Are the Oceans Becoming More Acidic?

Guest essay by Clyde Spencer

“There’s nothing so absurd that if you repeat it often enough, people will believe it.” – William James

Introduction

Are the oceans becoming “more acidic,” as is often claimed in recent research papers and trumpeted by the news media? I’ll explore that in the following discussion. However, there are two parts to the answer, so I’ll provide the response in two parts.

Part 1

I’ll start with some basic chemistry to be sure that everyone is at the same level. Some of the water molecules in pure water will naturally dissociate into hydrogen (hydronium) ions (cation) and hydroxyl ions (anion) in equal amounts. The amount is very small: 0.0000001 or 10^-7 moles per liter of each; this is defined as a neutral solution. If a quantity of a base, such as sodium hydroxide (NaOH) is added, there will be a decrease in the hydrogen ions (with the creation of some water molecules) and an increase in the hydroxyl ion concentration; this is called an alkaline solution. If a quantity of an acid, such as hydrochloric acid (HCl) is added, there will be an increase in the concentration of hydrogen ions. If both NaOH and HCl are added in appropriate quantities, then the solution can still be neutral, but it will also be saline. What usually isn’t appreciated by those who aren’t chemists is that the concentration of the two ions varies inversely. That is to say, the product of the concentration of the two ions is a constant, approximately 10^-14. The use of the hydrogen ion concentration alone is a convenience that implies that the hydroxyl ion is also present in inverse proportion to the hydrogen ion. Either ion could be used to characterize the chemical activity of the solution. The important point, which is usually overlooked in discussions of the chemistry of seawater, when talking about the hydrogen ion concentration, is that it is the ratio of hydrogen ions to hydroxyl ions that determines the chemical behavior. If the ratio is one, then the solution is neutral, neither acidic nor basic. If the ratio is less than one, the solution is alkaline, and vice versa.

The hydrogen ion concentration is expressed with the pH scale universally, which avoids having to deal with extremely small numbers. The logarithmic pH scale was developed to make the handling of ionic dissociation changes in an aqueous solution, with a range of more than 14 orders of magnitude, less cumbersome. In addition, dealing with very small numbers in a denominator (as when calculating percentage change) can lead to the false impression that a change has greater significance than is warranted. A pH decline from 8.2 to 8.1, the commonly claimed recent change in seawater, (http://www.whoi.edu/fileserver.do?id=165564&pt=2&p=150429) amounts to a change of -1.2% on the pH scale (-0.1/8.2) and it would take a change of about -15% to reach neutrality (pH = 7). It is disingenuous to cite an equivalent change of 30% in the untransformed active hydrogen ion concentration (http://www.huffingtonpost.ca/alex-mifflin/oceans-co2-seafood_b_7286392.html) without noting the percentage change required to reach even neutrality, let alone an actual significant acid condition. The alleged change that has occurred in hydrogen ion concentration is actually about 25% (The value usually cited is rounded up considerably!). It would take a change of nearly 1500% in the hydrogen ion concentration to reach neutrality. That is, there has been a percentage change of about 1.7% (25/1500) of hydrogen ion concentration necessary to reach neutrality. Inflating numbers and not putting them in context seems to me to be an act of hyperbole that raises a question of objectivity.

Solutions with a pH of less than 7 have been called acidic, and greater than 7, alkaline or basic, for more than 100 years. Acids and bases have different properties. The hydrogen ion concentration of an alkaline solution, which experiences a lowering of pH, moves in the direction of acidity, but does not become acidic until neutrality is first reached and then passed. A pH converging on 7 – from either end of the pH range – is referred to as neutralizing. One might think that referring to alkalinity would be the obvious way to refer to the state of an alkaline solution. Unfortunately, alkalinity was given a very specific definition that is related to buffering capacity, or the resistance to pH change when an acid is added to an alkaline solution. Although, some pool and spa-chemical manufacturers commonly label their pH-increasing product, sodium bicarbonate, as “pH/alkalinity up.” The phrases “less alkaline,” “reduced base,” or “de-alkalizing” would convey more information than ‘acidification.’ That is, one would immediately understand that seawater is alkaline and the pH is decreasing. The term lowered or decreased ‘basicity’ might be used alternatively, although it tends to be used to describe how chemically ‘basic’ a particular anion is. Even saying “decreasing towards neutrality” has merit over the shorter, casual ‘acidifying.’ One could legitimately say that an alkaline solution experiencing a reduction in pH is becoming less caustic; however, the expression is nearly as loaded as “more acidic.” Carbonation is a term that has been suggested as an alternative to ocean acidification. I could support this because it isn’t a pejorative word and it speaks to the process that is of concern, namely the chemical changes resulting from the absorption of carbon dioxide.

The pH scale as it should be presented

 

(https://www.climate.gov/news-features/features/upwelling-crisis-ocean-acidification)

Note that fresh water, slightly buffered with sodium bicarbonate, super-saturated with CO₂ (Club Soda), has an initial pH of about 5 when first opened. After reaching equilibrium with CO₂ in the air (becomes ‘flat’), it has a pH of about 8. Tap water, with some calcium and magnesium carbonates present (hard water), has a pH of about 7.5 to 8.0. Putting that same tap water in a soda charging-bottle and super-saturating it with CO₂ yields a solution with a pH of about 6, initially. Freshly opened commercial 7 UP™, with both carbonic acid (H₂CO₃) and citric acid has a pH of about 3!

A commonly held misconception is that as the carbon dioxide (CO₂) levels in the atmosphere increase, thus increasing the dissolved CO₂ in the oceans, the weak carbonic acid builds up, which causes the oceans to be acidic (http://www.theverge.com/2015/7/20/9007807/winners-announced-in-competition-to-develop-tech-for-measuring-ocean). According to Gattuso (2011), the resident carbonic acid that is created is less than 0.3% of the free aqueous CO₂ in solution. In actuality, the carbonic acid that forms has a lifetime of about 26ms; it turns into a hydronium ion (which may react with other anions present) and a buffering bicarbonate anion (http://newscenter.lbl.gov/2015/06/16/unravelling-the-mysteries-of-carbonic-acid/). This creates a buffering system whose behavior is much more complex than generally appreciated (http://ion.chem.usu.edu/~sbialkow/Classes/3600/Overheads/Carbonate/CO2.html ).

I recently reviewed my college oceanography text, the classic, The Oceans (Sverdrup, Johnson, and Fleming, 1963). I found no use of the terms “acidification,” “acidifying,” or “more acidic” except in association with the laboratory procedure of titration for determining pH or CO₂ content, where the end goal was to produce a solution that was an acid. The authors consistently refer to the chemical condition of the oceans with pH, alkalinity, salinity, and chlorinity.

Sverdrup et al. (1963), remark that buffered solutions resist changes from alkaline to acid condition. Further, “This property is of vital importance to the marine organisms, mainly for two reasons: (1) an abundant supply of carbon can be available in the form of carbon dioxide for the use of plants in the synthesis of carbohydrates without disturbance to the animal life that may be sensitive to small changes in pH, and (2) in the slightly alkaline habitat the many organisms that construct shells of calcium carbonate (or other calcium salts) can carry on this function much more efficiently than in a neutral solution.” That is to say, less energy is expended to create and maintain shells in alkaline water.

It shouldn’t come as any great surprise that CO₂ bubbling up on the floor of the ocean in various locations is deleterious to the organisms normally found there. In a similar manner, CO₂ diffusing up through the ground has killed trees on the margin of Long Valley Caldera in California. These are exceptional circumstances that, in my opinion, are not good analogues for what might happen to the oceans in general if the CO₂ content of the atmosphere were to even double. The volcanic CO₂ vents create a situation where the water is super-saturated with CO₂ compared to the partial-pressure of the CO₂ in the atmosphere, similar to the behavior of soda water. That would be an impossible widespread situation as long as CO₂ remains a minor constituent of the atmosphere. Oceanic CO₂ vents are a poor model for the real world impact of CO₂ increases in the atmosphere because of the strong buffering capacity of seawater. Thus, the concerns expressed in NOVA’s recent Lethal Seas are considerably exaggerated!

When TV programs, such as Lethal Seas, state that CO₂ is “raising the ocean’s acidity,” I’m sure that the typical viewer is going to incorrectly assume that the oceans are acidic. Thus, rather than being educated, viewers are being misled! I may be paranoid, but it seems to me that when people substitute pejorative words for neutral words that are more accurate they are not being objective and possibly have an agenda other than communicating facts. Thus, when the TV program PBS NOVA talks about the oceans “becoming more acidic,” it raises questions about the objectivity of the writers and producers. The practice of being a disinterested observer seems to have been abandoned.

It is appropriate to use “more” or “increasing” when referring to an open-ended scale like temperature. However, if the temperature were to change from -10° to -5° C, people would look at you as if you were daft if you said that it was becoming hotter. It has to be hot before it can become hotter. I will allow that it is sometimes casually said – indeed often jokingly – that it has become warmer, but saying that it is ‘less cold’ or that the temperature had simply increased would be more accurate and is what I would expect in a scientific article. However, when there are natural or defined boundaries for measurement systems, then you cannot have more of nothing. The current use of “more acidic” for a decreasing pH of alkaline water is like saying that a block of ice that has increased in temperature slightly (say from -10° to -5° C) has become ‘more liquid’ while it is still obviously solid! In the same manner, something that is not acidic cannot logically become more acidic. It is, at best, grammatically careless to use those words.

Let’s see if I can further illuminate this point. Assume that you are observing a beam of cyan (blue-green) light (for purposes of analogy, let’s call it alkaline). You then add increasing amounts of red light (acid) to it. It is correct to say that the red component (hydrogen ions) is increasing. However, anyone observing the mixed beam would initially only see cyan light, with the saturation of the hue decreasing as more red light is added. Therefore, it would be sophistry to maintain that the beam was becoming more red when it was clearly still cyan. Only after becoming colorless would the light start to become perceptibly red (acidic). Then, there would be no disagreement that adding more red light (acid) would cause the beam to become more red (acidic) in appearance.

Note that when the term “ocean acidification” is used in recent publications, it is usually followed by the parenthetical explanation that it means a decrease in pH. I would suggest that is tacit acknowledgement that the readership – well-educated or not – is not expected to understand what “ocean acidification” means. That is because it is a new, inappropriate term!

The current use of “Ocean Acidification (OA),” has no historical precedent. Using Google Scholar to search for the first use of the term, it apparently was first used in the title of a Nature article in 2000; it subsequently showed up in numerous articles in 2005 and continues to today.

One of the first publications to use the misnomer “Ocean Acidification,” is a 2005 Royal Society (RS) article with twelve contributors. It refers to the UNESCO Symposium on the Ocean (2004), wherein some authors use the term “Ocean Acidification.” The RS article seems to be the primary source of many of the statements regarding the claimed decline in surface pH. Interestingly, their list of formal definitions does not include “Ocean Acidification” or “OA,” and it unconventionally claims pH is the acidity of a solution instead of the original, long-standing, hydrogen-ion concentration. That is, Krauskopf (1967), and others, formally define pH as follows: “The negative logarithm of the hydrogen-ion concentration.” (I was pleased to find that a current, local high school chemistry text (Sarquis & Sarquis, 2015) uses the conventional definition.) Only one of the RS article’s 162 citations uses the word “acidification” in the title of the article. Therefore, while this Royal Society publication is not the first to use OA, it is apparently a watershed event in what appears to be an attempt to change scientific vocabulary. They had a measure of success in introducing Newspeak because since 2005 there have been numerous published articles using “ocean acidification,” as well as NOAA websites.

The unconventional use of the term “acidity” in recent oceanography publications can be confusing, particularly when comparing the ‘acidity’ of ocean water with the real problems of Acid Rain and Acid Mine Drainage, and characterizing the water as having low acidity or high acidity. There should be consistency between different scientific disciplines that invoke chemistry.

The reason that the English language has so many words is to communicate subtleties. A reduction in vocabulary, such as avoiding using the word alkalinity, prevents communicating those nuances. Indeed, it may lead to false impressions. One of the regular contributors to the comment section of The Conversation blog was of the opinion that sinks in the limestone in Florida might be caused by the oceans having become ‘acidified.’ Therefore, whether the oceans are alkaline or acid is a distinction that makes a difference. Words should be used correctly and precisely so that meaning isn’t obscured and information reduced.

I have to ask why any writer would choose a phrase that conveys less information, is potentially confusing, and is tacitly pejorative. I can think of a few: The writer, 1) knows little about chemistry; 2) is sloppy and/or careless; 3) has little regard for communicating effectively; 4) is “ideologically motivated” and wants to heighten concern by using pejorative words (think disfigured by acid splashed in one’s face); or, 5) is unthinkingly using terminology used by others who were influenced by one or more of the preceding.

What is at issue here really is why some scientists would adopt a term that is a poor choice for communication. “Increasing acidification” is ambiguous at best and misleading at worst. It says something about the direction of change, but leaves readers clueless about where on the pH scale the change is taking place. For me, it implies that a solution is already acidic and is becoming more so. For laymen, it probably is similarly interpreted since most probably don’t know what a base is and what the differences in the characteristics of acids and bases are. However, writing for a well-educated audience, one can reasonably assume that most of the readers know the characteristics.

Therefore, I don’t know whether the current generation of oceanographers is less competent then their teachers were, or if they are embellishing for reasons they best know. If one is willing to distort facts to promote an ideology, then it really doesn’t matter how expert one is.

Scientists have a responsibility to not only do research, but to communicate it to their peers in clear, unambiguous ways. It helps if the communication is clear enough that even journalists can understand. I’m concerned that ideological motivations are taking precedence over good writing. In science, an ace beats a full house any day. Therefore, I’m not impressed that current scientists make the same mistake of describing alkaline sea water as becoming more acidic. Consequently, I’m asking for clarity in scientific writing so that people actually understand and not go away with the mistaken impression that the oceans are acidic.

In summary, recent research publications are using a term (OA) that is technically incorrect, misleading, and pejorative; it could not be found in the oceanography literature before about 15 years ago.

Part 2

Recent journal articles and media headlines have used “more” or “increasing” with “acid” as a root for a supposed lowering of the average pH of the oceans from 8.2 to 8.1. I say, “supposed,” because Sverdrup et al. (1963) state, “The pH encountered in the sea is between about 7.5 and 8.4.” Elsewhere, they say, “The pH of water in contact with the air will vary between about 8.1 and 8.3, depending upon the temperature and salinity of the water and the partial pressure of carbon dioxide in the atmosphere.” Similarly, the well-known Stanford geochemist Konrad Krauskopf (1967) states, “The pH of ocean water sampled near the surface is almost always between the narrow limits of 8.1 and 8.3. Locally and temporally it may deviate from this range, but by and large the pH stays surprisingly constant.” Thus, the claimed recent decline to an average pH of 8.1 from 8.2 is within the bounds of the typical range observed more than 50 years ago. Further, the claim has been made that the average pH of the oceans in pre-industrial times was 8.2; yet values as high as 8.3 were reported commonly 50 years ago. Thus, it would seem that neither the claimed values of 8.1 currently, or 8.2 150 years ago are credible. Unfortunately, the claims for the current “average” don’t specify whether the mean, median, or mode is intended. So much for ‘science!’

Interestingly, the Royal Society article (2005), mentioned in Part 1, has conflicting information. It states that, “The surface waters of the oceans are slightly alkaline, with an average pH of about 8.2, although this varies across the oceans by ±0.3 units because of local, regional and seasonal variations.” [p. 1]. They cite a paper (Caldeira & Wickett, 2003) that claims a decline of 0.1 pH units from pre-industrial times; there are no error bars provided to accompany this claimed value. They call this hydrogen ion increase (-0.1 pH) a “considerable increase,” after just acknowledging that the oceans typically vary ±0.3 units! In another contradiction, they re-state [p. 6], the above claim of a current average of 8.2 units; they then immediately cite a personal communication from Sabine [p. 7] that the average is 8.08 units. Table 1, [p. 13] lists an average surface pH of 8.07! What is the correct value? None of these claims provides estimates of the standard deviation. The paper also mentions the onerous 30% increase in hydrogen ion concentration, despite not stating a definitive value for the current average ocean-surface pH, nor the pre-industrial pH!

Caldeira and Wickett (2003) base their claims on modeling and literature review. Krauskopf (1967) has some cautionary observations for modelers. He remarks,

“These numbers give us a quantitative expression of the variation in the solubility of CaCO₃ that we predicted from qualitative arguments in the last chapter. Our next move obviously should be to compare the theoretically derived numbers with actual measured concentrations of CaCO₃ in natural solutions. The comparison is easily made, but it turns out to be most disillusioning. Concentrations of CaCO₃ in natural waters are extremely variable and only rarely come close to the numbers predicted in the last few paragraphs. Low concentrations can be plausibly explained as the result of failure of solutions to reach equilibrium with solid carbonate. In many natural waters, however, the discrepancy is in the opposite direction; concentrations are embarrassingly high, much higher than can be accounted for even with generous assumptions about temperature, CO₂ pressure, and acidity.”

He then goes on to offer possible explanations for these anomalously high values. This is important because pH is not the only factor to consider with respect to bioavailability of carbonate. However, the point is that predictions from modeling may be wrong and it is imperative to actually test these predictions. One last point that Karauskopf (1967) makes that modelers should take to heart is, “Seawater is a concentrated and exceedingly complex solution, containing electrolytes in great variety plus an abundance of living and dead organic material. The ordinary laws of dilute solution cannot be applied, or at best need great modification. The chemistry of seawater can be described fairly satisfactorily in general terms, but details about the behavior of even so simple a substance as CaCO₃ remain obscure.” [My emphasis]

Despite the claims of a recent lowering of pH (without associated ranges or uncertainties), there is a lack of good evidence that there has actually been any significant changes in the pH of seawater. Indeed, Wallace (2015) takes strong exception to the claim because most of the historical data on ocean pH are not used. Apparently, According to Wallace (2015), the claim of pH decline is based on hindcasts from a computer model.

NOAA (2015) goes to some trouble to explain why it chooses not to use pH data acquired before 1989. One rationalization (Telford, 2015) for dismissing historical data is that poor sampling protocol renders the historical data useless and therefore only modern data are reliable and useful. Telford further complains that older data are not gridded. The only good data that one has is the measured data. Interpolating with a gridding program is a best-guess (with lots of implied assumptions) at intermediate-point values, but it doesn’t actually provide additional data for averaging. However, the same complaint could be made about all temperature data, and especially the historical data. Similarly, CO₂ data are questionable. However, this creates a Catch 22 situation where any claims made about recent changes in measured quantities cannot then be validated by historical measurements!

It was formerly a truism that CO₂ is well mixed in the atmosphere and a single monitoring station at Mauna Loa was adequate to understand what was happening. However, recent OCO-2 satellite observations have shown that belief to be false. [See below] Incidentally, photosynthesis is essentially shut down in the northern hemisphere during Fall and, therefore, CO₂ is not being scrubbed from the atmosphere by land plants. Yet, one doesn’t see much effect of fossil fuel burning (except possibly in China) of the magnitude assumed to be active. Notice too, that the CO₂ is high around southern Greenland, yet the water pH is high!

Source: NASA/JPL-Caltech

Even taking the different color schemes into account, here is very little resemblance between the measured surface CO₂ and the estimated pH of the surface seawater. That is, in part, because the effects of atmospheric CO₂ are overwhelmed by photosynthetic activity, which consumes dissolved CO₂, and upwelling from deep, cold, oxygen-depleted waters enriched in CO₂ from the decomposition of organic matter. See the graphic immediately below and compare with the above. Note also that the Scientific American graphic gives no acknowledgment to the alkalinity of the seawater despite the pH range shown being entirely in the region of basic solutions!

Without a dense, random or uniform sampling grid, modern CO₂ data (Mauna Loa post-1958, pre-OCO-2) are no better than the historical ocean pH data, or for that matter, quantitatively no better than historical temperature data! Clearly, when and where one samples will determine the observed pH. Because the question at hand is to what extent anthropogenic CO₂ is influencing surface water pH, regions of coastal upwelling should be excluded from calculating averages and trends because it is not anthropogenic CO₂ that creates low pH in upwelling along coastal areas. However, none of the stated values indicate whether this is done; presumably, upwelling is included in calculated ‘averages.’

We have more detailed spatial sampling, and more precision in pH measurements today; however, it doesn’t seem that there are any fundamental changes in our understanding. Concerns about instrumental precision are misplaced when there is disagreement about whether the average surface pH is 8.2 or 8.07, however. It is well known that the pH of seawater varies with dissolved CO₂; CO₂ decreases with increasing temperature, thus increasing pH. The pH also increases with salinity, which increases with increased evaporation rates (which in turn increases with increasing temperature), relative humidity, and windiness. Thus, there are observed diurnal, seasonal, and weather-related variations that perhaps weren’t fully appreciated 50 years ago. However, Sverdrup et al. (1963) remark that it is only under exceptional conditions, such as in hydrogen sulfide-rich ‘dead zones,’ that ocean water will even reach a pH of 7. Thus, current researchers are talking about ‘acidifying’ when seawater is unlikely to ever actually become acidic. In any event, considering that seawater is highly buffered, and it isn’t trivial to calculate carbonate precipitation and outgassing in such a highly buffered complex solution with several environmental variables, the forecast of an average pH of 7.8 – 7.9 in 85 years (https://www.iaea.org/ocean-acidification/download/OA20Facts_Nov.pdf) should probably be taken with a grain of salt [Please pardon the pun.].

Historical pH data are rejected by government agencies, thus depriving us of any measured benchmarks. The claim that the surface-water of the oceans has declined in pH from 8.2 to 8.1, since the industrial revolution, is based on sparse, contradictory evidence, at least some of which is problematic computer modeling. Some areas of the oceans, not subject to algal blooms or upwelling, may be experiencing slightly lower pH values than were common before the industrial revolution. However, forecasts for ‘average’ future pH values are likely exaggerated and of debatable consequences. The effects of alkaline buffering and stabilizing biological feedback loops seem to be underappreciated by those who carelessly throw around the inaccurate term “ocean acidification.” Claims should be examined carefully for unstated assumptions.

References

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Anon [Ed], (2005), Ocean acidification due to increasing atmospheric carbon dioxide; The Royal Society, UK, 59 pp.: http://eprints.uni-kiel.de/7878/1/965_Raven_2005_OceanAcidificationDueToIncreasing_Monogr_pubid13120.pdf

Caldeira, Ken and Wickett, Michael E., (2003), Oceanography: anthropogenic carbon and ocean pH; Nature, Vol. 425 Issue 6956, p. 365: http://www.nature.com/nature/journal/v425/n6956/abs/425365a.html

Doney, Scott C., (2006), The dangers of ocean acidification; Scientific American, p. 58-65.

Feely, Richard A., Sabine, Christopher L., and Fabry, Victoria J., (2006), Science Brief: Carbon dioxide and our ocean legacy; Ocean Legacy: http://www.pmel.noaa.gov/pubs/PDF/feel2899/feel2899.pdf

Gattuso, J-P, and Hansen, L., [ed.], (2011), Ocean acidification; Oxford Univ. Press, NY, p. 2.

Krauskopf, Konrad B., (1967), Introduction to geochemistry; McGraw-Hill, New York, NY, 721 pp.

NOAA, (2015): http://www.pmel.noaa.gov/co2/story/Quality+of+pH+Measurements+in+the+NODC+Data+Archives

Sarquis, Mickey and Sarquis, Jerry L., (2015), Modern chemistry; Houghton Mifflin Harcourt, NY, p. 475.

Sverdrup, H. U., Johnson, Martin W., and Fleming, Richard H., (1942, 1963), The oceans their physics, chemistry, and general biology; Prentice Hall, Englewood Cliffs, NJ, 1060 pp.

Telford, Richard, (2015), Musings on quantitative palaeoecology: Not pHraud but pHoolishness: https://quantpalaeo.wordpress.com/2014/12/26/not-phraud-but-phoolishness/

UNESCO, (2004), Symposium on the ocean in a high-CO2 world; Paris, France, 10-12 May 2004

Wallace, Michael, (2015), http://wattsupwiththat.com/2015/03/31/ocean-ph-accuracy-arguments-challenged-with-80-years-of-instrumental-data/