Development of the marine CO 2 sink in the future

2.6 Development of the marine CO₂ sink in the future Key questions concerning the future development of the oce-anic carbon sink include: Is the steep rise in atmospheric CO₂ concentration just starting – we assume that the basic signal is governed by the ocean, but is this indeed the case? How can we check whether the feedback processes really work and where are the most sensitive regions, which may hold surprises?

How large are the feedbacks in relation to mitigation options and emission reduction plans – do we have to revise emission reduction policies?

We use the latest European coupled carbon cycle climate mod-els in order to make the best possible prediction of the CO₂ airborne fraction in the future. These predictions are based on the sources and sinks of CO₂ estimated by the ocean modules, but also the terrestrial fluxes estimated by the land modules.

For year 2000, the coupled models result in a global oceanic net sink for anthropogenic carbon of around 2.3 GtC/yr. We consider the time frame until year 2100 (“IPCC time frame”) and 100 years beyond, until 2200. We are carrying out fully coupled climate/ocean carbon cycle simulations using realistic scenarios for future anthropogenic CO₂ emissions, including new process knowledge on biogeochemical feedbacks. These simula-tions will give the most realistic estimates of future transient CO₂ source-sink distributions currently available. The largest in-crease of the anthropogenic CO₂ build up in the atmosphere has occurred over the past few years and the rate of CO₂ emissions is expected to continue to increase (Raupach et al., 2007).

Changes in the greenhouse effect and climate evolution in the coming decades and centuries will critically depend on human action with respect to emissions. The purpose of the predic-tive model runs in estimating the repartitioning of carbon, in the coming decades and centuries until 2200, is to make best possible projections of: the overall airborne fraction of CO₂ (the ratio of the annual increase in atmospheric CO₂ to the combined annual CO₂ emissions from fossil fuel burning and cement manu-facture combined). To achieve this the model must predict the evolution of the oceanic uptake kinetics, taking into account all climatic and environmental changes in an integrated way, in-cluding modelling how the oceanic carbon sink depends on the carbon fluxes to and from the land biosphere. Such a budgeting approach, which is based on best possible process knowledge, can only be carried out with prognostic models. These are mod-els whose framework of mathematical equations can be reliably integrated forward in time on the basis of given initial con-ditions. As it is difficult to consider regional carbon budgets, because one would have to know all carbon fluxes at all open boundaries, such a modelling approach can only be based on global Earth system models (coupled ocean-atmosphere climate models, that include biogeochemical modules for ocean and land biogeochemistry). Within CarboOcean-IP, modelling future scenarios using Earth system models is an important

synthesis-ing tool, where the results from the data collection, the process determination, and the model performance assessment can all be brought together to make realistic forecasts for a given set of CO₂ emission scenarios.

Friedlingstein et al. (2006) published an intercomparison of 11 Earth system models (general circulation models and coupled models of intermediate complexity) for carbon fluxes, land-atmosphere as well as ocean-land-atmosphere, for the A2 SRES IPCC emission scenario (IPCC, 2000) until year 2100 (C4MIP project).

The models revealed a broad range of different time evolutions for the concentration of atmospheric CO₂. For the ocean, all models showed a continuous flux of excess CO₂ into the ocean.

However, this flux decreased as the prevailing CO₂ concentra-tion in the atmosphere increased (with the excepconcentra-tion of one model). This implies that the oceans will continue to act as a CO₂ sink; however, the uptake per additional unit of CO₂ emitted will slow down. The result will be an accelerating rate of CO₂ build up in the atmosphere. The reason for this change has still to be analysed in detail. The land uptake predicted by many of the models declined to possibly zero in year 2100, while some models even predict that the land will become a CO₂ source to the atmosphere after initially acting as an increasing sink due to CO₂ fertilisation of the land biosphere.

In CarboOcean-IP, we chose five model systems to further ana-lyse the ocean’s role in future CO₂ uptake (model systems COS-MOS/Max Planck Institute of Meteorology, IPSL-LSCE, Hadley Centre, CCSM NCAR/Bern, BCM). These five model systems in-clude several new developments: for example, in the Bergen Climate Model BCM, the biogeochemical ocean model HAMOCC (Maier-Reimer et al., 2005) was implemented and converted to communicate with the isopycnal ocean model MICOM, resulting in a new model type within the interactive carbon cycle climate models. Further progress beyond the previous state of the art was made through implementation and use of new parameteri-sations (such as particle dynamics Gehlen et al., 2006). All the Earth system models used show a future reduction in the oce-anic sink in response to climate change, but with considerable differences between the models (Fig. V.24). These differences can be attributed to changes in mixed layer depth, temperature changes, changes in ocean circulation, and related changes in biogeochemical cycling of carbon, nutrients, and oxygen. In ad-dition to analysis of the reaction of the CO₂ airborne fraction to changes in climate and oceanic biogeochemical feedbacks, the ocean acidification due to marine CO₂ uptake was also studied.

CarboOcean-IP includes an analysis of the feasibility of deliber-ate carbon storage in the ocean. It is by no means the goal of CarboOcean-IP to explore this as a realistic means of climate mitigation; rather our aim is to contribute to the discussion with respect to a few key scientific questions. The two primary questions addressed are: what is the dispersion process when

2.6 Development of the marine CO ₂ sink in the future

anthropogenic CO₂ is injected purposefully into the deep-water column? How does the injected CO₂ spread at a larger scale?

To address the first question, we carried out a suite of labora-tory experiments with a sophisticated high-pressure tank. The results of these experiments show that injected CO₂ can rela-tively quickly rise up to shallower layers because of high droplet rise rates (droplets without hydrate skin) in the water column (Bigalke et al., 2008) (Fig. V.25). Modelling has progressed through the further development of a process model, now ca-pable of realistically simulating the spread of directly injected CO₂ in all three directions and the simulation of the dispersal of CO₂ into the water column out of a “CO₂ lake” on the ocean floor. First studies with a high resolution ocean general circula-tion model indicated that the details of the upwelling in the Southern Ocean critically depend on model resolution and that significant differences for the predicted CO₂ injection efficiency can be expected, depending on the resolution used (Lachkar et al., 2007).

Fig. V.25: Various technological mitigation options are currently under public debate. One example is “storing” CO₂ on the ocean floor to keep it out of the atmosphere. CarboOcean-IP includes an analysis of the feasibility of such deliberate carbon storage in the ocean to provide a critical quality check on this proposal. The results of these experiments show that injected CO₂ can relatively quickly move to shallower water depths layers. This figure is derived from pressure chamber measurements. It shows the droplet rise rates of liquid CO₂ versus droplet radius at pressure and temperature conditions inside and outside the field of hydrate stability of deliberately injected CO₂ (Bigalke et al., 2008). Droplets without a hydrate skin (triangles) can rise signifi-cantly more quickly through the water column than those with a hydrate skin (circles). Reprinted with permission from Bigalke, N. K.; Rehder, G.;

Gust, G. Experimental Investigation of the Rising Behavior of CO2 Droplets in Seawater under Hydrate-Forming Conditions. /Environ. Sci. Technol., (2008) 42 (14), 5241–5246. 10.1021/es800228j. Copyright (2008) Ameri-can Chemical Society.

Fig. V.24: We assume that the steep rise in atmospheric CO₂ concentration is just starting and that the basic signal is governed by the ocean- but how do we make the best possible predictions of the future atmospheric CO₂ con-centration? To tackle this problem, five model systems were chosen within CarboOcean-IP to analyse the ocean’s role in future CO₂ uptake. In this figure, the mean atmospheric CO₂ concentration is simulated (black line) by the BCM-C model as compared to the range (grey shading) from other C4MIP (Friedlingstein et al., 2006) models. (Source: Tjiputra et al., in prep.)

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Atmospheric CO2 [ppm]

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2.6 Development of the marine CO ₂ sink in the future

Robust findings:

The ocean will continue to respond to further CO₂ additions in the atmosphere by absorbing CO₂. However, climate change and rising CO₂ concentrations in the atmosphere and ocean will gradually reduce the oceans ability to keep up with additional greenhouse gas loads. This will lead to a gradual decrease in the sink efficiency of the ocean and thus a temporary huge increase in the rate of growth of atmospheric CO₂. This increase will depend on the future amount of CO₂ emitted, the change in ocean circulation, and related biogeochemical processes. Climate model runs which account for an interactive carbon cycle show an accelerated climate change as compared with less realistic models which are based on physics only.

Remaining key questions

The science of accurate quantification of the physical and biogeochemical feedback processes to future carbon emissions is in its infancy. Even the reason for the natural positive feedback of the marine carbon cycle to climate change is not yet clear (glacial-interglacial changes). Prognostic Earth system models need to be systematically calibrated to measurements from modern and paleo-times in order to increase their accuracy for future predictions.