by Judith Curry
This thread discusses sections 10.3 and 10.4 in Alan Longhurst’s new book Doubt and Certainty in Climate Science.
The topic of ocean acidification is one for which I don’t have any expertise, beyond following some news items on the topic. So I found the two sections on ocean acidification in Longhurst’s book to be especially helpful. Below are some excerpts from these two sections (no italics used here for direct quotations):
10.3 Acidification of sea water – uncertainty levels
One of the major concerns now expressed is that increasing atmospheric CO2 may soon come to affect those marine organisms, large and small, which incorporate carbonate into their exoskeletal structure. Although the dissolution of carbonate sediments will buffer pH changes by adding alkalinity and so restore some of the buffering/uptake capacity of the oceanic CO2 system, it is the biological effect that has taken our attention and this has been confidently described as an inevitable disaster for marine ecosystems.
The consequences of this process are only now beginning to be understood in all their complexity, and I suggest that doubt and certainty concerning the long-‐term future of some marine organisms may be appropriate in about equal proportions. These potential problems came to our attention only relatively recently compared with other concerns about changing climate. Until the 2005 Royal Society report on the potential effects of ocean acidification by atmospheric carbon dioxide, no more than a handful of studies on the subject were published annually; subsequently, the floodgates opened and ‘acidification of seawater’ rapidly became headline news. This term, perfectly proper chemically, had been very rarely used previously in this context, and its use has undoubtedly assisted in bringing the issue to our attention. The pH of ocean water is everywhere, and at all depths, higher than 7.0 and therefore basic, and even the most extreme anthropogenic climate change scenarios do not suggest that the pH of ocean water will fall below neutrality, since the reserves of carbonate in the ocean are far too great for that.
But, because ocean surface water is naturally alkaline, atmospheric CO2 does readily pass into solution, at rates determined by the pCO2 gradient and by physical factors including water temperature, wind speed and surface roughness. Of the anthropogenic carbon dioxide that was emitted into the atmosphere during the 19th and 20th centuries, approximately 48% has been dissolved in the oceans, where it is not uniformly distributed. Regions influenced by the formation of deep and intermediate water masses in the North Atlantic dominate the column inventory, yet <10% occurs deeper than 1500 m.
The first consequence of the solution of CO2 in seawater is the formation of carbonic acid, but this immediately dissociates to form bicarbonate. Over geological time scales, this process is buffered by the terrestrial carbonate cycle so that a balance tends to be maintained between carbonate weathering ashore, sedimentation of biogenic carbonates as chalk or limestone in the ocean that has been recorded in several locations.
So, to understand the reactions of marine organisms having carbonate incorporated in the skeletal material to the solution of CO2 in ocean water, we must look not only at changing pH, but also at the ambient level of carbonate saturation of seawater. Decreasing pH may erode skeletal material, and calcite under-‐saturation may constrain the rate of production of skeletal material, the whole entering a delicate balance. If the rate of change of pH is faster than the rate of equilibrium of carbonate saturation dynamics under-‐saturation of calcite may result with potentially stressful consequences for calcifying marine organisms; unfortunately, rates of change of pH at present are significantly faster than during periods in the distant past when atmospheric CO2 concentrations increased much more slowly. The faster rate of change in pH in today’s ocean must result in greater relative changes in calcite saturation than during CO2 increases during geological time. This critical observation must be considered carefully when remarking, as is often done, that many extant marine organisms have passed successfully through ancient episodes of high atmospheric CO2 concentrations: it is essential that changing calcite saturation should be considered when predicting the consequences of changing pH of ocean water, otherwise conclusions drawn concerning the consequences of pH changes may not be correct.
Near the surface, we expect to find the highest pH values, with progressively lower values downwards. At the surface of the Pacific, values run from pH 8.05 in the tropics to pH 7.6 in the Gulf of Alaska while, at 1000m in high latitudes and 250m at the equator, water of around pH 7.5 occurs at mid-‐depths; a similar pattern occurs in the Atlantic, where near-‐surface water in the Arctic regions reaches pH 8.2 compared with about 7.9 in the African upwelling regions in low latitudes. Near-‐surface, the pCO2 difference between gas and water phases controls gas CO2 exchange across the surface, while in the interior of the ocean pCO2 values are controlled by respiration and carbonate dissolution.
Some of the alarming reports concerning ‘corrosive sea water have been based on observations of commercial shellfish cultures in which the oysters have failed to produce normal shells; this syndrome has popularly been ascribed to changing pH of ocean water, especially on the Oregon and California coasts. But this is incorrect, because the failure to produce normal shell material is due to very low levels of calcite saturation that results in abnormal calcification of larvae and adult shells that appear to be eroded. Here, it is not pH that is involved but rather calcite saturation in a usually complicated environmental situation in which river water quality is involved as well as that of coastal sea water. Yet it is likely that it is calcite undersaturation, rather than pH itself that is the principal agent of change in the ecology of small calcifying organisms.
Hermatypic coral reefs in shallow water are commonly presented as one of the most obvious victims of decreasing pH of the oceans, but consider the facts: the daily range of pH experienced by near-‐surface reef corals should, by this logic, prevent their continued existence – we have long known, but apparently forgotten, that, where water circulation is relatively limited on the reef top of the Great Barrier Reef, “CO2 in the water is depleted by photosynthesis during the hours of daylight, while the O2 content rises to as much as 250% saturation and the pH rises to 8.9. At night, photosynthesis ceases, O2 may fall to as low as 18% saturation and the pH drops to 7.8”.
We now know that although all these dangers are real, and cannot be ignored, the probable outcome is more nuanced than was once thought. The relief (if you can call it that) comes from experimental evidence that shows that the reaction of organisms to low pH water is not as simple and direct as might be assumed from first principles; to some extent, this is due to the fact that populations of organisms tend not to be homogenous genetically, but to include individuals having a rather wide range of potential response to their naturally-‐variable environment.
Unfortunately, it is not easy to disentangle the effects of pH and calcite saturation from changes in stratification, nutrient availability and temperature during this warm episode. But I think that those who now deeply worried about the observed changes in pH and calcite saturation in today’s ocean might well take some comfort from a reading of the palaeontological literature, into which I have no more than dipped my toe.
10.4 Experimental evidence for acidification effects
The concerned reader may well be excused for being confused by recent studies concerning problems with coral decline on the Great Barrier Reef. The story starts with a study published in 2009 by De’Ath and others who suggested that calcification in massive Porites colonies on the Great Barrier Reef had declined by as much as 14% since 1990, associated with a decline of about the same magnitude in linear colony growth; it was held that this was an unprecedented decline compared with rates over the last 400 years; although the cause was not established, increasing temperature stress and a declining saturation state of aragonite was suspected.590 This report resulted in alarmism at the BBC and local media and the public were assured that “coral growth could hit zero by 2050”. In any case, this result appeared to contradict an earlier study that had concluded that, far from declining, coral growth on the Barrier reef had increased by up to 4% in warm periods of increasing temperature during the 20th century.591
But even more confusing the publication of a later paper by De’Ath, who reported that the decline in growth had really been caused by a 27-‐year period of strong tropical cyclones (48%), by crown-‐of-‐thorns starfish predation (42%), and by bleaching (10%). Associated with this was the good news that the estimated rate of recovery of coral cover in the absence of these factors would be about 3% p.a., and also that in northern regions, where the three destructive factors had minimal effect, there was no significant decline in coral cover.592 In the light of the studies discussed above it would appear that a reduced rate of calcification is, at least at present, a negligible factor in whatever it is that ails Great Barrier Reef corals. It is not helpful to suggest, as some have done, that the Barrier Reef of 2050 will be no more than rubble of carbonate rock.
In the light of the studies discussed above it would appear that a reduced rate of calcification is, at least at present, a negligible factor in whatever it is that ails Great Barrier Reef corals. It is not helpful to suggest, as some have done, that the Barrier Reef of 2050 will be no more than rubble of carbonate rock.
The entire subject of the response of the marine ecosystem to increasing levels of atmospheric CO2 is in such an early stage of investigation that I believe it is not yet possible to achieve any level of certainty about what the future holds for the marine ecosystem, but one has to conclude that alarmism is premature. It seems clear from these few examples of recent studies that our opinion on the consequences for marine biota of increasing ocean acidification should be more nuanced than it was 10-‐15 years ago.
Fortunately, we now have an increasing body of experimental evidence for large marine organisms, done at realistic pH levels, that suggests two major conclusions.598 First, as noted above, adult fish appear to be little affected by water of rather low pH although there do appear to be serious but subtle consequences for brain function and hence behaviour pattern. This has mostly been investigated in tropical reef teleosts, relatively easy to handle experimentally, among which responses to olfactory, optical and auditory stimuli have been found to become inappropriate: cues for larval settlement, for prey and for predator recognition, and for habitat landscape may be misinterpreted. These are subtle effects, in a variety of species, for which failure of a single receptor function in the brain is shown to be responsible.
But, once again, one must recognise that these findings are all based on relatively short-‐term experimentation and that they deliver no prediction of the probability that species may evolve an appropriate response, by selection of genotypes from the existing range, if the pH of ambient water changes as slowly as it is doing in the ocean at the present time. In fact, there is some evidence, reported by Branch et al., to support that this suggestion; when adults of the shore fish Amphiprion melanopus are exposed to near-‐future levels of CO2, their young show reduced size and growth rate when grown at similar levels, but this is not the case for the young of adults that have been exposed to very high CO2 levels: epigenetic changes in gene expression would appear to be responsible for this result.
But if one thing is sure in climate science, it is that without a global economic meltdown or pandemic, atmospheric CO2 will continue to increase and the pH of ocean water will continue to change accordingly: to count on anything else is to put too much credence on the common-‐sense of the human animal. And of all the topics that I have reviewed for this book, it is the consequences of the changes in ocean pH and calcite saturation that will accompany the inevitable increase in atmospheric CO2 that are the most worrying, and the most likely to cause consequences to biota. I do not venture to suggest what these consequences will be, although I agree with the comment from a reliable source that “we are entering an unknown territory of marine ecosystem change”.
I find these sections on ocean acidification to be an invaluable reference. Please confine your discussion to topics raised in Chapter 10.