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.

Three years of quasi-continuous atmospheric ¹⁴CO₂ observations in Heidelberg (Germany) have been used together with continuous CO measurements to determine the CO/fossil fuel CO₂ ratio in a regional polluted area. Comparison with bottom-up information on fossil fuel CO₂ and CO emissions for the respective catchment area shows that large discrepancies (up to 60%) between inventory data and observations exist. Therefore both, a lot of care and reliable emissions inventory data are necessary if CO shall be used as a quantitative surrogate for fossil fuel CO₂.

The role of the Southern Ocean as a source or a sink for CO₂ in the modern ocean is heavily disputed, its interannual variability is unknown, and its control on atmospheric CO₂ during glaciations is suspected but still not understood nor quantified.  We estimate the variability of the air-sea CO₂ fluxes in the Southern Ocean for the 1992-2003 period using the spatio-temporal distribution of atmospheric CO₂ measurements from 12 stations in the Southern Ocean and 43 stations worldwide.  Our results show basin-scale variability of ±0.1 to 0.3 PgC/y that are related to physical variability in the Southern Ocean.

The ocean margins form the transition zone between terrestrial and open ocean areas and represent up to 30% of total ocean productivity, yet their role in the global carbon cycle is ill quantified. In order to address this issue, a bi-weekly time-series program was established in Santa Monica Bay in January 2003 to measure the seasonal evolution of the upper ocean carbon cycle at this coastal site. Our measurements reveal a strong seasonal cycle with an amplitude in salinity normalized dissolved inorganic carbon (DIC) reaching nearly 200 µmol/kg and pCO₂ changes of more than 200 µatm. The seasonal cycle of DIC is characterized by a maximum in late winter/early spring, which is caused by upwelling bringing high DIC concentrations from the upper thermocline during this time of the year. The concomitant supply of high levels of nutrients fuels an intense bloom, whose strength varies from year to year in response to large interannual variations in upwelling. In 2003 and 2004, substantial surface DIC decreases were observed under nitrate depleted conditions i) right after the occurrence of upwelling, and i) about three months after upwelling. This implies that during these times, either organic matter production occurred with a very high stoichiometric C:N ratio and/or an additional source of new nitrogen existed that supplied nitrogen without supplying DIC. The seasonal cycle of pCO₂ follows that of DIC with a late winter/early spring maximum, whose levels far exceed that of the atmosphere, and a summer-time minimum with undersaturated pCO₂ values. Annually, Santa Monica Bay acts as a weak to moderate sink for atmospheric CO₂. We suggest that this is mainly due to biological production and in part driven by the uptake of anthropogenic CO₂.

Over the last decade the TransCom group has coordinated a number of intercomparisons. The latest project focuses on the diurnal and synoptic behaviour of transport models.  The poster will describe the experiment, introduce the participating models and present a sample of preliminary results.

We report on the set-up of and first experiences with a medium-precision CO₂ concentration monitoring network in Europe, linked to existing flux towers. The system is to be embedded in an integral GHG monitoring system to be developed for the Netherlands and into the CABOEUROPE effort to quantify the European carbon balance. The proof of concept has not been fully satisfactory as yet, but work continues.

The air-sea gas exchange rate is important for modeling and verifying ocean CO₂ uptake, but remains subject to considerable uncertainty. The widely assumed quadratic or cubic dependence of the exchange rate on windspeed together with the latitudinal pattern of mean windspeed implies that exchange is much faster at high compared with low latitudes. This should affect the pattern of ocean uptake of bomb carbon-14 as well as the rate of decline of and latitudinal gradients in atmospheric Δ¹⁴CO₂. We evaluate the constraints on the windspeed dependence of the exchange rate offered by available isotopic measurements, discuss the major uncertainties, and suggest observational strategies to reduce these uncertainties.

Measurements of atmospheric O₂/N₂ ratio and CO₂ concentration are presented over the period 1989 to present from the Scripps Institution of Oceanography global flask sampling network. The data are used to make estimates of land and ocean sinks over various time scales. The oceanic and land biotic sinks are estimated to be 1.9±0.6 (ocean) and 1.2±0.8 Pg C/yr (land) over the period Jan. 1990-Jan. 2000 and 2.2±0.5 (ocean) and 0.5±0.7 Pg C/yr (land) over the period Jan. 1993-Jan. 2003. These estimates make allowance for oceanic O₂ and N₂ outgassing based on observed changes in ocean heat content and estimates of the relative outgassing per unit warming. The recent ocean sink is consistent, to within the uncertainties, with estimates of the accumulation of anthropogenic CO₂ in the ocean since 1800, assuming the oceanic sink varied over time as predicted by a box-diffusion model. The possibility that the ocean sink is being reduced slightly by climate feedbacks, as predicted by some models, is not ruled out, however.

Measurements of atmospheric CO₂ began in 1957-1958 at a wide range of locations, including at fixed stations, on ice floes, on oceanic expeditions, and on aircraft flights, with logistical and financial support provided by the International Geophysical Year (IGY) program. Although the measurement effort was reduced in scope immediately following the IGY, today, measurements are made at more than 100 locations.  Over this same time interval, emissions of CO₂ from fossil fuel combustion increased from 2.3 thousand million metric tons per year (GtC/yr) in 1958 to 7.1 GtC/yr in 2003 [Marland et al., 2005, and personal communication].  More than 90% of this CO₂ was released into the northern hemisphere where it lingered before mixing fully world-wide.  The atmospheric CO₂ concentration, in response, rose faster in the northern hemisphere than in the southern, the interhemispheric difference increasing from near zero during the IGY to about 3 parts per million (ppm) in 2003. For all northern hemisphere stations where our program has measured CO₂, the gradient changes relative to the South Pole are generally proportional to the rate of fossil fuel CO₂ emissions, disregarding seasonal and short term interannual variability in the CO₂ data.  Here, we use this fact to diagnose how the carbon cycle has evolved over the past half century.