A prudent planetary boundary for geological carbon storage

Storing carbon for centuries to millennia in geological formations will be required if the world is to achieve net-zero CO2 emissions, and an even more critical feature of mitigation strategies if net negative CO2 or net-zero greenhouse gas emissions are to be achieved in order for global mean surface temperature to decline. The technical potential for carbon storage is commonly assumed to be vast, with estimates of available storage of around 10,000-40,000 Gt CO2 in the scientific literature. We reassess that assumption, providing a new spatially explicit estimation of carbon storage potential in sedimentary basins consistent with the principle of harm prevention which can help guide policy makers when updating their climate pledges and stay within safe planetary boundaries. 

 

We begin with current estimates of sedimentary basin volume and systematically apply a number of prudent, precautionary spatial and volumetric risk exclusions. These include minimum depths of ~1 km to ensure cap rock seal, maximum depths of ~2.5 km to avoid bedrock and limit potential seismic activation of deep rooted faults, areas with more than “moderate” historic seismic activity, environmental protection areas including the polar circles, offshore areas with >300m water depth based on current practices in the oil and gas industry, and built-up areas of human settlement under a high-population future scenario. Combining all of our risk spatial layers, we find that global storage potential declines from 11,314 Gt GO2 to 1,550 Gt CO2 of which 70% is onshore. 

 

We classify countries into four categories combining their historical contributions to cumulative emissions and their available prudent carbon storage potential. We find that number of countries with strong per-capita contributions to historical emissions also can potentially play a strong role in storing carbon in the future (e.g., USA, Australia, Saudi Arabia) whereas others have a strong responsibility but low storage capacity (e.g., the EU) implying the need to utilize storage outside their borders.

 

We then compare our prudent storage potential with mitigation pathways assessed by the IPCC. We find that, if carbon storage injection rates were to be held constant at their respective levels at the time of CO2 net-zero, scenarios in line with the 1.5C limit of the Paris Agreement would allow for approximately 250 years of continued storage time, whereas scenarios with a 50% chance of limiting warming to 2C would have approximately 100 years of storage capacity remaining. However, scenarios in general tend to increase their use of storage beyond net-zero CO2 in order to counterbalance continued fossil fuel use or to draw down temperature levels beyond their peak. Extrapolating geologic storage usage forward, we find that nearly all IPCC-assessed scenarios limiting warming to 2C or less would reach our assessed planetary boundary before the year 2200.

 

Our analysis has broad implications for national mitigation plan development and suggests a need for countries to explicitly state their plans for geologic carbon storage as they develop the next round of their Nationally Determined Contributions.