Key Uncertainties in the Recent Air‐Sea Flux of CO2

Dk Woolf ,F Girard‐Ardhuin,Jamie D Shutler ,Lonneke Goddijn-Murphy ,Craig Donlon ,Philip D Nightingale ,Ian Ashton ,John Prytherch ,Antoine Grouazel ,Ricardo Torres ,Fabrice Ardhuin ,Thomas Holding ,Jean-François Piollé ,Andrew J Watson ,Ute Schuster ,Bertrand Chapron ,J A Hanafin ,Jacek Piskozub ,Peter E Land ,Mark Warren ,Margaret J Yelland ,Iwona Wróbel ,Ben Moat ,Frédéric Paul

doi:10.1029/2018gb006041

Abstract

AbstractThe 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.

Highlights

The air‐sea flux of carbon dioxide is a critical part of the climate system (Ciais et al, 2013; Rhein et al, 2013) and a major factor in the development of the ocean
We investigate if the growing knowledge of both CO2 concentrations in the upper ocean and the air‐sea gas transfer velocity are yet sufficient to reduce the uncertainty in the global flux
This pair of calculations show that the combined effect of the thermal and haline skins is an increase in global net flux of −0.35 Pg C/year

Summary

Introduction

The air‐sea flux of carbon dioxide is a critical part of the climate system (Ciais et al, 2013; Rhein et al, 2013) and a major factor in the development of the ocean (e.g., ocean acidification). Takahashi et al (2009) estimate a sink of −2.0 ± 1.0 Pg C/year “normalized to the year 2000” based on a calculated contemporary air‐sea flux of −1.6 ± 0.9 Pg C/year corrected by an estimated preindustrial flux of +0.4 ± 0.2 Pg C/year. From these three estimates, there is a high level of confidence in a global uptake between −1.0 and −3.2 Pg C/year (Rhein et al, 2013). Note that we will always represent a net downward flux as a negative value, for more consistency with the convention for uptake

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Discussion

Conclusion