Since the beginning of the industrial revolution the release of the acid carbon dioxide (CO2) from human activities has resulted in atmospheric CO2 concentrations that have increased from approximately 280 to 385 parts per million (ppm). The atmospheric concentration of CO2 is now higher than experienced on Earth for at least the last 800,000 years and probably over 20 million years, and is expected to continue to rise at an increasing rate, leading to significant temperature increases in the atmosphere and oceans in the coming decades. The oceans have absorbed approximately 525 billion tons of carbon dioxide from the atmosphere, or about one third of the anthropogenic carbon emissions released. This absorption has benefited humankind by significantly reducing the greenhouse gas levels in the atmosphere and minimizing some of the impacts of global warming. However, the ocean's uptake of carbon dioxide is having negative impacts on the chemistry and biology of the oceans. Hydrographic surveys and modeling studies have revealed that the chemical changes in seawater resulting from the absorption of carbon dioxide are lowering seawater pH. The pH of ocean surface waters has already decreased by about 0.1 units from an average of about 8.21 to 8.10 since the beginning of the industrial revolution. Estimates of future atmospheric and oceanic carbon dioxide concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) CO2 emission scenarios and coupled ocean-atmosphere models, suggest that by the middle of this century atmospheric carbon dioxide levels could reach more than 500 ppm, and near the end of the century they could be over 800 ppm. This would result in an additional surface water pH decrease of approximately 0.3 pH units by 2100.
The ocean absorbs approximately 1/3rd of the CO2 emitted to the atmosphere from the burning of fossil fuels (1). However, this valuable service comes at a steep ecological cost - the acidification of the ocean. As CO2 dissolves in seawater, the pH of the water decreases, which is called "acidification".
Since the beginning of the industrial revolution, ocean pH has dropped globally by approximately 0.1 pH units.
Past and present variability of marine pH. Future predictions for years shown on the right-hand side of the figure are model-derived values based on IPCC mean scenarios. From Pearson and Palmer (2), adapted by Turley et al. (3) and from the Eur-Oceans Fact Sheet No. 7, "Ocean Acidification - the other half of the CO2 problem", May 2007 (4).
While these pH levels are not alarming in themselves, the rate of change is cause for concern. To the best of our knowledge, the ocean has never experienced such a rapid acidification. By the end of this century, if concentrations of CO2 continue to rise exponentially, we may expect to see changes in pH that are three times greater and 100 times faster than those experienced during the transitions from glacial to interglacial periods. Such large changes in ocean pH have probably not been experienced on the planet for the past 21 million years (5).
When the acid CO2 reacts with seawater, the reduction in alkaline seawater pH also reduces the availability of alkalizing carbonate ions, which play an important role in shell formation for a number of marine organisms such as corals, marine plankton, and shellfish. This phenomenon, which is commonly called "ocean acidification," could have profound impacts on some of the most fundamental biological and geochemical processes of the sea in coming decades. Some of the smaller calcifying organisms are important food sources for higher marine organisms. Declining coral reefs due to increases in temperature and decreases in carbonate ion would have negative impacts on tourism and fisheries. Abundance of commercially important shellfish species may also decline and negative impacts on finfish may occur. This rapidly emerging scientific issue and possible ecological impacts have raised serious concerns across the scientific and fisheries resource management communities.
When CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions into solution. Acidity is a measure of the hydrogen ion concentration in the water, where an increase in hydrogen leads to an increase in acidity (and a decrease in the pH scale used to quantify acidity). These hydrogen ions then combine with carbonate ions in the water to form bicarbonate. Carbonate ions are the basic building blocks for the shells of many marine organisms. Thus the formation of bicarbonate through this chemical reaction removes carbonate ions from the water, making them less available for use by organisms. The combination of increased acidity and decreased carbonate concentration has implications for many functions of marine organisms, many of which we do not yet fully understand.
The details of the reactions look like this:
When CO2 dissolves in seawater, carbonic acid is produced via the reaction:
This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate:
The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases). This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold.
One result of the release of hydrogen ions is that they combine with any carbonate ions in the water to form bicarbonate:
This removes carbonate ions from the water, making it more difficult for organisms to form the CaCO3 they need for their shells.
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The oceans are not, in fact, acidic, but slightly basic. Acidity is measured using the pH scale, where 7.0 is defined as neutral, with higher levels called "basic" and lower levels called "acidic". Historical global mean seawater values are approximately 8.16 on this scale, making them slightly basic. To put this in perspective, pure water has a pH of 7.0 (neutral), whereas household bleach has a pH of 12 (highly basic) and battery acid has a pH of zero (highly acidic). However, even a small change in pH may lead to large changes in ocean chemistry and ecosystem functioning. Over the past 300 million years, global mean ocean pH values have probably never been more than 0.6 units lower than today (6). Ocean ecosystems have thus evolved over time in a very stable pH environment, and it is unknown if they can adapt to such large and rapid changes.
Figure reproduced from the Pew Charitable Trust Policy Brief "Carbon Dioxide and Our Ocean Legacy", by Feely, Sabine, and Fabry (7). |
What can we expect in the future ?
Based on the emissions scenarios of the Intergovernmental Panel on Climate Change and general circulation models, we may expect a drop in ocean pH of about 0.4 pH units by the end of this century, and a 60% decrease in the concentration of calcium carbonate, the basic building block for the shells of many marine organisms (8).
Changes in atmospheric CO2 under the "business as usual" scenario to the year 2100 and associated changes in ocean pH and carbon chemsitry. Adapted from Wolf-gladrow et al., 1999 (9).
Today, the surface ocean is saturated with respect to calcium carbonate (including its several mineral forms, i.e., high-magnesium calcite, aragonite, and calcite), meaning that under present surface conditions these minerals have no tendancy to dissolve and that there is still enough calcium and carbonate ions available for marine organisms to build their shells or skeletons. Colder and deeper waters are naturally undersaturated with respect to calcium carbonate, where the water is corrosive enough to dissolve these minerals. The transition between saturated surface waters and undersaturated deep waters is called the saturation horizon. Because of the increase in CO2 entering into the ocean from the atmosphere, the saturation horizons for calcium carbonate have shifted towards the surface by 50-200 meters compared with their positions before the industrial revolution (10). This means that the zone occupied by undersaturated deep waters is growing larger and the zone occupied by the saturated surface waters is growing smaller.
By 2050, this saturated surface zone will begin to completely disappear in some areas of the ocean. High-latitude surface waters, already naturally low in calcium and carbonate ion concentration, will be the first to have undersaturated surface waters with respect to aragonite, with undersaturations for the calcite phase of calcium carbonate expected to follow 50-100 years later (11).
The figure below by Feely et al. (12) shows aragonite saturation levels from before the industrial revolution to 2100 and how these saturation levels affect the growth of both shallow and deep corals (models based on the work of Orr et al., 2005, (11)). Before the industrial revolution, we see large bands of the tropical ocean that are optimal for growth. By 2040, these same bands are only adequate, and by 2100 most areas are only marginal at best.
Many scientists believe that stabilizing atmospheric CO2 concentration at 550 parts per million (ppm) may avoid the worst impacts from climate change. Atmospheric concentration of CO2 is currently ~380 ppm and, if no precautionary action is taken, is expected to reach 550 ppm by the middle of this century. However, if we consider the impacts of CO2 on ocean chemistry and ecosystems rather than on climate considerations alone, there are strong arguments to be made for a lower stabilization target of 450 ppm.
In order to prevent changes that would lead to undersaturation of aragonite and put marine ecosystems at risk, it has been suggested that the average pH of surface waters should be prevented from dropping by more than 0.2 units below the pre-industrial value. Stabilization of atmospheric CO2 concentrations at 450 ppm by the year 2100 would lead to a pH decrease of about 0.17; stabiliztaion at 540 ppm by the year 2100 would lead to a decrease of 0.23 pH units. With stabilization at 450 ppm, about 7% of the Southern Ocean will still become undersaturated with respect to aragonite. At 550 ppm, about half of the Southern Ocean will be undersaturated (13, 14, 15).
(1) Sabine, C.L., et al. (2004), The Oceanic Sink for Anthropogenic CO2, Science, v305, 367-371.
(2) Pearson, P. and M. Palmer (2000), Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, 406, 695 - 699.
(3) Turley, C., et al. (2006), Reviewing the Impact of Increased Atmospheric CO2 on Oceanic pH and the Marine Ecosystem, in Avoiding Dangerous Climate Change, 65-70, Cambridge University Press.
(4) Eur-Oceans Fact Sheet No. 7 (2007), "Ocean Acidification - the other half of the CO2 problem".
(5) Priorities for Research on the Ocean in a High-CO2 World (2004) from the international science symposium The Ocean in a High-CO2 World.
(6) Caldeira, K. and M.E. Wickett (2003), Anthropogenic carbon and ocean pH, Nature, 425, 365.
(7) The Pew Charitable Trust Science Brief (2006), "Carbon Dioxide and Our Ocean Legacy", by R.A. Feely, C.L. Sabine, and V.J. Fabry.
(8) Feely, R.A. et al. (2004), Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans, Science, v305, 362-366.
(9) Wolf-Gladrow et al. (1999) Direct effects of CO2 concentration on growth and isotopic composition of marine plankon. Tellus B, 51, 461-476.
(10) Doney, S.C. (2006), The Dangers of Ocean Acidification, Scientific American, 58-65.
(11) Orr, J.C. et al. (2005), Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, Nature, 437, 681-686.
(12) Feely, R.A., J. Orr, V.J. Fabry, J.A. Kleypas, C.L. Sabine, and C. Landgon (2006): Present and future changes in seawater chemistry due to ocean acidification. AGU Monograph on "The Science and Technology of Carbon Sequestration," in press.
(13) The Future Oceans - Warming up, Rising High, Turning Sour (2006), A Special Report of the German Advisory Council on Global Change.
(14) Cao, L. and K. Caldeira (2007), Ocean acidification and atmospheric CO2 stabilization, Geophysical Research Letters, Vol. 34, L05607, doi: 10.1029/2006GL028605.
(15) Caldeira, K. and M.E. Wickett (2005), Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean, J. Geophys. Res., 110, C09S04, doi: 10.1029/2004JC002671.
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