© 2005 Nature Publishing Group Anthropogenic ocean acidification overthe twenty-first century and its impact oncalcifying organismsJames C. Orr1, Victoria J. Fabry2, Olivier Aumont3, Laurent Bopp1, Scott C. Doney4, Richard A. Feely5,Anand Gnanadesikan6, Nicolas Gruber7, Akio Ishida8, Fortunat Joos9, Robert M. Key10, Keith Lindsay11,Ernst Maier-Reimer12, Richard Matear13, Patrick Monfray1†, Anne Mouchet14, Raymond G. Najjar15,Gian-Kasper Plattner7,9, Keith B. Rodgers1,16†, Christopher L. Sabine5, Jorge L. Sarmiento10, Reiner Schlitzer17,Richard D. Slater10, Ian J. Totterdell18†, Marie-France Weirig17, Yasuhiro Yamanaka8& Andrew Yool18Today’s surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxideconcentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonatesaturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals andsome plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of theocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for futureemissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to becomeundersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, thisundersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When livepteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, theiraragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitudeecosystems could develop within decades, not centuries as suggested previously.Ocean uptake of CO2will help moderate future climate change, butthe associated chemistry, namely hydrolysis of CO2in seawater,increases the hydrogen ion concentration [Hþ]. Surface ocean pHis already 0.1 unit lower than preindustrial values. By the end of thecentury, it will become another 0.3–0.4 units lower1,2under the IS92ascenario, which translates to a 100–150% increase in [Hþ]. Simul-taneously, aqueous CO2concentrations [CO2(aq)] will increase andcarbonate ion concentrations ½CO223 will decrease, making it moredifficult for marine calcifying organisms to form biogenic calciumcarbonate (CaCO3). Substantial experimental evidence indicates thatcalcification rates will decrease in low-latitude corals3–5, which formreefs out of aragonite, and in phytoplankton that form their tests(shells) out of calcite6,7, the stable form of CaCO3. Calcification rateswill decline along with ½CO223 owing to its reaction with increasingconcentrations of anthropogenic CO2according to the followingreaction:CO2þ CO223þ H2O ! 2HCO23ð1ÞThese rates decline even when surface waters remain supersaturatedwith respect to CaCO3, a condition that previous studies havepredicted will persist for hundreds of years4,8,9.Recent predictions of future changes in surface ocean pH andcarbonate chemistry have primarily focused on global averageconditions1,2,10or on low latitude regions4, where reef-building coralsare abundant. Here we focus on future surface and subsurfacechanges in high latitude regions where planktonic shelled pteropodsare prominent components of the upper-ocean biota in the SouthernOcean, Arctic Ocean and subarctic Pacific Ocean11–15. Recently, it hasbeen suggested that the cold surface waters in such regions will beginto become undersaturated with respect to aragonite only whenatmospheric CO2reaches 1,200 p.p.m.v., more than four times thepreindustrial level (4 £ CO2) of 280 p.p.m.v. (ref. 9). In contrast, ourresults suggest that some polar and subpolar surface waters willbecome undersaturated at ,2 £ CO2, probably within the next 50years.ARTICLES1Laboratoire des Sciences du Climat et de l’Environnement, UMR CEA-CNRS, CEA Saclay, F-91191 Gif-sur-Yvette, France.2Department of Biological Sciences, California StateUniversity San Marcos, San Marcos, California 92096-0001, USA.3Laboratoire d’Oce´anographie et du Climat: Expe´rimentations et Approches Nume´riques (LOCEAN), CentreIRD de Bretagne, F-29280 Plouzane´, France.4Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1543, USA.5National Oceanic and AtmosphericAdministration (NOAA)/Pacific Marine Environmental Laboratory, Seattle, Washington 98115-6349, USA.6NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NewJersey 08542, USA.7Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, California 90095-4996, USA.8Frontier Research Center for Global Change, Yokohama236-0001, Japan.9Climate and Environmental Physics, Physics Institute, University of Bern, CH-3012 Bern, Switzerland.10Atmospheric and Oceanic Sciences (AOS) Program,Princeton University, Princeton, New Jersey 08544-0710, USA.11National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA.12Max Planck Institut fu¨rMeteorologie, D-20146 Hamburg, Germany.13CSIRO Marine Research and Antarctic Climate and Ecosystems CRC, Hobart, Tasmania 7001, Australia.14Astrophysics andGeophysics Institute, University of Liege, B-4000 Liege, Belgium.15Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania 16802-5013, USA.16LOCEAN, Universite´Pierre et Marie Curie, F-75252 Paris, France.17Alfred Wegener Institute for Polar and Marine Research, D-27515 Bremerhaven, Germany.18NationalOceanography Centre Southampton, Southampton SO14 3ZH, UK. †Present addresses: Laboratoire d’Etudes en Ge´ophysique et Oce´anographie Spatiales, UMR 5566 CNES-CNRS-IRD-UPS, F-31401 Toulouse, France (P.M.); AOS Program, Princeton University, Princeton, New Jersey 08544-0710, USA (K.B.R.); The Met Office, Hadley Centre, FitzRoyRoad, Exeter EX1 3PB, UK (I.J.T.).Vol 437|29 September 2005|doi:10.1038/nature04095681© 2005 Nature Publishing Group Changes in carbonateWe have computed modern-day ocean carbonate chemistry fromobserved alkalinity and dissolved inorganic carbon (DIC), relying ondata collected during the CO2Survey of the World Ocean CirculationExperiment (WOCE) and the Joint Global Ocean Flux Study(JGOFS). These observations are centred around the year 1994,and have recently been provided as a global-scale, gridded dataproduct GLODAP (ref. 16; see Supplementary