Abstract
<jats:p>Abstract. Climate policy treats carbon dioxide as the single lever on the climate. We suggest a second one has been overlooked: the marine biodiversity that builds and maintains the ocean surface microlayer (SML) and that pollution is now degrading. Water vapour, not CO₂, is the largest single contributor to the natural greenhouse effect and the strongest feedback on warming. The ocean supplies most of that vapour across the 71% of the planet it covers, and the thin biogenic lipid-and-surfactant film phytoplankton spread over the sea surface helps govern how readily it evaporates and forms aerosol. That same film gathers lipophilic "forever" chemicals, together with the microplastic and black-carbon soot they cling to, to levels far above the water beneath, where they turn toxic to the plankton that hold the ecosystem together. An automated image pipeline sized and classified 51,547 particles larger than 20 μm in 352 georeferenced surface samples from eight sailing vessels crossing the trade-wind North Atlantic. Zooplankton were absent from 93.8% of stations. Black carbon appeared at 89.8%, microplastic particles at 69.9%, microfibres at 22.4% and phytoplankton at 81.8%; 99.2% of everything counted measured below 200 μm. A separate 110-image, GPS-tagged crossing returned zero zooplankton at every point on the track, and a plankton net towed at the surface by day and by night confirmed the scarcity directly, returning roughly 0.2 to 0.5 animals per cubic metre – far below any productive ocean. In short, this surface layer is stripped of animal plankton and steeped in combustion and plastic residue. We trace what that may mean for the microlayer, for water-vapour feedback and cloud formation, and for ocean pH, and argue that cutting pollution has to sit beside cutting carbon.</jats:p>