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

Abstract

[1] 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 CO 2 emissions (SRES A2 until year 2100 and subsequent linear decrease to zero until year 2300, corresponding to a total release of 5100 GtC). Atmospheric CO 2 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 CO 2 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 N 2 O 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 pCO 2 . 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 CO 2 increase since the preindustrial era.

The Origin and Limits of the Near Proportionality between Climate Warming and Cumulative CO2 Emissions

“…The inorganic ocean carbon cycle is simulated following the protocols of the ocean carbon cycle intercomparison project (Orr et al 1999). Ocean biology is simulated using a nutrient-phytoplankton-zooplanktondetritus ecosystem model (Schmittner et al 2008). Ocean sedimentary processes are simulated using an oxic-only model of sediment respiration (Archer 1996).…”

Winners and losers: Ecological and biogeochemical changes in a warming ocean

“…[5] Model studies suggest that reduced rates of nutrient supply in a future ocean will lower productivity [Schmittner et al, 2008;Steinacher et al, 2010;Marinov et al, 2010;Bopp et al, 2013;Bopp et al, 2001]. Taucher and Oschlies [2011], however, suggest that this "indirect" effect might be counteracted by the direct effect of increased growth rates due to increased temperatures.…”

“…[4] Published numerical projections of future oceans suggest very different changes in global integrated primary production in future scenarios [Sarmiento et al, 2004;Schmittner et al, 2008;Steinacher et al, 2010;Marinov et al, 2010;Taucher and Oschlies, 2011;Bopp et al, 2005;Bopp et al, 2013], even disagreeing on the sign of the net change. Locally, there are only select regions of general agreement between models, with other regions of disparate signs of changes [Bopp et al, 2013].…”

Presentation, calibration and validation of the low-order, DCESS Earth System Model (Version 1)

“…On the other hand, time scales that can be addressed by Earth System Models are restricted by their complexity and spatial resolution, given available computational resources. For example, comprehensive Earth System Models of high complexity and spatial resolution built around AtmosphereOcean General Circulation Models can nowadays be integrated over thousands of years and have been extended to include land and ocean biogeochemical cycling, important over these time scales (Cox et al, 2000;Doney et al, 2006;Schmittner et al, 2008). Only very recently have comprehensive Earth System Models of intermediate complexity and spatial resolution been developed that can be integrated over tens of thousands of years and that have been extended to include ocean sediments, important over these longer time scales (Ridgwell and Hargreaves, 2007;Brovkin et al, 2007).…”