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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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Ocean acidification: global warming's evil twin

What the science says...

Select a level... Basic Intermediate

Ocean acidification threatens entire marine food chains.

Climate Myth...

Ocean acidification isn't serious

'Our harmless emissions of trifling quantities of carbon dioxide cannot possibly acidify the oceans. Paper after paper after learned paper in the peer-reviewed literature makes that quite plain. Idso cites some 150 scientific sources, nearly all of them providing hard evidence, by measurement and experiment, that there is no basis for imagining that we can acidify the oceans to any extent large enough to be measured even by the most sensitive instruments.' (Christopher Monckton)


Have you heard of ocean acidification? Does it mean that if you go swimming in the sea, you are liable to dissolve? No. You'll be OK because you are not a calcifying organism, such as a mollusc, a coral or a sea-urchin.

So why is ocean acidification serious? Because it can potentially lead to massive collapse of marine food-chains. Let's take a look at what the term means.

The pH scale, which measures acidity and alkalinity of water-based chemical solutions, runs from 0 (highly acidic) to 14 (highly alkaline), with pH 7 being the neutral halfway point. Importantly, the scale is logarithmic, meaning that a jump of one point towards zero means a tenfold increase in acidity.

Acidification simply means lowering the pH value from any point on the pH scale towards zero. It's similar to the way we talk about temperatures. If the pH of a solution shifts from 9 to 8, that is acidification, even though the pH is still on the alkaline side of neutral. Likewise, if the temperature rises from -40°C to -15°C, it has noticeably warmed, even though it's still darned cold.

Now, typical seawater is slightly alkaline at around pH 8.1. Rainwater, which always contains dissolved carbon dioxide (the old name for which was 'carbonic acid gas'), has a more acidic pH of around 5.6. You have likely visited or watched footage of spectacular caves, have you not? All carved out by carbonic acid, dissolving solid limestone over many thousands of years.

Carbonic acid is not only present dissolved in raindrops. It also forms by the dissolving of carbon dioxide at the air-water interface of our oceans. The more carbon dioxide in the air, the more goes into the oceans, driving their pH from 8.1 downwards. Now, the huge problem this creates, well before we get anywhere near the neutral value, is as follows.

Many marine organisms build and maintain their protective shells or skeletons from 'biogenic' calcium carbonate. The word biogenic means made by living things. These creatures extract the calcium and carbonate ions dissolved in seawater and combine them together. Under normal conditions, such calcium carbonate is stable in shallow waters. That's because dissolved carbonate ions are present in such high concentrations that the waters are said to be saturated with them.

But if seawater pH falls, even by a small amount, the concentration of dissolved carbonate ions falls. When that happens, biogenic calcium carbonate becomes more soluble and can start to dissolve. Depletion in dissolved carbonate ions thus makes it harder for such organisms to maintain their protective or skeletal structures. In the worst case scenario, the rate of calcium carbonate dissolution is faster than its formation. When that happens, mass-mortality of calcifying organisms can occur.

We're talking about critters that underpin entire marine food-chains here. Things from near-microscopic calcifying plankton to shellfish, lobsters and crabs the seafood we eat in other words. That's why ocean acidification is deadly serious.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Not all of the CO2 emitted by human industrial activities remains in the atmosphere. Between 25% and 50% of these emissions over the industrial period have been absorbed by the world’s oceans, preventing atmospheric CO2 buildup from being much, much worse. But this atmospheric benefit comes at a cost.

As ocean waters absorb CO2 they become more acidic. This does not mean the oceans will become like the acids one encounters in a chemistry lab. However, marine life can be highly sensitive to slight changes in pH levels and any drop in pH is an increase in acidity, even in an alkaline environment. Worse, the pH scale is logarithmic, meaning that for each single-digit decline in pH, acidity (defined as hydrogen ion activity) rises tenfold.

Surface seawater pH has been relatively stable over recent geological time, fluctuating between cold glacial periods (pH 8.3) and warmer interglacials (pH 8.2). But since the Industrial Revolution, average seawater pH has dropped towards a recent figure of less than 8.06, an approximately 30% increase in acidity (fig. 1). This is a faster change than any over the past 50 million years (Rhein et al, 2013, available from IPCC here).

Decline in ocean pH

Fig. 1: Decline in ocean pH measured at the Aloha station (in the Pacific Ocean off Hawaii) and yearly mean surface seawater pH reported on a global scale Source: European Environment Agency (Copernicus Marine Service).

Because of its inextricable link with CO2 emissions, this rate of acidification is projected to accelerate even further through the 21st century under a business-as-usual scenario with potentially catastrophic impacts to marine ecosystems (Bindoff et al. 2019 (PDF from IPCC)). These trends are becoming clearer globally.

According to the IPCC's Sixth Assessment Report (AR6), there is, " a very likely rate of decrease in pH in the ocean surface layer of 0.016 to 0.020 per decade in the subtropics and 0.002 to 0.026 per decade in subpolar and polar zones since the 1980s. Ocean acidification has spread deeper in the ocean, surpassing 2000 m depth in the northern North Atlantic and in the Southern Ocean (fig. 2)."

 Spread of ocean acidification from the surface into the depths

Fig. 2: Spread of ocean acidification from the surface into the depths since pre-industrial times. (a) Map showing the three transects used to create the cross sections shown in (b), showing the vertical sections of the changes in pH between 1800–2002 due to anthropogenic CO2 emissions; the darker the colours the greater the change. Contour lines are their contemporary values in 2002. Graphic sourced from IPCC AR6. (Lauvset et al. 2020).

Such changes in ocean chemistry, if allowed to occur, will be irreversible for many thousands of years. The biological consequences could last much longer.

How do we know that? Through the geological record. When mass-extinctions have occurred, most of them are tied-into unimaginably severe episodes of volcanism, at a scale never witnessed by humans. But the carbon footprint of such cataclysms has in fact been similar to our own. And what do we see as a consequence of such events? The fossil record shrinks in terms of its biodiversity and there are what we call 'reef-gaps', periods of several million years during which coral reefs - large highly diverse colonies of corals and myriad other species - were to all intents and purposes absent.

The reason why reef-gaps occur at such times is because as surface waters become more acidic, it becomes more difficult for corals, shellfish and other calcifying organisms to form and maintain the hard calcium carbonate skeletons or shells necessary for their survival. When things start getting really bad, that calcium carbonate dissolves away as fast as it can be deposited - that means curtains for such critters.

 Life-forms at deadly risk from the acidification of near-surface ocean waters.

Fig. 3: just some of the life-forms at deadly risk from the acidification of near-surface ocean waters.

Coral reefs provide a home for more than 25% of all oceanic species, so you can see why this matters so much. Some calcifying organisms, such as the tiny pteropods (fig. 3), underpin many oceanic food chains: take them out of the system and down those food-chains come crashing. Many communities around the world, constituting millions of people, are at the apex of such food-chains, relying on seafood as part of a healthy diet. You should now be able to see the problem. Like a thief in the night, ocean acidification is creeping up on us, while we sleep on in blissful unawareness.

Last updated on 25 June 2023 by John Mason. View Archives

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Fact brief

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Question: Is the ocean acidifying?

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Question: Is ocean acidification from human activities enough to impact marine ecosystems?

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Comments 26 to 50 out of 100:

  1. Most references I see on coral bleaching list increased temperature as the main stress likely to cause it. Reading the wiki on Coral bleaching seems to offer a contradiction Not really. Coral reef bleaching can be induced by a number of conditions- as listed in the Wiki page. The bleaching events I have previously linked to are truly massive in scale and correspond with anomalously warm, and sustained, sea surface temperatures. Hence the ability to predict these large bleaching events in advance. If you pore over the peer-reviewed literature you will see a lot of debate about the cause in earlier years, however as global warming, and warming ocean temperatures have continued, the evidence identifying warming SST's has strengthened. Try reading the first link I provided at @ 19. The lead author, Eakin, has published a lot of work on corals, and the study gives a good overview.
  2. That seems to say that CO2 is required for photosynthesis. Yes, same for "sea plants" as it is for land based ones. The carbon in the CO2 molecule provides the basis for carbohydrates which are synthesized using energy from the sun.
  3. #24: "Coral bleaching is a vivid sign of corals responding to stress," Let's walk down your stress list point by point: a. increased (most commonly), or reduced water temperatures -- emphasis added b. increased solar irradiance (photosynthetically active radiation and ultraviolet band light) -- not happening, unless you live under an ozone hole c. changes in water chemistry (in particular acidification) -- yes, that happens when CO2 concentration increases and is the point of this thread c. starvation caused by a decline in zooplankton -- do they die due to lowered pH? d. increased sedimentation (due to silt runoff) -- local effects only e. pathogen infections -- chicken and egg here, increased stress leads to greater susceptibility to infection f. changes in salinity -- most likely local (we'd notice it globally) g. wind -- random, at best h. low tide air exposure -- sea level is rising i. cyanide fishing -- whatever that means So the winner is ... most commonly temperature, acidification "CO2 appears to be essential for coral photosynthesis?" Corals are Cnidaria ... (animals). We don't do photosynthesis.
  4. Muoncounter: you might want to read replies from Rob Painting. Coral is a symbiotic organism, basically an algae inside the animal. The algae provides energy for the coral from photosynthesis. Chris Shaker
  5. I remain curious as to why coral are so temperature sensitive, considering that they have evolved during millions of years of the glacial cycle? As of 1950, we were still 4.5C short of the temperature high achieved during the previous warming phase. The glacial cycle causes large climate changes. Why have coral not evolved to be more flexible about temperature? Chris Shaker
  6. Muoncounter - We don't do photosynthesis. The corals in shallow water reefs are a symbiosis between the coral polyp and photosynthesizing algae. The algae provide nutrients to the polyp via photosynthesis and in exchange get shelter and protection. Typically too warm waters (above the photo-symbiont's tolerance) leads to a shut down in photosynthesis and because they are no longer getting food, the polyps expel the algae, hence the "bleaching" of colours which the algae produce. So although the polyp may not photosynthesize, it is certainly dependent on it for survival.
  7. I remain curious as to why coral are so temperature sensitive Chris, were the Earth (and sea temperatures) not warming so rapidly, it's likely that coral would be able to adapt, by acquiring less temperature sensitive photo-symbionts. Like other changes in the Earth system, it's the speed of change that is the problem.
  8. Rob I've been reading about dramatic temperature changes in the recent past, documented from temperature proxy data retrieved from the ice cores. "Following this abrupt shift, as much as 20 degrees Fahrenheit (10 degrees Celsius) of warming occurred over the subsequent decades—a change that ultimately resulted in at least 33 feet (10 meters) of sea-level rise as the ice melted on Greenland." "The ice core showed the Northern Hemisphere briefly emerged from the last ice age some 14,700 years ago with a 22-degree-Fahrenheit spike in just 50 years, then plunged back into icy conditions before abruptly warming again about 11,700 years ago. Startlingly, the Greenland ice core evidence showed that a massive "reorganization" of atmospheric circulation in the Northern Hemisphere coincided with each temperature spurt, with each reorganization taking just one or two years, said the study authors." These seem like pretty massive and rapid climate changes to me. Chris Shaker
    Response: This discussion has drifted off the topic of this thread. Everybody please start commenting on the appropriate threads.
  9. I responds to this over on #148 of "we're heading into an ice age".
  10. scaddenp: I put the sources detailing recent dramatic temperature shifts here, because we're talking about the inability of coral to deal with dramatic temperature shifts. It seemed surprising to me. Chris Shaker
  11. These seem like pretty massive and rapid climate changes to me. Indeed, they would affect any coral reefs growing around Greenland. I'm not aware there was, but if there was. Moderator: There isn't a coral rebuttal or post (yet). I didn't realize all those coral "arguments" existed.
  12. Chris Shaker, The changes in tropical temperatures during the glacial/inter-glacials was much smaller than the global average, "probably" 1-2 degrees change. Most of the large temperature swings occurred at high latitudes and especially the Northern Hemisphere where continental ice sheets formed. This small change is temperature in the tropics is why the tropical rainforests were not greatly affected by the ice ages. Precessional changes (wobbles in the Earth's rotation) which affect the amount of solar radiation at the equator are thought to have had a greater significance. Localized coral bleaching is not a new phenomenon, however mass coral bleaching is, certainly within the last several hundred thousand years.
  13. Rob: Do we have good temperature proxy data from the tropics? Sea floor cores, or such? Do the glacial cycles show up there, too? Chris Shaker
  14. Follow up from here. Excellent link posted by RSVP. Ocean CO2 concentrations on the increase, as of 2006 Odd, the largest changes in the Atlantic are in the northern hemisphere, the Indian is worse in the south, but the Pacific seems even.
    Response: I take back the 'excellent link' opinion. The website has an extensive page in denial of AGW.
  15. I'm not understanding this ocean acidifying-ocean CO2 feedback question. Oceans are indeed acidifying as they absorb atmospheric CO2, as the figure above shows. From Caldeira and Wickett 2005: The SRES pathways considered here produce global surface pH reductions of about 0.3 to 0.5 pH units by year 2100. ... Atmospheric emissions of 5000 Pg C and 20,000 Pg C produce global surface pH reductions of 0.8 and 1.4 pH units, respectively by year 2300. We depend on these CO2 sinks to take up to 50% of our fossil fuel waste product out of the atmosphere each year. There's evidence that ocean sinks are weakening, as in LeQuere et al 2007: Based on observed atmospheric carbon dioxide (CO2) concentration and an inverse method, we estimate that the Southern Ocean sink of CO2 has weakened between 1981 and 2004 by 0.08 petagrams of carbon per year per decade relative to the trend expected from the large increase in atmospheric CO2. We attribute this weakening to the observed increase in Southern Ocean winds resulting from human activities ... How is it possible for oceans to simultaneously absorb and emit CO2?
  16. RSVP, Coming in from the cool ice data thread. Three factors of ten is ten times ten times ten or one thousand times different. The pH affects the different ion distribution as Stephen Baines points out. The pH in fresh water with CO2 dissolved is around 5 and the pH in the ocean is around 8 or 1000 times less hydrogen ions. This affects the carbonate concentration and therefore the solution of CO2. N2 and O2 are not pH sensitive. That is part of why Henry's law seems to not work according to your calculations. As I said here and Alec said here, carbon dioxide is soluble in water and O2 and N2 are not. Because CO2 is soluble in water its properties are different from O2. The calculations are very difficult (I have not done them) and to get better than a qualatative answer you will have to read the peer reviewed literature. My understanding is that research continues to estimate how much CO2 the ocean can absorb before it is saturated. (Currently the ocean is absorbing CO2 as the concentration increases according to Henry's law. The warming of the ocean is not as important today as it was in the past) CO2 running out: stop speculating about processes you do not understand. We better pray that the CO2 in the ocean never outgasses enough to run out. There is an enormous amount of CO2 in the ocean and the surrounding land would start to dissolve as the CO2 outgasses. That would be Venus for sure.
  17. @ muoncounter. "I'm not understanding this ocean acidifying-ocean CO2 feedback question." As I understand it RSVP is claiming (incorrectly) that there can be no CO2 oceanic solubility feedback during northern latitude warming phases of Milenkovitch cycles because outgassing from warming oceans would have resulted in similar relative increases in CO2, N2 and O2. Summing up succinctly, RSVP fails to understand that the pool of exchangeable CO2 in the ocean is very large relative to that in the atmosphere (for a number of reasons), while the opposite is true for N2 and O2. As a consequence, outgassing from a warmer ocean affects atmospheric CO2 concentrations significantly, but it does not affect N2 and O2 concentrations in any measureable way. As far as I can tell, pH only comes into the discussion because of its effect on the relative abundance of carbonic acid, bicarbonate and carbonate ions (and protons) as temp changes the solubilty of CO2.
  18. #42: "outgassing from a warmer ocean affects atmospheric CO2 concentrations significantly" I'm not referring to RSVP's mathiness. Papers are practically screaming that the oceans are sucking up CO2 to what will become dangerous levels (at least to plankton) in not very many years. From McNeil and Matear 2008, Southern Ocean acidification via anthropogenic CO2 uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate ion (CO3-2) to levels where calcium carbonate (both aragonite and calcite) shells begin to dissolve. ... Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038. On the other hand, the language of outgassing is very complicated, as in Lovenduski et al 2006: In contrast, there is a simultaneous anomalous uptake of anthropogenic CO2 during a positive phase of the SAM in the southernmost regions of the Southern Ocean, due to increased upwelling of deep, older waters and their subsequent exposure to higher atmospheric CO2 levels. The anthropogenic uptake only slightly mitigates the natural outgassing from the Southern Ocean, so that a positive SAM is associated with anomalous outgassing of contemporary CO2. In a future characterized by higher atmospheric CO2, however, positive phases of the SAM may be associated with a greater oceanic uptake of anthropogenic CO2. And they say clouds are complicated beasts?
  19. Ah, I see your problem. The simple (probably non-informative answer) answer is that the ocean is heterogeneous. Depending on where you are it can be a source or a sink for CO2. What increasing atmospheric CO2 concentration has done is to shift that balance uniformly toward the sink side of things. Still, there are large areas where upwelling of deep, CO2 rich water and limited phytplankton growth (often due to relatively low Fe availability) results in surface waters that are supersaturated with CO2. This is particularly true in the Eastern Equatorial Pacific because the cold CO2-rich waters warm significantly after reaching the surface, making them even more supersaturated. It's the largest natural source of CO2 on the planet. In the Southern Ocean things are more complicated because the deep water upwells vigorously there and can be subducted beneathe warmer, light water after moving northward, or can sink to great depths if it gets particularly cold and salty after moving south. I need to read the paper when I have time, but the quote you cite seems to have something to do with the fact that under positive SAM and steady state CO2 conditions, vigorous upwelling brings CO2 rich water continually to the surface where it can lead to net evasion of CO2 on the whole. However, in some places there may net uptake of CO2, or net evasion depending on initial CO2, the evolution of water temperature and mixing with surrounding waters. Under higher atmospheric CO2 levels, though, the net flux would be into the ocean across the Southern Ocean because, while the atmopsheric concentration is changing, the concentration in the upwelled water stays constant because it reflects past atmospheric conditions 200-1000 years ago. If that difference is high enough, then the same vigorous upwelling during SAM could actually enhance net storage of CO2 by continuously bringing new undersaturated water to the surface across the entire region where it can soak up CO2 and then sink to depth again.
  20. michael sweet #41 "stop speculating about processes you do not understand" Thanks for answering my question. In reading 41, 42, 43, and 44 it becomes clear that the problem is not simple.
  21. Muoncounter - How is it possible for oceans to simultaneously absorb and emit CO2? The temperature of the ocean is heterogeneous too. Generally the flux of CO2 to the atmosphere originates from the tropical regions where the ocean is warmer and CO2 less soluble. The uptake is occurring in the larger region of cooler waters where CO2 is more soluble. Much more complicated than that of course, as others have already pointed out, but why over-complicate matters?. The overall effect of human fossil fuel combustion is to increase CO2 dissolved into the oceans at a rate that is unprecedented. Papers are practically screaming that the oceans are sucking up CO2 to what will become dangerous levels (at least to plankton) in not very many years. From McNeil and Matear 2008 The Arctic Ocean is projected to reach aragonite (more soluble form of calcium carbonate) undersaturation within a decade, meaning the waters will be corrosive to calcifying marine organisms that make their shells from aragonite. Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model "Aragonite undersaturation in Arctic surface waters is projected to occur locally within a decade and to become more widespread as atmospheric CO2 continues to grow. The results imply that surface waters in the Arctic Ocean will become corrosive to aragonite, with potentially large implications for the marine ecosystem, if anthropogenic carbon emissions are not reduced and atmospheric CO2 not kept below 450 ppm." It amazes me how little attention is being given to such a serious issue. It will have profound effects for life on Earth, and the changes to ocean chemistry are pretty much irreversible for many ten of thousands of years.
  22. Muoncounter - "How is it possible for oceans to simultaneously absorb and emit CO2?" Actually a simple source of confusion may be the separation of natural and anthropogenic CO2 that is made the Luvenduski paper. It helps to know that net flux of CO2 into the ocean really reflects the balance between CO2 influx and outflux, just like temperature of the ocean reflects a net energy balance. So there can be a net flux of anthropogenic CO2 in (as all of it is in the atmosphere), but a net total flux of CO2 out due to local imbalances between pCO2 in water and air. It can be hard to parse that out in words without being confusing, as that text you quote makes quite clea! But fig 3 does a good job. The complex pattern of "natural" CO2 influx and outflux (Fig 3b)reflects upwelling of CO2 rich water, temp changes, downwelling (well constrained) as well as sea surface exhange and phytoplankton growth/sinking (both less well constrained) on the CO2 balance. The contemporary pattern differs in that influx of CO2 has increased regionwide due to anthorpogenic CO2. The spatial pattern of that increase is shown in Fig3c. That's the shift toward sink state I mentioned. One big unknown in all of this is if the biological pump in this region may respond to changing CO2 in this region. People are working on various aspects of that as we speak.
  23. As for whether clouds are trickier than oceanic CO2 exchange, you'd have to say the Luvenduski paper does an outstanding job of reproducing the mesocale SST variability across the Southern Ocean (fig 2). I have pretty good confidence in their prediction of the underlying physical variables that largely constrain CO2 flux. Clouds are WAY more difficult, I think.
  24. Lovenduski's Fig 4 captures in time series format the changes going on south of 35S latitude. For 'natural flux', the trend turned from - (sink) to + (source) in the mid 80s; a warming sign? But when she adds in 'anthropogenic' to create the 'net flux', the long term trend is driven downwards towards a stronger sink. Hence this patch of southern ocean is, on balance, soaking up CO2 and acidifying. But the picture changes in other parts of the world: --from Lamont Doherty --from NOAA PMEL So anyone who says 'atmospheric CO2 increase is solely due to ocean outgassing' is all wet.
  25. Glad someone has the patience and time to post those images! "So anyone who says 'atmospheric CO2 increase is solely due to ocean outgassing' is all wet." On that we should be able to agree. Physical and chemical considerations as well the evidence from stable isotopes, physical measurements, times series of pH and pCO2 pattern/trends are in agreement and unequivocal.

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