Researchers caution about reliance on carbon dioxide removal

Carbon dioxide removal (CDR) is a key component of many climate change mitigation scenarios. Nearly all models in the latest report from the United Nations Intergovernmental Panel on Climate Change (IPCC) rely to some extent on CDR to keep warming below 2 °C and reach net-zero carbon emissions.

It makes sense then that governments have pledged to use approximately 10 million km² of territory for land-based CDR methods, like afforestation, reforestation, and bioenergy with carbon capture and storage (BECCS). But a new study warns that relying on projects of such a large scale would have a huge impact on both people and the environment (Science 2024, DOI: 10.1126/science.adj6171) .

For example, the IPCC estimates that BECCS could remove up to 11.3 billion metric tons (t) of CO₂ per year. Afforestation and reforestation, which rely on newly planted trees to sequester CO₂, could remove an additional 10.0 billion t of the greenhouse gas annually.

However, to achieve the full mitigation potential of these methods, countries would have to collectively convert 29 million km² of land to bioenergy crops or trees—approximately three times the area of the US—the study says. Such a large shift in land use could push over 300 million people into food insecurity, the researchers calculate. It could also affect freshwater supply or conflict with existing conservation efforts.

“Biodiversity, freshwater use, food security should not be an afterthought when we think about CDR. It should be the starting point,” says Alexandra Deprez, a research fellow at the Institute for Sustainable Development and International Relations (IDDRI) and one of the study’s authors.

In estimates of the mitigation potential of CDR, the IPCC considers only technical and economic constraints that might prevent the technology from being widely implemented. “The models used to produce these scenarios are not good at imposing sustainability limits,” says David Morrow, director of research for the Institute for Carbon Removal Law and Policy at American University, who was not involved in the Science study.

One reason for this exclusion is that the field of research is still emerging, Deprez says.

But that means policymakers relying on the IPCC report might overestimate how much the world can safely depend on CDR to meet the Paris Agreement climate goals. When Deprez and colleagues factored in sustainability risk, they found that the mitigation potential of CDR is far below what the IPCC states in its report.

The researchers found the combined mitigation potential for afforestation, reforestation, and BECCS is only around 2.6 billion t of CO₂ per year under scenarios that pose low sustainability risk. For medium-risk scenarios, the mitigation potential rises to around 5.1 billion t.

The paper “nicely encapsulates some important ideas that are well-known within the CDR community that might not be widely known in the broader climate community,” Morrow says. “These are ideas worth disseminating.”

CDR can still prove useful in offsetting hard-to-abate emissions, such as those from cement and steel plants, but it “can only be a marginal solution,” Deprez says. At the end of the day, “the main focus needs to be reducing emissions as much as possible across every sector.”