Are we CO2 storage capacity limited?

The recent publication in Nature regarding the constraints on global CO2​ storage capacity, while executed within its self-defined parameters, presents a narrative that fundamentally misrepresents the feasible potential of carbon storage, and by extension carbon capture and storage (CCS), direct air capture (DAC), and bioenergy CCS (BECCS). By imposing overly restrictive criteria on a couple of key parameters and excluding mineral storage, the study severely underestimates the potential of CO2​ storage. The reductionist view of available capacity, particularly in the public communication, risks creating a premature sense of scarcity. This rebuttal argues that the technical availability of CO2​ storage is not the limiting factor for large-scale CCUS deployment and that the distraction created by this capacity debate is unhelpful to the real limiting factor: a lack of long-term, durable, and adequate incentives.

A core issue with the study is that even though the authors artificially constrain technical storage capacity, they still misplace the conclusion. The remaining capacity is thousands of gigatons, enough to help lower temperatures by 0.7C °C, and as we know, every tenth of a degree counts.

Considering current global annual CO2​ emissions are around 40GtCO2​, even a “severely constrained” capacity of, say, 1,400 GtCO2​ represents well over three decades of total global emissions, and more crucially, many decades of addressable industrial and power sector emissions suitable for capture. When viewed through the lens of a 2∘C or 1.5∘C decarbonization pathway, CCUS deployment is expected to scale up gradually over the next few decades, reaching its peak contribution in the second half of the century. The available capacity, even under the paper’s restricted lens, is far greater than the projected cumulative CO2​ that can realistically be captured and stored by 2050 or likely even 2100. The debate over the absolute GtCO2​ number, therefore, becomes a statistical exercise rather than a practical constraint on deployment planning. The planet has, for all intents and purposes, plenty of geologic capacity to meet climate mitigation needs in this century.

A significant methodological flaw lies in the utilization of overly restrictive depth criteria to limit the calculated storage volume. While the authors are correct to consider and include a minimum depth at which CO2 becomes a supercritical, dense fluid that descends instead of ascends like CO2 does as a gas, the necessary temperature (31.1 °C) and pressure (7.38 Mtpa) are typically met by the time one reaches 800m down. Also, while minimum depth constraints are necessary to ensure the CO2​ remains in a supercritical, dense phase, the implementation of arbitrary maximum depth limits fundamentally lacks a sound geological or engineering basis.

The study appears to rely on historical precedents or generalized economic thresholds rather than the maximum pressure and temperature limits of modern drilling and injection technology. Modern oil and gas operations routinely drill and complete wells to depths exceeding 4,000 meters (13,000 feet) in both onshore and offshore settings. The technical capacity to characterize, drill, and inject CO2​ into formations at these greater depths is demonstrably mature.

3. Exclusion of Mineral Carbonation Undermines the True Potential

This study focuses exclusively on geologic storage in deep saline aquifers and depleted reservoirs. It notes but neglects the mature and highly permanent potential of mineral carbonation. Mineral storage (or carbon mineralization) involves reacting CO2​ with naturally occurring silicate minerals (like basalt or peridotite) to form stable, solid carbonate minerals. This process provides arguably the most durable form of carbon sequestration, mimicking natural weathering at an accelerated pace.

While the engineering challenges related to energy input and reaction kinetics remain, pilot projects on peridotites are up and running in the Gulf, and Carbix has stored over 100,000 tons year in Iceland. More importantly, the capacity associated with global mafic and ultramafic rock formations is theoretically orders of magnitude larger than saline aquifer capacity, effectively rendering the capacity question moot from a geological standpoint. Any study aiming to represent the “technical limits” of sequestration must, at a minimum, include the recognized global potential of mineral storage as a critical long-term component.

The study, like most large-scale global assessments, is fundamentally limited by the availability of high-quality geological data. The resulting capacity numbers are, by definition, based on known and characterized sedimentary basins. This limitation is particularly egregious when assessing the global potential, as immense swathes of the planet remain underexplored for CO2​ storage potential.

This is especially true across the continent of Africa (save for Nigeria, South Africa, and Kenya) and in many parts of Asia and South America. These regions contain large, deep sedimentary basins that, while well-known to geologists, lack the dense network of seismic surveys and well data necessary to transition from “theoretical” capacity to “effective” or “practical” capacity estimates. The absence of data does not equate to the absence of storage.

The communication surrounding constrained capacity estimates often conflates two distinct issues: global storage capacity and commercial site-specific availability.

1. Global Capacity is a geological maximum: the total volume of pore space that could theoretically hold CO2​. This is shown to be abundant.

2. Commercial Availability in a specific country or region is an economic and regulatory reality: the volume of storage that can be proven, permitted, financed, and brought online at a specific industrial hub today.

The true bottleneck is not that the globe has limited CO2​ storage; it is the immense effort and time required to convert a theoretical basin into a commercial, permitted injection site—a process that involves years of seismic acquisition, characterization drilling, regulatory review, monitoring, public engagement, and most importantly, financial incentives to capture the CO2 in the first place.

Finally, the focus on storage capacity availability is a classic case of misplaced scrutiny. The primary limiting factor for large-scale CCUS deployment is the long-term market signal and financial viability of the capture and storage process itself.

Capture facilities, especially industrial sources and power plants, require multi-billion-dollar investments and multi-decade commitments. These investments will not materialize unless there is absolute regulatory and market certainty that the cost of carbon will remain sufficiently high, or the value of captured and stored CO2​ (via tax credits, like 45Q in the U.S., or contract-for-difference schemes) is guaranteed for the lifespan of the asset. The technical capacity of a storage reservoir 1,000 kilometers away is irrelevant if the chemical plant or cement factory cannot secure the finance to build the capture unit in the first place. Resolving the long-term price certainty for carbon management is the necessary and sufficient condition for accelerating CCUS deployment, not proving the 1000th GtCO2​ of geological capacity. As a final point of perspective, the study also misses that while CCS might only add 1200-1500 Gt CO2 mitigation capacity by their measures, that’s more than the 800-1000 Gt CO2 difference between 1.5 and 2.0 °C in the Paris Agreement, for which it could provide a critical buffer.

This study, and the communication and press following its release, do not accurately represent the technical limits of CO2​ storage capacity, in any real sense. Employing a narrow and technically questionable set of constraints, particularly regarding depth and the omission of mineral storage, it offers a deeply conservative lower-bound estimate rather than a true reflection of the planet’s vast geological potential. The subsequent narrative, suggesting a capacity bottleneck, is a critical distraction from the real issues with CCUS deployment, which are overwhelmingly economic, regulatory, and infrastructural. The focus must be shifted from the false premise of storage scarcity to the genuine challenge of creating predictable, durable policy and long-term market signals necessary to finance, deploy, and govern capture technology at the requisite scale.