Acidifying the oceans

It is well known that CO2 in the atmosphere has risen from about 275 ppm (.0275%) to 375 ppm (.0375%) since the Industrial Revolution began in the 1800s. This extra CO2, added to the atmosphere by the burning of fossil fuels, has contributed to the observed rise in global temperatures of 0.6 degrees C via the greenhouse effect. What is less well known, and is discussed in detail in a March 2006 article in Scientific American called "The Dangers of Ocean Acidification", is that a tremendous amount of the CO2 emitted by fossil fuel burning winds up in the oceans. The oceans have absorbed 48% of all the CO2 emitted since 1800, according to a study published by Sabine et al. in 2004 in Science. Without the action of the oceans to absorb so much of our waste gases we've pumped into the atmosphere, Earth would be a seriously toasty planet right now.

The price paid
The oceans are paying a price for this service, though. When CO2 dissolves into the ocean, it creates carbonic acid--the same acid found in soda pop. The oceans have dissolved so much CO2 during the past 150 years that the acidity of the oceans' surface waters has substantially increased. The pH, which decreases as acidity increases, used to range from 8.0 to 8.3 for the oceans' surface waters before the Industrial Revolution. This has now dropped a full 0.1 units 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 0.3 units by the year 2100 as the oceans continue to acidify. 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.

Losers
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. These effects are unlikely to be beneficial to fish, though--high levels of CO2 are sometimes used by researchers to euthanize fish.

Winners
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. 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."

Jeff Masters

References

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.