By: Nick Davies & Anastasia O'Rourke
Emperor Augustus championed this maxim. Today, it resonates in the field of carbon dioxide removal, directing our attention to the slow geochemical processes that could reequilibrate the global carbon cycle.
Climate change is predominantly a result of the destabilizing effect that human activity continues to have on the global carbon cycle. The combustion of fossil fuels since the Industrial Revolution has led to a massive transfer of carbon from its slow domain—the highly durable carbon stores in rocks and geologic reservoirs—to its fast domain, where carbon cycles rapidly through the atmosphere, oceans, vegetation, soils, and freshwater.[1] Today, almost one sixth of the carbon dioxide (CO2) in the fast domain of the global carbon cycle is a result of human activity.[2]
Fossil fuel emissions are far exceeding the geochemical processes that might counterbalance them. Yearly fossil fuel emissions continue to grow, reaching a new high of 10.2 billion metric tons of carbon (GtC) in 2024. Meanwhile, the natural flux returning carbon to the slow domain is only sequestering 0.3 GtC per year.[3] The emerging field of geochemical carbon dioxide removal (CDR) is seeking to emulate and accelerate the slow natural processes behind this flux in order to correct for the growing imbalance that fossil fuel combustion is causing in the global carbon cycle.
At present, the geochemical CDR field is at an inflection point. On one hand, it is still far from making a meaningful contribution to the reequilibration of the global carbon cycle; the atmosphere-to-geosphere carbon flux from CDR activities (largely attributed to geochemical CDR) in 2023 was a million times smaller than the opposite carbon flux from fossil fuel combustion.[3]
On the other hand, experimental deployments and commercial pilots are transforming geochemical CDR from a nascent outsider into a central pillar of the broader CDR landscape. Results from this year’s XPrize Carbon Removal competition are a testament to the growing confidence in geochemical CDR; of the twenty finalists, nine employed geochemical approaches. What’s more, geochemical CDR companies won the $50 million Grand Prize (MATI Carbon) and the $5 million Third Runner-Up Prize (UNDO Carbon).
As geochemical CDR approaches continue to gain attention, the need for a clear understanding of these different technologies has become apparent. We have noticed that sometimes different pathways within the broad geochemical CDR space get confused, and as a result, have the potential to be misunderstood and their impacts conflated.
In this post, we lay out a framework to understand and categorize approaches within the field of geochemical CDR. In upcoming posts, we’ll introduce a life cycle assessment (LCA) tool developed to quantify the capture efficiency of reactor-based mineralization systems—a geochemical CDR approach discussed here—under site-specific conditions. We will also share our takeaways from applying this tool to the US’s largest deposit of olivine, a mineral highly suitable for geochemical CDR.
Coming to terms with geochemical CDR
At a fundamental level, geochemical CDR focuses on harnessing two related natural CO2-capturing processes: mineralization and chemical weathering. While these processes generally share the same combination of reactants–CO2, water, and an alkaline mineral rich in magnesium or calcium–they diverge in how and where carbon is ultimately stored.[5] Indeed, mineralization forms solid carbonate minerals at the reaction site, whereas chemical weathering (also called enhanced weathering, EW) produces dissolved bicarbonate ions that are eventually stored in ocean reservoirs.[4]
Breaking it down further
Geochemical CDR approaches can be further divided according to the environment and conditions in which they occur. Mineralization pathways are generally categorized as in situ or ex situ depending on whether they involve mineralization within natural geologic formations or in mined or waste feedstocks beyond those formations. Enhanced weathering pathways, by contrast, span reactor-based, agricultural, and marine settings. The taxonomy below offers a visual overview of the principal geochemical CDR approaches, while the glossary in the table provides definitions, co-benefits and example companies.
Companies featured in the glossary are non-exhaustive and for illustrative purposes only.
Figure 1: Taxonomy of geochemical CDR approaches
Table 1: Glossary of geochemical CDR approaches
This holistic overview of geochemical CDR approaches is intended as a foundation for a more rigorous understanding of the field; additional conceptual lenses are necessary to add color and contrast to its delineations. For example, geochemical CDR approaches can be situated along a spectrum ranging from open systems—those that harness naturally occurring processes over broad spatial and temporal scales—to closed systems that depend on engineered, industrial environments. It is also crucial to distinguish between capture-only approaches, which require an external input of concentrated CO₂, and complete capture-and-storage systems. Equally important is clarifying whether a given geochemical CDR pathway achieves net atmospheric carbon removal or contributes to emissions mitigation within an overall CO₂-emitting system.[5]
Festina lente—make haste slowly. Effective deployment of geochemical CDR depends on careful groundwork. Taking the time to establish a clear framework and a common language within the field of geochemical CDR, particularly as project development accelerates, will contribute to the environmental impact of the field as a whole.
Equally important are analytical tools that can help assess how different geochemical CDR approaches perform in a particular regional context. In our next post on the subject of geochemical CDR, we’ll share how we underwent the process of designing an LCA tool that can evaluate the performance of reactor-based mineralization approaches in site-specific scenarios.
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