Partitioning Uncertainty in Ocean Carbon Uptake Predictions

The oceans have absorbed a large fraction of anthropogenic carbon dioxide emissions, having consequences for ocean biogeochemistry and ecosystems. Simulations with Earth System Models can be used to predict the future evolution of ocean carbon uptake in the coming decades and beyond, but there is substantial uncertainty in these model predictions, particularly on regional scales. Uncertainty can be separated into three component parts: (1) uncertainty due to internal variability, (2) uncertainty due to model structure, and (3) uncertainty due to emission scenario. These uncertainties are not constant, but instead vary with prediction lead time and the scale of spatial averaging. Here, we isolate and quantify the evolution of these three sources of prediction uncertainty in ocean carbon uptake over the next century using output from two sets of ensembles from the Community Earth System Model (CESM) along with output from models participating in the Fifth Coupled Model Intercomparison Project (CMIP5). We show that prediction uncertainty in ocean carbon uptake is relatively high for short lead times (~5 years), achieves a minimum at medium lead times (~20 years), and is again relatively high for long lead times (~80 years). The dominant source of prediction uncertainty in globally-integrated ocean carbon uptake for the next few decades is internal variability, whereas, emission scenario uncertainty contributes the most to the total prediction uncertainty by the end of the century. On regional scales, and in highly variable regions, such as the California Current System, prediction uncertainty in ocean carbon uptake is dominated by internal variability for the next 70 years.