Theme 3: Biogeochemical impacts and feedbacks
What do we do? - Science

 

Theme leader: James Orr james.orr (at) lsce.ipsl.fr CEA

 

WP 9 - From process studies to ecosystem models

WP 10 - Future changes in ocean carbonate chemistry

WP 11 - Biogeochemical impacts

WP 12 - Assessment and Earth system feedbacks

 

In regards to future changes in carbonate chemistry, the Royal Society (2005) concludes that “the magnitude of this [ocean] acidification can be predicted with a high level of confidence.” This holds true across open ocean surface waters based on model studies published so far (Caldeira & Wickett, 2005; Orr et al., 2005). However, those studies were not able to assess future changes in the Arctic Ocean, where rapid climate change is already under way. Nor has there been adequate assessment of shelf areas, such as those bordering Western Europe. Regional or “shelf-sea” models are just beginning to be used, but these have additional uncertainties due to shallow coastal sediment processes (Blackford & Gilbert, 2007). There are also greater uncertainties in subsurface waters (Fig. 1).

 

We hypothesize that the Arctic Ocean may be the ultimate hotspot. Our recent preliminary analysis with one model suggests that, under the IPCC SRES A2 scenario, all Arctic surface waters will become undersaturated with respect to aragonite by 2050, sooner even than the Southern Ocean (Fig. 2.). Even calcite would start to dissolve by 2100. This could make the Arctic’s surface layer hostile to all calcifying pelagic organisms as well as put at risk the Arctic shelf’s entire benthic ecosystem, which is dominated by bivalve molluscs During EPOCA, five Earth system models are used to help assess the large uncertainties due to the effects of climate change, particularly where they are largest such as in the Arctic (Fig. 3).

Figure 3 also suggests that other areas of immediate concern are the European shelf and the Canary current region. These areas may be particularly vulnerable because as eastern boundary upwelling systems, their surface waters are heavily influenced by deep waters that are rich in CO2 and poor in CO32-. During EPOCA, three high-resolution regional models are used to help evaluate future changes in these areas that are vital to Europe.

 

More generally, ocean acidification will alter ocean biogeochemistry and fundamental ecosystems and these changes will feedback on climate. During EPOCA, we use the same global and regional ocean models as well as sediment and Earth system models to assess how ocean acidification will impact ocean biogeochemistry, fundamental ecosystems, and outgassing of climate reactive gases, and in turn how these changes will feedback on climate. The overarching question of Theme 3 are:

To what extent will ocean acidification alter ocean carbonate chemistry, biogeochemistry, and marine ecosystems over the next 200 years, and how will that feed back on climate?

To project future changes in ocean biogeochemistry as well as in climate, there is but one choice—we must rely on models. In Theme 3, we use models to improve our understanding of how ocean acidification will alter ocean biogeochemistry and ecosystems and how that will feed back on climate. Simulations are made in a suite of regional- to global-scale models. Ocean carbon cycle models couple a physical circulation component (ocean general circulation model—OGCM) to components describing ocean biogeochemistry and fundamental ecosystems (phytoplankton and zooplankton functional types). During EPOCA, we use five global-scale OGCMs in two configurations: (1) where each is forced only by external data (boundary conditions) and (2) where each is coupled to an atmospheric general circulation model (AGCM) and to a sea-ice model to predict how climate change affects and is affected by ocean acidification. Often, such coupled models also include a terrestrial biosphere component and are termed either coupled carbon-climate models or Earth system models. We also zoom in on changes in waters bordering Europe by using three regional models for (1) the Northwest European shelf, (2) the entire Western European shelf seas, and (3) the Canary Current upwelling system. Regional model simulations are planned over the 1958 to 2058 period (±50 years), whereas global model runs are longer (±200 years). Additionally, Theme 3 modellers compare these results to longer term effects determined with an Earth System Model of Intermediate complexity (EMIC).

In WP10, we determine the possible future changes in ocean pH, CaCO3 saturation states, and other carbonate chemistry variables for a given set of atmospheric CO2 pathways. The models are evaluated by comparing simulated results with relevant carbonate chemistry data collected during the recent past and present, particularly those data collected during EPOCA (WP2 and WP3). Model uncertainties are assessed by comparing models to one another, wherever domains overlap. The five independent global models provide an uncertainty range over the entire open ocean. The three regional model configurations are used to provide the first complete assessments of future carbonate chemistry changes in near-coastal waters that lie within the immediate vicinity of Europe. Analysis of model results identify critical regions where variability is large or where pH and carbonate saturation states decrease most rapidly, for example to a point where surface waters first become corrosive to aragonite (Fig. 3.) as well as calcite. Sensitivity tests are made in individual models to determine how simulated changes in carbonate chemistry are altered by different emission scenarios, physical model forcing, and improved horizontal model resolution (i.e., coarse, eddy permitting, and eddy resolving simulations with one global model and two European shelf sea models). Special commitment simulations are made with another global model to quantify the inertia of past carbon emissions (relative to a given reference year, for example 2020) in terms of their impact on future ocean carbonate chemistry. In other words, the inertia inherent in the ocean and in the climate system means that the past has an effect on the future, i.e., that we are committed for some time to further decline in ocean pH and [CO32-] even if, for instance, we could stop all emissions immediately.

In WP11, we study how ocean acidification impacts the carbon cycle as a whole. We also study how acidification affects other key elemental cycles that are tightly coupled to that of carbon, namely the nitrogen and sulphur cycles, each of which includes production and outgassing of other climate relevant gases (DMS and N2O). For these studies, we use forced ocean models to account for how ocean acidification will alter biogeochemical and ecosystem components. The current biogeochemical and ecological parametrizations of these models are expanded and improved by exploiting new process knowledge and parametrizations gained from Theme 2 synthesis efforts in WP9. Resulting model improvements are evaluated by comparing simulated and observed distributions for (i) dissolved biogeochemical properties (e.g., O2 and nutrients) and (ii) geographical distribution of calcifying organisms (data from WP3). Uncertainties are assessed by comparing models and by making sensitivity tests in individual models. One such sensitivity test assesses how simulated ecosystem structure and function may differ due to the intensity of carbon emissions (using a suite of IPCC scenarios). We also focus on other critical factors within the photic zone (i.e., the surface sunlit layer) including stoichiometry of carbon fixation and export production, CaCO3 production, and production of climate-relevant gases. Further below the surface, WP11 studies how ocean acidification affects aphotic zone biogeochemistry, particularly remineralisation of organic carbon, particle aggregation and ballasting, CaCO3 dissolution, and the inorganic-to-organic carbon rain ratio. In WP11, we also use marine sediment models to quantify the changing role of marine sediments as sources and sinks of macro- and micro-nutrients (N, P, Si and Fe) under future ocean acidification and climate change scenarios. The impact of these acidification-induced changes in nutrient supply are further investigated in terms of their effects on shelf and open ocean ecosystems and biogeochemistry.

In WP12, the focus is on how climate change will affect ocean acidification and in turn on how ocean acidification will feedback on climate. This task requires the use of Earth system models. These tools help gather together understanding gained from other work packages, putting them in the true future context where climate, ocean biogeochemistry, and ocean ecosystems will be changing simultaneously. The objectives of WP12 are (i) to quantify links and feedbacks between carbon emissions, ocean acidification, and climate in coupled, state-of-the-art Earth system models; (ii) to explore ocean acidification under a range of mitigation and non-mitigation scenarios; and (iii) to determine how ocean acidification will simultaneously affect ocean productivity and climate reactive gases as well as the climate system. To address these concerns in WP12, simulations are made in five Earth system models developed independently in France, Germany, Norway, Switzerland and the UK. These five groups incorporate new process-based knowledge acquired in Theme 2, synthesized by WP9, and tested beforehand in the forced ocean models in WP11. Sensitivity tests are made to isolate effects due to climate change as well as how increasing atmospheric CO2 drives changes in ocean biogeochemistry and ecosystems that in turn feedback on climate. Uncertainties are assessed by comparing models. There stands a good chance that real ocean and climate system behaviour are bracketed by the range of WP12 predictions, given the large diversity among these Earth system models (Fig. 4.), not only in terms of their ocean biogeochemical and ecosystem components, but also in terms of their climate system components (atmosphere, ocean, and ice models). By the end of EPOCA, WP12 will be able to demonstrate the extent to which the new, acidification-sensitive model parametrizations will alter current Earth system model estimates in terms of changes in climate and fluxes of greenhouse gases.

 

References

Blackford J. C. & Gilbert F. J., 2007. pH variability and CO2 induced acidification in the North Sea. Journal of Marine Systems 64:229-241.

Caldeira K. & Wickett M. E., 2003. Anthropogenic carbon and ocean pH. Nature 425(6956):365.

Friedlingstein P. et al., 2006. Climate-carbon cycle feedback analysis: results from the C4MIP model intercomparison. Journal of Climate 19:3337-3353.

Orr J. C. et al., 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437:681-686.

 

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