Transient tracer-based Investigation of Circulation and Thermal Ocean Change (TICTOC)

Lead Research Organisation: University of Reading
Department Name: National Centre for Atmospheric Science

Abstract

Carbon dioxide produced by burning fossil fuels such as oil and gas is building up in the atmosphere and causing the planet to warm. The oceans have absorbed more than 90% of the heat trapped on the planet to date. However, this heating also causes the ocean to expand, leading to rising sea level and consequently to an increased risk to and vulnerability of people and industries located near the coast. Understanding how much sea level will change into the future allows us to plan accordingly the defences we need to install in order to safeguard the infrastructure and livelihoods of our coastal communities.

Warming of the ocean is not geographically uniform however, as ocean currents move heat around the globe. This leads to contrasting changes in ocean temperature and sea level (affecting coastal communities and assets). By the end of the century, some regions may experience very large sea level rises of up to a metre while others will see far less (or even a lowering).

This science program will use observations made from research ships and computer models of the ocean to understand where the ocean takes up heat from the atmosphere and how ocean currents transport and redistribute that heat. To study ocean currents we need a 'tracer' - something that is placed in and moves with the flow, like a chemical dye. Although not intentionally for this purpose, three varieties of tracers have been added to the atmosphere since the 1950s and have since gradually been absorbed into the ocean, and redistributed by ocean currents. These are radioactive carbon (produced by mid 20th century nuclear bomb tests), chlorofluorocarbons (historically used in refrigerators and aerosol cans, and which caused an expansion of the Ozone Hole) and more recently sulphur-hexafluoride (formerly found in tennis balls but now predominantly used in electrical industries as an insulator). These tracers have entered the ocean as distinct pulses at different times, forming a fortuitous experiment we can now observe. We will use high-precision equipment to measure these tracers in the Atlantic and Southern Oceans and collaborate with international partners to track their global fate. We will use these observations to estimate the rate at which heat is being absorbed and re-distributed throughout the ocean and to assess and improve climate model predictions of regional sea level rise.

Planned Impact

TICTOC will ultimately result in improved regional sea level prediction. This will enable society to decide how and where to prioritise spending on mitigating the effects of sea level rise e.g. by installing flood defences. For this to occur, the results of TICTOC need to be rapidly communicated to the key decision making bodies. Many of these ultimately ground their decisions in the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Hence the IPCC is a key route to Impact for TICTOC. TICTOC will influence the IPCC assessment in two ways:

- By producing policy-relevant knowledge about ocean heat uptake and sea level rise, based on observations and on the CMIP6 models, which will provide the basis for projections in the Sixth Assessment Report (due 2021). To be considered in this report, results will need to be published in the peer-reviewed scientific literature by the likely 2020 deadline, based on the previous timeline. TICTOC investigators from each institute have a track-record of producing papers that are cited in IPCC reports. In addition, investigator Gregory was a coordinating lead author of the sea-level chapter of the IPCC Third Assessment Report (2001), a lead author of the ocean observations and projections chapters of the Fourth (2007), and a lead author of the sea-level chapter of the Fifth (2013), in each case with responsibility for sea-level projections.
- By providing new diagnostics and constraints to inform and improve the next generation of ocean and coupled climate models, which will be used to make improved projections during the 2020s. This will be delivered through the evaluation of OMIP (Ocean Model Intercomparison Project, TICTOC investigator Nurser is part of the co-ordination panel), and FAFMIP (Flux Anomaly Forced Model Intercomparison Project, TICTOC investigator Gregory is part of the co-ordination panel) model runs, both components of the CMIP6 model intercomparison. TICTOC investigators at the NOC are part of the team that provide the ocean model for inclusion in the UK coupled climate modelling contribution by the UK Met Office (TICTOC project partners).


In addition, nationally, TICTOC will influence UK non-government and government stakeholders with whom the Met Office engages including providing sea level projections. The stakeholders include energy companies, infrastructure management, flood risk assessment (EA), UK consultancies, academic institutes etc. The main government stakeholders are BEIS (which includes the function of what was DECC), DEFRA and EA. The PI, McDonagh, will, with project partner Palmer at the Met Office, use the results from TICTOC, particularly the improved sea level predictions, to advise these stakeholders. The main route for this advice will be through UK climate projections (UKCP). The last UKCP was done in 2009 (UKCP09), with the next one due in March 2018 (UKCP18). The results from TICTOC will feed into the subsequent round of UKCP or possibly as an update to UKCP18 if a clear impact can be demonstrated.

TICTOC will expedite communication to the academic community throughout the project, as well as through the usual conferences, seminars and meetings, by interacting with the Programme Advisory Board and inviting key academics to the Royal Society Event that we plan for end of the project in 2021. We will further engage stakeholders by inviting them to this event as well.

Publications

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Zanna L (2019) Global reconstruction of historical ocean heat storage and transport. in Proceedings of the National Academy of Sciences of the United States of America