The application of finely ground silicate minerals to croplands and forests, with the aim of enhancing the rate of natural CO₂ consuming weathering reactions, is receiving attention as a part of climate change mitigation strategies. Yet considerable uncertainty surrounds the quantification of CO₂ removal associated with Enhanced Weathering, and its potential efficacy remains undemonstrated outside of the laboratory. Here, I discuss how the geochemical insights garnered from decades of natural weathering studies provide a pathway towards a strategy for ‘Monitoring, Reporting and Verification’ of CO₂ sequestration. These natural weathering studies have also produced an understanding of what limits silicate weathering in different settings, which can be used to shed light on how deployment strategies, and specifically application sites, can be optimised.

In this work we study critical questions which determine the scale and viability of ocean alkalinity enhancement (OAE): Which coastal locations are able to sustain a large flux of alkalinity at minimal pH and aragonite saturation changes? What is the interference distance between adjacent OAE projects? How much CO₂ is absorbed per unit of alkalinity added? How quickly does the induced CO₂ deficiency equilibrate with the atmosphere?
Using the LLC270 (0.3deg) ECCO global circulation model we find that the steady-state OAE rate varies over 1–2 orders of magnitude between different coasts and exhibits complex patterns and non-local dependencies which vary from region to region. Neighboring OAE sites can exhibit dependencies as far as 400 km or more. We show that near-coastal OAE has the potential to scale globally to several GtCO₂/yr of drawdown with conservative pH constraints, but only if the effort is spread over the majority of available coastlines.
Depending on the location, we find a diverse set of equilibration kinetics, determined by the interplay of gas exchange and surface residence time. Most locations reach an uptake-efficiency plateau of 0.6–0.8mol CO₂ per mol of alkalinity after 3–4 years, after which there is little further CO₂ uptake. Some locations (e.g. Hawaii) take significantly longer to equilibrate (up to 8–10 years), but can still eventually achieve high uptake. If the alkalinity released advects into regions of significant downwelling (e.g. around Iceland) up to half of the OAE potential can be lost to bottom waters.

The seafloor functions as a substantial long-term (>100 yr) carbon sink and reservoir in the form of sedimentary organic particles. Human activities can modify the seafloor's natural carbon sequestration capacity by disturbing the upper sediment layers and thereby altering biogeochemical processes and releasing previously trapped carbon. Yet, the overall magnitudes of the changes in sedimentary carbon pools induced by these processes are not well known and could not be adequately considered in environmental impact assessments so far.

In this study, we quantify anthropogenic disturbances of sedimentary carbon sequestration in the North Sea using a combination of measurement data and numerical modeling. In particular, we examine the effects of bottom-contacting fisheries, sediment extraction, and material dumping. By resolving spatial and temporal patterns of both natural and anthropogenic drivers, we identify areas particularly vulnerable to degradation, representing the most detailed large-scale estimates of human impacts on North Sea sediments to date. Our results indicate that while the impacts of human activities on sedimentary carbon sequestration are comparable in magnitude to natural sedimentation processes, the resulting carbon benefits are considerably lower than previously estimated.

Although remaining uncertainties need to be further confined and missing processes such as ecosystem feedbacks considered, our findings can serve as a useful basis for the consideration of sedimentary carbon disturbance in the context of marine spatial management plans.

The tropics have the potential to capture large amounts of CO2 through enhanced weathering of mafic rocks. This negative emissions technology can be articulated with the needs of the agricultural sector, in particular the large irrigated tropical cropland in the tropics. High-yield agricultural soils in the tropics are acidic. Their acidity is traditionally controlled with lime, which in turn emits CO2. Crushed mafic rocks can be used instead of lime, with the added benefit of not only avoiding the lime emissions, but also capturing CO2. In tropical Colombia, there are extensive plantations of African oil palm, sugar cane, rice, banana, and corn. Nearby, there are large open-cast mining operations that produce massive volumes of mafic and ultramafic rocks as waste product. We are characterizing mafic and ultramafic rocks closest to the potential application sites, as well as the products of their weathering under natural conditions. We are also conducting field experiments with natural soils from areas of active afforestation, as well as in soils degraded by cattle ranching. Both were previously covered by rain forests. In these experiments we are evaluating the reaction rates and efficiencies of mafic and ultramafic rocks under tropical conditions in natural soils. We aim to establish the technical and scientific basis for a process by which the forestry, mining, agricultural, and energy industries in the tropics can reduce their operational carbon footprint, and eventually offer carbon capture bonds in the international market.

It is now well known that limiting the warming of the planet to 1,5°C by the end of the century will require some degree of carbon dioxide removal (CDR) and marine CDR technologies have emerged as a potential solution to mitigate climate change. While marine CDR can be piloted domestically, its maximum efficiency requires international coordination due to transboundary effects. However, the current international legal framework, including the UNFCCC and its descendants, lacks a dedicated regime for CDR, resulting in a plethora of potentially applicable sources of law with different answers on the legality of CDR. Emphasizing the challenges and necessities of coordinating CDR activities on a global level, this presentation addresses the unrealistic optimism of current law to over-rely on clear cut answers from natural sciences. It will explain how the traditional approach of the law needs to be revised to adapt to the scientific realities of climate action. This analysis should provide a reflection on the need for a wider societal debate to solve “trade-offs” issues that lawyers alone are not in a position to solve.