A team of researchers led by the University of Cambridge has developed a new framework to assess whether carbon removal portfolios can effectively limit global warming over centuries. The approach distinguishes between buying credits to offset risk and claiming actual net-negative emissions, offering a way to evaluate long-term climate benefits. Published in Joule, the study presents a method to integrate both nature-based and technology-driven approaches into cohesive climate strategies aimed at stabilising temperatures over the long term.

According to the researchers, the findings could transform how carbon removal is managed, showing that reforestation and soil carbon projects can play a meaningful role when combined with more permanent technological solutions. This challenges the widespread belief that only geological storage, such as deep underground carbon injection, can secure durable climate benefits. However, the team warns that existing schemes like California’s forest carbon offsets programme may be underprepared for risks extending beyond the next few decades.

The study introduces the idea of maintaining a “carbon buffer” to manage long-term storage risks. Researchers recommend storing roughly two tonnes of carbon for every tonne offset in portfolios that include nature-based solutions, arguing that such buffers are adequate in most cases. Yet, for portfolios dominated by nature-based projects, they suggest that far larger buffers—up to nine tonnes for each tonne emitted—might be necessary. These extreme ratios, they caution, would make some portfolios economically unsustainable.

Lead author Dr Conor Hickey of Cambridge’s Department of Land Economy explains that companies such as Microsoft and Meta are already investing billions in carbon removal portfolios, but lack clear methods to judge their long-term impact. Hickey says the new risk management framework provides one of the first reliable ways for portfolio managers to ensure that their strategies contribute to sustained temperature stabilisation. He adds that nature-based carbon storage, when properly balanced with technological methods, has a more significant role in climate mitigation than critics typically acknowledge.

Co-author Professor Myles Allen of the University of Oxford stresses that proper climate stability requires geological storage in the long run. He notes that to meet the Paris Agreement’s temperature goals, all offset schemes should aim to transition entirely to carbon dioxide removal with geological storage by mid-century. While biological approaches like afforestation and biochar are cost-effective and more accessible, they face higher reversal risks from factors such as wildfires or land-use changes.

The researchers’ framework shows that increasing carbon removal today can compensate for future risks, ensuring climate stability for centuries. Their analysis supports blending costly but permanent methods, such as Direct Air Capture, with cheaper nature-based options. By incorporating well-designed buffers and diversification, they argue, carbon portfolios can balance affordability with durability—helping societies reach and maintain net zero while preserving flexibility in how carbon is captured and stored.

More information: Conor Hickey et al, Carbon storage portfolios for the transition to net zero, Joule. DOI: 10.1016/j.joule.2025.102164

Journal information: Joule Provided by University of Cambridge