Constraining the EvoLution of the southern Ocean-carbon Sink (CELOS)

The Southern Ocean (SO) plays a key role for climate and climate change. It is the largest oceanic sink of anthropogenic CO2, absorbing each year between 5 and 10 percent of the global CO2 emissions from human activities. The evolution of the Southern Ocean CO2 sink this century will therefore play an important role for modulating the pace of climate change.

Several factors can influence the efficiency of the Southern Ocean CO2 sink. These include the rate and level of change of CO2 in the atmosphere, the associated changes in climate including warming and winds, and the changes in marine ecosystems and biogeochemistry that occur within the ocean in response to anthropogenic drivers. CELOS contribute to understanding how each of these sets of processes will impact the Southern Ocean CO2 sinks, and to determine how to keep track of its evolution with metrics.

In particular, winds in the Southern Ocean have been observed to increase in the past 50 years. This increase has been linked to the change of stratospheric ozone, but models that project climate change also attribute the increase in winds to the increase in greenhouse gases. The relative evolution of greenhouse gases and ozone recovery could therefore be important this century, and this has not yet been explored.

CELOS proposes a unique contribution in the form of a limited set of model simulations that go beyond what is already done, and an analysis that explores scientific boundaries in a way never done before.

The new simulations explore three elements in detail.

First, simulations will be done with the NEMO global ocean model in two configurations, one of low physical resolution (2 degrees) and one of relatively high resolution (0.25 degrees). The two resolutions mostly differ in their representation of eddies. In the low resolution, eddies are parameterised, and it is hypothesised that this parameterisation dampens the variability of the Southern Ocean CO2 sink. In the high resolution, the parameterisation of eddies is removed, and eddy activity responds explicitly to changes in ocean density gradients. This enables more mechanistic responses to changes in winds.

Second, the role of ozone will be isolated in future projections. The UKESM will pursue simulations that they have already done, turning off interactive ozone up to year 2015, to the end of this century. They will also do one further simulation, fixing interactive ozone at its 1985 level, which assumes no recovery in ozone. Therefore, we will have three set of forcing, one with no ozone, one with ozone but no ozone recovery, and one with best estimated ozone recovery. This set will nicely border the possible evolution of ozone this century.

Third, simulations will be done to test the ecosystem and biogeochemical processes identified in CUSTARD and PICCOLO. Five variants are proposed initially, covering iron availability and how changes in surface ecosystems translate into changes in carbon export to the deep ocean. However, there is potential for additional simulations through the interactions with the RoSES and ORCHESTRA communities during annual meetings.

Using these new simulations and existing model results from the UKESM, we will then proceed into the development of metrics to keep track of future changes. The metrics work will build on the sink rate as a measure of the efficiency of the carbon sinks in the future. It will use the identification of fingerprints of changes to identify how to keep track of the evolution of specific processes this century. It will pay particular attention to the overall efficiency of the sink and to the changes mediated by marine ecosystems and resulting carbon export.

Finally, the new insights of CELOS will be used to make recommendations for future developments, use and analysis of the UKESM.

The main beneficiaries outside of the academic community for our project are listed below, along with how they might benefit from CELOS work. What will be done is listed in the Pathways to Impact.

The Intergovernmental Panel on Climate Change (IPCC). The IPCC assesses the scientific, technical and socio-economic information relevant for the understanding of the risk of human-induced climate change. CELOS will provide key information on the evolution of the Southern Ocean (SO) carbon sink this century, published in peer-reviewed paper, and therefore suitable for assessments. This work will show scientific advances compared to IPCC AR5 and AR6 concerning at least: for the first time, the plausible changes in the SO CO2 sink caused by changes in marine ecosystems, and the relative impact of ozone recovery versus greenhouse gases on the SO CO2 sink.

National and International Policymakers: National and international policymakers will benefit from results of CELOS to inform policy on climate change, in particular through information on the likely evolution of the SO CO2 sink this century, and importantly the potential risks to positive feedbacks on climate change. This information is relevant to determine what pathways are realistic for given climate targets, and the risks associated with these pathways. It will also help, eventually, to determine what pathways are no longer achievable. Targeted policymakers intervention will focus on the UN Framework Convention on Climate Change (UNFCCC) that regulates the Paris Agreement on climate change.

The media and the public: CELOS researchers are very involved in promoting a good public understanding of science, particularly climate change and the carbon cycle. The work of CELOS will serve as a basis for supporting active discussions on what to do about climate change, and how the earth is responding to it. This broad effort will be made if publications come out in high-impact journals, as well as with the discussion conference requested to the Royal Society.