Vincent Gitz

Amplifying effects of land‐use change on future atmospheric CO2 levels

We constructed a model to analyze the interactions between land-use change and atmospheric CO2 during the recent past and for the future. The primary impact of the conversion of forested lands to cultivated lands is to increase atmospheric CO2, via losses of biomass and soil carbon to the atmosphere. This increase is likely to continue in the next decades, but its magnitude can vary according to each land-use scenario. We show that this first-order effect is further amplified by the correlated diminution of terrestrial sinks, because when croplands replace forests, the turnover time of excess carbon in the biosphere decreases, and hence the sink capacity of terrestrial ecosystems decreases. This effect acts to further increase by up to 100 ppm the CO2 level reached by 2100, and it is ofthe same order of magnitude, although smaller, than climate-carbon feedbacks. Uncertainties on the magnitude of this land-use induced effect are large, because of uncertainties in the sink role of terrestrial ecosystems in the future and because of uncertainties inherent to the modeling of land-use induced carbon emissions. Such an extra rise in atmospheric CO2 is however partially offset by the ocean reservoir and by sinks operating over undisturbed, pristine ecosystems, suggesting that conserving pristine forests with long turnover times might be efficient in mitigating the greenhouse effect

Remarks on the Use of 13C and 18O isotopes in Atmospheric CO2 to Quantify Biospheric Carbon Fluxes

This chapter uses the mass-conservation equations for CO 2 and its isotopomers 13 CO 2 and CO 18 O that can be used to infer globally biospheric and oceanic net fluxes in the case of 18 C, and gross terrestrial biospheric fluxes in the case of 18 O. The quantitative use of atmospheric measurements of 13 C and 18 O in CO 2 to better constrain those fluxes requires knowledge of various processes specific to each isotopomer. The chapter is divided into two parts, one on each isotope. For 13 C, it reviews existing work that calculated isofluxes either using global estimates or derived isofluxes from spatially and temporally explicit models. In addition, it estimates the magnitude of new isofluxes that were not addressed in former studies. These cover the effects of biomass burning, rock weathering and volcanism, and the oxidation of reduced carbon gases into CO 2 within the atmosphere.