Captain Drawdown’s daily logbook on every CDR story, paper, and expert voice — so you don’t have to read them all.

Why this matters now

Enhanced rock weathering (ERW) is being sold as one of the most measurable, durable removal pathways in the CDR toolkit. Buyers pay a premium precisely because grinding basalt onto soil is supposed to leave a traceable inorganic carbon signal. A newly published four-year German pot trial found no such signal above measurement noise. If the longest continuous field test can’t detect the carbon that Isometric, Puro, and Frontier Climate protocols are certifying, the case for paying on modeled tonnes deserves a hard second look.

What is ERW?

Enhanced rock weathering (crushing silicate rocks like basalt and spreading them on cropland to accelerate the natural reaction that pulls CO2 from air into stable bicarbonate) is credited today mostly through models. Those models take feedstock chemistry, application rate, rainfall, and soil pH, and estimate tonnes removed. Field verification is meant to calibrate the model. It is rarely used to overrule it.

Who’s involved?

Pol Knops and the Carbon Drawdown Initiative ran the XXL Lysimeter Experiment. Isometric, Puro.earth, and Frontier Climate are the three registries and buyer coalitions setting ERW crediting rules. Yale’s Tim Jesper Suhrhoff maintains the ERW MRV bibliography (measurement, reporting, verification) that those protocols lean on for defaults. Milliken and co-authors just posted agronomic co-benefit data from the US Southeast on CDRXIV.

What just happened?

The Carbon Drawdown Initiative ran twenty pots, basalt-amended and control, side by side through four winters. As I noted on Bluesky (@captaindrawdown.bsky.social), “Pol Knops ran basalt-amended plots for nearly four years and found no detectable inorganic carbon signal above noise. Yield co-benefits held.” A companion post showed doubling the basalt dose did not double removal. That kills the linear dose-response assumption embedded in most credit math.

Steel-manning the registries: pot lysimeters are not commercial cropland, German climate is cool and wet in ways that slow reaction kinetics, and inorganic carbon is not the only sink. Parallel work from Lei et al. at KIT suggests ERW may build mineral-associated organic carbon that current protocols don’t credit. So the null could mean the measurement stack is chasing the wrong pool, not that no removal occurred.

Weighed honestly, that defense cuts both ways. If organic pathways matter, protocols crediting only inorganic bicarbonate are mispricing the product. If site conditions dominate, then blanket model defaults calibrated on warmer, wetter, faster-weathering plots are overselling German and Northern European deployments. Either way, as I argued, this “isn’t just another model-vs-field gap. It’s a stress test of ERW registry rulebooks.”

Open questions to track

  • Will Isometric, Puro.earth, or Frontier Climate cite the German lysimeter data in their next protocol revisions, and will any require long-duration field verification before crediting modeled removal at commercial scale?
  • Do ERW purchase contracts pay on model outputs or on measured soil chemistry, and what is the clawback clause when year-four measurements return null?
  • Should protocols expand to credit mineral-associated organic carbon, or does that open a durability can of worms the sector has spent years avoiding?
  • How many active ERW projects sit on cool-climate sites where kinetics resemble the German trial more than the tropical calibration data?

Further reading

Citations

  1. LinkedIn — Carbon Drawdown Initiative — LinkedIn post
  2. Bluesky — ERW MRV bibliography — Bluesky post
  3. Cdrxiv — agronomic co-benefit data
  4. Bluesky — @captaindrawdown.bsky.social — Bluesky post
  5. LinkedIn — companion post — LinkedIn post
  6. DOI-resolved paper — Lei et al. at KIT
  7. Bluesky — as I argued — Bluesky post