Key Uncertainties in the Recent Air‐Sea Flux of CO₂

Abstract

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 databases. The effect of sparse sampling on the calculated fluxes was investigated by a bootstrap method, that is, treating each ship cruise to an oceanic region as a random episode and creating 10 synthetic data sets 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/year is proposed, where the uncertainty derives primarily from uncertainty in the transfer velocity.

The FluxEngine air–sea gas flux toolbox: simplified interface and extensions for in situ analyses and multiple sparingly soluble gases

“…1b of Shutler et al (2016). Woolf et al (2019) shows that ignoring vertical temperature and salinity gradients for 2010 results in a 0.35 Pg C (12 %) bias or underestimate in the oceanic sink of CO 2 .…”

“…At the time of writing, the toolbox and resulting data have been used to quantify regional and global uncertainties (e.g. Wrobel and Piskozub, 2016;Wrobel, 2017;Woolf et al, 2019;Shutler et al, 2019), evaluate the impact of gas transfer processes on regional and global gas exchange (e.g. Ashton et al, 2016;Pereira et al, 2018), evaluate the European shelf sea CO 2 gas fluxes and sinks (Shutler et al, 2016) and investigate biological and physical controls of air-sea exchange (Henson et al, 2018).…”

“…The impact on the net gas fluxes of not performing this reanalysis on a relatively large time series of CO 2 measurements through the north and south Atlantic is demonstrated within case study 1 (Sect. 3.1), whereas the impact on global net integrated gas fluxes has been analysed by Woolf et al (2019).…”

“…All inputs to FluxEngine were provided as monthly averages with a 1 • × 1 • resolution. The additional input data were wind speed data from WAVEWATCH III reanalysis forcing field (WAVE-WATCH III development group, 2016), sea surface temperature from Reynolds Optimum Interpolation Sea Surface Temperature dataset (OISST; Reynolds et al, 2007), salinity data from the NOAA World Ocean Atlas (Zweng et al, 2018), xCO 2 from the GLOBALVIEW-CO2 dataset (GLOBALVIEW-CO2, 2013), and f CO 2 data from the SO-CAT derived sea-to-air CO 2 flux reference dataset for 2010 (Woolf et al, 2019). Since the Zappa et al (2007) relationship was parameterised in low to moderate wind speeds and in shallow marine environments, a mask was set in the configuration file to constrain the calculation to grid cells with wind speeds less than 10 m s −1 and shelf sea water depths between 20 and 200 m; then the analysis repeated with depths between 20 and 500 m. These depth ranges were chosen to be consistent with previous studies (e.g.…”

“…This advised formulation enables nearsurface temperature and salinity gradients to be included. For reasoning and examples of this advice, please see Woolf et al (2016Woolf et al ( , 2019 and Sect. 1b of Shutler et al (2016).…”