A lack of clarity about what “counts” as removing carbon dioxide from the atmosphere is jeopardizing the success of global efforts to stop climate change, according to a new analysis. The study suggests that removed carbon must be stored, or sequestered, for at least 1,000 years in order to avoid greater-than-expected warming.
Despite the massive efforts to decarbonize the global economy that are underway, some sources of carbon emissions are difficult or impossible to eliminate, and will likely continue for the foreseeable future. But if an amount of carbon dioxide equivalent to these so-called residual emissions can be removed from the atmosphere and stored away for the long term, then the world can achieve net-zero emissions and limit climate change.
“Carbon dioxide removal is not the most important, but an essential part of any net-zero emissions target,” says study team member Cyril Brunner, a climate scientist at ETH Institute for Atmospheric and Climate Science in Zurich, Switzerland.
Broadly speaking, there are two ways to sequester carbon removed from the atmosphere: Natural carbon sequestration (plants absorb carbon dioxide from the air and store it in their tissues) which lasts on the order of perhaps 100 years, and forced carbon sequestration (e.g., injecting carbon dioxide into underground rock formations or sinking blocks of carbon to the bottom of the ocean) which last for at least 1,000 years.
International carbon sequestration schemes and agreements aren’t always consistent about what it means to store carbon for the long term. And climate models generally treat all forms of carbon sequestration as equivalent.
But the time scale of carbon sequestration matters a lot, Brunner and his colleagues demonstrate.
The researchers used a simplified climate model to find out how different durations of carbon storage would affect total warming. Only 1,000-year carbon storage strategies will avoid further warming from residual carbon emissions, the researchers report in the journal Communications Earth & Environment.
The 1,000-year strategies sequester carbon for about as long as it takes for carbon to be naturally recycled out of the atmosphere. Shorter-term strategies that only hold carbon for 100 years will result in the re-release of that carbon long before the emissions that are being offset would leave the atmosphere naturally. The result of these short-term strategies: excess, unaccounted-for emissions, and more warming.
For example, suppose there is a need for atmospheric carbon removal to offset 6 gigatons per year of residual carbon dioxide emissions. If that carbon is sequestered for only 100 years, its re-release would result in an additional 1.1 °C of warming by the year 2500 compared to permanent carbon storage, the researchers calculated.
The findings are “not really” surprising, according to Brunner. “The basis for our conclusions has been well-known for more than a decade,” he says. But the new analysis lays bare the contradictions of the current approach in stark numbers for the first time.
“That does not mean that carbon dioxide removal with shorter storage durations than 1,000 years is not useful, quite the opposite,” Brunner says. Natural carbon sequestration strategies can improve air and water quality and safeguard biodiversity, and might also be useful to offset emissions of shorter-lived greenhouse gases such as methane. “Studying the climate outcome [of this strategy] will be a topic of further research,” Brunner says.
Source: Brunner C. et al. “Durability of carbon dioxide removal is critical for Paris climate goals.” Communications Earth & Environment 2024.
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