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Global CO2 emissions per unit annual increase in atmospheric CO2 concentration show no significant trend over the 49 year timescale of the available data (1960-2008). The mean emissions/concentration-growth ratio over the period was 15.5 Gte CO2 ppmv–1. Current central estimates are that CO2 concentration will rise by 345 ppmv over the 21st century, during which CO2-driven warming is projected to be 1.56 K, suggesting that, on a centennial scale, CO2 concentration must rise by 223 ppmv, or 3450 Gte CO2, to cause 1 K of warming. Since total global CO2 emissions from 1960-2008 were 975 Gte CO2, the CO2-driven contribution to the 0.66 K measured global warming over the period was 0.28 K. However, on currently-accepted central estimates, the CO2-driven warming over the period was almost two-thirds higher, at 0.46 K. The ratio of CO2 emissions to concentration change – useful as an independent constraint on climate sensitivity – suggests that CO2-driven warming in the 21st century may be little more than 1 K. In the short term and perhaps also in the long, climate sensitivity may lie below the values found in the general-circulation models.