Drawing down carbon from the air and stashing it in underground rock formations has been framed as an essential way to slow and reverse global warming. But new research published Wednesday in the journal Nature finds there are far fewer suitable places to do this than previously thought.

After screening out “risky” areas, like those that are vulnerable to earthquakes, a team of researchers from Europe and the U.S. found that the Earth can only safely store about 1,460 gigatons of injected carbon in its sedimentary basins. This is an order of magnitude less than previous estimates, and — if you convert stored carbon to an estimated impact on the climate — only enough to cut global warming by about 0.7 degrees Celsius (1.3 degrees Fahrenheit), not the 6 degrees C (10.8 degrees F) described in other research. 

Carbon storage “can no longer be considered an unlimited solution to bring our climate back to a safe level,” one of the study’s co-authors, Joeri Rogelj, said in a statement. “Geological storage space needs to be thought of as a scarce resource that should be managed responsibly to allow a safe climate future for humanity.” Rogelj is director of research at the Grantham Institute on climate change and the environment at Imperial College London.

Carbon storage, for the sake of the paper, refers to the injection of carbon dioxide into underground reservoirs where it theoretically can’t contribute to climate change. There are two broad ways to get this carbon: first, by capturing it at the point of emission — say, the smokestack of a fossil fuel-powered cement factory — and second, by sucking it out of the ambient atmosphere.

According to the United Nations’ Intergovernmental Panel on Climate Change, or IPCC, the world’s foremost authority on the topic, at least some carbon storage will be necessary to achieve international climate targets. 

But the amount needed is dependent on a number of factors, including how much countries plan to slash emissions versus “offsetting” them, especially from hard-to-decarbonize sectors, and whether they intend to blow past 1.5 or 2 degrees C (2.7 or 3.5 degrees F) of global warming and then return to a more liveable temperature by removing carbon from the atmosphere. The latter is a contentious idea known as “overshoot,” and it would necessitate more carbon pulled out of the air and stored. Some IPCC scenarios involving substantial overshoot assume up to 2,000 gigatons of carbon storage by the year 2100.

According to the study’s authors, no previous global or regional estimate of the Earth’s technical carbon storage potential has taken into account key risk factors that would make some areas undesirable for storage. Starting from an estimate of all potentially available storage sites, their analysis cuts out areas that are too shallow, too deep, and too prone to earthquakes, as well as environmentally protected areas and areas near where people live. This reduces the total available capacity for carbon storage from 11,780 gigatons to just 1,460 gigatons of CO2, 70 percent of it on land and 30 percent on the seafloor.

The authors used an existing conversion rate from the IPCC to translate that gigaton number to about 0.4 to 0.7 degrees C (0.7 to 1.3 degrees F) of reduced global warming. 

They also noted some geographical disparities in the potential for carbon storage: While some historical climate polluters such as the U.S. and Canada have lots of space to safely stash carbon, others in Europe don’t. If those countries intend to make carbon storage a significant piece of their climate mitigation plans, they will likely have to look for locations in countries that have done little to contribute to climate change, potentially in Africa.

Sally Benson, an energy science and engineering professor at Stanford University who was not involved in the new research, said its findings should not be seen as “alarming” or “dramatic.” As described in the paper, IPCC scenarios that give the world a 50 percent chance of limiting global warming to 1.5 degrees Celsius by the end of the century would require sequestering about 9 gigatons of carbon per year (assuming that net-zero emissions are achieved around 2050). That means it could be more than 160 years before the world reaches the safe carbon storage limit calculated in the study. 

“What that tells me is that this is kind of good news,” Benson said. “Somebody has taken the most conservative of possible approaches to looking at this capacity and concluded, from my perspective, that there’s a lot of capacity relative to what we need.”

The study authors note that the need for storage could continue after their theoretical limit is reached, especially if countries keep needing to offset residual emissions from agriculture or the burning of fossil fuels in some sectors. Climate tipping points could also release more carbon dioxide into the atmosphere than anticipated, necessitating a greater-than-expected need for carbon removal. 

But Benson said these risks are too far in the future and that “we need to use all of the technologies available as quickly as possible.”

Both Benson and another independent expert — Jennifer Wilcox, a professor of chemical engineering and energy policy at the University of Pennsylvania’s Kleinman Center for Energy Policy — said the paper’s central estimate for safe and prudent carbon storage is likely too conservative. Wilcox told Grist it “undercounts what carefully pressure-managed projects can safely deliver.” 

But Naomi Oreskes, a professor in the history of science at Harvard University, held the opposite opinion. Oreskes said the paper fails to consider governmental, economic, and scientific challenges to actually deploying carbon storage at scale. “When you take those factors into account,” she said, “the potential for carbon storage, particularly in the crucial next decade, is even less.”

Despite significant hype around the technology, only about 0.05 gigatons of CO2 are currently stored via point-of-emission carbon capture each year. So far, most of these carbon capture projects inject carbon into the ground to aid the extraction of even more oil and gas, in a process known as “enhanced oil recovery.” And only 0.00001 gigatons of CO2 are removed from the ambient air each year. That’s less than the stated annual greenhouse gas emissions of Bowdoin College, a small liberal arts school in Maine.

“This new information is consistent with a broader pattern we have observed, of overstating the promise of ‘solutions’ that sidestep the central issue of reducing fossil fuel use,” Oreskes said.