Missions

Project context
This recruitment is part of the Priority Research Programme (PPR) Ocean & Climate, supported by the CNRS and Ifremer, set up by the French government for a period of 7 years, from 2021 to 2028, to address research issues relating to the advancement of knowledge on climate and ecology, the blue economy, law, geostrategy, the global management of socio-ecosystems and the well-being of societies.

This post-doctorate focuses on a major issue identified by the PPR: estimating the effectiveness and assessing the impacts of ocean-based carbon dioxide removal techniques (or marine Carbon Dioxide Removal, mCDR). The Paris Agreement aims to limit global warming to below 2°C, ideally 1.5°C (UNFCCC, 2015), requiring zero net CO2 emissions by 2050 (IPCC, 2018). This requires a rapid, deep and strong reduction in greenhouse gases (GHGs), and implies negative emissions technologies such as Carbon Dioxide Removal (CDR). Ocean-based approaches are attracting growing interest, particularly from industrial players and start-ups, which are proliferating in this field. However, numerous reports written by scientific bodies have highlighted the lack of hindsight and the many gaps in knowledge surrounding these technologies.

Among the marine CDRs (mCDRs), ocean alkalinity enhancement (OAE) involves locally increasing the alkalinity of surface marine waters, by dissolving alkaline materials or using electrochemical methods, to convert some of the dissolved CO2 into bicarbonate and carbonate ions and thus reduce the partial pressure of oceanic CO2. The OAE leads to a new equilibrium in the carbonate system resulting from the interaction of mixing, transport and biogeochemical processes. The changes in air-sea fluxes induced by these approaches are difficult to measure directly and numerical models therefore seem necessary to estimate their quantification in the short and long term and on different spatial scales (Ho et al., 2023). The use of these models, in parallel with experimental studies, could also help to identify and assess, prior to the implementation of mCDR techniques, any modifications to the ecosystem and biogeochemical cycles that might be induced by these mCDR techniques. Several modelling studies on the feasibility and effectiveness of such OAE technologies have been carried out at global (Iliyna et al., 2013; Köhler et al., 2013; Keller et al. 2014, Kwiatkowski et al., 2023) and regional (Mongin et al., 2021; Butenschön et al., 2021; Wang et al., 2023) scales. Some of these studies suggest that these OAE solutions are technically feasible and show their potential in terms of CDR. Currently, such high-resolution regional studies, representing sub-mesoscale processes and/or coastal dynamics likely to influence the effectiveness of mCDRs, have rarely been developed to evaluate these approaches (Fennel et al., 2023).

The Mediterranean, a climate change hotspot, is characterised by high absorption of anthropogenic CO2 and high alkalinity. The study of Butenschön et al. (2021) suggests that by adding calcium hydroxide along shipping lanes, CO2 absorption could almost double in 30 years and slow acidification. As part of ongoing projects (ANR POPNCO, PPR Riomar), high-resolution coupled physical/biogeochemical simulations (lower than 450 m horizontal resolution), forced by coupled simulations over the entire basin (6-8 km), are being implemented over periods of more than 20 years in the north-western Mediterranean. We propose to use this climate modelling work as a basis for simulating OAE scenarios.

Main tasks
The proposed postdoctoral project aims to assess the net increase in atmospheric CO2 uptake, the mitigation of acidification trends, and the effectiveness of CDR associated with two OAE approaches in the Mediterranean: one approach through additional alkalinity fluxes at coastal points and the second along shipping routes by commercial vessels. To this end, coupled high-resolution regional physical-biogeochemical modelling over the period 2030-2050 of an SSP5-8.5 scenario will be used at two spatial scales: (1) over the entire basin at a resolution of 6-8 km and (2) over the north-western region at a resolution of less than 450 m.

Activités

The work proposed for this post will consist of:
- Implementing mCDR scenarios in the coupled physical/ biogeochemical model and setting up mCDR simulations - Three alkalinisation scenarios will be simulated: alkalinisation (1) by inputs at coastal stations (e.g. sewage treatment plants or other continental inputs), (2) by inputs along shipping routes by commercial vessels and (3) by both types of inputs combined. The scenarios will be carried out for 2030-2050 on the scale of the Mediterranean basin (low-resolution model) and on the scale of the north-western basin (high-resolution model).

- Assessing the effectiveness of OAEs - The CDR and acidification mitigation of the three alkalinisation scenarios will be calculated as the variation in air-sea CO2 fluxes and pH trends obtained in the OAE scenarios compared with a reference simulation without alkalinisation. Spatial and temporal variability in CDR and acidification mitigation effectiveness will be examined, by calculating the temporal evolution of (i) the ratio of the cumulative increase in air-sea CO2 flux to the cumulative alkalinity added, and (ii) the ratio of the change in mean surface pH relative to the baseline simulation to the cumulative alkalinity added.

- Evaluating the capacity of observation networks to quantify mCDR - In this project, an estimate of the 'limit of use' of observation services for evaluating mCDR methods could be estimated using a metric constructed from the comparison of the differences between the impact of alkalinization scenarios (difference between the reference simulation and the alkalinization scenarios) and the uncertainties associated with the observation systems.

Compétences

Profile and skills required
- PhD in oceanography/geosciences.
- Experience in biogeochemical oceanography would be appreciated.
- Experience in numerical modelling and data processing
- Competence in programming and analysis tools (Fortran, Python, Matlab or similar)
- Ability to work collaboratively
- Autonomy and organisational skills

Contexte de travail

LEGOS is a joint research center (University of Toulouse, CNES, CNRS-INSU, IRD) comprising ~130 staff. LEGOS conducts multidisciplinary research into the global environmental system, drawing on ocean physics, marine geochemistry and biogeochemistry, hydrology and glaciology.

The postdoctoral researcher will join the LEGOS ECOLA team where he/she will work with Caroline Ulses in collaboration with Thibaut Wagener from MIO, the two project leaders. They have solid complementary expertise in high-resolution coupled physical-biogeochemical modelling, the Mediterranean carbonate system and observation methods. Weekly meetings with the 2 project leaders and 2 one-week stays per year at the MIO will be organised to discuss the progress of the postdoc.

The researcher will also interact with researchers and engineers from the ECOLA team (in particular the sub-team developing the model used), from other institutions as part of the PPR Ocean and Climate, and the Med-CORDEX initiative, working on various aspects of the project, such as climate projections, high-resolution modelling and ocean observations. He/she will travel in France and abroad to attend meetings and conferences on the mCDR theme.

Contraintes et risques

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