A new study led by researchers at the University of Cambridge has proposed a method to evaluate whether carbon removal portfolios that mix technology-based and nature-based solutions can help stabilize global temperatures over centuries. The research, published in the journal Joule, examines how companies can combine more expensive technologies that store carbon deep underground with less costly, more accessible options such as forest projects and biochar.
The study outlines a framework that allows companies to assess the effectiveness of their carbon removal strategies over the long term. The researchers also clarify the distinction between purchasing credits to manage risk and claiming net-negative emissions.
The team found that nature-based carbon removal projects—like planting new forests or restoring existing ones—could be effective in addressing climate change when used alongside other removal techniques. This challenges the idea that only permanent technological storage is sufficient for climate goals.
Lead author Dr Conor Hickey, Assistant Professor in Energy and Climate at the University of Cambridge’s Department of Land Economy, said: “Tech giants like Microsoft and Meta are collectively spending billions on carbon removal portfolios to offset their growing carbon footprints. 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 added: “Our risk management approach offers one of the first reliable measures for portfolio managers targeting long-term temperature stabilisation. 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.”
The study also points out that some carbon removal portfolios, such as California’s forest carbon offsets programme, may not have enough funding to cover risks beyond a few decades. The authors recommend a “buffer” of about two tonnes of stored carbon for every tonne offset in portfolios containing nature-based solutions, stating this is generally sufficient for managing long-term risks. However, portfolios that rely heavily on nature-based offsetting may require much higher buffers—up to nine tonnes for every tonne emitted—due to greater uncertainties.
Professor Myles Allen, a co-author from the University of Oxford, stated: “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.”
Current market incentives tend to favor biological projects such as forestry or biochar because they are cheaper and more available. These methods capture CO₂ by storing it in trees or soil but carry higher risks of reversal from land use changes or wildfires. These are often seen as temporary solutions because the carbon may not remain stored for long periods.
In contrast, technology-based solutions like Direct Air Capture (DAC) are more permanent but face challenges due to high costs and energy demands. DAC, when paired with deep underground storage, offers more secure carbon containment.
The new risk management framework developed by the research team calculates how much additional CO₂ removal is necessary to ensure stable temperatures for different types of storage portfolios over centuries. Their analysis suggests that, in some cases, increasing carbon removal now—using a mix of nature-based and technological methods—can provide coverage for long-term storage risks.
Hickey explained: “Removing more carbon now can effectively cover carbon storage risk for centuries, and this can be done with a mix of nature and tech, as long as the right buffers are built in. Portfolios can combine expensive permanent solutions like DAC with lower-cost nature-based options like planting trees – matching society's willingness to pay while still contributing to temperature stabilisation goals.”
He added: “Our approach enables strategic carbon storage choices based on current availability, while targeting long-term temperature stabilisation. It provides buyer flexibility while valuing lower-risk storage options, something today's market lacks.”
By 2050, the UK aims to reach net zero emissions, with geological storage expected to play a key role in handling any ongoing CO₂ emissions. New guidance in the UK and EU requires projects to meet a minimum 200-year permanence standard.