The contemporary air-sea flux of CO2 is investigated by the use of an air-sea flux equation, with
particular attention to the uncertainties in global values and their origin with respect to that
equation. In particular, uncertainties deriving from the transfer velocity and from sparse upper
ocean sampling are investigated. Eight ...
The contemporary air-sea flux of CO2 is investigated by the use of an air-sea flux equation, with
particular attention to the uncertainties in global values and their origin with respect to that
equation. In particular, uncertainties deriving from the transfer velocity and from sparse upper
ocean sampling are investigated. Eight formulations of air-sea gas transfer velocity are used to
evaluate the combined standard uncertainty resulting from several sources of error. Depending
on expert opinion, a standard uncertainty in transfer velocity of either ~5% or ~10% can be
argued and that will contribute a proportional error in air-sea flux. The limited sampling of upper
ocean fCO2 is readily apparent in the Surface Ocean CO2 Atlas (SOCAT) databases. The effect
of sparse sampling on the calculated fluxes was investigated by a bootstrap method; i.e. treating
each ship cruise to an oceanic region as a random episode and creating 10 synthetic datasets by
randomly selecting episodes with replacement. Convincing values of global net air-sea flux can
only be achieved using upper ocean data collected over several decades, but referenced to a
standard year. The global annual referenced values are robust to sparse sampling, but seasonal
and regional values exhibit more sampling uncertainty. Additional uncertainties are related to
thermal and haline effects and to aspects of air-sea gas exchange not captured by standard
models. An estimate of global net CO2 exchange referenced to 2010 of -3.0 ± 0.6 Pg C yr-1 is
proposed, where the uncertainty derives primarily from uncertainty in the transfer velocity.