EAPS

Professor Gruber (Professor of Environmental Physics at ETH Zurich) will give a special lecture as part of the Climate Symposium week.

Title:
Recent variability and trends of the ocean carbon sink.

Abstract:
The community-driven syntheses of global data sets for both the surface and interior ocean, gives us the opportunity to develop an observation- based perspective on the recent variability and trends of the global ocean carbon sink. Here, we synthesize and compare two estimates, one based on surface pCO2 data (Landschützer et al., 2016) providing monthly resolved global air-sea CO2 fluxes for the period from 1982 through 2018, and one based on ocean interior data providing an estimate of the oceanic accumulation of anthropogenic CO2 between the 1990s and the mid-2000s (Gruber et al., 2019). The surface data suggest the presence of a substantial amount of decadal variability in the ocean carbon uptake around a trend that is largely consistent with expectations based on the increase in atmospheric CO2. These decadal variations originate mostly from the extratropical oceans and especially the Southern Ocean, with the latter mostly being driven by changes in ocean temperature and circulation, induced by a reorganization of the weather systems in the southern hemisphere. The ocean interior data cannot resolve the detailed temporal evolution, but provide a time-integrated perspective. These data suggest a global increase in the inventory of anthropogenic CO2 of 34±4 Pg C between 1994 and 2007, which is equivalent to an annual uptake rate of 2.6±0.3 Pg C yr-1. This contrasts with an average uptake rate of only 1.3 Pg C yr-1 over this period inferred from the surface ocean pCO2 data. Even if we account for the loss of about 0.6 Pg C yr-1 stemming from the steady-state balance between river input and burial, the surface ocean data still indicate substantially less carbon uptake than the ocean interior data constraints. This discrepancy can be resolved if we accept a loss of extra natural carbon form the ocean of about 0.7 Pg C yr-1 as a result of climate variability and change. Hints of such decadal variations can be seen in the ocean’s interior, especially in the mid to high-latitudes of the Southern Hemisphere. These results thus portray an ocean carbon sink that is more variable than expected from models, indicating that the ocean may respond more sensitively to changes in future climate than currently modeled.