Martin Heimann Christian Rdenbeck and Manuel Gloor

Ever since the importance of the CO2 gas for the radiative balance of the atmosphere was recognized, the mechanisms, both natural and anthropogenic, that control its concentration have been an active area of study. The possibility of a direct anthropogenic influence on atmospheric CO2 concentration from the emissions of CO2 from fossil-fuel burning and cement production was discussed by Arrhenius more than 100 years ago (Arrhenius 1896, 1903). Interestingly, Arrhenius predicted an atmospheric increase of only about 6 parts per million (ppm) over the 20th century, contrasting strongly with the 75 ppm that have been reconstructed from ice core measurements. Arrhenius's low prediction was partly due to assumed constant anthropogenic emissions of only 0.7 petagrams of carbon per year (PgC y-1). In addition, he assumed perfect equilibration between the atmosphere and the entire ocean, hence neglecting the finite time needed to mix the anthropogenic excess CO2 into the interior of the ocean.

Reliable atmospheric measurements of CO2 in the Southern and Northern Hemispheres were started about 45 years ago (Keeling 1960). Early calculations with simple ocean box models calibrated against observations of natural and bomb-produced radiocarbon showed that a significant fraction of the anthropogenic CO2 is taken up by carbon sinks, primarily in the oceans and possibly also on land (Keeling 1973; Oeschger et al. 1975). Indeed, in the late 1970s, it appeared that the global fossil-fuel carbon emissions were almost balanced by the atmospheric increase and the calculated ocean uptake (Siegenthaler and Oeschger 1978). Thereafter, the recognition of significant additional emissions from changes in land use, in particular deforestation in the tropics (Woodwell et al. 1978), started the quest for the "missing sink" to balance the global budget. Additional information from ice cores on the variability of CO2 in the past and the evidence of changes in the atmospheric stable isotope ratios subsequently demonstrated that changes in terrestrial sources and sinks also play a role and may constitute a sizable fraction of the "missing sink."

Since this first phase of carbon cycle research, many new techniques and observations have provided a wealth of heterogeneous information on the global carbon cycle. Many of the different identified sources and sinks of atmospheric CO2 remain relatively elusive, however, and the processes that control them, especially the terrestrial exchange fluxes, are still poorly understood. In order to improve this knowledge, many recent efforts have been directed at estimating the regional patterns of carbon exchange and their variability, to better identify and quantify individual source processes and to assess their driving mechanisms. The establishment of the Kyoto Protocol has added a second motivation for the regional quantification of sources and sinks of CO2. Independent and scientifically credible estimates of regional carbon balances must be established to corroborate carbon source and sink estimates as reported by individual countries.

In the sections that follow, we briefly summarize the existing methods to establish regional carbon budgets, comment on the global carbon balance, look at the direct anthropogenic imprint, and present a summary of regional estimates based on a recent inversion study (Rödenbeck et al. 2003).

Methods

Generalizing, one may distinguish four main approaches to estimating regional distribution and variability of surface-air carbon fluxes, which, in practice, may also be combined in various ways.

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