Variations in atmospheric carbon dioxide, CO2, provide unique, large-scale constraints on the global carbon cycle. At interannual timescales, atmospheric CO2 variations primarily reflect variations in terrestrial ecosystem carbon fluxes and have been linked to climatic drivers such as temperature and precipitation. In this talk, I will discuss how measurements of the temporal evolution of CO2 in different latitude bands may be used to account for global fire emissions and to quantify the sensitivity of terrestrial ecosystem fluxes to temperature and drought stress.
Several recent studies have identified the temperature dependence of tropical net ecosystem exchange (NEE) as a primary driver of terrestrial ecosystem variability by analyzing a single globally averaged time series of CO2 anomalies. By simulating atmospheric CO2 from simple proxies for ecosystem processes, we found that NEE responses to temperature, NEE responses to drought stress, and fire emissions all contributed significantly to CO2 variability, suggesting that no single mechanism is the dominant driver. In fact, the sum of drought and fire contributions to CO2 variability exceeded direct NEE responses to temperature in both the northern and southern hemispheres. Accounting for fires, the sensitivity of tropical NEE to temperature stress decreased by 25% to 2.9 ± 0.9 Pg C y-1 K-1. Our findings provide a more mechanistic understanding of processes that control contemporary carbon cycling and are an impetus to accurately implement these controls in Earth System Models to ensure improved prediction of long term feedbacks between climate change and the carbon cycle.