Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink
Estimates in RECCAP2 and Recommendations for Future Studies
Abstract
The ocean is a major carbon sink and takes up 25-30% of the
anthropogenically emitted CO2. A state-of-the-art method to quantify
this sink are global ocean biogeochemistry models (GOBMs) but their
simulated CO2 uptake differs between models and is systematically lower
than estimates based on statistical methods using surface ocean pCO2 and
interior ocean measurements. Here, we provide an in-depth evaluation of
ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As
sources of inter-model differences and ensemble-mean biases our study
identifies the (i) model set-up, such as the length of the spin-up, the
starting date of the simulation, and carbon fluxes from rivers and into
sediments, (ii) the ocean circulation, such as Atlantic Meridional
Overturning Circulation and Southern Ocean mode and intermediate water
formation, and (iii) the oceanic buffer capacity. Our analysis suggests
that the late starting date and biases in the ocean circulation cause a
too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean
biogeochemistry biases might also cause simulated anthropogenic fluxes
to be too low but the current set-up prevents a robust assessment. For
simulations of the ocean carbon sink, we recommend in the short-term to
(1) start simulations in 1765, when atmospheric CO2 started to increase,
(2) conduct a sufficiently long spin-up such that the GOBMs reach
steady-state, and (3) provide key metrics for circulation,
biogeochemistry, and the land-ocean interface. In the long-term, we
recommend improving the representation of these metrics in the GOBMs.