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 likelihood of the observed trend given the distribution of synthetic trends during these two periods. Over the decade 1990-1999, three of the four observation-based products exhibit small negative trends in globally integrated sea-air CO2 flux (i.e., enhanced ocean CO2 absorption with time) that are highly probable (44-72% chance of occurrence) in their respective synthetic trend distributions. Over the decade 2000-2009, however, three of the four products show large negative trends in globally integrated sea-air CO2 flux that are somewhat improbable (17-19% chance of occurrence). Our synthetic ensembles suggest that the largest observation-based positive trends in global and Southern Ocean CO2 flux over 1990-1999 and the largest negative trends over 2000-2009 are somewhat improbable (<30% chance of occurrence). Our approach provides a new understanding of the role of internal and external processes in driving sea-air CO2 flux variability.