Freshwater transport from warm to cold ocean regions amplifying faster than all model estimates

Warming-induced global water cycle changes pose a significant challenge to global ecosystems and human society. The magnitude of historical water cycle change is uncertain due to a dearth of direct rainfall and evaporation observations, particularly over the ocean where 80% of global evaporation occurs. Air-sea fluxes of freshwater and river run-off imprint on ocean salinity at different temperatures, such that warmer regions tend to be saltier and cooler regions tend to be fresher. In this work, we track observed salinity trends in the warm, salty fraction of the ocean from 1970 to 2014, and infer the global poleward transport of freshwater over this period. Since 1970, 46 - 77 x10^12 m^3 of freshwater has been transported poleward from the warmest fraction of the ocean. No model in the current generation of climate models (the 6th Climate Model Intercomparison Project; CMIP6) replicates this transport, with the closest model underestimating transport by 2 - 4 times. We trace the climate model biases to a weaker than expected surface freshwater flux intensification, just 0 - 4% in CMIP6 models compared to an estimated 3 - 7.5% in observations.