Climatic Responses of Tropical Ecosystems Control Variations in Atmospheric CO2 Growth Rates

An understanding of the response of the atmospheric CO2 growth rate (CGR) to climate variations is a prerequisite for accurate assessments of the impacts of climate changes on the global carbon cycle and the vulnerability of ecosystems to future climatic extremes. Previous studies have highlighted the occurrence and intensity of El Niño-Southern Oscillation (ENSO) as important drivers of CGR variability, but the underlying biogeophysical mechanisms governing such connections remain unclear. In this study we identify a strong and persistent coupling (r2≈0.5) between interannual variations of CGR and tropical land-surface air-temperature (LSAT) over the past 50 years, with a 1° C tropical LSAT anomaly leading to a 3.5±0.6 PgC/yr CGR anomaly. We infer that the LSAT-CGR coupling is mainly contributed by the additive responses of heterotrophic respiration (Rh) and net primary production (NPP) to temperature interannual variations in tropical ecosystems. In contrast to results from global terrestrial biosphere models, we find the interannual coupling between CGR and tropical land precipitation to be weaker and less consistent (r2≈0.2) than with temperature, likely resulting from the subtractive responses of tropical Rh and NPP to precipitation anomalies that cancel each other in net ecosystem exchange (NEE). Variations in other climate variables (e.g. incident solar radiation) and major disturbances (e.g., volcanic eruptions) in the tropics or elsewhere may induce brief reductions in the CGR-LSAT coupling; however, the relationship is robust and shows full recovery within a few years.