may depend greatly on the saturation states of carbonate there, and although pH influences these states, the relationship is not so simple that the other parameters can be ignored. In summary, I think you raise interesting questions, but I don't think you can assume that corals persisting in low pH conditions in "location X" mean that corals in "location Y" will be fine when exposed to similar pH levels. You may also want to check on whether or not your beloved Galapagos corals are expected to be exposed to further decreases in pH.
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  10. I'd recommend reading Biogeosciences (An Interactive Open Access Journal of the European Geosciences Union). It has a nice Public Peer-Review & Interactive Public Discussion process and the papers are published under the Creative Commons Attribution 3.0 License, both Discussion Papers (BGD) & Final Revised Papers (BG). In most cases there is also some Supplement. This is how all scientific publication should look like.
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  11. Berényi Péter (...) Galapagos coral reefs are still well and alive. With emphasis on 'still', I suppose. There was indeed large mortality of coral reefs at Galapagos during El Niño 1998. During large extinction events corals are the ones who suffers the most as in e.g. the Cretaceous–Tertiary extinction event. Just because corals did not become extinct as a class so far in no way precludes that we can not drive them to extinction.
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  12. I have seen a fair amount of discussion on the effect of atmospheric CO2 on ocean surface pH. Unfortunately there has been a confounding between deep ocean upwelling and acidification from atmospheric sources. The ocean pH decreases with depth, with a minimum of around pH 7.6 at a depth of about 800 meters. When this deep water upwells to the surface it mixes and reduces surface pH. Areas that are subject to upwelling are a function of the thermohaline circulation and wind conditions near coastal regions: Here is an example of a commonly referenced research of ocean surface pH drops attributed to atmospheric CO2 in a region subject to upwelling - Wootton (2008) It is vitally important for the cause of sound science to look at all causes for ocean pH changes and accurately represent their relative impacts. Otherwise, this "advocacy science" will cast doubt on the whole community.
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  13. #61 Jacob Bock Axelsen at 21:51 PM on 11 April, 2010 There was indeed large mortality of coral reefs at Galapagos during El Niño 1998 Yes. But as I've already mentioned, pH during an El Niño event gets higher. The problem is not too much CO2, just the opposite. In fact in 1998 NTCO2 (Salinity-Normalized Total Inorganic Carbon) in eastern parts of the Pacific got extremely low, which implies higher than normal pH.
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  14. BP @63 -- Didn't you have anything to say about my previous comment? You're at it again, saying: 1. El Niño is associated with higher pH, and 2. El Niño is bad for Galapagos corals, therefore 3. Higher pH is bad for Galapagos corals. This is a fallacious argument, because you're not accounting for impacts other than pH that are associated with El Niño (e.g., increased temperature associated with bleaching). Somewhat aside from this problem with logic, there is another specific issue here, within point 1: you say total inorganic carbon is low and "implies higher than normal pH". In my previous comment I tried to point out that the negative effects of low pH are exerted largely through making carbonate unavailable to shell-building creatures. Carbonate ion is inorganic, so low total inorganic carbon may imply carbonate undersaturation as much as it implies low pH.
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  15. guinganbresil @62: I was wondering about that too (@23), so thanks for the map. Of course, circulation patterns are affected by AGW and coastal areas often receive excess nutrient input. It may be hard to distinguish direct acidification from CO2 emissions from indirect via changes in ocean currents from other anthropogenic sources through decomposition. But we know how much CO2 is getting dumped into the atmosphere, and we know pretty well how much of this is absorbed by the ocean, and therefore some attribution of pH change to various causes should be possible.
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  16. I mentioned @59 that carbonate availability was the main negative impact of ocean acidification. I just came across this about kelp and this about promotion of bacteria at the EPOCA blog. I guess this is a rapidly growing field and we'll be learning a lot about other impacts in the future. PS. I remember something called ATOC that was going to measure global warming in the oceans and I also remember reading somewhere that ocean acidification was going to make the ocean louder (with consequent effects on cetaceans). Ah, the EPOCA site again. Anybody else 'hear' of other potential impacts?
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  17. Berényi Péter ... pH during an El Niño event gets higher. The problem is not too much CO2, just the opposite. In fact in 1998 NTCO2 (Salinity-Normalized Total Inorganic Carbon) in eastern parts of the Pacific got extremely low, which implies higher than normal pH. Nice graph. However, coral bleaching is when the corals are vacated of algae, the socalled Zooxanthellae, due to the fact that either photosynthetic pigment is lost or cellular adhesion is disrupted altogether mostly due to Heat Shock. pH drops only exacerbates this or may act alone. In your example, pH elevations under rising temperatures apparently do not alleviate this - which seems rather logical.
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  18. John, have you considered posting this as a reply to the argument "Ocean acidification isn't going to happen"?
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    Response: I was planning to do a synthesis of a number of papers into a single post but rather than let perfect be the enemy of good, for now I've used this post to add the 107th skeptic argument "ocean acidification isn't going to happen". Thanks for the suggestion.
  19. For BP, a paper on Eastern Tropical Pacific coral reefs and how upwelling of low pH, low carbonate water makes them more vulnerable to erosion. As I tried to say earlier, your oversimplification leads you astray (but the details are a bit different and more interesting than I had guessed).
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  20. I don't disagree with any of the statements in this paper but I do take issue with the graph. I've just read some very detailed arguemtnts refuting the inaccuracies in Ian Plimer's book. One of the major concerns wah his improper use of graphs. I think it behooves us to also take care with the graphs presented. If you look at the graph presented herein, supposedly showing acidification of the ocean and increasing CO2 (present level is I believe 380 ppm) this graph does no such thing. The graph should reflect the text and vice versa. A small point perhaps but but we should be consistent.
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    Response: The purpose of Figure 1 is to show that in the past, when CO2 changed, pH levels changed accordingly. I think that's fairly clearly explained in the preceding paragraph.

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