AtmosphericCO2 and the global continental carbon cycle

Today, the atmospheric concentration of carbon dioxide is around 400 parts per million (ppm), whereas it was 280 ppm before the beginning of the ^18th century, as shown by the analysis of air bubbles occluded in the Antarctic ice. This_CO2 content is increasing, mainly due to the use of fossil fuels, which has accumulated to around 350 billion tonnes of carbon (GtC) since the beginning of the industrial era. Global emissions are increasing every year, reaching an annual injection of around 10 GtC.

To calculate the carbon balance of the atmosphere, other human activities such as deforestation and soil degradation must also be taken into account. Several techniques can be used to reconstruct the history of these activities, with flows of around 1 to 2 GtC per year since the middle of the ^20th century.

By taking the difference between the sum of anthropogenic emissions and the atmospheric stock, it is possible to deduce a global flow of_CO2 diffusing towards the other reservoirs of the carbon cycle. As the oceanic sink has been quantified by several independent techniques (see previous years' lectures), it is now possible to calculate the sink to the terrestrial biosphere. Since the 1960s, this sequestration has increased from around 1 to 3 GtC per year, but with significant inter-annual variability (from virtually zero flux in some years, to over 4 GtC in others).

The penetration of carbon into the terrestrial biosphere and soils can be monitored by analyzing and modeling natural and anthropogenic (thermonuclear) carbon-14. This approach confirms the importance of the biospheric sink over the last century.

An independent method is based on the analysis of_CO2 and _O2 contents in the same atmospheric air samples. Unlike diffusion and physico-chemical dissolution processes in the ocean, the biosphere's fertilization effect is accompanied by a net flow of _O2 into the atmosphere. The " trajectory" of the atmosphere in the _O2-CO2 plane has been virtually rectilinear since the early 1990s, but it differs from the evolution expected from the simple stoichiometry of combustion reactions. Taking data and theory into account, we can calculate both oceanic and biospheric sinks, the latter averaging around 1 GtC per year for the period 1991 to 2011.

The biospheric pumping calculated by the _O2-CO2 method corresponds to a net flux from the atmosphere to the biosphere. It is therefore equivalent to the difference between the gross biospheric flux (≈ 2 to 3 GtC/year, see above) and the anthropogenic flux from deforestation and land degradation (≈ 1 to 2 GtC/year) over the same period. The compatibility of these independent estimates cannot hide the existence of large uncertainties in this carbon balance, linked in particular to significant inter-annual variability in pumping by the biosphere.