Rising natural aerosols drive marine radiative forcing as pollution declines

Atmospheric aerosol particles are the leading uncertainty in climate forcing because they both reflect and absorb sunlight. Over the oceans, this influence is poorly constrained because long-term, composition-resolved observations are rare and natural sources vary widely. Using 2008–2019 aerosol records from interhemispheric observatories at Mace Head (Ireland) and Kennaook/Cape Grim (Tasmania), we show that increasing natural aerosol concentrations are driving increasingly negative marine aerosol forcing. After removing seasonality and quantifying trends across the concentration distribution, we find that at Mace Head historical declines in ammonium-sulfate and combustion-derived organics have largely levelled off, and the net anthropogenic forcing trend is now indistinguishable from zero because reduced scattering is offset by weaker black/brown carbon absorption. By contrast, increases in sea-salt, biogenic sulfate and marine organics drive a robust cooling trend (median −0.03 W m ⁻² decade ⁻¹ ; −0.05 W m ⁻² decade ⁻¹ at background levels). At Kennaook/Cape Grim, decreasing black carbon alongside increasing biogenic sulfate indicates that this shift is not confined to the North Atlantic. Meanwhile, extreme aerosol episodes linked to wildfire smoke, marine blooms and long-range transport also increase. These results show that evolving natural aerosol sources must be represented in climate models to correctly attribute and project near-term marine forcing.