Future expansion of agriculture and pasture acts toamplify atmospheric CO2 levels in response to fossilfuel and land-use change emissions

The expansion of crop and pastures to the detriment of forests results into an increase in atmospheric CO2. A first obvious cause is the loss of forest biomass and soil carbon during and after conversion. A second, generally ignored cause, is the reduction of the residence time of carbon when for example forests or grasslands are converted to cultivated land. This decreases the sink capacity of the global terrestrial biosphere, and thereby may amplify the atmospheric CO2 rise due to fossil and land-use carbon release. For the IPCC-A2 future scenario, characterized by high fossil and high land-use emissions, we show that the land-use amplifier effect adds 61 ppm extra CO2 in the atmosphere by 2100 as compared to former treatment of land-use processes in carbon models. Investigating the individual contribution of each of the 6 land-use transitions (forest - crop, forest - pasture, grassland -crop) to the amplifier effect indicates that the clearing of forest and grasslands to arable lands explains most of the CO2 amplification. The amplification effect is 50% higher than in a previous analysis by the same authors which did not consider neither the deforestation to pastures nor the ploughing of grasslands. Such an amplificationeffect is further examined in sensitivity tests where the net primary productivity is considered independant of atmospheric CO2. We also show that land-use changes which have already occurred in the recent past have a strong inertia at releasingCO2, and will contribute to about 1/3 of the amplification effect by 2100. These results suggest that there is an additionnal atmospheric benefit of preserving pristine ecosystems with high turnover times.