Systematic anomalies in the recent global atmospheric CO2 concentration

Atmospheric CO 2 is the major contributor to climate change and ocean acidification resulting from human activities. Over recent decades of improving measurement precision of CO 2 , of its isotopic composition and of air mass history, highly systematic variation has emerged in data selected for minimal exposure to regional air–surface exchange and maximum spatial representation. Between 2009 and 2015, anomalous behaviour is observed in air samples collected in baseline conditions from globally distributed sites. It is evident in atmospheric CO 2 amount, in its stable carbon isotope ratio, in individual sample data and in annually averaged data, and in the interhemispheric difference between primary baseline sites. The behaviour is also anomalous with respect to the wide-spread CO 2 response to El Niño-Southern Oscillation. It is clearly observed as a residual from either the 10-year running mean or smooth exponential curves that describe the CO 2 increase, and isotopic decrease, due to anthropogenic emissions. In Southern Hemisphere annual data there is uniformity throughout 71° of extratropical latitudes, and the isotopic data exclude an ocean-atmosphere contribution. The 2009–2015 anomaly is globally apparent in our data, but not previously reported in conventional global carbon budget studies using CO 2 growth rate changes where it is less evident. It is preceded by the Global Financial Crisis and bracketed by unprecedented activity in interhemispheric exchange indices. The robust and precise anomaly in Southern Hemisphere baseline data provides an unusual opportunity to calibrate causal studies with conventional CO 2 transport models used to verify anthropogenic CO 2 emission estimates and air-surface exchange.