Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2

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.