Climate change caused by rising levels of atmospheric carbon dioxide (CO2) is now widely recognized. But the other side of the equation—the massive absorption of CO2 by the ocean—has received far less attention. The planet’s seas quickly absorb 25 to 30 percent of humankind’s CO2 emissions and about 85 percent in the long run, as water and air mix at the ocean’s surface. We have “disposed” of 530 billion tons of the gas in this way, and the rate worldwide is now one million tons per hour, faster than experienced on earth for tens of millions of years. We are acidifying the ocean and fundamentally changing its remarkably delicate geochemical balance. Scientists are only beginning to investigate the consequences, but comparable natural changes in our geologic history have caused several mass extinctions throughout the earth’s waters.
That careful balance has survived over time because of a near equilibrium among the acids emitted by volcanoes and the bases liberated by the weathering of rock. The pH of seawater has remained steady for millions of years. Before the industrial era began, the average pH at the ocean surface was about 8.2 (slightly basic; 7.0 is neutral). Today it is about 8.1.
Although the change may seem small, similar natural shifts have taken 5,000 to 10,000 years. We have done it in 50 to 80 years. Ocean life survived the long, gradual change, but the current speed of acidification is very worrisome. Emissions could reduce surface pH by another 0.4 unit in this century alone and by as much as 0.7 unit beyond 2100. We are hurtling toward an ocean different than the earth has known for more than 25 million years.
About 89 percent of the carbon dioxide dissolved in seawater takes the form of bicarbonate ion, about 10 percent as carbonate ion, and 1 percent as dissolved gas. Modern marine life has evolved to live in this chemistry. A wide variety of organisms use carbonate ion to manufacture their skeletons: snails, urchins, clams, crabs and lobsters. And notably, it forms the calcified plates of microscopic phytoplankton that are so abundant and crucial to the entire marine food chain. Meanwhile carbon dioxide levels influence the physiology of water-breathing organisms of all kinds, which for most creatures has been optimized to operate in a narrow range of dissolved CO2 and ocean pH.
We are now carrying out an extraordinary chemical experiment on a global scale. Our fossil-fuel emissions raise the dissolved CO2 levels in the ocean, which reduces carbonate ion concentrations and lowers pH. The ocean’s sunlit surface layer (the top 100 yards or so) could easily lose 50 percent of its carbonate ion by the end of this century unless we reduce emissions dramatically. Marine animals will find it harder to build skeletons, construct reefs, or simply to grow and breathe. Compared with past geologic events, the speed and scale of this conversion is astonishing.
We therefore have a dilemma. The ocean’s absorption of CO2 helps to keep atmospheric change in check. For decades, climate scientists described the uptake as a blessing for society, and ocean chemists hoped that calcium carbonate sediments on the seafloor would dissolve in sufficient quantities to offset a drop in pH. But research has shown that the rate at which sediments dissolve cannot possibly keep pace with the far faster rate of acidification. Society can continue to depend on the ocean for help, but the cost is a rising threat to all marine life.
Although our understanding remains murky, the fossil record shows that ocean life has suffered massive extinctions during periods of rapidly rising carbon dioxide levels. Marine animals’ metabolic functions are typically tuned to narrow, internal pH ranges. In addition to reducing the calcification of skeletons, more acidic water will acidify body fluids, likely raising respiratory stress and depressing metabolism.
See what we're tweeting about
19 Comments
Add CommentThis claim of ocean absorption of CO2 causing reduced levels of carbonate ions has to be news to every trained chemist.
Reply | Report Abuse | Link to thisHow can adding CO2 to seawater possibly cause a reduction in carbonate ions? When you add CO2 to water some of it reacts with the water to form acidic protons and carbonate ions. The reaction is:
CO2 + H20 = 2H+ + CO3--
The equilibrium constant equation is:
[CO2] = k x [H+] x [H+] x [CO3--] where k is the dissociation or equilibrium constant and the bracketed terms represent the concentrations of those entities.
Adding CO2 must increase the concentration of carbonate ions.
The only way carbonate ion concentration can be reduced is if acid is added from some other source or if carbonate is removed by some other process.
This is basic solution chemistry. Why give credence to unexplained chemical nonsense?
You've neglected the amphiprotic nature of CO3-. ie, HCO3- + H2O --> CO32- + H3O+ and HCO3- + H2O > H2CO3 + OH-. The question is what are those equilibriums, and what are their corresponding strengths compared to the atmospheric pressure of co2.
Reply | Report Abuse | Link to thisOur bodies expose our blood to the same co2. Our blood has the exact same salinity. Anyone notice the mass retardation? So you think you might have a little acid reflux? Math scores dropping lately? Bankers doing dumb stuff? Having a little antibiotic resistant bacteria? You humans have a nice day.
Reply | Report Abuse | Link to thisPraxis, the bicarbonate equilibrium doesn't make any difference to the equilibrium between CO2 and carbonate ion. It is already accounted for in the carbonate equilibrium constant.
Reply | Report Abuse | Link to thisPut simply, there is no way that adding CO2 to water reduces carbonate concentration. It may well reduce Calcium concentrations and that may affect shell growth for all I know but the argument in this article about carbonate concentrations is just utter nonsense.
The principle problem is that dissolved CO2 is dissolving carbonate in the form of shells or plankton skeletal structures. The exact chemical equilibria involved may not be known, but the damage is there.
Reply | Report Abuse | Link to thisThat is totally absurd, eco-steve. More CO2 implies greater carbonate concentrations which will inhibit rather than encourage shells from dissolving. You will have to look deeper for the cause, assuming that the damage has more reality than the present ludicrous explanation.
Reply | Report Abuse | Link to thisHow in the world can SciAm publish an article that doesn't even make sense? This is ridicuous...
Reply | Report Abuse | Link to thisok, i feel a little silly, but I rescind my previous comment, sort of. The author does not do a good job explaining what he means by 'the carbonate concentration will decrease'. It would have been accurate to state that the carbonate ion concentration relative to total dissolved carbon decreases with increasing CO2 absorption.
Reply | Report Abuse | Link to thisYou were right the first time, enochdames. What mechanism or evidence is there that carbonate ion concentration relative to anything decreases with increasing CO2 absorption?
Reply | Report Abuse | Link to thisThe equilibrium constant I quoted establishes a linear relationship between CO2 and carbonate ion concentrations.
This article is chemical nonsense, pure and simple.
the equation you wrote is valid, but it is one of many in the aqueous carbonate system. take a look at this link (http://lawr.ucdavis.edu/classes/ssc102/Section5.pdf) and check out the figure on the 5th page. In short, the carbonate ion is a conjugate base of the bicarbonate ion, and increasing acidity decreases its relative concentration
Reply | Report Abuse | Link to thisThat's true, enochdames, but since both carbonate and bicarbonate conentrations increase with increasing CO2 how is that relevant to shell formation issues?
Reply | Report Abuse | Link to this(I acknowledge my previous post was hasty in that the linearity only exists if acidity is unchanged.)
Reply to Alanw: No doubt it may not seem logical, but increasing dissolved CO2 in seawater preferentially dissolves the shells of marine organisms. This is the conclusion after many tests. Experiment is always more valid that theoretical guesswork....
Reply | Report Abuse | Link to thiseco-steve, fine if that is true in which case they should present the data and let it speak for itself rather than advance a pseudo-explanation that is scientifically absurd.
Reply | Report Abuse | Link to thisWhy in this article would you even put this sentance "some 250 million years ago massive volcanism is thought to have caused ocean acidification and other factors that left 90 percent of marine species dead."
Reply | Report Abuse | Link to thisAs the next paragraph admits the process that brought about 90% extinction rates is on a scale and of a nature completely unrelated to the CO2 acidification process that is under discussion in the rest of the article.
The general need by environmental activists to exaggerate what is going on and get sensationalist about things suggests to me a weakness in the real supportive data. It's about time we saw less hype and more science.
Seems to me the larger concern here is the effect rising ocean acidity will have not on larger shellfish or snails, but on diatoms. Diatoms produce most of the oxygen in the atmosphere. Collapse of their populations could have drastic effects on all oxygen-breathing life on the planet.
Reply | Report Abuse | Link to thisDespite your thoughts, CO2 can't be a good source to rising the acidity of the ocean...
Reply | Report Abuse | Link to thisDespite your thoughts, CO2 can't be a good source to rising the acidity of the ocean...
Reply | Report Abuse | Link to thisThis has to be about the most "unscientific" drivel I have read in years.
Reply | Report Abuse | Link to thisFACT 1: The solubility of a gas - any gas - in a liquid - any liquid, is inversely proportional to the temperature of the liquid, (assuming more or less constant atmospheric pressure). This is one of the so-called "Gas Laws".
Therefore the "amount" of CO2 (regardless of source) dissolved in the ocean is DIRECTLY dependent on the TEMPERATURE of the water, and on NO other factor .
FACT 2: If indeed the global temperatures are rising (including the oceans) - as some would have us believe, then the total amount of dissolved CO2 will - MUST - DECREASE, not INCREASE, and atmospheric levels of CO2 WILL increase.
Conversely, if global temperatures are decreasing, as some argue, the levels of dissolved CO2 WILL increase, and levels of atmospheric CO2 WILL decrease.
Since there would have been a vastly higher level of dissolved CO2 in the oceans during the last Ice Age than now, and since every species alive now in the oceans survived that, preposterous arguments about "man-made ocean acidification" causing damage can be seen for what they are - asinine, entirely non-scientific attempts by the clueless to jump on the AGW gravy train.
There seems to be some confusion among people regarding the effect of increasing CO2 concentration in the atmosphere on the oceans.
Reply | Report Abuse | Link to thisThe first effect to look at is the direct effect of increasing the atmosphere's CO2 concentration on the ocean. The amount that dissolves in a liquid is described by Henry's Law.
"At constant temperature the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid"
This is basically saying that the higher the concentration of the gas the more will dissolve in the liquid and conversely the more gas dissolved in the liquid the less gas the liquid will absorb. If the liquid has a very high level of gas and its partial pressure is higher than the surrounding atmosphere then the liquid will release that gas.
How temperature effects solubility is actually quite complicated but can be simplified; At around room temperature, increasing the temperature reduces the solubility of the gas. For sea water it takes an increase of 16K to half the amount of gas the liquid can absorb. As CO2 has increased by 70% over the last 200 years it would take an increase of 10K to get the amount of CO2 in the seas back to how it was. So far, due to the seas immense weight and therefore heat capacity the seas have increased in temperature by far less than 1K. Therefore as has been measured over the past decades we will expect CO2 levels in the sea to increase.
As CO2 increases so the pH decreases. As more CO2 is added more bicarbonate and carbonate is added to the oceans. As more CO2 is added bicarbonate becomes more favoured and carbonate less so. As sea shells are made of carbonate increasing the CO2 levels causes the shells to dissolve more readily.