[1] This paper documents a global Bayesian variational inversion of CO 2 surface fluxes during the period . Weekly fluxes are estimated on a 3.75°× 2.5°(longitudelatitude) grid throughout the 21 years. The assimilated observations include 128 station records from three large data sets of surface CO 2 mixing ratio measurements. A Monte Carlo approach rigorously quantifies the theoretical uncertainty of the inverted fluxes at various space and time scales, which is particularly important for proper interpretation of the inverted fluxes. Fluxes are evaluated indirectly against two independent CO 2 vertical profile data sets constructed from aircraft measurements in the boundary layer and in the free troposphere. The skill of the inversion is evaluated by the improvement brought over a simple benchmark flux estimation based on the observed atmospheric growth rate. Our error analysis indicates that the carbon budget from the inversion should be more accurate than the a priori carbon budget by 20% to 60% for terrestrial fluxes aggregated at the scale of subcontinental regions in the Northern Hemisphere and over a year, but the inversion cannot clearly distinguish between the regional carbon budgets within a continent. On the basis of the independent observations, the inversion is seen to improve the fluxes compared to the benchmark: the atmospheric simulation of CO 2 with the Bayesian inversion method is better by about 1 ppm than the benchmark in the free troposphere, despite possible systematic transport errors. The inversion achieves this improvement by changing the regional fluxes over land at the seasonal and at the interannual time scales. Citation: Chevallier, F., et al. (2010), CO 2 surface fluxes at grid point scale estimated from a global 21 year reanalysis of atmospheric measurements,

We present an estimate of net ecosystem exchange (NEE) of CO₂ in Europe for the years 2001 through 2007. It is derived with a data assimilation that uses a large set of atmospheric CO₂ mole fraction observations (<70 000) to guide relatively simple descriptions of terrestrial and oceanic net exchange, while fossil fuel and fire emissions are prescribed. Weekly terrestrial sources and sinks are optimized (i.e., a flux inversion) for a set of 18 large ecosystems across Europe in which prescribed climate, weather, and surface characteristics introduce finer scale gradients. We find that the terrestrial biosphere in Europe absorbed a net average of 2212165 TgC yr22121 over the period considered. This uptake is predominantly in non-EU countries, and is found in the northern coniferous (221294 TgC/yr) and mixed forests (221230 TgC yr22121) as well as the forest/field complexes of eastern Europe (221285 TgC yr22121). An optimistic uncertainty estimate derived using three biosphere models suggests the uptake to be in a range of 2212122 to 2212258 TgC yr22121, while a more conservative estimate derived from the a-posteriori covariance estimates is 2212165±437 TgC yr22121. Note however that uncertainties are hard to estimate given the nature of the system and are likely to be significantly larger than this. Interannual variability in NEE includes a reduction in uptake due to the 2003 drought followed by three years of more than average uptake. The largest anomaly of NEE occurred in 2005 concurrent with increased seasonal cycles of observed CO₂. We speculate these changes to result from the strong negative phase of the North Atlantic Oscillation in 2005 that lead to favorable summer growth conditions, and altered horizontal and vertical mixing in the atmosphere. All our results are available through http://www.carbontracker.e

Abstract. The CO 2 source and sink distribution across Europe can be estimated in principle through inverse methods by combining CO 2 observations and atmospheric transport models. Uncertainties of such estimates are mainly due to insufficient spatiotemporal coverage of CO 2 observations and biases of the models. In order to assess the biases related to the use of different models the CO 2 concentration field over Europe has been simulated with five different Eulerian atmospheric transport models as part of the EU-funded AE-ROCARB project, which has the main goal to estimate the carbon balance of Europe. In contrast to previous comparisons, here both global coarse-resolution and regional higherresolution models are included. Continuous CO 2 observations from continental, coastal and mountain sites as well as flasks sampled on aircrafts are used to evaluate the models' ability to capture the spatiotemporal variability and distribution of lower troposphere CO 2 across Europe. 14 CO 2 is used in addition to evaluate separately fossil fuel signal predictions. The simulated concentrations show a large range of variation, with up to ∼10 ppm higher surface concentrations over Western and Central Europe in the regional models with highest (mesoscale) spatial resolution.The simulation -data comparison reveals that generally high-resolution models are more successful than coarse models in capturing the amplitude and phasing of the observed Correspondence to: C. Geels (cag@dmu.dk) short-term variability. At high-altitude stations the magnitude of the differences between observations and models and in between models is less pronounced, but the timing of the diurnal cycle is not well captured by the models.The data comparisons show also that the timing of the observed variability on hourly to daily time scales at lowaltitude stations is generally well captured by all models. However, the amplitude of the variability tends to be underestimated. While daytime values are quite well predicted, nighttime values are generally underpredicted. This is a reflection of the different mixing regimes during day and night combined with different vertical resolution between models. In line with this finding, the agreement among models is increased when sampling in the afternoon hours only and when sampling the mixed portion of the PBL, which amounts to sampling at a few hundred meters above ground. The main recommendations resulting from the study for constraining land carbon sources and sinks using high-resolution concentration data and state-of-the art transport models through inverse methods are given in the following: 1) Low altitude stations are presently preferable in inverse studies. If high altitude stations are used then the model level that represents the specific sites should be applied, 2) at low altitude sites only the afternoon values of concentrations can be represented sufficiently well by current models and therefore afternoon values are more appropriate for constraining large-scale sources and sinks in combination with tr...

Abstract. We present inverse modelling (top down) estimates of European methane (CH 4 The hypothesis of significant natural emissions is supported by the finding that several inverse models yield significant seasonal cycles of derived CH 4 emissions with maxima in summer, while anthropogenic CH 4 emissions are assumed to have much lower seasonal variability. Taking into account the wetland emissions from the WETCHIMP ensemble, the top-down estimates are broadly consistent with the sum of anthropogenic and natural bottom-up inventories. However, the contribution of natural sources and their regional distribution remain rather uncertain.Furthermore, we investigate potential biases in the inverse models by comparison with regular aircraft profiles at four European sites and with vertical profiles obtained during the Infrastructure for Measurement of the European Carbon Cycle (IMECC) aircraft campaign. We present a novel approach to estimate the biases in the derived emissions, based on the comparison of simulated and measured enhancements of CH 4 compared to the background, integrated over the entire boundary layer and over the lower troposphere. The estimated average regional biases range between −40 and 20 % at the aircraft profile sites in France, Hungary and Poland.

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