|Figure 1. Changes in surface oceanic concentration of CO2 (left, in micro-atmospheres), and pH (right) from three locations. Blue is at 29°N, 15°W in the Canary Islands; green is at 23°N, 158°W in the Hawaiian Islands, and red is 31°N, 64°W at Bermuda. The mean seasonal cycle was removed from the data, and the thick black line is smoothed to not include any information less than 1/2 year in period. Note that as CO2 has risen, the pH of the oceans has fallen as the waters become more acid. Image credit: IPCC 2007: The Physical Basis for Climate Change.|
The price paid
The oceans are paying a price for this service, though. When CO2 dissolves into the ocean, it creates carbonic acid. The oceans have dissolved so much CO2 during the past 150 years that the acidity of the oceans' surface waters has substantially increased. Before the Industrial Revolution, pH of the ocean surface waters ranged from 8.0 to 8.3 (pH decreases as acidity increases). Ocean pH has dropped a full 0.1 units since then, to the 7.9 to 8.2 range. Unless significant cuts in CO2 emissions are realized in the next few decades, the pH will fall another 0.14-0.35 units by the year 2100 as the oceans continue to acidify, according to the Intergovernmental Panel on Climate Change (IPCC) 2007 Synthesis Report. A 2005 report by the Royal Society of the UK projects the decrease by 2100 will be 0.5 pH units, and notes that it will take more than 10,000 years for the ocean to return to its pre-1800s acidity level.
Higher acidity in the ocean creates problems for a number of organisms. Corals and other creatures that build shells out of calcium carbonate are particularly vulnerable, since they cannot form their shells if the acidity passes a critical level--their shells will dissolve. Several shell-building planktonic organisms, such as coccolithophorids, pteropods, and foraminifera, form an important basis of the food chain in the cold waters surrounding Antarctica. The effect of ocean acidification is more pronounced at colder temperatures, and it is believed that these important micro-organisms will die out or be forced to move to warmer waters in order to survive in the coming decades. What this will mean to the birds, fish, marine mammals, and humans that depend on the oceans for their livelihood is unknown. Major die-offs of many species are quite possible, which would have serious impacts for nations such as Chile, where marine-related activities provide more jobs than any other sector of the economy. The effects on the Atlantic are expected to be delayed several decades compared to the Southern Hemisphere oceans, but are still expected to be significant by the end of the century.
Corals in tropical and subtropical waters will not dissolve in the more acidic waters, but the increased acidity will cause them to grow more slowly. When this added stress is added to the already significant impacts of coral bleaching from global warming, pollution, and destruction due to dynamiting of reefs to harvest fish, the outlook for coral reefs this century is exceedingly bleak. About one-third of the world's coral reefs have already been damaged or destroyed in the past century, with another one-third at serious risk of destruction by 2030.
The effect of higher oceanic acidity and CO2 levels on higher organisms such as fish, birds, and sea mammals is largely unknown. A 2008 study found that purple sea urchins are unable to build their spiny shell in acid water. Fish are also likely to be adversely impacted, since high levels of CO2 are sometimes used by researchers to euthanize fish.
Higher dissolved CO2 in the oceans will benefit a number of species. For example, many higher plants such as sea grasses use dissolved CO2 directly to help them grow, and should prosper from higher CO2 levels in the ocean, just as many plants on land are expected to benefit from higher atmospheric CO2 levels. Some types of phytoplankton will probably benefit as well, although laboratory studies on this are not conclusive. Other species of phytoplankton will likely be unaffected. The Royal Society of the UK report concluded, "the increase of CO2 in the surface oceans expected by 2100 is unlikely to have any significant direct effect on photosynthesis or growth of most micro-organisms in the oceans."
What the future holds
Ocean life can adapt to higher acidity. One study (Spivack et al., 1993) found that pH levels in the ocean 7.5 million years ago were about 7.4, well below today's pH. The big concern with the current increase in acidity and drop of ocean pH levels is that it is being compressed into such a short period of time. Past changes in oceanic acidity have presumably occurred over tens of thousands of years, giving time for life to adapt. A July 2006 study, Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, put out by 50 of the world's leading experts in ocean chemistry, warned that modern sea life will probably adapt poorly to more acidic waters. This is because the oceans have not been as acidic as they now are for at least 650,000 years, and probably millions of years beyond that. Modern ocean life has evolved for a great deal of time under balanced ocean conditions, and the current change may occur so fast that a partial collapse of the food chain in some regions may occur. One note of optimism: similar concerns were voiced when the Antarctic ozone hole opened up, exposing phytoplankton in the Southern Hemisphere oceans to a rapid and unprecedented increase in levels of damaging ultraviolet radiation. It was widely feared that this increase in UV light would destroy enough phytoplankton to trigger a collapse of the food chain in the waters off of Antarctica. This has not happened. One study (Smith et. al., 1992) found a 6-12% decrease in phytoplankton during the time the ozone hole opens up, typically about 10-12 weeks of the year. So, at least in this one case, the marine ecosystem was able to adapt to a rapid, unprecedented change and not collapse.
As is the case with many aspects of human-caused climate change, the dangers are enormous, but poorly understood. In the words of the Dr. Doney's Scientific American article, "dramatic alterations in the marine environment appear to be inevitable." The Royal Society's article cautions, "research into the impacts of high concentrations of CO2 in the oceans is in its infancy and needs to be developed rapidly." The report goes on to state, "Ocean acidification is a powerful reason, in addition to that of climate change, for reducing global CO2 emissions. Action needs to be taken now to avoid the risk of irreversible damage to the oceans. We recommend that all possible approaches be considered of prevent CO2 reaching the atmosphere. No option that can make a significant contribution should be dismissed."
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Sabine et al., "The Oceanic Sink for Anthropogenic CO2", Science, 305, 367-371, 16 July 2004.
Smith, R., B. Prezelin, K. Baker, R. Bidigare, N. Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D. Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion: Ultraviolet radiation and phytoplankton biology in Antarctic waters", Science, 255, 952, 1992.
Spivack, A.J., You, C., and H.J. Smith, "Foraminiferal boron isotope ratios as a proxy for surface ocean pH over the past 21 Myr", Nature, 363, 149-151, 13 May 1993, doi:10.1038/363149a0.