Abstract
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.
Highlights
Atmospheric CO2 concentrations have risen over the last 200 years at an accelerating rate, in response to increasing anthropogenic CO2 emissions
This paper has offered two main conclusions, the first being that the total airborne fraction is increasing at r(aE)≈0.2% y−1, with probability ≈0.9 of a positive trend
The immediate significance is that since 1959, growth in natural sinks has fallen slightly behind growth in total emissions. This conclusion needs to be interpreted with regard for three factors: (1) there is uncertainty both from the statistics of interannual variability and from imprecisely determined emissions from land use change; (2) the result does not imply that “sinks are weakening”, but rather shows that growth in sinks has not kept pace with growth in emissions; (3) the airborne fraction is a simple, robust diagnostic property of the carbon cycle which can provide the above conclusions but cannot partition trends in sinks between land and ocean – this requires additional information, supplied in Fig. 5 by modelled estimates of the ocean sink (Le Quereet al., 2007)
Summary
Atmospheric CO2 concentrations have risen over the last 200 years at an accelerating rate, in response to increasing anthropogenic CO2 emissions. Volcanic events are 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. 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 This interannual variability in the CO2 growth rate is important for two reasons: it indicates mechanisms that govern the land and ocean CO2 sinks, and it masks important longerterm trends in the CO2 growth rate with strong variability at higher frequencies. We introduce an extended form of the Kaya identity which combines the biophysical and anthropogenic drivers of CO2 growth, and use it both to diagnose the drivers of past trends and offer some indicative estimates of future CO2 growth rates
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