Increasing levels of atmospheric carbon dioxide (CO₂) from anthropogenic sources are of growing concern due to their impact on the global climate system. Current concentrations of atmospheric CO₂ are much larger than pre-industrial values found in polar ice core records of atmospheric composition dating back 650,000 years. Research has shown that there is a strong link between increasing atmospheric CO₂ concentrations and the warming climate. Currently, it is thought that the ocean system annually absorbs up to 30% of the CO₂ released by burning fossil fuels. This exchange of CO₂ between the ocean and atmosphere contributes to the mitigation of the anthropogenic climate impacts through absorption of excess atmospheric CO₂ by the oceans. However, it is not clear whether this ocean carbon 'sink' is increasing or decreasing, and considerable spatial and temporal inter-annual variation appears to occur. Furthermore, dissolved CO₂ forms a weak acid, so as the amount of CO₂ in the oceans increases, the more acidic they become. Understanding these exchanges is clearly of importance for understanding the global carbon cycle and for climate modelling. This means that the estimation of global and regional air-sea CO₂ fluxes is a key goal of climate research.

The rate at which CO₂ exchanges between the ocean and the atmosphere (termed air-sea CO₂ flux) is related to wind speed, temperature, sea state and any surface processes such as biological activity. Earth Observation (EO) techniques are well placed to study these fluxes and are potentially the only way of reliably monitoring global air-sea CO₂ fluxes. However, there remain large uncertainties in the current parameterisations of EO-derived air-sea gas interactions, which can have profound effects on the resulting CO2 flux estimates. For example, current EO estimates of net global air-sea CO₂ fluxes calculated using different satellite wind speed and gas exchange parameterisations vary from 1.2 - 2.7 Gt C yr^-1. These are estimates valid for the open ocean and do not include the majority of coastal regions. Despite accounting for just 7% of the world ocean surface, coastal regions play an important role in the global carbon cycle and in buffering human impacts on the marine system. One of the major sources of uncertainty in the calculation of air-sea CO₂ fluxes on a global scale is the parameterisation of the gas transfer velocity, which controls the transfer of gas molecules across the air-water interface.

The OC-flux project is exploiting the synergy of the sensors onboard Envisat to investigate the measurement-uncertainties in current EO retrievals of air-sea CO₂ fluxes. To achieve this, a multi-year global time series of coincident MERIS, AATSR and RA2 data has been generated. These data, all collected by sensors onboard Envisat, are firstly being used to assess the accuracy of existing gas transfer velocity parameterisations, which underpin the EO-derived CO₂ flux estimates. This work is also exploiting over 2000 in situ data collected from all over the world within a number of different scientific research cruises, including coastal data.

It is known that surface biology can dampen surface turbulence, thus potentially reducing the air-sea CO₂ flux. However, it is not currently known whether this dampening of the water surface can be detected by spaceborne altimeters, and thus accounted for in altimeter-retrieved estimates of air-sea CO₂ flux. Unfortunately, the lack of (spatially and temporally) coincident altimeter and appropriate measurements of biology has precluded any specific investigation of these effects. Therefore, the OC-flux data are also being used to investigate the impact that biological slicks can have on altimeter-derived CO₂ fluxes.

The methods developed for Envisat data within OC-flux are applicable to the proposed Sentinel-3 suite of sensors, meaning that future satellite missions will be able to study air-sea CO₂ fluxes. Furthermore, the global datasets generated for this work are suitable for evaluating the performance of hydrodynamic ecosystem models which are used to predict marine carbon budgets.