Summary
Monitoring of background atmospheric CO2 concentration in recent decades has revealed interannual growth rate variations (IAV) that are linked to the El Nino Southern Oscillation (ENSO). However, the nature of this link is not well understood. Different approaches investigating the relative contributions of oceanic and terrestrial carbon fluxes have drawn conflicting conclusions. Studies using 13C in atmospheric CO2 have generally called for large, and in some cases opposing, contributions from both oceanic and terrestrial exchange. Recent ocean-based studies suggest that variability in air-sea fluxes is too small to account for the atmospheric signal. Some climate-driven terrestrial ecosystem models are able to accommodate the CO2 IAV through an ENSO effect on imbalance of photosynthesis/respiration in tropical forests. Air samples obtained from CSIRO GASLAB’s global flask sampling network are routinely analysed for multiple trace gases and isotopes. High precision measurements of CO2 and its isotope 13C, CH4, H2 and CO since 1992 show strong inter-species correlation in IAV, thus providing a new window on understanding the nature of the forcing. We explore the implications of this correlation, in terms of common sources and sinks and possible covariations forced by ENSO-related climatic variability. The multi-species signature points to one mechanism as a likely major influence, and is supported by several independent lines of evidence gathered from ground, aircraft and satellite-based observations. (Author abstract).