The oceanic response to carbon emissions over the next century: investigation using three ocean carbon cycle models

Abstract

study of coupled atmospheric carbon dioxide and the biosphere (Cox et al., 2000, Nature, 408, 184-187) found alarming sensitivity of next-century atmospheric pCO 2 (and hence planetary temperature) to uncertainties in terrestrial processes. Here we investigate whether there is similar sensitivity associated with uncertainties in the behaviour of the ocean carbon cycle. We investigate this important question using three models of the ocean carbon cycle of varying complexity: (1) a new three-box oceanic carbon cycle model; (2) the HILDA multibox model with high vertical resolution at low latitudes; (3) the Hadley Centre ocean general circulation model (HadOCC). These models were used in combination to assess the quantitative significance (to year 2100 pCO 2 ) of potential changes to the ocean stimulated by global warming and other anthropogenic activities over the period 2000-2100. It was found that an increase in sea surface temperature and a decrease in the mixing rate due to stratification give rise to the greatest relative changes in pCO 2 , both being positive feedbacks. We failed to find any comparable large sensitivity due to the ocean.

Characterizing post-industrial changes in the ocean carbon cycle in an Earth system model

“…They do not add substantially to the uncertainty in projections of anthropogenic carbon in the ocean (Figs 9g and h). This is consistent with recent studies that also find very small impacts of increased river inputs on the ocean carbon cycle (Chuck et al, 2005; Cotrim da Cunha et al, 2007). Similarly, the temperature dependence of marine metabolic rates is not important (Figs 9e and f).…”

The role of ocean transport in the uptake of anthropogenic CO<sub>2</sub>

“…In the study of oceanic uptake of anthropogenic CO 2 , there has been a history of investigating the effect of climate change on the ocean carbon cycle (e.g., Maier-Reimer et al, 1996;Sarmiento et al, 1998;Joos et al, 1999;Chuck et al, 2005;Friedlingstein et al, 2006;Zickfeld, 2007;Plattner et al, 2008), and recently on the potential effects of ocean acidification (e.g., Heinze, 2004;Gehlen et al, 2007;Ridgwell et al, 2007b). In this study we investigate the role of ocean transport in CO 2 uptake for a suite of models, including ocean carbon cycle models participating in OCMIP and recently developed Earth system models.…”

The long-term legacy of fossil fuels

“…An earlier version of this model (without a dynamic lysocline) was used to calculate impacts of different potential anthropogenic changes to the ocean on the rise in atmospheric CO 2 (Chuck et al, 2005, which can be consulted for model equations and parameter values) and for most scenarios gave broadly comparable results to the other models used in that study. While appropriate for the task here, that is, calculating the post-fossil fuel ocean chemical equilibrium, because of its simplicity the model predictions of transient behaviour (as the ocean moves to the new equilibrium) should be treated with caution.…”

“…The model (Chuck et al, 2005) is an attempt to capture the most significant dynamics of the medium-to long-term behaviour of the ocean carbon cycle in the simplest possible physical structure. It consists of three vertically stacked boxes, the upper one (100 m deep) representing the euphotic zone, the middle one (400 m deep) containing water down to the average depth of maximum annual mixing, and the deep box (3230 m deep) representing all deep ocean water that only connects with the surface via the slower processes of upwelling, ocean circulation and diffusion.…”