Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 3 2010 10.5194/bg-7-979-2010 http://www.biogeosciences.net/7/979/2010/ http://www.biogeosciences.net/7/979/2010/bg-7-979-2010.html http://www.biogeosciences.net/7/979/2010/bg-7-979-2010.pdf 979 1005 2010-03-11 Projected 21st century decrease in marine productivity: a multi-model analysis M. Steinacher steinacher@climate.unibe.ch F. Joos T. L. Frölicher L. Bopp P. Cadule V. Cocco S. C. Doney M. Gehlen K. Lindsay J. K. Moore B. Schneider J. Segschneider Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland Oeschger Centre for Climate Change Research, University of Bern, Zähringerstrasse 25, 3012 Bern, Switzerland Laboratoire du Climat et de l'Environnement (LSCE), L'Orme des Merisiers Bât. 712, 91191 Gif sur Yvette, France Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307, USA Dept. of Earth System Science, University of California, Irvine, CA 92697, USA Institute of Geosciences, University of Kiel, Ludewig-Meyn-Str. 10, 24098 Kiel, Germany Max-Planck-Institut für Meteorologie, Bundesstrasse 53, 20146 Hamburg, Germany Changes in marine net primary productivity (PP) and export of particulate organic carbon (EP) are projected over the 21st century with four global coupled carbon cycle-climate models. These include representations of marine ecosystems and the carbon cycle of different structure and complexity. All four models show a decrease in global mean PP and EP between 2 and 20% by 2100 relative to preindustrial conditions, for the SRES A2 emission scenario. Two different regimes for productivity changes are consistently identified in all models. The first chain of mechanisms is dominant in the low- and mid-latitude ocean and in the North Atlantic: reduced input of macro-nutrients into the euphotic zone related to enhanced stratification, reduced mixed layer depth, and slowed circulation causes a decrease in macro-nutrient concentrations and in PP and EP. The second regime is projected for parts of the Southern Ocean: an alleviation of light and/or temperature limitation leads to an increase in PP and EP as productivity is fueled by a sustained nutrient input. 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