Take on a podcast episode from The Land & Climate Podcast, originally published Fri, 26 Ju. Listen: https://www.landclimate.org/podcast/

TL;DR

  • David Ho (Hawaiʻi, IPCC lead author) argues marine CDR isn’t ready — most techniques carry too many known unknowns to deploy. Fair and specific.
  • Enthusiasm has cooled since ~2023 across CDR broadly, not just ocean. Matches what practitioners are seeing.
  • Ocean iron fertilization: nutrient robbing effect only visible at scales we’ll never test. Models flag it, deployers dismiss it. Useful framing of the epistemic trap.
  • Direct ocean removal is being added to IPCC methodology as a “political decision” despite research gaps. First time I’ve seen it stated this bluntly by a lead author.
  • Governance gap: equilibration happens outside the deploying country’s EEZ — nobody’s worked out who claims the credit. Underrated issue.

Bertie Harson-Bredinsky hosts David Ho of the University of Hawaiʻi at Mānoa, fresh off the 4th International Conference on Carbon Dioxide Removal in Milan and now a lead author on the IPCC methodology report covering CDR and CCUS. The conversation is a sober tour of marine CDR — iron fertilization, ocean alkalinity enhancement, direct ocean removal, and blue carbon — from someone who’s moved from cautiously curious to openly skeptical. Listen here: landclimate.org/podcast.

The most substantive thread is Ho’s taxonomy of why marine CDR isn’t ready, and it’s not the usual cost complaint. Biotic techniques (iron fertilization) manipulate ecosystems directly; the phytoplankton bloom gets eaten and respired back to CO₂ in the surface ocean, and the nutrient-robbing effect on downstream regions only shows up in models — models the pro-deployment camp waves away. Abiotic techniques (ocean alkalinity enhancement, enhanced weathering) look more durable — “tens to hundreds of thousands of years” — but the scale is daunting and the cost, in Ho’s estimate, runs to “trillions of dollars” for meaningful deployment. His sharpest point lands on direct ocean removal: because you have to acidify seawater to strip the CO₂ out, you’re mobilizing a permanently-sequestered inorganic carbon pool. “If you don’t make sure that the CO₂ that you strip out of seawater is buried somewhere for 100,000 years, then you’ve made the problem worse.” That’s a durability inversion worth sitting with.

The second thread worth your time is governance. Ho notes the London Protocol prohibits ocean iron fertilization deployment but not research — and says nothing about alkalinity additions. More novel: because deployment happens in a country’s EEZ but ocean-atmosphere equilibration happens elsewhere (including other EEZs or the high seas), a country claiming the removal in its Nationally Determined Contribution is effectively consuming another country’s future removal capacity. This isn’t in most credit-accounting conversations yet and probably should be.

For adjacent context: Ho’s 2023 Nature piece arguing CDR isn’t a current climate solution has been downloaded 100,000+ times and remains the reference text for the mitigation-deterrence critique. On the buyer side, Frontier and Isometric have been building out ocean alkalinity enhancement protocols with startups like Planetary and Ebb Carbon — worth reading alongside Ho’s skepticism, because the offtake side is proceeding as if measurement, reporting, and verification (MRV) is a solved-in-principle problem while Ho is saying the underlying science isn’t there. Also flagged in the show notes: the November 2025 npj Ocean Sustainability paper on three challenges to marine CDR.

Useful if you’re a buyer evaluating ocean pathways, an MRV designer working on marine protocols, or a policy person tracking how IPCC methodology gets built under political pressure. Skip if you want optimism.