One of the more immediate problems with the rapid increase in atmospheric CO2 concentration due to human activities is the sudden shift in ocean acidity. The natural dynamic equilibrium exchange of carbon between the atmosphere and the ocean has existed for eons. For a vast majority of that time, there was little disruption in that exchange for although pH levels have oscillated between 7.3 and 8.2 such oscillation occurred over millions of years at a slow and steady pace.1 However, the excess CO2 that is being released into the atmosphere in the last 200 years, largely due to burning fossil fuels and deforestation, has accelerated oceanic uptake of atmospheric CO2 in effort to maintain the carbon concentration equilibriums. This additional uptake over a much shorter time frame than that of the past has created a concern regarding the adaptation and survival ability of oceanic flora and fauna.
Recall that when CO2 dissolves in water it forms carbonic acid eventually leading to the reactionary increase in ocean acidity (additional hydrogen atoms are contributed from the breakdown of carbonic acid). In fact CO2 absorption has reduced surface pH (increased acidity) by approximately 0.1 in the last decade after over 100 million years of steady decrease in acidity.1,2,3 Calcium carbonate becomes thermodynamically less stable as oceanic acidity increases, due to reducing concentrations of carbonate, a result tied to the increase in CO2 concentration, increasing the metabolic cost to organisms when constructing calcium carbonate-based infrastructure (shells and skeletons).
In fact the Southern Ocean near Antarctica is already experiencing significant acidification far beyond anywhere else in the world and this increase is having a negative influence on the ability of G. bulloides to build their shells.4 Similar results in calcification rates have also been seen in the Arabian Sea for other similar calcium carbonate shell builders.5 This negative influence limits the ability of the ocean to expand CO2 uptake from the atmosphere without increasing acidity due to reduction in sedimentation burial. In addition this infrastructure instability disrupts a variety of different and important food chains.
There are some that believe nature will be able to adapt to these acidity changes because the Arctic and Southern Ocean regions have life that is more used to higher acidity conditions, but this mindset comes off as rather naïve optimism. The most popular example relating to this belief is the analogy that a 5 degree average temperature increase in Phoenix will not faze its residence as much as a 5 degree average temperature increase in Siberia (or some other cold region). However, such an example irrationally seems to mitigate the fact that pH exists on a log scale, thus even small changes are significant to a given life form, hence why most life, without special ‘millions of years in the making’ adaptation, can only exist within a very small range of pH. It does not matter whether one lives in Phoenix or Siberia, dealing with consistent 120 degree temperatures that arise suddenly is a burden that will have significant influence on livelihood. In fact over 65 million years ago ocean acidification was linked to mass extinctions of calcareous marine organisms,6 it would be foolish to assume that a more rapid acidity increase would fail to replicate this extinction in due time.
Another influencing factor when considering ocean acidity is ocean temperature. It is common chemistry that gas solubility in a liquid decreases as temperature increases because increasing temperature increases available kinetic energy which increases molecule movement. Greater molecule movement increases the probability of bond breaking which reduces the ability of the gas to remain in solution. There is no argument that global temperatures both on land and in the ocean are increasing; therefore, as these temperatures go up it is likely that the overall capacity of the ocean to absorb excess CO2 from the atmosphere will decrease and cause the ocean to release CO2 into the atmosphere.
Releasing CO2 into the atmosphere until a new equilibrium is achieved may slightly increase ocean pH (lower ocean acidity), but the problem is that the magnitude or timing of such a reaction is completely unknown. The ocean cannot be viewed so simply as a giant beaker of water sitting on a bench in a laboratory, so applying any simplistic CO2 solubility curve to determine when any switch from sink to source for the ocean may occur is naïve. Regardless of when the ocean begins to naturally decrease in acidity due to decreased CO2 solubility, it is reasonable to believe that ocean acidity levels will not naturally drop below their present level. Therefore, despite the potential for CO2 release at some point in the future, the issue of ocean acidity still needs to be addressed in the near future.
The rate of calcium carbonate precipitation is an important element in determining the sink capacity of the ocean and the total expected acidity change because calcium carbonate has a tendency to be removed through gravitational settling.1 Considering this removal due to calcium carbonate precipitation is important because despite the total sum of dissolved carbon species (DIC) decreasing, the remaining carbon shifts its balance in favor of pure CO2 (aq) increasing the higher partial pressure of CO2 in the ocean.1 The reason for the shift is the loss of CO3 which drives the aqueous carbonate equilibrium reaction [CO2 (aq) + CO32- + H2O ↔ 2HCO3] to the left to compensate.1
However, dissolution of calcium carbonate, frees more carbonate ions, resulting in an opposite shift reducing oceanic concentration of CO2 enhancing atmospheric CO2 acquisition. Basically precipitation of carbonate reduces CO2 uptake from the atmosphere whereas dissolution of carbonate increases CO2 uptake from the atmosphere. Remember that because both the carbonic acid and the calcium carbonate are in equilibrium, loss of reactionary species typically CO32-, will be influenced by Le Chatlier’s principal.
However, a second factor in this process of CO2 exchange must be considered, the interaction and association between particulate organic carbon and calcium carbonate concentration shifts.7,8 A decrease in calcium carbonate reduces the rate and effectiveness of moving particulate organic carbon to deeper waters, thus weakening the biological pump portion of oceanic CO2 absorption method.1 This result reduces the total CO2 sink capacity of biological denizens of the ocean like phytoplankton.9,10 So an impasse exists in that does decreasing the concentration of calcium carbonate increase oceanic sink capacity or decrease oceanic sink capacity? Currently there is no good answer to that question.
Regardless of the correct answer it cannot be debated that the ocean is becoming more acidic due to an increased rate in uptake of atmospheric CO2, the only thing up for debate is the rate of acidity change. Also it is important to note that any geo-engineering strategy to ward of atmospheric global warming that does not result in the removal of CO2 from the atmosphere will have no ability to reduce the rate of acidification. In fact such geo-engineering methods may actually increase ocean acidity by delaying any CO2 release from the ocean due to temperature increases.
Unfortunately the atmospheric-oceanic exchange is the not the only contribution to increased ocean acidity. Increasing surface temperatures have destabilized methane hydrate stored in sediments beneath the seabed throughout various portions of the ocean. Due to the accelerated warming in the Arctic, most of the new methane hydrate destabilizations are originating in the Arctic and Southern Oceans and areas along the continental shelf.11 The good news/bad news aspect of this destabilization is that most of the methane is absorbed/dissolved in an upper layer of the ocean before it is able to fully escape into the atmosphere, thus only a small percentage of this released methane will immediately influence global warming. Unfortunately it does not stay as methane for long in the ocean as methanotrophs interact with this methane converting it into CO2 not only further increasing ocean acidity, but also increasing the concentration of CO2 in the ocean which will eventually cause the ocean to become a source of atmospheric CO2 instead of a sink for atmospheric CO2. However, unlike the solubility change due to temperature increase scenario, the acidity will not go down because for all of the CO2 released more methane will be converted to CO2 to take the place of the released CO2. The final side detriment to this methane hydrate release is that in the process of converting the methane to CO2 the methanotrophs use oxygen which creates the high probability for hypoxic or anoxic conditions within the localized region of ocean.
With the continuing increase in acidity and the negative influence it seems to be having on oceanic fauna, a remediation strategy is needed before permanent damage occurs. Unfortunately the best option, significantly reducing the concentration of CO2 released into the atmosphere from human based sources, which would eventually reverse the process of ocean CO2 absorption, is decades away, if it happens at all; therefore an alternative stabilizing strategy needs to be considered.
The most straightforward means to reduce ocean acidity would be to speed the removal of unassociated (free) CO2 from the ocean. Reducing free CO2 would in turn reduce the probability of carbonic acid formation and the resultant equilibrium shifts. One of the first ideas that comes to mind would be iron fertilization, but unfortunately iron fertilization does not appear to be as useful as advertised,12,13 especially where it counts in the Southern and Arctic Oceans.
Another idea that has gained some backing in recent years is thermally decomposing limestone into CO2 and calcium oxide and then depositing the calcium oxide into the ocean to facilitate a chemical reaction to sequester CO2. When dumped into the ocean the calcium oxide reacts with water forming calcium hydroxide. Finally the calcium hydroxide reacts with free dissolved CO2 in the ocean creating calcium bicarbonate. The three primary chemical reactions governing this strategy are shown below.
Unfortunately there are some concerns that significantly reduce the viability of this option. First, the reaction rate between calcium hydroxide and CO2 is contingent on many factors, similar to most chemical reactions most notably pH, pressure and temperature. Also CO2 within the ocean is still rather dilute, which further lowers the probability of reaction. Realistically it is logical to anticipate some inefficiency or non-reaction from the total amount of calcium hydroxide. Therefore, an estimate of 1.6 to 1.8 moles of CO2 reacted per 1 mole of calcium hydroxide in the ocean seems more realistic.
Although some of the benefit was lost, so far so good as the process still removes more CO2 than it generates, right? Not necessarily, second most of the proponents of this strategy play-down that the limestone needs to be processed and that requires significant amounts of heat energy (800-900 C). It takes approximately 2.67 GJ (741.67 kw-h) to calcinate 1 ton of limestone.
Looking at general power sources that could provide that energy, coal typically produces 1 ton of CO2 per 1000 kw-h of electricity, which would put the process on the edge of being carbon positive/negative unless calcium hydroxide reaction efficiency was higher than anticipated, so coal is out. Oil cannot be used because of a continuing dwindling supply and the emission profile is not much better as although it remains carbon negative (approximately 1300 kw-h per 1 ton of CO2) the ratio drops to about 0.1-0.3 tons of CO2 per ton of calcium hydroxide). Natural gas is a bit better, depending on the total efficiency of the combustion, 2,000 to 2,500 kw-h of electricity per 1 ton of CO2 emitted; however, using natural gas would still require the generation of 0.297 to 0.371 tons of CO2, which would take a bite out of the overall CO2 absorption. Overall it does not seem to matter whether the source of the energy provider is stranded or not because the resultant CO2 emission would put unacceptable economic burden on the overall removal ability of the process. Therefore, it appears that a zero carbon emission energy source will have to be used to generate the calcium oxide from limestone to ensure appropriate economical action.
The additional CO2 produced aside, another problem is the shear cost of the electricity to run the process. For example in the United States using an average of 11 cents per kW-h conversion of 1 ton of limestone would cost $81.58. Taking that cost and expanding it to calculate the cost of removing 1 net ton of CO2 from the ocean assuming a zero carbon emission source is used to generate the power, zero transportation emissions (highly unlikely) and a high efficiency reaction of 1.8 moles of CO2 from the ocean and it would currently cost approximately $102 to remove 1 net ton of CO2 from the ocean with this process, just for the energy required for the limestone conversion alone. Assuming that 100% of all of the CO2 produced from the limestone conversion reaction were sequestered the cost would drop to $45.32 per net ton of CO2, but that cost is not absolute because of the cost uncertainty associated with the capture and sequestration processes. Finally in order to generate a reasonably pure CO2 product stream during the limestone reaction either co-firing with limestone, fuel/electricity source and oxygen or separation of the heating/calcine reactions will have to take place increasing the probability of greater cost.
Overall the initial calculations project the limestone strategy to not be economically attractive. However, clearly it is worth paying a price to avoid severe and detrimental climate change, which would cost much more in the long run. Unfortunately the cost may not be the only problem with the limestone strategy. Currently there has been very little practical application of the limestone strategy through discussion of exactly how the calcium oxide would be distributed throughout the ocean. This lack of discussion is a problem because it is very unrealistic to expect a widespread distribution strategy to be successful largely because first the ocean is rather huge. Second, the transport emissions associated with widespread distribution would almost certainly switch the process from slightly carbon negative to definitely carbon positive, and that does not include the associated transportation costs. Remember there are no viable zero emission planes, zero emission ships would probably be too inefficient in transport time and the cost to create infrastructure for new zero emission trains would be backbreaking.
Based on these two problems it appears that the best option is a localized release. Initially this result may seem beneficial because of the large limestone deposits located at Nullarbor Plain, Australia. However, localized distribution also has a significant problem, rate of deposit. If too much calcium oxide is depsoited into the ocean over too short a timeframe and/or area then it is highly possible that a pH shift in the opposite direction would occur in that localized region significantly reducing the biodiversity of the deposit region. If too little calcium oxide is deposited into the ocean then the overall CO2 removal process would be far too slow to make any real difference in averting climate change or reducing ocean acidity and the entire process itself could be viewed as a waste of time and money. Therefore, a proper despoit rate over a given localized area would need to be determined, a determination that appears to be difficult to do in the lab. Also even if a proper balance was determined, another lingering problem is that although the ocean does mix, calcium oxide interaction over a small localized region would probably induce faster reaction over ocean mixing and atmospheric uptake which would further reduce the efficiency of the reaction and the prospect of it being carbon negative. Overall it appears that the total amount of limestone, energy and cost required for the above strategy provide too great of obstacles to make this an effective strategy for remediation of atmospheric or oceanic CO2 at the current time.
With more natural methods lacking efficiency perhaps a more technological strategy to facilitate CO2 removal is necessary. One advantage with the deployment of such a solution is that it does not need to be scaled-up to reduce the acidity of the entire ocean. Due to the non-uniformity of oceanic flora and fauna, technologies can be designed and applied to higher density and biologically and/or commercially important regions to reduce localized levels of acidity to increase survivability. Note that it is important that any technology eliminate the acidity at the localized region not simply divert it elsewhere further increasing the acidity levels at non-targeted regions of the ocean because due to ocean mixing such a strategy would not achieve the desired goal, even in the short-term. At a later time such a device will be proposed here at the Bastion of Reason that will hopefully provide a means to reduce ocean acidity.
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