The dominant role of semi-arid lands in the trend and variability of the land CO2 sink

Abstract:

Since the 1960s terrestrial ecosystems have acted as a substantial sink for atmospheric CO₂, sequestering about one quarter of anthropogenic emissions in an average year. Variations in this land carbon sink are also responsible for most of the large interannual variability in atmospheric CO₂ concentrations. While most evidence places the majority of the sink in highly productive forests, the location and the processes governing the interannual variations are still not well characterised. Here we evaluate the hypothesis that the mean, long-term trend and the variability in the land CO₂ sink are respectively dominated by geographically distinct regions. Using information from global CO₂ records, an ensemble of terrestrial ecosystem models and an empirical observation-based product of gross primary productivity, we show that whereas the mean sink is dominated by tropical forests, the trend and interannual variations of the sink are dominated by semi-arid ecosystems occupying the transition between moist forests and deserts. The contribution of both global land and semi-arid ecosystems to global CO₂ interannual variations are driven almost equally by precipitation and temperature, suggestive of drought. ENSO, and especially its positive phase, El Niño, appears as the main control of variations in semi-arid ecosystems, covarying with extreme NBP events across the semi-arid zone, although the sign of the response differs between locations.