Global oxygen distributions at the Earth's surface.

Available oxygen (O2), described by its partial pressure ( [Formula: see text] ), governs key aerobic processes like dissolution, respiration, and oxidation. It makes up a practically constant fraction of the partial pressure of dry air (pd), which declines at constant pressure (p) with rising water vapor pressure (e) according to Dalton's law (p = pd + e). Since sea level p does not increase equatorward away from temperate zones, tropical humidity-with e reaching several percent of p-suppresses both pd and [Formula: see text] by equal proportions. Oxygen's molar fraction ( [Formula: see text] ) reflects this, dropping by thousands of ppm in the tropics, unlike the humidity-invariant dry-air fraction ( [Formula: see text] ), which varies inversely with carbon dioxide over mere dozens of ppm. Past O2 studies eliminated water vapor and analyzed dry air expressing [Formula: see text] , which was useful in constraining the carbon cycle but misrepresented O2 abundances and gradients. Using ERA5 reanalysis data, we estimate molar fractions of dry air (χd) to map global [Formula: see text] , revealing strong meridional gradients from the highly aerobic winter pole to the relatively hypoxic tropics. Reckoning with humidity's control over pd and χd clarifies key features of atmospheric O2 cycling such as cross-equatorial exchanges, air-sea fluxes, and the physical mechanisms of gas transport. This challenges transport models based exclusively on diffusion and highlights the hydrological cycle's role in propelling air away from water vapor sources, significantly conveying not only O2, but also greenhouse gases like methane and nitrous oxide.