The presence of vertical temperature and salinity gradients in the upper ocean and the occurrence of variations in temperature and salinity on time scales from hours to many years complicate the calculation of the flux of carbon dioxide (CO₂) across the sea surface. Temperature and salinity affect the interfacial concentration of aqueous CO₂ primarily through their effect on solubility with lesser effects related to saturated vapor pressure and the relationship between fugacity and partial pressure. The effects of temperature and salinity profiles in the water column and changes in the aqueous concentration act primarily through the partitioning of the carbonate system. Climatological calculations of flux require attention to variability in the upper ocean and to the limited validity of assuming “constant chemistry” in transforming measurements to climatological values. Contrary to some recent analysis, it is shown that the effect on CO₂ fluxes of a cool skin on the sea surface is large and ubiquitous. An opposing effect on calculated fluxes is related to the occurrence of warm layers near the surface; this effect can be locally large but will usually coincide with periods of low exchange. A salty skin and salinity anomalies in the upper ocean also affect CO₂ flux calculations, though these haline effects are generally weaker than the thermal effects.

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 air–sea flux of greenhouse gases [e.g., carbon dioxide (CO₂)] is a critical part of the climate system and a major factor in the biogeochemical development of the oceans. More accurate and higher-resolution calculations of these gas fluxes are required if researchers are to fully understand and predict future climate. Satellite Earth observation is able to provide large spatial-scale datasets that can be used to study gas fluxes. However, the large storage requirements needed to host such data can restrict its use by the scientific community. Fortunately, the development of cloud computing can provide a solution. This paper describes an open-source air–sea CO₂ flux processing toolbox called the “FluxEngine,” designed for use on a cloud-computing infrastructure. The toolbox allows users to easily generate global and regional air–sea CO₂ flux data from model, in situ, and Earth observation data, and its air–sea gas flux calculation is user configurable. Its current installation on the Nephalae Cloud allows users to easily exploit more than 8 TB of climate-quality Earth observation data for the derivation of gas fluxes. The resultant netCDF data output files contain >20 data layers containing the various stages of the flux calculation along with process indicator layers to aid interpretation of the data. This paper describes the toolbox design, which verifies the air–sea CO₂ flux calculations; demonstrates the use of the tools for studying global and shelf sea air–sea fluxes; and describes future developments.

Climatologies, or long-term averages, of essential climate variables are useful for evaluating models and providing a baseline for studying anomalies. The Surface Ocean CO₂ Atlas (SOCAT) has made millions of global underway sea surface measurements of CO₂ publicly available, all in a uniform format and presented as fugacity, fCO₂. As fCO₂ is highly sensitive to temperature, the measurements are only valid for the instantaneous sea surface temperature (SST) that is measured concurrently with the in-water CO₂ measurement. To create a climatology of fCO₂ data suitable for calculating air–sea CO₂ fluxes, it is therefore desirable to calculate fCO₂ valid for a more consistent and averaged SST. This paper presents the OceanFlux Greenhouse Gases methodology for creating such a climatology. We recomputed SOCAT's fCO₂ values for their respective measurement month and year using monthly composite SST data on a 1° × 1° grid from satellite Earth observation and then extrapolated the resulting fCO₂ values to reference year 2010. The data were then spatially interpolated onto a 1° × 1° grid of the global oceans to produce 12 monthly fCO₂ distributions for 2010, including the prediction errors of fCO₂ produced by the spatial interpolation technique. The partial pressure of CO₂ (pCO₂) is also provided for those who prefer to use pCO₂. The CO₂ concentration difference between ocean and atmosphere is the thermodynamic driving force of the air–sea CO₂ flux, and hence the presented fCO₂ distributions can be used in air–sea gas flux calculations together with climatologies of other climate variables.

The seasonality of the mass specific scattering and backscattering by different components of the suspended particulate matter (SPM) was investigated at a coastal station in the Western English Channel. This study considered different components of the SPM, including inorganic and organic suspended matter, chlorophyll a and six phytoplankton groups. The data set comprised a 9-year time series of observations at station L4. Scattering variability was best explained by the particulate organic matter content, with significant differences between spring–summer and autumn–winter. Particulate backscattering variability was best explained by the inorganic fraction, with no clear seasonal variation. The temporal variations of the spectral slopes of these optical properties were related to the particle composition and size distribution slope.