Human drivers of variability in the atmospheric CO2 growth rate

Abstract:

Humans directly or indirectly influence atmospheric carbon dioxide variability on multiple time scales. While fossil fuel combustion and land use emissions are known to be the primary agents driving the long-term secular increase in atmospheric CO₂, variability on interannual time scales has been attributed primarily to canopy-scale responses of terrestrial ecosystems to temperature and drought stress. Recent work by Wang et al. (2014) identifies a 2-fold increase in the sensitivity of the CO₂ growth rate to interannual variations in temperature and attributes this trend to changing interactions between temperature and soil moisture. Here we provide an independent assessment of this trend using a different filtering approach, and describe several novel mechanisms by which humans have exerted direct control on changes in the CO₂ growth rate variability over the past 50 years. These mechanisms include: 1) increasing use of fire by humans in dry and wet tropical forests, where fire behavior and fire spread is sensitive to El Nino-Southern Oscillation (ENSO) and other climate modes, 2) modifications to ecosystems that increase their vulnerability to fire and drought stress, 3) land use responses to economic growth cycles, and 4) covariance between atmospheric transport variability and increasing regional (and interhemispheric) gradients of fossil fuel CO₂. We conclude by reviewing how human and climate controls on CO₂ growth rate variability are likely to change in future decades for different climate change scenarios.