If you follow the Seattle Times, or ocean acidification as an emerging issue, you’ve likely seen Craig Welch’s “Sea Change” series published last month, a series that is impressive in terms of scope and depth of coverage of the issue of ocean acidification, with its possible impacts on marine life and our economy. I’ve been following how this series of articles has been reverberating around the internet and among thought leaders, and had a draft post put together on its distribution through syndicated media, how it passed through twitter, and how it may be influencing policy and science. I couldn’t get the post “just right” and it remains in draft form–and I’m somewhat glad it did because it delayed things until Cliff Mass posted a rather scathing critique of the Seattle Times on his blog this past Wednesday.
Cliff Mass’s critique raises some interesting points, and highlights the difficulties of communicating complex science through articles like what Craig Welch put together. If you have interest, I’d highly recommending reading both authors’ works. In short, Craig Welch and his team spent the better part of a year traveling and researching the impacts of increased atmospheric carbon dioxide concentrations on sea life–the premise being that increased carbon dioxide concentrations in the atmosphere drive the pH of ocean waters down, due to gas exchange with surface waters and the formation of a carbonic acid as a result. That acidification may result in impacts to sea life, and Craig Welch and his team wanted to document and highlight those potential impacts. One of his articles details the problems oyster growers here in the Pacific Northwest have had with failure of spawning events due to–as Craig Welch puts it–decreased pH of the waters those oysters are spawning in. He even goes on to characterize those waters as “corrosive” with the idea behind that label being the decreased ability of oysters to build calcium carbonate shells in such waters.
Cliff Mass jumped all over the Seattle Times in his critique, arguing that there is no demonstrated connection between atmospheric CO2, decreased ocean pH, and the problems the oyster industry is facing. He put together a series of data plots that show the decrease in ocean pH attributed to increased carbon dioxide emissions is about 0.1, while the natural variability of pH in coastal waters in the Northwest is much greater than that decrease, ranging from 0.6 to 1 pH units. He argues that this natural variability — being much greater than the decrease in pH due to increased carbon dioxide in the atmosphere — essentially dwarfs the pH decrease due to carbon dioxide in the atmosphere and therefore “it seems highly unlikely that changes in atmospheric CO2 are driving the large observed changes of ph (sic) of our local waters,” and as a result, atmospheric CO2 increases do not explain the problems being faced by the oyster industry.
So, who has it right? Craig Welch and the Seattle Times, or Cliff Mass?
The answer is neither party. Craig Welch–most likely because he was trying to translate complex science and chemistry into a form that was understandable to the public–used pH as a proxy for the real driver of the recent failures of oyster spawning events in the Pacific Northwest [EDIT Craig Welch rightly points out that he covers the issue of aragonite saturation in the oyster piece here, and in a previous article here]. Cliff Mass–most likely because he is an atmospheric scientist and not an aquatic chemist–missed diving into the detailed relationship between pH and the driver of the recent failures of oyster spawning events in the Pacific Northwest.
What is that driver? Well, the work of George Waldbusser et al. strongly suggests the important driver in terms of water chemistry isn’t pH but aragonite saturation state–a measure of the thermodynamic ability to form aragonite (a form of calcium carbonate present in oyster shells) from seawater. Above a value of 1, formation of aragonite is favored; below 1, dissolution of aragonite is favored. Aragonite saturation state is defined by the relative presence of calcium ions and carbonate ions in seawater–the building blocks of calcium carbonate. Carbonate is a form of inorganic carbon, and inorganic carbon in seawater comes in various forms, one being carbonate (CO3), another being dissolved CO2, and another being bicarbonate (HCO3-). These forms of inorganic carbon cannot all be measured directly, so researchers rely on models that use measured parameters in seawater (temperature, salinity, dissolved CO2, pH, or alkalinity) to calculate the carbonate ion concentration in seawater and then they can calculate aragonite saturation state.
So, seawater pH in one sense can be seen as a rough proxy of aragonite saturation state, which is why I think Craig Welch and others use pH when talking about ocean acidification impacts on oysters. As noted above, seawater pH can be influenced by atmospheric CO2 concentrations. But, seawater pH is also influenced by other processes, such as photosynthesis and respiration involving organic carbon, which is the likely source of the larger pH variations Cliff Mass discusses in his blog post. This is why seawater pH is a rough proxy at best for aragonite saturation state. Therefore, to be precise, Craig Welch should be referencing aragonite saturation state, at least when talking about impacts to calcium carbonate forming organisms.
Head spinning? See why science communicators need to simplify things? I have a Ph.D. in aquatic chemistry (with knowledge atrophied by years of being a lawyer) and it took me a good hour to go back through all of the above and try and distill things down for you. I THINK I have it right–but the lawyer in me puts that caveat on things, and I welcome comments if I missed anything.
Now that we are done with the aragonite saturation state primer, we can get at Cliff Mass’s contention that the oyster failures that have been observed are NOT due to rising CO2 levels in the atmosphere:
Can we state with any degree of certainty that the oyster spawning failures are due to a decreased aragonite saturation state? The answer at this point–largely based on Waldbusser’s work and work at the Whiskey Creek hatchery in Oregon–seems to be yes, there is both laboratory research and field data that support that conclusion. Can we then go on to attribute the decreased aragonite saturation state to increased CO2 concentrations in the atmosphere? The answer there is yes as well, at least based on the recent work by Katherine Harris and others that looks at the complex interplay of upwelling, freshwater inputs, biological processes, and atmospheric gas exchange on aragonite saturation state. That work shows that surface aragonite saturation states in modern upwelled surface waters along the Oregon coast are lower (0.66 to 3.9) compared to pre-industrial rates (1.0 to 4.7), a drop attributed to increased carbon dioxide in the atmosphere and gas exchange with upwelled surface waters. That work goes into extreme detail trying to tease out the different drivers of aragonite saturation state. It is worth noting that gas exchange is only one of those drivers, but appears to be the factor driving the drop in aragonite saturation states from pre-industrial levels. As I note above, aragonite dissolution is favored for values below 1, and oysters appear to need values of ~1.7 or above to thrive–so the drop from pre-industrial levels may be significant in oyster survival.
So, can we state that increased CO2 concentrations in the atmosphere are the possible cause of the problems seen in the oyster industry? Yes, because atmospheric CO2 is impacting aragonite saturation state-but not because of drops in pH in surface waters. That is an over-simplification of the issue.
Putting all this together, and refereeing Cliff Mass and Craig Welch, I think Craig Welch oversimplified in a necessary manner to communicate complex science. Cliff Mass–not surprisingly–looked at the issue from an atmospheric perspective and missed this issue of aragonite saturation state, although to his credit, he was just looking at the question of whether pH changes due to increased carbon dioxide was driving the observed problems in oyster production.
Okay. I’m well beyond my word limit for a blog post. What are the impacts of this debate? Cliff Mass has provided fodder for people skeptical of climate change impacts, which I don’t think was his intention (he was driving at scientific integrity and communication in the media). If you look at comments on other media coverage on this issue, you can already see that the Cliff Mass critique is being used by others to discredit the connection between rising CO2 in the atmosphere and impacts to the shellfish industry. That is unfortunate, because, for the reasons I state above, I think we are seeing impacts that are directly attributable to rising CO2 in the atmosphere. The story just isn’t simple.