Precise, accurate systematic measurements of atmospheric CO₂ at Mauna Loa, Hawaii and South Pole were started by C. D. Keeling from Scripps Institution of Oceanography in the late 1950s (Keeling et al., 1976a; Keeling et al., 1976b). The 1750 globally averaged abundance of atmospheric CO₂ based on measurements of air extracted from ice cores and from firn is 278 ± 2 ppm (Etheridge et al., 1996). Globally averaged CO₂ mole fractions since the start of the instrumental record are plotted in Figure 2.1. The main features in the contemporary CO₂ record are the long-term increase and the seasonal cycle resulting from photosynthesis and respiration by the terrestrial biosphere, mostly in the Northern Hemisphere (NH). The main contributors to increasing atmospheric CO₂ abundance are fossil fuel combustion and land use change (Section 6.3). Multiple lines of observational evidence indicate that during the past few decades, most of the increasing atmospheric burden of CO₂ is from fossil fuel combustion (Tans, 2009). Since the last year for which the AR4 reported (2005), CO₂ has increased by 11.7 ppm to 390.5 ppm in 2011 (Table 2.1). From 1980 to 2011, the average annual increase in globally averaged CO₂ (from 1 January in one year to 1 January in the next year) was 1.7 ppm/yr (1 standard deviation = 0.5 ppm/yr; 1 ppm globally corresponds to 2.1 PgC increase in the atmospheric burden). Since 2001, CO₂ has increased at 2.0 ppm/yr (1 standard deviation = 0.3 ppm/yr). The CO₂ growth rate varies from year to year; since 1980 the range in annual increase is 0.7 ± 0.1 ppm in 1992 to 2.9 ± 0.1 ppm in 1998. Most of this interannual variability in growth rate is driven by small changes in the balance between photosynthesis and respiration on land, each having global fluxes of ~120 PgC/yr (Chapter 6).