CR2025_20 Seaweed cultivation for climate change mitigation: what would the net effect on atmospheric composition and climate be?

Lead Supervisor: James Weber, Department of Meteorology, University of Reading

Email: j.m.weber@reading.ac.uk

Co-supervisors: Bill Collins, Department of Meteorology, University of Reading; Fiona O’Connor, Met Office

The wide scale cultivation of seaweed, over regions of 10³-10⁶ km², has been proposed as a means to capture large quantities of atmospheric CO₂ (e.g. Doumeizel et al., 2020), thus slowing climate change. However, the presence of seaweed will alter the reflectivity (albedo) of the ocean, affecting the atmosphere’s energy budget (Bach et al., 2021). In addition, certain seaweeds are known to emit halogenated organic (halocarbon) compounds into the atmosphere (e.g. Leedham et al 2013). These compounds can react chemically in the atmosphere. In the lower atmosphere (troposphere), these reactions affect the Earth’s energy budget by perturbing the greenhouse gases methane and ozone (Li et al., 2023) with implications for other climatically important components including tiny airborne particles called aerosols and clouds (O’Connor et al., 2021). In the upper atmosphere (stratosphere), these halocarbon species can contribute to ozone destruction via catalytic cycles (e.g. Solomon., 1999).

Thus, assessment of the full climate impact of seaweed cultivation extends beyond its CO₂ removal and must involve quantification of the net impact on atmospheric composition. Analysis of the impact on stratospheric ozone is also necessary given its critical role for life on Earth. These areas have not been assessed in an Earth System model before and this project aims to address this gap in the knowledge by answering the following key questions:

1. How would emissions from seaweed cultivation affect atmospheric composition, including non-CO₂ greenhouse gases, air quality and stratospheric ozone?
2. How does this composition change influence the energy budget of the atmosphere?
3. How does seaweed cultivation affect ocean reflectivity and thus the energy budget of the atmosphere?
4. What is the net impact on climate of seaweed cultivation, and how could it be optimised?

The project will be the first to assess the impact of emissions from widespread seaweed farming as a climate change mitigation strategy on atmospheric composition and climate in an Earth System model. It will provide a more comprehensive assessment of the true potential of this nascent mitigation strategy by considering its full impact on the Earth’s energy budget, rather than focusing solely on carbon, following approaches applied recently to terrestrial forestation (Weber et al., 2024). That work showed that changes to aerosol and non-CO₂ greenhouse gases (ozone and methane), following the emission of organic compounds from vegetation, along with reductions in terrestrial albedo were influential in determining the net climate impact of forestation.

In an analogous approach, the student will use literature measurements of the concentrations/emissions of important halocarbon such as methyl bromide (CH₃Br), a potent ozone depleting substance, from seaweed to develop halocarbon emissions (with uncertainty ranges) from different cultivation scenarios. The scenarios will involve varying the size (e.g. regional vs global) and location (e.g. tropical vs mid-latitude, Atlantic vs. Pacific) of cultivation and be informed by biogeochemical limitations on seaweed growth. These emissions will be applied in United Kingdom Earth System Model (UKESM) simulations to examine the impact on atmospheric composition (including stratospheric ozone and methane) and, along with changes to ocean reflectivity, the impact on the Earth’s energy budget and climate.

A successful project will provide a significant contribution to the scientific understanding of this climate change mitigation strategy, which is gaining significant interest, providing policy makers and other stakeholders with a more comprehensive view of its advantages and disadvantages and reducing the chance of there being unintended consequences.

Training opportunities: 

The student will take taught modules depending on their academic background. (e.g., Atmospheric Chemistry and Transport, Climate Change, Introduction to Numerical Modelling). This project will provide training in the key cross-disciplinary skills of running models, handling large Earth observational datasets and interpreting their data. The student will attend NCAS training courses Introduction to the Unified Model, Introduction to UKCA and Climate Modelling Summer School Science.

The student will have the opportunity to visit the Met Office and collaborate with members of the Earth System and Mitigation Science team collaborating and attend national/international summer conferences such as EGU. 

Student profile:

This project would be suitable for students with a degree in mathematics, physics, chemistry or a closely related environmental or physical science.

Co-Sponsorship details:

The project will be applying for co-sponsorship from the Met Office.

Reference:

• Doumeizel, V. et al. Seaweed Revolution: A Manifesto for a Sustainable Future (Lloyd’s Register Foundation, 2020).
• Bach, L.T. et al. Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt. Nat Commun 12, 2556 (2021).
• Leedham Elvidge, E. C et al. Emission of atmospherically significant halocarbons by naturally occurring and farmed tropical macroalgae, Biogeosciences, 10, 3615–3633 (2013).
• Li, Q., Meidan, D., Hess, P. et al. Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions. Nat Commun 14, 4045 (2023).
• O’Connor, F. M et al.: Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1, Atmos. Chem. Phys., 21, 1211–1243 (2021)
• Solomon, S.: Stratospheric ozone depletion: A review of concepts and history, Review of Geophysics 7(3), pp.275-316 (1999).
• Weber, J. et al: Chemistry-albedo feedbacks offset up to a third of forestation’s CO₂ removal benefits. Science 383, 860-864 (2024).