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 CO 2 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. Atmospheric CO2 concentrations have risen over the last 200 years at an accelerating rate, in response to increasing anthropogenic CO2 emissions. The resulting CO2 disequilibrium has led to uptake of CO2 from the atmosphere by land and ocean CO2 sinks, which currently remove over half of all anthropogenic emissions and thereby provide a strong negative (stabilising) feedback on the carbon-climate system (Gruber et al., 2004; Sabine et al., 2004). The CO2 airborne fraction (the fraction of total emissions from fossil fuels and land use change accumulating in the atmosphere) has averaged 0.43 since 1959, but has increased through that period at about 0.2% y-1 (Canadell et al., 2007). These interdecadal trends in CO2 growth rate and the airborne fraction are the outcome of a race between two groups of forcing factors: the social, economic and technical drivers of anthropogenic emissions (including population, wealth and the carbon intensity of the economy), and the biophysical drivers of trends in land and ocean sinks. The CO2 growth rate also varies strongly at interannual (∼1 to ∼10 y) time scales, through mainly biophysical mechanisms. Fluctuations in CO2 growth rate correlate with the El-Niño-Southern-Oscillation (ENSO) climate mode (Keeling and Revelle, 1985; Keeling et al., 1995; Jones and Cox, 2005), because the terrestrial carbon balance in tropical regions is tilted from uptake to release of CO2 during dry, warm El-Niño events (Zeng et al., 2005; Knorr et al., 2005). Volcanic events are also significant: the CO2 growth rate decreased for several years after the eruption of Mt. Pinatubo in June 1991 (Jones et al., 2001), probably because of increased net carbon uptake by terrestrial ecosystems due to higher diffuse solar radiation (Gu et al., 2003) and cooler temperatures (Jones and Cox, 2001) caused by volcanic aerosols. © Author(s) 2008.