Global high‐precision atmospheric Δ14CO2 records covering the last two decades are presented, and evaluated in terms of changing (radio)carbon sources and sinks, using the coarse‐grid carbon cycle model GRACE. Dedicated simulations of global trends and interhemispheric differences with respect to atmospheric CO2 as well as δ13CO2 and Δ14CO2, are shown to be in good agreement with the available observations (1940–2008). While until the 1990s the decreasing trend of Δ14CO2 was governed by equilibration of the atmospheric bomb 14C perturbation with the oceans and terrestrial biosphere, the largest perturbation today are emissions of 14C‐free fossil fuel CO2. This source presently depletes global atmospheric Δ14CO2 by 12–14‰ yr−1, which is partially compensated by 14CO2 release from the biosphere, industrial 14C emissions and natural 14C production. Fossil fuel emissions also drive the changing north–south gradient, showing lower Δ14C in the northern hemisphere only since 2002. The fossil fuel‐induced north–south (and also troposphere–stratosphere) Δ14CO2 gradient today also drives the tropospheric Δ14CO2 seasonality through variations of air mass exchange between these atmospheric compartments. Neither the observed temporal trend nor the Δ14CO2 north–south gradient may constrain global fossil fuel CO2 emissions to better than 25%, due to large uncertainties in other components of the (radio)carbon cycle.

[1] We re-assess the constraints that estimates of the global ocean excess radiocarbon inventory (I E ) place on air-sea gas exchange. We find that the gas exchange scaling parameter a q cannot be constrained by I E alone. Non-negligible biases in different global wind speed data sets require a careful adaptation of a q to the wind field chosen. Furthermore, a q depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed-and inventorynormalized gas exchange parameter a q N which takes into account these biases and which is easily adaptable to any new estimate of I E . Our study yields an average estimate of a q of 0.32 ± 0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wanninkhof (1992). We calculate a global annual average piston velocity for CO 2 of 16.7 ± 2.9 cm/hr and a gross CO 2 flux between atmosphere and ocean of 73 ± 10 PgC/yr, significantly lower than results from previous studies. Citation: Naegler, T., P. Ciais, K. B. Rodgers, and I. Levin (2006), Excess radiocarbon constraints on air-sea gas exchange and the uptake of CO 2 by the oceans, Geophys. Res. Lett., 33, L11802,

[1] D 14 CO 2 observations are increasingly used to constrain recently added fossil fuel CO 2 in the atmosphere. We use the LMDZ global atmospheric transport model to examine the pseudo-Lagrangian framework commonly used to determine recently added fossil fuel CO 2 (CO 2ff ). Our results confirm that D 14 CO 2 spatial variability in the Northern Hemisphere troposphere is dominated by the effect of CO 2ff , whereas in the Southern Hemisphere, ocean CO 2 exchange is more important. The model indicates that the free troposphere, at 3-5 km altitude, is a good choice for ''background,'' relative to which the recently added fossil fuel CO 2 can be calculated, although spatial variability in free tropospheric D 14 CO 2 contributes additional uncertainty to the CO 2ff calculation.Comparison of model and observations suggests that care must be taken in using high-altitude mountain sites as a proxy for free tropospheric air, since these sites may be occasionally influenced by (polluted) boundary layer air, especially in summer. Other sources of CO 2 which have D 14C different than that of the atmosphere contribute a bias, which, over the Northern Hemisphere land, is mostly due to the terrestrial biosphere, whereas ocean CO 2 exchange and nuclear industry and natural cosmogenic production of 14 C contribute only weakly. The model indicates that neglecting this bias leads to a consistent underestimation of CO 2ff , typically between 0.2 and 0.5ppm of CO 2 , with a maximum in summer. While our analysis focuses on fossil fuel CO 2 , our conclusions, particularly the choice of background site, can also be applied to other trace gases emitted at the surface.Citation: Turnbull, J., P. Rayner, J. Miller, T. Naegler, P. Ciais, and A. Cozic (2009), On the use of 14 CO 2 as a tracer for fossil fuel CO 2 : Quantifying uncertainties using an atmospheric transport model,

Abstract. Emissions of sulphur hexafluoride (SF6), one of the strongest greenhouse gases on a per molecule basis, are targeted to be collectively reduced under the Kyoto Protocol. Because of its long atmospheric lifetime (estimated as 800 to 3200 years), the accumulation of SF6 in the atmosphere is a direct measure of its global emissions. Examination of our extended data set of globally distributed high-precision SF6 observations shows an increase in SF6 abundance from near zero in the 1970s to a global mean of 6.7 ppt by the end of 2008. In-depth evaluation of our long-term data records shows that the global source of SF6 decreased after 1995, most likely due to SF6 emission reductions in industrialised countries, but increased again after 1998. By subtracting those emissions reported by Annex I countries to the United Nations Framework Convention of Climatic Change (UNFCCC) from our observation-inferred SF6 source leaves a surprisingly large gap of more than 70–80% of non-reported SF6 emissions in the last decade. This suggests a strong under-estimation of emissions in Annex I countries and underlines the urgent need for independent atmospheric verification of greenhouse gases emissions accounting.