Anthropogenic and biophysical contributions to increasing atmospheric CO₂ growth rate and airborne fraction

Abstract. We quantify the relative roles of natural and anthropogenic influences on the growth rate of atmospheric CO₂ and the CO₂ airborne fraction, considering both interdecadal trends and interannual variability. A combined ENSO-Volcanic Index (EVI) relates most (~75%) of the interannual variability in CO₂ growth rate to the El-Niño-Southern-Oscillation (ENSO) climate mode and volcanic activity. Analysis of several CO₂ data sets with removal of the EVI-correlated component confirms a previous finding of a detectable increasing trend in CO₂ airborne fraction (defined using total anthropogenic emissions including fossil fuels and land use change) over the period 1959–2006, at a proportional growth rate 0.24% y⁻¹ with probability ~0.9 of a positive trend. This implies that the atmospheric CO₂ growth rate increased slightly faster than total anthropogenic CO₂ emissions. To assess the combined roles of the biophysical and anthropogenic drivers of atmospheric CO₂ growth, the increase in the CO₂ growth rate (1.9% y⁻¹ over 1959–2006) is expressed as the sum of the growth rates of four global driving factors: population (contributing +1.7% y⁻¹); per capita income (+1.8% y⁻¹); the total carbon intensity of the global economy (−1.7% y⁻¹); and airborne fraction (averaging +0.2% y⁻¹ with strong interannual variability). The first three of these factors, the anthropogenic drivers, have therefore dominated the last, biophysical driver as contributors to accelerating CO₂ growth. Together, the recent (post-2000) increase in growth of per capita income and decline in the negative growth (improvement) in the carbon intensity of the economy will drive a significant further acceleration in the CO₂ growth rate over coming decades, unless these recent trends reverse.