How will future ocean acidification impact the ocean carbon cycle and could it further amplify rising CO₂ and climate change?

A key facet of ocean acidification is the impact that changes in the chemistry of the ocean may have on marine algae, particularly those that make shell material out of the mineral calcium carbonate as this will become less easy to precipitate as ocean acidification intensifies in the future. If the rate of carbonate production by algae living at the surface slows, there will be less dense shell material to help weigh down the fluffier organic matter and the food supply to organisms on the seafloor may be reduced. Weakening the transport of organic carbon from the surface to the deep ocean would also act to increase further atmospheric CO2 concentrations. Other impacts of ocean acidification may include changes in the amount and nutritional content of organic matter produced in the ocean, and loss of nitrate, all of which would affect the nutrient and carbon cycles in the global ocean.

There are large uncertainties surrounding how the carbon cycle works and the importance of carbonate shells in the sinking of organic matter, and hence, uncertainties in the impacts of acidification on the global ocean. Our first aim is to achieve a much better understanding of the modern ocean carbon cycle – a prerequisite to making reliable future predictions. We will do this by using vast datasets describing what the ocean ‘looks’ like today and running computer models of ocean carbon cycling and climate thousands of times. Together, this will allow us to improve our model and also estimate some of the uncertainty in our predictions. The second aim of the project will build directly on this to make predictions of the range of potential changes we might see in ocean carbon and nutrient cycles in the future, and whether these changes will affect the degree of future warming by emitting greenhouse gases back to the atmosphere.

The outputs of this programme will:

• Feed into the cross-government Climate Change Adaptation programme
• Make a significant contribution to the Living With Environmental Change programme
• Provide evidence to the IPCC 5th Assessment Report on Climate Change
• Provide information to marine bioresource managers, policy makers negotiating CO₂ emissions reduction and other stakeholders
• Inform parties interested in exploring how the geochemical consequences of ocean acidification varies in time and space

The results of the project will be 3-fold:

• Better constraints on the magnitude of carbonate production by calcifying algae in the surface ocean in the modern ocean as well as the controls on their distribution.
• Improved understanding of the role of carbonate shells in controlling the sinking of organic matter in the ocean.
• Estimates of the uncertainty in future projections of impacts of ocean acidification on marine carbon and nutrient cycles, taking into account a range of potential future fossil fuel CO₂ emissions as well as prevailing uncertainty in the sensitivity of climate to CO₂.

The first two results will be primarily utilised by other scientists, in particular climate modellers and marine biologists. The third result of the project will be made available to a range of scientists, policy makers, and the general public, as follows:

Firstly, we will set up a dedicated web portal (the ‘Online Global Ocean Acidification Viewer’), creating a web interface to allow users to interrogate model simulations and answer specific questions about the impacts of ocean acidification and changing carbonate chemistry in the future. Thus we will make the ocean acidification projections available not only to UK Ocean Acidification Research Programme members and other scientists, but also to the general public and policy makers. Our model projections and portal will also make it possible to identify CO₂ emissions or atmospheric concentration thresholds that avoid certain impacts together with a measure of the uncertainty in where key ocean acidification ‘thresholds’ might lie.

Secondly, we will hold two ocean acidification modelling workshops. These will not only be open to UK Ocean Acidification Research Programme participants, but also to BIOACID, EPOCA, and MEECE participants, and help foster closer links between the various EU OA programmes. These workshops will provide training in using and interpreting model projections of ocean acidification and instil an essential appreciation of model limitations.

Finally, we will make findings available for use in policy documents, for example, the Marine Climate Change Impacts Partnership, to help disseminate future projections and assessments of potential ocean acidification impacts to policy makers and a wider audience. Results and findings will also be disseminated through more popular scientific literature and publications.