Global Atmospheric Carbon Dioxide Cycle
The mean residence time of carbon dioxide in the atmosphere is in the order of only 3-4 years. Accordingly, about 25-30% of the atmospheric CO2 inventory of today about 800 PgC (1 PgC = 1015gC) are annually exchanged with the biosphere (ca. 120 PgC/yr) and the ocean surface waters (ca. 78 PgC/yr, Naegler, 2009). Compared to this large gross exchange of carbon between reservoirs, the total yearly net perturbation fluxes are smaller by almost two orders of magnitude: In the last 10 years, the average input of CO2 into the atmosphere from burning of fossil fuels and cement production summed up to about 9.0 PgC/yr, and, in addition, about 1 PgC/yr were released in the course of deforestation and land use change. Although the effect of these emissions on the atmospheric CO2 concentration is clearly observed and well documented (see e.g. NOAA), it sums up to an average increase of only 4.4 PgC/yr in the atmospheric burden during the last decade. The remaining fraction is buffered away about equally by the world oceans and the biosphere (see Figure). This partitioning of anthropogenic CO2 between buffer reservoirs is determined by the dynamics of their internal mixing as well as by the strength of their gross carbon exchange with the atmosphere and leads to a residence time of excess man-made CO2 in the atmosphere as large as several hundred years. Despite intensive research in the last decades, the uncertainty of the gross exchange rates between carbon reservoirs is still in the order of ±20% (see e.g. Levin and Naegler, 2009; Naegler, 2009). It thus remains difficult to univocally quantify the repartition of net uptake between the biosphere and the oceans, and thus, to predict the fate of anthropogenic CO2 emissions (Ballantyne et al., 2012, Nature 488, 70-72, doi:10.1038/nature11299). The situation is further complicated by the fact that the share of excess CO2 taken up by the biosphere and oceans varies from year to year and will further change in the near future under possibly changing climatic conditions. This concerns in particular the terrestrial biospheric reservoir, which has only a small carbon inventory (about 2000 PgC) compared with the oceans (38,000 PgC). Moreover, this reservoir consists of living plants and dead soil organic matter, which are highly vulnerable and already today strongly perturbed by human activity.
For more detailed information on the global carbon cycle and its recent changes see global carbon project.
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