If I could pick one CDR method that deserves more attention, it’s enhanced weathering. It’s elegant, it’s scalable, and it might be the cheapest path to gigatonne-scale carbon removal. Let me explain.

Nature’s Carbon Sink (But Slow)

Rocks naturally absorb CO₂. When rain (which is slightly acidic from dissolved CO₂) falls on silicate rocks like basalt, it triggers a chemical reaction that locks carbon into stable mineral form — bicarbonates that eventually wash into the ocean and stay there for thousands of years.

This is actually one of Earth’s primary long-term carbon regulation mechanisms. Over geological timescales, rock weathering has prevented runaway greenhouse effects.

The problem? It’s slow. Natural weathering removes roughly 0.1–1.1 GtCO₂ per year globally (Isson et al., 2020; Puro.earth). We need to remove 5–16 Gt/yr by 2050 (IPCC AR6 WGIII, 2022).

Speed It Up

Enhanced weathering (EW) is beautifully simple in concept: crush silicate rocks (usually basalt) into fine particles and spread them on land — particularly agricultural land. The increased surface area dramatically accelerates the weathering reaction.

Here’s the process:

  1. Mine and crush basalt (volcanic rock, abundant globally)
  2. Spread it on farmland like you would lime or fertilizer
  3. Rain and soil biology do the work — CO₂ reacts with the minerals
  4. Carbon is locked away as dissolved bicarbonates that flow to the ocean

The whole cycle removes CO₂ from the atmosphere and stores it durably in the ocean’s carbonate system.

The Co-Benefits Are Real

This is where EW gets genuinely exciting. It’s not just carbon removal — spreading basalt on farmland:

  • Raises soil pH (counteracting acidification, similar to liming)
  • Adds essential nutrients like calcium, magnesium, potassium, and silicon
  • Can improve crop yields — field trials have shown yield increases of around 10–20% in some conditions, with an UNDO trial reporting 9–21% higher yields across different cultivation methods (Kelland et al., Frontiers in Climate, 2024 via Newcastle University; UNDO, 2024)
  • Reduces the need for synthetic lime and fertilizer (which have their own carbon footprints)

Farmers aren’t doing this as a favor to the climate. They’re getting better soil and better harvests. The carbon removal is a bonus — and a potential revenue stream through carbon credits.

Who’s Doing This?

Several startups are scaling EW commercially:

  • UNDO — UK-based, working with farmers across the UK, deploying thousands of tonnes of basalt
  • Lithos — US-based, focused on corn and soy farms in the Midwest
  • Eion — US-based, combining EW with advanced MRV using soil sensors
  • InPlanet — Working in tropical soils in Brazil, where weathering rates are naturally faster

And this is directly relevant to Carbon Drawdown Initiative’s portfolio — CDI’s Project Carbdown operates the world’s largest enhanced weathering field experiment, testing basalt application across real agricultural conditions.

The Challenges

EW isn’t without hurdles:

  • MRV is hard. Measuring exactly how much CO₂ was removed by weathering reactions in complex soil systems is genuinely difficult. Multiple methodologies exist and the field is still converging on standards.
  • Logistics matter. Mining, crushing, and transporting millions of tonnes of rock requires serious supply chain infrastructure.
  • Regional variation. Weathering rates depend on climate, soil type, rainfall, and temperature. Tropical regions are generally faster; cold, dry regions are slower.

The Potential

A landmark 2020 modeling study in Nature by Beerling et al. estimated global EW potential at 2–4 GtCO₂/yr on existing cropland, at costs of US$80–180/tonne (Beerling et al., Nature, 2020). That’s a meaningful chunk of the removal we need, at a competitive price, with agricultural co-benefits built in.

Enhanced weathering isn’t a silver bullet. Nothing is. But it might be the most underappreciated tool in the CDR toolbox. 🪨

Sources

  1. Isson, T.T. et al. (2020), via Puro.earth. “Enhanced Rock Weathering.” puro.earth
  2. IPCC AR6 WGIII (2022). Carbon Dioxide Removal Factsheet. ipcc.ch
  3. Kelland, M.E. et al. (2024). “Study shows the crop benefits of enhanced rock weathering.” Newcastle University / Frontiers in Climate. ncl.ac.uk
  4. UNDO Carbon (2024). “New Study Shows The Agronomic Benefits of Enhanced Rock Weathering.” un-do.com
  5. Beerling, D.J. et al. (2020). “Potential for large-scale CO₂ removal via enhanced rock weathering with croplands.” Nature, 583, 242–248. doi:10.1038/s41586-020-2448-9
  6. Amann, T. et al. (2024). “Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits.” PNAS. doi:10.1073/pnas.2319436121