Storing carbon deep underground has been presented as an almost limitless solution to the climate crisis. To understand the limits to this resource, we designed an experiment using GIS methods based on the principles of multicriteria volumetric exclusion zones to systematically reduce the available azimuthal extent of global sedimentary basins – large underground rock formations where layers of sand, mud, and other materials have built up over millions of years, making them prime locations for both fossil fuel deposits and potential carbon storage. To construct the global exclusion zone layers, we assessed the suitability of sedimentary basins for carbon storage by looking at risks such as CO₂ leaking back into the atmosphere, the possibility of triggering earthquakes during injection, contamination of groundwater supplies, proximity to population centres or protected ecosystems, and where deep oil and gas drilling occurs today.

We conducted rigorous rounds of debates to prioritise the exclusion zones based on either binary applicability value i.e. either include the exclusion layers as a whole or remove it completely e.g. protected zones or using multiple sensitivity bounds e.g. buffers around population centres. The finalised areal spread of sedimentary basins considering environmental, human health, and engineering factors were combined with sedimentary depth requirements and limits to realise a spatially explicit estimate of CO2 storage potential within global sedimentary deposits. When these factors are taken into account, the global storage capacity shrinks dramatically from optimistic upper estimates of up to 40,000 billion tons to about 1,460 billion tons.

Assessed CO2 storage potential per sedimentary basin and per country. (Left) interactive map depicting global sedimentary basins and their assessed potentials for different sensitivity cases with offshore and onshore attribution. (Right) interactive map depicting areal spread of global onshore and offshore sedimentary basins, with overlay of areal spread of assessed onshore and offshore sedimentary basins for central sensitivity. Additionally, a map of global country level political boundaries and their prudent geological storage attribution.

To examine what these storage limits mean for the planet’s ability to cool down after overshooting temperature goals, we assess how our much cooling potential could be realized using the Transient Response to Cumulative CO2 Emissions (TCRE), one of the core scholarly tools behind understanding carbon budgets. If geological storage capacity would be exclusively used for CO2 removal and no further emissions would be produced by other activities at that point, a maximum 0.7°C warming reversal is possible before we run out of prudent storage space. Larger engineering and industry estimates suggest much deeper drawdowns could be possible – 5 to 6°C, or even higher in some studies – but these analyses make optimistic assumptions about how much carbon we can store underground, assume idealized conditions that are unlikely to be realized, and are based on using all available storage space on planet Earth. We emphasize that such comparisons highlight the stark difference between what is technically imaginable and what can be safely achieved. We also caution that removing carbon may not reduce warming in the same way that emitting it causes warming, and that the climate system might not return to its earlier state even if global temperatures are brought back down. Many scenarios assessed by the Intergovernmental Panel on Climate Change (IPCC) for limiting warming below 2°C depend on carbon storage at levels that could deplete this prudent capacity by the year 2200.

In addition to the above, our work highlights questions of fairness and responsibility. Countries with the largest fossil fuel industries often have the greatest storage potential but also bear the greatest historic responsibility for emissions. By framing carbon storage as a finite global resource, our study calls for international cooperation and careful planning. Policymakers will need to decide how to balance the competing demands of ongoing fossil fuel use with the need to remove carbon from the atmosphere to protect future generations. While carbon storage remains an important part of climate solutions, it should be treated like any scarce resource – with transparency, fairness, and a long-term vision.

References