Alternate Histories: Synthetic Large Ensembles of Sea‐Air CO 2 Flux Global Biogeochemical Cycles

We use a statistical emulation technique to construct synthetic ensembles of global and regional sea-air carbon dioxide (CO2) flux from four observation-based products over 1985–2014. Much like ensembles of Earth system models that are constructed by perturbing their initial conditions, our synthetic ensemble members exhibit different phasing of internal variability and a common externally forced signal. Our synthetic ensembles illustrate an important role for internal variability in the temporal evolution of global and regional CO2 flux and produce a wide range of possible trends over 1990–1999 and 2000–2009. We assume a specific externally forced signal and calculate the rank of the observed trends within the distribution of statistically