about half of the net 6 billion metric tons of anthropogenic carbon dioxide
emissions produced each year.
Source: Intergovernmental Panel on Climate Change
Figure 11.3 Global carbon cycle (billion metric tons carbon) Figure 11.4
shows that for the past 2,000 years, the atmospheric concentrations of CO 2 ,
CH 4 , and N 2 O—three important, long-lived greenhouse gases—have
increased substantially since around the time of the beginning of the
Industrial Revolution (approximately 1750), and the rates of increase in the
levels of these gases has been dramatic. By analyzing gas bubbles in ice
core samples, scientists discovered that CO 2 , for instance, never increased
more than 30 ppm during any previous 1,000-year period, but has already
risen by 30 ppm in the past 20 years.