a critical emergiNg prOblem FOr the OceaN ScieNceS

Author Posting. © Oceanography Society, 2009. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 22 no. 4 (2009): 16-25.

S p e c i a l i S S u e F e at u r e

OceaN acidiFicatiON abStr act
Over a period of less than a decade, ocean acidification-the change in seawater chemistry due to rising atmospheric carbon dioxide (CO 2 ) levels and subsequent impacts on marine life-has become one of the most critical and pressing issues facing the ocean research community and marine resource managers alike.The objective of this special issue of Oceanography is to provide an overview of the current scientific understanding of ocean acidification as well as to indicate the substantial gaps in our present knowledge.Papers in the special issue discuss the past, current, and future trends in seawater chemistry; highlight potential vulnerabilities to marine species, ecosystems, and marine resources to elevated CO 2 ; and outline a roadmap toward future research directions.In this introductory article, we present a brief introduction on ocean acidification and some historical context for how it emerged so quickly and recently as a key research topic.

backgrOuNd
When we burn gasoline in our cars, use electricity from burning natural gas or coal at power plants, or chop down and burn tropical forests for new agricultural land, we release carbon dioxide (CO 2 ) gas into the air.The quantity of carbon released by human activities is enormous.For 2008, the most recent year for which we have published data, total human CO 2 emissions were about 10 billion tons of carbon annually (equivalent to one million tons per hour or, on a per capita basis, ~ 0.2 kg person -1 h -1 ; note that 1 billion tons equals 1 Pg or 1 x 10 15 g).Of this amount, 8.7 ± 0.5 billion tons originates from fossil fuel combustion and cement production and another 1.2 ± 0.7 billion tons from deforestation (Le Quéré et al., 2009).The cumulative human CO 2 emissions over the industrial era now amount to close to 560 billion tons.A little less than half of this anthropogenic CO 2 remains in the atmospherecertainly enough to be of grave concern FOr the OceaN ScieNceS (Keeling, 1960);present values (387 ppm) are already more than 37% greater than pre-industrial levels (~ 280 ppm) (Feely et al., 2009;Tans, 2009).If fossil fuel consumption continues unabated, it could double or triple before the end of this century (Tans, 2009).The current rapid rise in atmospheric CO 2 is as much as 30 times faster than natural rates in the geological past, and present levels are higher than at anytime in at least the last 850,000 years and likely several million years (Kump et al., 2009).
As atmospheric CO 2 rises, thermodynamics and air-sea gas transfer processes drive some of the extra CO 2 into ocean surface waters, leading to substantial shifts in seawater acid-base chemistry and, importantly, the chemical speciation of the large reservoir of inorganic carbon dissolved in seawater (Figure 1).In aggregate, these chemical changes are termed "ocean acidification." The basic principles for these reactions have been appreciated for some time, and even before the start of the Mauna Loa record, Revelle and Suess (1957) argued that the ocean would quite effectively remove a large fraction of fossil fuel CO 2 from the atmosphere (see also Bolin and Eriksson, 1959).
The basic chemistry of ocean acidification was first described in the early 1970s, based on early models of CO 2 exchange at the air-sea interface and the thermodynamics of the carbon system in seawater (Broecker et al., 1971(Broecker et al., , 1979;;Broecker and Takahashi, 1977;Fairhall, 1973;Zimen and Altenhein, 1973;Whitfield, 1974;Skirrow and Whitfield, 1975;Pytkowicz and Small, 1977).Although these early authors all presented calculations to show that CO 2 emissions would likely cause undersaturation with respect to aragonite and calcite at some point, their estimates of when this might happen varied greatly because of a lack of agreement on carbon system equilibrium in seawater at that time.As more laboratory and field results were published in the 1980s, it became clear that the high-latitude regions of the ocean would first become undersaturated with respect to aragonite sometime in the twenty-first century (Feely and Chen, 1982;Mucci, 1983;Byrne et al., 1984;Feely et al., 1984Feely et al., , 1988) ) and that tropical regions would remain supersaturated with respect to these minerals throughout the twenty-first century.
By the 1990s, an intense effort of ship-based surveys and ocean time series was underway to quantify the ocean's role in the climate system as a sink for human-released CO 2 (Sabine et al., 2004).Now, the alteration of seawater chemistry from the invasion of excess CO 2 into the ocean is also clear from these ongoing field observations (Dore et al., 2009;Fabry et al., 2009;Feely et al., 2009;Hauri et al., 2009).The analytical methods for seawater carbonate chemistry are now well established (Dickson et al., 2007).Coordinated observational strategies for monitoring ocean acidification (and its potential biological impacts; see below) are underway (Feely et al., in open review), applying traditional techniques as well as new approaches that employ satellite remote sensing (Gledhill et al., 2009;Balch and Utgoff, 2009) and autonomous platforms such as floats and gliders (Johnson et al., 2009;Bishop, 2009).Note that the increase in oceanic cO 2 over the past 17 years is consistent with the atmospheric increase within the statistical limits of the measurements.Mauna Loa data courtesy of Pieter Tans, National Oceanic and Atmospheric Administration/Earth System Research Laboratory (http://www.esrl.noaa.gov/gmd/ccgg/trends);Hawaii Ocean Time-Series (HOT)/ALOHA data courtesy of David Karl, University of Hawaii (http://hahana.soest.hawaii.edu;see also Dore et al., 2009) Concerns also arose about how marine ecosystems might respond to ocean warming and changes in circulation caused by alterations in the planet's radiative balance from the elevated CO 2 in the atmosphere (Boyd and Doney, 2002).By contrast, the biological effects of rising atmospheric and oceanic CO 2 directly on marine life was a rather obscure topic through much of the 1980s and early 1990s, explored by only a few scientists.Several groundbreaking studies, specifically designed to test atmospheric CO 2 impacts, revealed potentially dramatic responses in corals and coral reef communities (e.g., Gattuso et al., 1998;Marubini and Atkinson, 1999;Kleypas et al., 1999;Langdon et al., 2000) and planktonic organisms (e.g., Riebesell et al., 2000).
Broad interest and visibility on the topic were spurred by an influential Royal Society report (Royal Society, 2005) and the 2004 Symposium on the Ocean in a High-CO 2 World (Orr et al., 2009).Toward the end of this decade, there is now striking evidence from the laboratory and field that many marine species may be affected by ocean acidification (e.g., see recent review papers: Kleypas et al., 2006;Fabry et al., 2008;Guinotte and Fabry, 2008;Doney et al., 2009;Kleypas and Yates, 2009).

biOlOgical impactS
Unlike the case for terrestrial plants, many marine phytoplankton species are not limited by aqueous CO 2 gas concentrations, having developed biochemical techniques for concentrating CO 2 inside their cells or by tapping into the much larger seawater pool of dissolved inorganic carbon (e.g., Tortell et al., 1997).
Recent work, however, suggests that photosynthesis rates of some cyanobacteria may be enhanced under elevated aqueous CO 2 , especially in conjunction with warming, and that there may be a wide range of possible effects on nutrient cycling, including increased nitrogen fixation rates (Hutchins et al., 2009).Phytoplankton growth may also be influenced by CO 2 -driven changes in acid-base chemistry and trace metal availability (Millero et al., 2009).
Similarly, the pH gradient across cell membranes is coupled to numerous critical physiological/biochemical reactions within marine organisms, ranging from such diverse processes as photosynthesis, to nutrient transport, to respiratory metabolism.The impact of ocean acidification (and changing pH gradients) on this biochemistry is barely understood (Figure 2).
Increased solubility of calcium carbonate minerals used as skeleton and shell material by corals (Cohen and Holcomb, 2009;Kleypas and Yates, 2009) and other pelagic and benthic calcifiers (Fabry et al., 2009a(Fabry et al., , 2009b;;Balch and Utgoff, 2009) generally results in a slowdown of the overall calcification process by mechanisms that are just beginning to be understood (Cohen and Holcomb, 2009).In fact, the response of calcifying organisms to ocean acidification may be more varied than first thought, as indicated in recent experiments showing elevated calcification rates for some taxa under higher CO 2 (Ries et al., 2009).Decreased calcification could have negative impacts on marine ecosystems, with consequent effects on local marine fisheries and coastal protection from storms.The abundance of commercially important shellfish species (i.e., clams, oysters, sea urchins) could also decline, which could have serious consequences for marine food resources (Cooley et al., 2009).

phySical impactS
A generally unappreciated physical impact of ocean acidification is the reduction of low-frequency sound adsorption because of the pH-dependent decline in dissolved borate ions (Brewer and Hester, 2009).As noted by Brewer and Hester (2009), the effect can be significant: "a decline in pH of only 0.3 causes a 40% decrease in the intrinsic sound absorption coefficient (α, dB km -1 )." Nevertheless, the environmental consequences of increased "noise" in the ocean, particularly with respect to whales and other marine mammals, is largely unknown.This more applied chemistry would be of strategic importance to both shipping and naval interests, particularly as it affects the integrity of ship hulls. 1 it is recognized that the paleosciences will continue to provide important contributions to unraveling the consequences of ocean acidification.however, this field entails another broad spectrum of scientific approaches not covered in this guide to best practices.The paleo community may find it beneficial to formulate guidelines and standardized protocols specific for research on past acidification events.

NatiONal aNd iNterNatiONal reSpONSe tO OceaN acidiFicatiON
as a greenhouse gas leading to climate change.The remainder is, at present, removed in roughly equal parts into the ocean and by land vegetation.Revelle and Suess (1957) wrote a prophetic view of our perturbations to the global carbon cycle: Thus human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future-a sentiment that may be especially true for ocean acidification.The build-up of excess CO 2 in the atmosphere is clearly evident in time series such as the one established in 1958 by Charles David Keeling from the top of Mauna Loa volcano in Hawaii, the longest atmospheric CO 2 instrumental record.When Keeling started making measurements, atmospheric CO 2 was about 315 parts per million (ppm) b y S c O t t c .d O N e y, W i l l i a m m .b a l c h , V i c t O r i a J .Fa b r y, a N d r i c h a r d a .F e e ly a critical emergiNg prOblem

Figure 1 :
Figure1: time series of atmospheric cO 2 at mauna loa (in parts per million volume, ppmv; red), surface ocean pcO 2 (μatm; blue) and surface ocean ph (green) at Ocean Station alOha in the subtropical North pacific Ocean.Note that the increase in oceanic cO 2 over the past 17 years is consistent with the atmospheric increase within the statistical limits of the measurements.Mauna Loa data courtesy of Pieter Tans, National Oceanic and Atmospheric Administration/Earth System Research Laboratory (http://www.esrl.noaa.gov/gmd/ccgg/trends);Hawaii Ocean Time-Series (HOT)/ALOHA data courtesy of David Karl, University of Hawaii (http://hahana.soest.hawaii.edu;see alsoDore et al., 2009) Along with sound propagation, light propagation might also be affected.In a "decalcified" ocean, devoid of the ubiquitous calcium carbonate particles such as microscopic coccoliths, light scattering and attenuation would be reduced, resulting in deeper euphotic zones.This scenario could have consequences for such biogeochemical aspects as export Scott C. Doney (sdoney@whoi.edu) is Senior Scientist, Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.William M. Balch is Senior Research Scientist, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME, USA.Victoria J. Fabry is Professor, Department of Biological Sciences, California State University, San Marcos, San Marcos, CA, USA.Richard A. Feely is Senior Scientist, Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, WA, USA.

bOx 3 .
guide tO beSt pr acticeS iN OceaN acidiFicatiON reSearch aNd data repOrtiNg by u l F r i eb e Sel l , Vi c tO r ia J .Fab ry, l iNa h aN S S O N , aNd Je aN-pierre gat tuSO Growing concern about the effects of ocean acidification on marine organisms and ecosystems has stimulated a wide range of research activities over the past few years.With new national and international programs recently started and others still in preparation, research aimed at detecting potential effects of ocean acidification on various processes and organisms will increase in the coming decade.Due to the crosscutting nature of the scientific problem, research on ocean acidification brings together a spectrum of disciplines, from paleo-and chemical oceanography, to marine biogeochemistry and climate modeling, to marine ecology, physiology, and molecular and evolutionary biology.The various scientific communities have their own disciplinary heritage, and frequently use specific terminology, research approaches, and methodologies.Moreover, with new funding opportunities now becoming available for ocean acidification research, many researchers, postdoctoral investigators, and PhD students with no or limited previous experience in ocean acidification research will enter the field.To ensure comparability of the vast amount of data generated in this rapidly expanding field of marine sciences and to achieve the highest possible data quality, it is important to agree on standardized protocols for observational and experimental approaches, carbonate chemistry manipulations and measurements, and data reporting.In November 2008, the European Project on Ocean Acidification (EPOCA) and the International Oceanographic Commission (IOC) of UNESCO, organized an international workshop in Kiel, Germany, production (Balch and Utgoff, 2009).It is not a stretch of the imagination to assume that the physical chemistry of seawater will change such that rates of redox reactions associated with metal oxidation and electolysis would change.
Over the last several years, the rapid pace of scientific discoveries has led to a dramatic rise in the visibility of, concern about, and urgency of ocean acidification.Nongovernmental organizations are issuing communiqués and declarations, and research programs on ocean acidification are being launched around the world at national and international levels (e.g., the European Project on Ocean Acidification [EPOCA] in the European Union; Gattuso et al., 2009; http://www.epoca-project.eu/;Orr et al., 2009).EPOCA has also developed a Guide for Best Practices in Ocean Acidification Research and Data Reporting for conducting research on ocean acidification under controlled laboratory conditions (Box 3; Riebesell et al., in press).In the United States, a key scientific workshop on the topic was organized under the Ocean Carbon and Biogeochemistry (OCB) Program (http://us-ocb.org/;Fabry et al., 2009b).Following the passage of the Federal Ocean Acidification Research And Monitoring (FOARAM) Act of 2009 (Public Law 111-11), the United States is formulating its own strategy for an integrated national research program on Ulf Riebesell (uriebesell@ifm-geomar.de) is Professor, Biological Oceanography, Leibniz Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany.Victoria J. Fabry is Professor, Department of Biological Sciences, California State University, San Marcos, San Marcos CA, USA.Lina Hansson is Project Manager, European Project on Ocean Acidification (EPOCA), Laboratoire d'Océanographie, CNRS-Université Pierre et Marie Curie-Paris 6, Villefranche-sur-mer, France.Jean-Pierre Gattuso is Research Professor and Scientific Coordinator, EPOCA, Laboratoire d'Océanographie, CNRS-Université Pierre et Marie Curie-Paris 6, Villefranche-sur-mer, France." of the papers in this issue.Relevant subject matters range from seawater carbonate, to acid-base and physical chemistry, to organismal physiology, food web dynamics, and biogeochemistry.Time scales span the geological record for the last 500 million years to the recent historical past and extend to projections for the near-term future of the twenty-first century and beyond.Unlike many other human perturbations to the marine environment, ocean acidification is widely distributed and will influence many biogeographic regions, including open-ocean planktonic systems, coastal upwelling zones, coral reefs, high latitudes, benthic environments, and the deep sea.Ocean acidification is stimulating research in areas ranging from basic, unresolved questions on the biochemistry of biomineralization (shell and skeleton formation from carbonate minerals) to the socio-economic impacts on marine fisheries, aquaculture, and other ecosystem services.Unless there are dramatic changes in fossil fuel use, projected human-driven ocean acidification over this century will be larger and more rapid than anything affecting sea life for tens of millions of years.And the problem will be with us for a long time because it takes centuries to thousands of years for natural processes, primarily mixing into the deep-sea and increased dissolution of marine carbonate sediments, to remove excess carbon dioxide from the air.Future ocean acidification also will occur in conjunction with other human-driven stresses like global warming, pollution, overfishing, and coastal nutrient inputs.The solutions to ocean acidification are clear-slowing and eventually eliminating fossil fuel carbon emissions and, perhaps on longer time scales, developing approaches for removing excess carbon dioxide from the atmosphere.