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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 22, GB1013, doi:10.1029/2007GB002953, 2008

Future changes in climate, ocean circulation, ecosystems, and biogeochemical cycling simulated for a business-as-usual CO2 emission scenario until year 4000 AD

Andreas Schmittner

College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA


Andreas Oschlies

Leibniz Institute of Marine Sciences at the Christian-Albrechts University of Kiel (IFM-GEOMAR), Kiel, Germany


H. Damon Matthews

Planning and Environment, Concordia University, Montreal, Quebec, Canada


Eric D. Galbraith

Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey, USA


Abstract

A new model of global climate, ocean circulation, ecosystems, and biogeochemical cycling, including a fully coupled carbon cycle, is presented and evaluated. The model is consistent with multiple observational data sets from the past 50 years as well as with the observed warming of global surface air and sea temperatures during the last 150 years. It is applied to a simulation of the coming two millennia following a business-as-usual scenario of anthropogenic CO2 emissions (SRES A2 until year 2100 and subsequent linear decrease to zero until year 2300, corresponding to a total release of 5100 GtC). Atmospheric CO2 increases to a peak of more than 2000 ppmv near year 2300 (that is an airborne fraction of 72% of the emissions) followed by a gradual decline to ∼1700 ppmv at year 4000 (airborne fraction of 56%). Forty-four percent of the additional atmospheric CO2 at year 4000 is due to positive carbon cycle–climate feedbacks. Global surface air warms by ∼10°C, sea ice melts back to 10% of its current area, and the circulation of the abyssal ocean collapses. Subsurface oxygen concentrations decrease, tripling the volume of suboxic water and quadrupling the global water column denitrification. We estimate 60 ppb increase in atmospheric N2O concentrations owing to doubling of its oceanic production, leading to a weak positive feedback and contributing about 0.24°C warming at year 4000. Global ocean primary production almost doubles by year 4000. Planktonic biomass increases at high latitudes and in the subtropics whereas it decreases at midlatitudes and in the tropics. In our model, which does not account for possible direct impacts of acidification on ocean biology, production of calcium carbonate in the surface ocean doubles, further increasing surface ocean and atmospheric pCO2. This represents a new positive feedback mechanism and leads to a strengthening of the positive interaction between climate change and the carbon cycle on a multicentennial to millennial timescale. Changes in ocean biology become important for the ocean carbon uptake after year 2600, and at year 4000 they account for 320 ppmv or 22% of the atmospheric CO2 increase since the preindustrial era.

Received 5 February 2007; accepted 6 September 2007; published 14 February 2008.

Keywords: climate change; biogeochemical cycles; ecosystems.

Index Terms: 0414 Biogeosciences: Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912); 0429 Biogeosciences: Climate dynamics (1620); 0428 Biogeosciences: Carbon cycling (4806); 0439 Biogeosciences: Ecosystems, structure and dynamics (4815).

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Citation: Schmittner, A., A. Oschlies, H. D. Matthews, and E. D. Galbraith (2008), Future changes in climate, ocean circulation, ecosystems, and biogeochemical cycling simulated for a business-as-usual CO2 emission scenario until year 4000 AD, Global Biogeochem. Cycles, 22, GB1013, doi:10.1029/2007GB002953.