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Impacts of Anthropogenic CO2 on Ocean Chemistry and Biology

By Kathy Tedesco, Program Manager, NOAA/OGP
Richard A. Feely, NOAA/PMEL
Christopher L. Sabine, NOAA/PMEL
Cathrine E. Cosca, NOAA/PMEL

Last April, a NOAA/NSF/USGS-sponsored workshop at the USGS Center for Coastal Studies in St. Petersburg, Florida revealed potential future problems for marine ecosystems from ocean acidification (see workshop highlights). A group of fifty international experts discussed how the release of the huge amounts of carbon dioxide from fossil-fuel burning, land-use practices, and cement production will affect the chemistry and biology of the oceans. For 400,000 years prior to the industrial revolution, atmospheric CO2 concentrations remained between 200 to 280 parts per million (ppm). As a result of the industrial and agricultural activities of humans, current atmospheric CO2 concentrations are around 380 ppm, increasing at about 1% per year. Over the past two decades, only half of the CO2 released by human activity, the so-called “anthropogenic CO2,” has remained in the atmosphere; about 30% has been taken up by the ocean, and 20% by the terrestrial biosphere. The atmospheric concentration of carbon dioxide is now higher than experienced on Earth for at least the last 400,000 years, and is expected to continue to rise, leading to significant temperature increases by the end of this century.

Figure 1: Schematic diagram of the carbon dioxide (CO2) system in the atmosphere (top) and surface oceans (bottom), based on the various IPCC 2001 emission scenarios. The pre-industrial CO2 concentrations are for a surface ocean in equilibrium with an atmospheric CO2 level of 280 ppm. The 2x CO2 concentrations are for a surface ocean in equilibrium with an atmospheric CO2 level of 560 ppm. Current model projections indicate that this level could be reached sometime in the second half of this century. The atmospheric values are in units of ppm. The oceanic concentrations, which are for the surface mixed layer, are in units of µmol kg-1. Percent differences from the pre-industrial values are given in parentheses (modified after Feely et al., 2001).

The global oceans are the largest natural reservoir for this excess carbon dioxide, absorbing approximately one-third of the carbon dioxide added to the atmosphere by human activities each year, and over the next millennium, is expected to absorb approximately 90% of the CO2 emitted to the atmosphere. It is now well established that there is a strong possibility that dissolved CO2 in the ocean surface will double over its pre-industrial value by the middle of this century, with accompanying surface ocean acidity (pH) and carbonate ion (CO32-) decreases that are greater than those experienced during the transition from ice ages to warm ages. The uptake of anthropogenic CO2 by the ocean changes the chemistry of the oceans and can potentially have significant impacts on the biological systems in the upper oceans. Estimates of future atmospheric and oceanic CO2 concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) emission scenarios and general circulation models that include the biogeochemical cycles of carbon and nutrients, indicate that by middle of this century atmospheric CO2 levels could be reach over 500 ppm, and near the end of the century they could be over 800 ppm. Corresponding models for the oceans indicate that surface water acidity (pH) drop would be approximately 0.4 pH units, and the carbonate ion concentration would decrease almost 50 % by the end of the century. This surface ocean pH drop would be lower than it has been for more than twenty million years. A pH reduction of approximately 0.1 unit in surface waters has occurred already due to oceanic uptake of anthropogenic CO2.

Recent field and laboratory studies reveal that the carbonate chemistry of seawater has a profound negative impact on the calcification rates of individual species and communities in both planktonic (floating) and ocean bottom organisms. The calcification rate of nearly all calcium-secreting organisms investigated to date decreased in response to decreased carbonate ion concentration. This response holds across multiple taxonomic groups from single-celled organisms to reef-building corals. In general, when dissolved CO2 was increased to two times pre-industrial levels, a decrease in the calcification rate was observed, ranging from -5 to -50%. For example, decreased carbonate ion concentration has been shown to significantly reduce the ability of reef-building corals to produce their calcium carbonate skeletons, affecting growth of individual corals and the ability of the larger reef to maintain a positive balance between reef building and reef dissolution. Scientists have also seen a reduced ability to produce protective calcium carbonate shells in species of marine algae and planktonic molluscs, on which other marine organisms feed. Calcification probably serves multiple functions in calcifying organisms. Decreased calcification would presumably compromise the fitness or success of these organisms and could shift the competitive advantage towards non-calcifiers. Carbonate skeletal structures are likely to be weaker and more susceptible to dissolution and erosion. While long-term consequences are unknown, experimental results from enclosed laboratory experiments indicate that coral reef organisms do not acclimate to decreasing carbonate saturation states over several years. Thus, if calcifying organisms cannot adapt to the changes in seawater chemistry that will occur, the geographical range of some species may be reduced or may shift latitudinally in response to rising CO2. Based on the best available understanding, it appears that as levels of dissolved CO2 in sea water rise, the skeletal growth rates of calcium-secreting organisms will be reduced as a result of the effects of dissolved CO2 on ocean acidity and consequently, on calcification. The effects of decreased calcification in microscopic algae and animals could impact marine food webs and, combined with other climatic changes in salinity, temperature, and upwelled nutrients, could substantially alter the biodiversity and productivity of the ocean. As humans continue along the path of unintended CO2 sequestration in the surface oceans, the impacts on marine ecosystems will be direct and profound.

The following were some of the major conclusions of the workshop:

1. Ocean acidification is a predictable consequence of increased atmospheric carbon dioxide concentrations from human activities. Surface ocean chemistry CO2 and pH changes resulting from these activities can be predicted with a high degree of confidence.

2. Ocean acidification means that there would be concern over carbon dioxide emissions independently and apart from any possible effects of carbon dioxide on the climate system. Ocean acidification and climate change are both effects of CO2 emissions to the atmosphere, but they are completely different; ocean acidification depends on the chemistry of carbon dioxide; whereas climate change depends on temperature and freshwater changes resulting from the atmospheric carbon dioxide and other greenhouse gases.

3. If current trends in carbon dioxide emissions continue, the ocean will acidify to an extent and at rates that have not occurred for tens of millions of years. At present, ocean chemistry is changing at least 100 times more rapidly than it has changed in the 100,000 years preceding our industrial era.

4. Ocean acidification could be expected to have major negative impacts on corals and other marine organisms that build calcium carbonate shells and skeletons. When carbon dioxide reacts with seawater it forms carbonic acid, which is corrosive to calcium carbonate shells and skeletons. The impact is likely to be disruptions through large components of the marine food web. The potential for ecological adaptation is unclear at this time; however, both in today's ocean and over geologic time the rate of accumulation of shells and skeletons made from carbonate minerals shows a consistent relationship with ocean chemical conditions indicating that the success of these organisms is largely controlled by carbonate chemistry.

5. Research is needed to better understand the vulnerabilities, resilience, and adaptability of marine organisms and ecosystems. The science of understanding the biological consequences of ocean acidification, and placing these changes in a historical context, is in its infancy; initial information indicates that there is cause for great concern over the threat carbon dioxide poses for the health of our oceans.

Figure 2: Experimental setup for measuring E. huxleyi calcification under different environmental conditions: (A) Laboratory experiments with a "chemostat" [Sciandra, et al., 2003]; (B) Mesocosms in Norway, for studying effects of CO2 on marine plankton [Delille, et al., 2005; Engel, et al., 2005].

Figure 3a: Experimental setup used to control pCO2. (Langdon and Atkinson, in press) (larger image) Figure 3b: Mesocosm experiment for conducting calcification experiments on coral communities at the Hawaii Marine Biological Institute flume studies on coral communities. (Langdon and Atkinson, in press) (larger image)

Additional websites and reports:

Workshop on the Impacts of Increasing Atmospheric CO2 on Coral Reefs and Other Marine Calcifiers
Report on ocean acidification, Plymouth Marine Laboratory
The Ocean in a High CO2 World Symposium, May 10-12, 2004, UNESCO, Paris France
NOAA's Coral Reef Watch
Coral Reef Ecosystems Studies ­ Caribbean

Richard A. Feely, Christopher L. Sabine, and Cathrine E. Cosca
Pacific Marine Environmental Laboratory
National Oceanic and Atmospheric Administration
7600 Sand Point Way NE
Seattle, WA 98115-6349
(206) 526-6214 Phone
(206) 526-6744 FAX
Richard.A.Feely@noaa.gov
http://www.pmel.noaa.gov/co2/co2-home.html

10/3/05

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