Alternate histories: synthetic large ensembles of sea-air CO2 flux

Overview & Synopsis

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 modeled synthetic trends during these periods. Over the decade 1990-1999, three of 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 within one standard deviation of the mean in their respective synthetic ensembles. Over the decade 2000-2009, however, three products show large negative trends in globally integrated sea-air CO2 flux that have a low rate of occurrence in their synthetic ensembles. The largest positive trends in global and Southern Ocean flux over 1990-1999 and the largest negative trends over 2000-2009 fall nearly two standard deviations away from the mean in their ensembles. Our approach provides a new perspective on the important role of internal variability in sea-air CO2 flux trends, and furthers understanding of the role of internal and external processes in driving observed sea-air CO2 flux variability.