Researchers argue that restoring forests and improving soil carbon retention can play a significant long-term role when paired with durable underground storage. The findings, published in the journal Joule, challenge the assumption that only geological carbon storage can permanently mitigate climate change.
Lead author Dr Conor Hickey, Assistant Professor in Energy and Climate at Cambridge University's Department of Land Economy, said: "While companies and countries agree that increased investment in carbon removal is essential to reach net zero targets, they also want to understand whether carbon removal schemes can help stabilise global temperatures over the long term."
Hickey explained that the new "risk management approach" offers a quantitative way to determine how much extra CO2 must be removed to offset the risks of reversal in different storage portfolios. "It shows that nature-based carbon storage such as tree planting has a bigger role to play than critics assume when used as part of a diversified carbon removal portfolio," he added.
The study highlights the importance of "buffers" - extra carbon removed beyond direct emissions - to safeguard against re-release from natural disturbances. Portfolios with a moderate mix of technologies and ecosystems may need around two tonnes of carbon stored for every tonne offset, while high-risk portfolios reliant on forests may require up to nine.
Professor Myles Allen, co-author and Professor of Geosystem Science at the University of Oxford, emphasized the long-term shift required: "Durable net zero means geological net zero. To stabilise climate in line with Paris Agreement goals, anyone still relying on offsets must plan to shift entirely to carbon dioxide removal with geological storage by the middle of the century."
Nature-based methods such as tree planting or biochar creation remain more accessible and cost-effective but face challenges from land-use changes, fires, and shorter storage durations. In contrast, direct air capture (DAC) technologies - though energy-intensive and expensive - offer near-permanent storage when paired with underground geological formations.
The Cambridge team's risk management framework quantifies how much additional CO2 must be removed today to maintain climate stability for 300 to 1,000 years. Their results suggest that "removing more carbon now" can compensate for future losses and allow for flexible portfolio design combining nature and technology-based storage.
By integrating short-term biological methods with long-term geological solutions, the framework enables policymakers and corporations to meet permanence requirements, such as the UK's 200-year minimum standard for carbon storage projects.
Research Report:Carbon Storage Portfolios for the Transition to Net Zero
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