Warming and iron steepen ocean particle spectra without enhancing carbon export

Abstract Particle size structure sets how much of the carbon fixed at the ocean surface reaches depth. Two communities disagree about the sign of that link. Size-resolved ecosystem models expect steeper spectra — proportionally more small particles — to raise the export efficiency of primary production. We assemble 8,479 global UVP5 particle size-spectrum profiles (137 cruises, 75°S–81°N), co-locate them with Bio-ORACLE v3 environmental layers and match them to three global particle-export compilations and monthly satellite primary-production fields. Warming steepens the mesoplankton spectrum (−0.28 per SD of sea-surface temperature, P = 1.4 × 10 −8 , 56% of the shared variance). Iron-rich waters are steeper too (−0.21 per SD, P = 0.0013). Oxygen contributes no independent effect. The iron axis replicates against in situ GEOTRACES measurements. Against the model prediction, steeper spectra track lower export in two independent tests: e-ratio rebuilt on satellite NPP (+0.073 per SD, n = 6,539, P = 2.5 × 10 −5 ) and POC/²³⁴Th ratios (+0.065 per SD, n = 3,680, P = 1.1 × 10 −19 ). The signal the models describe is real, and it peaks in the micro-particle segment. Projected to the 2050s under SSP2-4.5 and SSP5-8.5, the median POC/²³⁴Th change is +1.9%, with declines beyond 15% in 14% of grid cells, concentrated in the North Atlantic subpolar gyre. The observational side of the dispute wins. Warming does not lower export everywhere. It lowers it in identifiable places, and those places can be mapped.