New Study Says Carbon Must Be Stored For 1000 Years
A study by Cyril Brunner, Zeke Hausfather, and Reto Knutti published in the journal Communications Earth & Environment on November 11, 2024, comes to a startling conclusion. In the abstract of the study, the authors write:
“Carbon dioxide removal is essential for achieving net zero emissions, as it is required to neutralize any residual CO2 emissions. The scientifically recognized definition of carbon dioxide removal requires removed atmospheric CO2 to be stored ‘durably.’ However, it remains unclear what is meant by durably, and interpretations have varied from decades to millennia. Using a reduced-complexity climate model, here we examined the effect of carbon dioxide removal with varying CO2 storage durations.
“We found that storage duration substantially affects whether net zero emissions achieve the desired temperature outcomes. With a typical 100-year storage duration, net zero CO2 emissions with 6 GtCO2 per year residual emissions result in an additional warming of 0.8 °C by 2500 compared to permanent storage, thus putting the internationally agreed temperature limits at risk. Our findings suggest that a CO2 storage period of less than 1000 years (emphasis added) is insufficient for neutralizing remaining fossil CO2 emissions under net zero emissions. These results reinforce the principle that credible neutralization claims using carbon dioxide removal in a net zero framework require balancing emissions with removals of similar atmospheric residence time and storage reservoir — e.g., geological or biogenic.”
Anthropocene Magazine puts the results of the study in the vernacular for those of us who are not climate scientists. It says 1,000-year strategies sequester carbon for about as long as it takes for the 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 will be 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, he explains. 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. More research is needed, he says.
The time scale of carbon sequestration matters a lot, Brunner and his colleagues suggest. Using a simplified climate model to find out how different durations of carbon storage would affect total warming, they found that only 1,000-year carbon storage strategies will avoid further warming from residual carbon emissions. Broadly speaking, there are two ways to sequester carbon removed from the atmosphere. Natural carbon sequestration, in which plants absorb carbon dioxide from the air and store it in their tissues, lasts about 100 years. Forced carbon sequestration, in which carbon dioxide is injected into underground rock formations or sunk in blocks to the bottom of the ocean, should last for at least 1,000 years.
The Takeaway
Sharp-eyed readers will notice the typical news story about carbon sequestration seldom mentions how long the carbon will be sequestered — a material flaw in any conversation about direct air capture. The assumption is that once sequestered, it will stay sequestered, but we all know about the word “assume” by now, don’t we? That suggests all the fancy promises we hear about sucking carbon dioxide out of the atmosphere and piping it underground are apt to be much too optimistic at best and deliberately misleading at worst.
The researchers are attempting to close a loophole here, but will anyone listen? Certainly, based on past experience, there is little reason to think the fossil fuel crowd wants to delve too deeply into the ramifications of this research. They have been lying to us for 7 decades and are not about to let little things like truth or accuracy interfere with their carbon capture dream.