FIGURE 6.1 Estimated global CO₂ emissions from fossil fuel sources, in gigatons (or billion metric tons). Based on data from Boden et al. (2009; available at http://cdiac.ornl.gov/trends/emis/tre_glob.html).

in Hawaii, far away from major industrial and population centers, and analyzing the composition of these samples to determine whether CO₂ levels in the atmosphere were increasing. Similar in situ measurements continue to this day at Mauna Loa as well as at many other sites around the world. The resulting high-resolution, well-calibrated, 50-year-plus time series of highly accurate and precise atmospheric CO₂ measurements (Figure 6.2), commonly referred to as the Keeling curve, is both a major scientific achievement and a key data set for understanding climate change.

The Keeling curve shows that atmospheric CO₂ levels have risen by more than 20 percent since 1958; as of January 2010, they stood at roughly 388 ppm, rising at an average annual rate of almost 2.0 ppm per year over the past decade (Blasing, 2008; Tans, 2010). When multiplied by the mass of the Earth’s atmosphere, this increase corresponds to 15.0 ± 0.1 Gt CO₂ added to the atmosphere each year, or roughly 45 percent of the excess CO₂ released by human activities over the last decade. The remaining 55 percent is absorbed by the oceans and the land surface. The size of these CO₂ “sinks” is estimated via both modeling and direct observations of CO₂ uptake in the oceans and on land. These estimates indicate that the oceans absorbed on average 8.4 ± 1.5 Gt CO₂ annually over the last decade (or 26 percent of human emissions), while the land surface took up 11.0 ± 3.3 Gt per year (29 percent), with a small residual of 0.3 Gt (Le Quéré et al., 2009).

A careful examination of the Keeling curve reveals that atmospheric CO₂ concentrations are currently increasing twice as fast as they did during the first decade of the record (compare the slope of the black line in Figure 6.2). This acceleration in the rate of CO₂ rise can be attributed in part to the increases in CO₂ emissions due to increasing