The primary aim of the Centre for Observation of Air-sea Interactions and Fluxes (CASIX) is to estimate accurately the air-sea fluxes of CO₂. Under CASIX, a high resolution ocean general circulation model, coupled to an ocean biogeochemistry model, has been used to provide estimates of surface ocean pCO₂ and air-sea fluxes of CO₂ for the year 2003. An initial global simulation was run at 1 degree horizontal resolution, providing boundary conditions for a limited area North Atlantic model at 1/3 degree resolution. Observed temperature and salinity data were assimilated into the model. Temporal variability in the resulting pCO₂ fields are compared to observations, and the primary production and pCO₂ results of the two different resolution runs are compared.

High-frequency atmospheric CO₂ measurements should become increasingly available by the end of this decade from a variety of sources, including low-Earth orbiting satellites. If of sufficient accuracy, these should allow the functioning of the global carbon cycle to be monitored at fine time/space resolutions using atmospheric transport inversions. Since traditional direct inversion methods (e.g., Bayesian synthesis) become computationally infeasible at these resolutions, we use an approximate method, variational data assimilation, to estimate surface CO₂ fluxes at spatial resolutions ranging from 10x10 degrees to 1x1 degrees and at time resolutions ranging from 2 weeks to 1 hour. We assess its performance using simulated data, including the effects of realistic transport and data errors.

A major challenge in reaching a better understanding of global change is the integration of global carbon observations at different scales, made in the atmosphere, ocean and terrestrial domains.  This is essential to optimize efforts supporting national, regional and international policy related to the global carbon cycle.  The partners of the Integrated Global Observing Strategy (IGOS-P) representing all players in carbon cycle research and monitoring recognised this and produced, with the help of an international panels of experts, published theme reports on the Carbon Cycle (IGCO) and on Atmospheric Chemistry (IGACO).  These themes contain recommendations on how to more effectively coordinate and fill gaps in global Earth observations. 

Regular vertical profiles over Europe were set up in 2001 as part of the AEROCARB and Carboeurope-IP projects at five locations: Griffin (56°36'N, 3°47'W, Scotland), Orléans (47°50'N, 2°30'E, France), Schauinsland (47°55'N, 7°55'E, Germany), Hegyhatsal (46°57'N, 16°39'E, Hungary), and Bialystok (53.20°N, 22.75°E, Poland). The objective of the program is to measure CO₂, CH₄, N₂O, SF₆, CO, ¹³C and ¹⁸O in CO₂ vertical profiles at a bi-weekly frequency using air samples taken up at several levels from 100m up to 3000 m above the ground surface. One liter flasks are sampled on board small aircraft using a standardised protocol. The samples are analysed at three laboratories (LSCE, MPI-BGC, IUP-UHEI) which are linked through regular intercomparison exercises. We have characterised for each site the CO₂ seasonal cycles within the atmospheric boundary layer (ABL: 14 to 20 ppm) and the free troposphere (FT: 10 to 13 ppm). From these signals we have calculated the difference between ABL and FT, known as the CO₂ 'jump', which will be compared to the simulations from atmospheric transport models. We have also calculated the offset between each airborne sampling site and the time series from Mace Head observatory, used as a maritime reference. For CO₂, the wintertime offsets at the lowest level of the average vertical profiles are ranging from 0 ppm in Scotland up to 10 ppm in all continental sites. Depending of the site the positive offset due to emissions from anthropogenic and biospheric processes may extend up to 300 to 1500 m agl. In summertime we observe a negative gradient in most of the sites with a typical decrease of 5 ppm between 2000m and 100m agl. The average vertical gradients will be compared to the ouput of atmospheric models, and will be analysed with regards to the other trace gas (CO, CH₄, and CO₂ isotopes).

CO₂ and methane are important greenhouse gases, both contributing in increasing amounts towards positive radiative forcing. It is hence important to gain maximum understanding of the carbon cycle in the atmosphere, and the scale of carbon trace gas sources and sinks, not only globally but also on a more regional level. The Orbiting Carbon Observatory (OCO) satellite, scheduled for launch in 2008, is designed for dedicated global mapping of CO₂. In order to investigate the usefulness of a variety of methods, including retrievals from satellite mapping, some preliminary inverse modelling using a Bayesian synthesis technique is performed using pseudodata generated to represent possible future measurement regimes. This study will focus on the ability of in-situ measurements within Australia to reduce the uncertainties in Australian continental CO₂ flux estimates. The specific measurements investigated include a Ghan railway transect between Adelaide (34.9°S, 138.6°E) and Darwin (12.5°S, 130.9°E), and a number of continuous permanent sites. The reduction in flux uncertainties from additional measurements compared to a background inversion is examined, from which it is concluded that measuring on the Ghan railway is potentially worthwhile for reducing uncertainties associated with flux estimates.

The Orbiting Carbon Observatory (OCO) mission will deliver space-based observations of atmospheric CO₂ with the potential to resolve many of the uncertainties in the spatial and temporal variability of carbon sources and sinks.  Our assessments of the measurement requirements for space-based remote sensing of atmospheric CO₂ conclude that the data must support retrievals of the column-averaged CO₂ dry air mole fraction, X_CO2, with precisions of 3 to 4 ppm to resolve the annually averaged gradients between the Northern and Southern hemispheres, but higher precision (1 to 2 ppm) will be needed to resolve East-West gradients and questions like the location and spatial extent of the Northern Hemisphere terrestrial carbon sink.  These conclusions are derived from the results of observational system simulation experiments (OSSEs) and synthesis inversion models [Rayner and O’Brien, 2001; O’Brien and Rayner, 2002; Rayner et al., 2002]. The X_CO2 precision requirements also considered the OCO mission design, the amplitude of X_CO2 spatial and temporal gradients, and the relationship between X_CO2 data precision and regional scale surface CO₂ flux uncertainties inferred from X_CO2 data.

Measurements of atmospheric O₂/N₂ ratios and CO₂ concentrations can be combined to form the tracer Atmospheric Potential Oxygen (APO), reflecting primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. [1998], we present a new set of APO observations including shipboard collections from the equatorial Pacific. Our data show a smaller interhemispheric gradient than observed in past studies and a substantial APO maximum around the equator. Following a modeling approach developed by Gruber et al. [2001], we compare these observations with APO fields generated by a set of oceanic and atmospheric models. Overall, our model results agree well with observations, but small differences suggest that modeled north-south transport may be too vigorous, air-sea fluxes may be too coarsely resolved in some regions, and seasonal trapping of surface fluxes may be excessive in some model locations.

GOSAT is a satellite to measure the column densities of CO₂ and CH₄ from space globally, and it is scheduled to be launched in 2008. It has a short wavelength infrared (SWIR) Fourier transform spectrometer (FTS) which measures both the ground surface scattered solar light over land and the right reflected light (sun-glint) over ocean. Column densities of CO₂ and CH₄ will be retrieved from the SWIR (i.e. 1.6 µm and 2.0 µm bands) data and the optical path length from oxygen A-band (0.76 µm). A cloud and aerosol sensor composed of three spectral image sensors (0.380, 0.678 and 1.62 µm) is equipped, viewing the wider area than FTS. This is a joint project among Ministry of Environment of Japan (MOE), National Insitutite for Environmental Studies (NIES) and Japan Aerospace Exploration Agency (JAXA).

We carried out airborne campaigns over Europe in order to analyze atmospheric CO₂ variability at the regional scale. Data reveal a higher standard variation in the planetary boundary layer (PBL) against a lower one in the free troposphere (FT), where the air is more well mixed. Ground data generally agree well with airborne measurements when done in the FT, but not in the PBL where they are exposed to local disturbances. Ground stations located in the FT are shown to be representative of a regional scale while PBL observatories provide only locally representative measurements.

The effect of rainfall on mass transfer across the air-water interface was investigated through the CO₂ absorption experiments in a turbulent open-channel flow with the free surface. The results show that the rainfall enhances both the turbulent mixing near the free surface on the liquid side and the CO₂ transfer across the interface. The mass transfer coefficient on the liquid side is well correlated by both the mean vertical momentum flux of rainfall, M, and the mean kinetic energy of rain droplets impinging on the unit area of the air-water interface, KEF. However, it was not concluded which of M and KEF is a better parameter for expressing the rainfall effects on the mass transfer. The comparison between the mass transfer coefficient obtained in this study and that obtained in wind-driven turbulence suggests that it is of great importance to consider the rainfall effect on the CO₂ exchange rate between the atmosphere and ocean in precisely estimating the global carbon cycle in a climate model.