A comprehensive analysis of air-sea CO2 flux uncertainties constructed
from surface ocean data products
Abstract
Increasing anthropogenic CO2 emissions to the atmosphere are partially
sequestered into the global oceans through the air-sea exchange of CO2
and its subsequent movement to depth, and this collective large-scale
absorption is commonly referred to as the global ocean carbon sink.
Quantifying this ocean carbon sink provides a key component for closing
the global carbon budget which is used to inform and guide policy
decisions. These estimates are typically accompanied by an uncertainty
budget built by selecting what are perceived as critical uncertainty
components based on selective experimentation. However, there is a
growing realisation that these budgets are incomplete and may be
underestimated, which limits their power as a constraint within global
budgets. In this study, we present a methodology for quantifying
spatially and temporally varying uncertainties in the air-sea CO2 flux
calculations and data that allows an exhaustive assessment of all known
sources of uncertainties, including decorrelation length scales between
gridded measurements, and the approach follows standard uncertainty
propagation methodologies. The resulting standard uncertainties are
higher than previously suggested budgets, but the components are
consistent with previous work, and they identify how the significance
and importance of key uncertainty components change in space and time.
For an exemplar method (the UEP-FNN-U method) the work identifies that
we can currently estimate the annual ocean carbon sink to an accuracy of
±0.72PgCyr-1 (1 standard deviation uncertainty). Due to this method
having been built on established uncertainty propagation and approaches,
it appears applicable to all data-product assessments of the ocean
carbon sink.