modeling, scientists have deduced that the ice ages were initiated by small recurring variations in the Earth’s orbit around the Sun.

Human activities have increased the concentration of CO₂ and certain other GHGs in the atmosphere. Detailed worldwide records of fossil fuel consumption indicate that fossil fuel burning currently releases over 30 billion tons of CO₂ into the atmosphere every year (Figure 2.3, blue curve). Tropical deforestation and other land use changes release an additional 3 to 5 billion tons every year.

Precise measurements of atmospheric composition at many sites around the world indicate that CO₂ levels are increasing, currently at a pace of almost 2 parts per million (ppm) per year. We know that this increase is largely the result of human activities because the chemical signature of the excess CO₂ in the atmosphere can be linked to the composition of the CO₂ in emissions from fossil fuel burning. Moreover, analyses of bubbles trapped in ice cores from Greenland and Antarctica reveal that atmospheric CO₂ levels have been rising steadily since the start of the Industrial Revolution (usually taken as 1750; see Figure 2.3, red curve). The current CO₂ level (388 ppm as of the end of 2009) is higher than it has been in at least 800,000 years.

FIGURE 2.3 CO₂ emissions due to fossil fuel burning (blue line and right axis) from 1800 to 2006 and atmospheric CO₂ concentrations (red line and left axis) from 1847 to 2008. For further details see Figures 6.2, 6.3, and 6.4. Based on data from Boden et al. (2009), Keeling et al. (2009), and Neftel et al. (1994).