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A team of researchers, led by Cambridge University, has now formulated a method to assess whether carbon removal portfolios can help limit global warming over centuries. The approach also distinguishes between buying credits to offset risk versus claiming net-negative emissions.

The study paves the way for nature-based carbon removal projects—such as planting new forests or restoring existing ones—to become effective climate change solutions when balanced with a portfolio of other removal techniques, according to researchers.

They say the findings, in the journal Joule, show how nature-based and technology-based carbon storage solutions can work together through the transition to net zero, challenging the notion that only permanent tech-based "geological storage" can effectively tackle climate change.

The study's authors point out that some carbon removal portfolios, such as California's forest carbon offsets program, may be severely underfunded for risks beyond the next few decades.

They call for a "buffer" of around two tons of stored carbon for every ton offset in portfolios containing nature-based solutions, noting that this is "sufficient in most cases" to manage long-term risks.

However, researchers say the most high-risk portfolios that rely heavily on nature-based offsetting might need extreme buffers of nine tons of carbon removed for every ton emitted. The authors caution against the use of such portfolios given the costs and uncertainties involved.

"Tech giants like Microsoft and Meta are collectively spending billions on carbon removal portfolios to offset their growing carbon footprints," said lead author Dr. Conor Hickey, Assistant Professor of Energy and Climate at Cambridge University's Department of Land Economy.

"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 stabilize global temperatures over the long term."

"Our risk management approach offers one of the first reliable measures for portfolio managers targeting long-term temperature stabilization," said Hickey. "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."

"Durable net zero means geological net zero," said Professor Myles Allen, a co-author on the paper and Professor of Geosystem Science at the University of Oxford.

"To stabilize 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 favor cheaper and more available 'biological' projects to pull carbon dioxide (COâ‚‚) from the atmosphere and store it, such as forestry, which locks carbon in trees, or biochar, where plant materials are heated to create a charcoal-like substance that traps carbon when incorporated into soil.

However, these methods carry a higher risk of carbon re-release, such as when land use changes or wildfires increase. They are often considered only a temporary solution—the carbon is not locked away for long enough to stem rising global temperatures.

Alternative tech-based solutions like Direct Air Capture (DAC) are proving hard to grow at scale when costs remain high and the process energy-intensive. Yet the permanence of carbon storage means this emerging technology is less vulnerable to reversal, such as through leakage. DAC can be combined with deep underground storage to lock the COâ‚‚ away.

For the latest study, the research team have developed a new "risk management framework" to accurately calculate the additional COâ‚‚ removal needed to keep temperatures stable over centuries for various storage portfolios.

Their analysis shows that in some cases, such as a high-risk portfolio dominated by forestry projects, the extra amount of CO₂ removal needed to make up for this risk doesn't change much—whether the timescale is 300 or even 1,000 years.

"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," said Hickey.

"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 stabilization goals."

"Our approach enables strategic carbon storage choices based on current availability, while targeting long-term temperature stabilization. It provides buyer flexibility while valuing lower-risk storage options, something today's market lacks," said Hickey.

By 2050, the UK aims to achieve net zero, with geological storage expected to play a major role in storing any ongoing COâ‚‚ emissions. Incoming UK and EU guidance states that projects must be subject to a minimum 200-year permanence requirement.