Caution Urged on Marine CO2 Removal Methods: Tasmanian Scientists

Limited understanding of basic ocean processes is hindering progress in marine carbon dioxide (CO2) removal, with the ongoing commercialisation of some approaches both premature and misguided, scientists say.

In a new paper published in Environmental Research Letters, a team of international researchers has reviewed the climatic effectiveness of four 'nature-based' techniques that use marine biological processes for CO2 removal. These include shellfish cultivation, seaweed farming, increasing whale populations through re-wilding, and coastal blue carbon captured by restoring seagrass, saltmarsh, and mangrove forests.

The study team included scientists from the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS), the University of East Anglia (UEA), Centre National de la Recherche Scientifique (CNRS), and the Institute for Sustainable Development and International Relations (IDDRI).

They concluded that, while these activities are highly worthwhile for their non-climatic benefits, they cannot provide a significant contribution to CO2 removal and risk being 'dead ends' for meaningful climate mitigation.

"To limit warming to less than 2°C requires both emissions reductions and CO2 removal, with a diverse range of potential approaches proposed to achieve billion-tonne annual CO2 removal rates within 30–50 years," lead author and IMAS oceanographer, Professor Philip Boyd, said. "But multiple techniques need to be developed and massively upscaled to achieve that goal."

While new methods are being proposed regularly, the study team says there are insufficient checks and balances in place. "This is particularly true for ocean-based CO2 removal, which is attracting greater interest as the constraints on land-based methods become increasingly apparent," Professor Boyd said.

The study team says the ocean-based approaches they've reviewed are being advocated not only by scientists but, in many cases, also by the private sector, without due diligence on the underpinning fundamental science.

"Proponents of these methods have an incomplete or incorrect grasp not only of how the ocean carbon cycle functions, but also the massive up-scaling needed to provide significant climatic benefits," said co-author Dr Phil Williamson, who is an Honorary Associate Professor at UEA's School of Environmental Sciences.

"Such upscaling brings other ocean processes into play that could cancel out the effectiveness of the proposed CO2 removal approach. In each case, misunderstanding and knowledge gaps affect the credibility of carbon offset schemes."

Professor Philip Boyd said the team considers that the non-climatic benefits of all these actions have the potential to greatly exceed their modest, or non-existent, possible contributions to ocean-based CO2 removal.

"Those advocating these approaches have given insufficient attention to basic constraints relating to ecosystem functioning and the ocean carbon cycle.

For example, they disregard the many processes that naturally return CO2 to the atmosphere, as well as the challenges of implementation at a climatically significant scale, whether carbon storage is secure, and the many difficulties in reliable quantification of climatic benefits," Professor Boyd said.

"There is a need for better communication of the basic criteria for viable CO2 removal through marine processes. Safety and durability, and being verifiable and scalable, should be considered when prioritising relevant R&D funding by Governments, as well as providing checks and balances for policymakers."

The study team has raised concerns over the 'opportunity cost' where the resources directed at these approaches could be better invested in reducing emissions – and in other land and ocean-based CO2 removal methods that are more likely to be safe, sustainable, durable, verifiable and scalable.

"We believe that the use of these four nature-based approaches for carbon offsets is more likely to represent greenwashing, rather than the 'climate heroes' some people claim," Dr Williamson said.

Cover image: Seaweed farming (Shutterstock)

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