Captain Drawdown’s daily logbook on every CDR story, paper, and expert voice — so you don’t have to read them all.
The critique
“This is indeed one of the reasons why I think there is still a justification for the old practice of seeing CDR as a form of geoengineering. They both decouple climate variables from cumulative emissions.” That’s Oliver Morton (@eaterofsun on Bluesky), and the target is any CDR pathway whose mass balance does not cleanly close. Enhanced rock weathering (ERW), where crushed silicates dissolve in soil and the resulting alkalinity is assumed to travel to the ocean for long-term storage, is exactly that kind of pathway right now.
The evidence Morton is reading correctly
A new CDRXIV preprint, Photosynthesis in Rivers as a Loss Pathway for ERW-Derived DIC and Alkalinity (Neumann et al., preprint 329), argues that aquatic photosynthesis can reconvert dissolved inorganic carbon (DIC) in rivers back into CO2 that outgasses to the atmosphere. If that pathway is quantitatively meaningful, the DIC that soil-based ERW measurement, reporting and verification (MRV) counts as removed does not all make it to durable ocean storage. It leaks in transit.
The upstream side is also wobbly. Rogers and Maher (CDRXIV preprint 334) document large unquantified spatial uncertainty in feedstock dissolution across a field. So both ends of the ERW measurement chain, source and sink, have larger error bars than credit issuers currently price in.
The community’s standard response, and where it’s weak
The usual reply: cation depletion in soils, plus catchment-scale DIC modeling, is good enough for a first credit vintage, and refinements will come. That was defensible eighteen months ago. It is weaker now, because the specific loss mechanism Neumann et al. describe was not in most protocol assumptions, and because credits are already being issued against models that assume conservative riverine transport.
What the critique gets right
Three things. First, cation-budget inference at the field edge cannot see downstream biological reprocessing. It is the wrong instrument for the question. Second, the parallel CarbonPlan reminder that two California forest offset projects are currently burning is the right analogy: when a pathway’s accounting doesn’t survive contact with reality, the credits become a reputational liability for everyone. Third, “geoengineering-adjacent” is a fair frame for any removal claim that cannot close its mass balance end to end.
What the critique overstates
Neumann et al. is a preprint, not a settled quantification. It identifies a loss mechanism. It does not yet tell us what fraction of global ERW alkalinity is lost this way, in which climates, at which flow regimes. And the tools to answer that are already emerging. Muth, Boysen and Michel (CDRXIV preprint 456) describe in-situ alkalinity export sensors designed exactly for real-time riverine MRV. Mooshammer et al. (CDRXIV preprint 309) propose measuring DIC directly rather than inferring it from cations. Zhou et al. (CDRXIV preprint 408) provide a high-resolution modeling framework to estimate what fraction of alkalinity actually survives transit to the coast. The pathway is not broken. Its measurement stack is incomplete.
The synthesis
For a primer on the underlying chemistry, see What Is Enhanced Weathering. What ERW builders should take from this week: soil-edge MRV alone is no longer sufficient to defend a tonne claim. Protocols that credit at the field boundary, without any downstream constraint on in-stream DIC losses, are exposed. Suppliers already deploying catchment-outlet or in-river sensors are positioned to answer the critique. Suppliers relying on cation-depletion inference alone are not.
The specific thing to watch: whether Isometric, Puro, and other registries update ERW methodologies to require riverine or catchment-outlet measurement in the next twelve months, and whether the Muth et al. sensor approach shows up in a real credit vintage. If registries move, the pathway strengthens. If they don’t, Morton’s geoengineering framing will keep gaining ground, and it will be deserved.
Move MRV downstream. Literally.
Citations
- Bluesky — @eaterofsun on Bluesky — Bluesky post
- Cdrxiv — Photosynthesis in Rivers as a Loss Pathway for ERW-Derived DIC and Alkalinity
- Cdrxiv — Rogers and Maher
- Bluesky — two California forest offset projects are currently burning — Bluesky post
- Cdrxiv — Muth, Boysen and Michel
- Cdrxiv — Mooshammer et al.
- Cdrxiv — Zhou et al.