Time of Emergence and Large Ensemble Intercomparison for Ocean Biogeochemical Trends

Anthropogenically forced changes in ocean biogeochemistry are underway and critical for the ocean carbon sink and marine habitat. Detecting such changes in ocean biogeochemistry will require quantification of the magnitude of the change (anthropogenic signal) and the natural variability inherent to the climate system (noise). Here we use Large Ensemble (LE) experiments from four Earth system models (ESMs) with multiple emissions scenarios to estimate Time of Emergence (ToE) and partition projection uncertainty for anthropogenic signals in five biogeochemically important upper‐ocean variables. We find ToEs are robust across ESMs for sea surface temperature and the invasion of anthropogenic carbon; emergence time scales are 20–30 yr. For the biological carbon pump, and sea surface chlorophyll and salinity, emergence time scales are longer (50+ yr), less robust across the ESMs, and more sensitive to the forcing scenario considered. We find internal variability uncertainty, and model differences in the internal variability uncertainty, can be consequential sources of uncertainty for projecting regional changes in ocean biogeochemistry over the coming decades. In combining structural, scenario, and internal variability uncertainty, this study represents the most comprehensive characterization of biogeochemical emergence time scales and uncertainty to date. Our findings delineate critical spatial and duration requirements for marine observing systems to robustly detect anthropogenic change. Plain Language Summary Man‐made climate change is causing physical, chemical, and biological changes in the ocean. We use Earth system models (climate models with an interactive carbon cycle) to estimate when these man‐made changes will be significantly larger than, and therefore distinguishable from, natural fluctuations in the climate and oceans. We find models agree that changes in sea surface temperature and the strength of the ocean carbon sink should already or will soon be detectable in the current observational record. Changes in the upper ocean biological cycling of carbon, photosynthetic activity, and salinity, however, are less certain and will take many more decades of monitoring in order for man‐made changes to potentially become visible. We examine sources of uncertainty inherent to projecting the ocean's future over the coming decades and find that uncertainty in the ocean's natural variability can be as important as uncertainty across different climate models