CO2 emissions to the atmosphere are caused primarily by fossil-fuel burning (~74 percent in 2004; IPCC, 2007b) and tropical deforestation (~22 percent), although recent work suggests that the contribution from deforestation has decreased to as little as 12 percent of CO2 emissions in 2008 (van der Werf et al., 2009a). Other contributors include industrial processes such as cement production. The primary sources of anthropogenic methane are energy production, ruminant animals, rice agriculture, landfills, and biomass burning (Denman et al., 2007). Natural sources of methane are dominated by wetlands and are approximately one-half the size of anthropogenic sources. Although understanding of anthropogenic N2O sources is incomplete, agriculture is likely the largest source because of the oxidation of nitrogen fertilizer and reduction of nitrite (see Table 7.7 in IPCC, 2007a). Natural sources are dominated by soils under natural vegetation and by microbial transformations of nitrogen compounds in the oceans and are thought to be roughly comparable in size to anthropogenic sources (Boumans et al., 2002; Nevison et al., 2004; Hirsch et al., 2006). The presence of CFCs, HFCs, SF6, and PFCs in the atmosphere is due almost entirely to human manufacture for a wide range of industrial applications in the latter half of the twentieth century (IPCC, 2007a).
Atmospheric abundances of greenhouse gases are best quantified by dry air mole fractions—the number of molecules of the gas in a set volume divided by the total number of molecules of dry air in the same volume (see Box 1.3). The mole fraction of CO2 in the atmosphere is currently 387 ppm (Figure 1.4), which is more than 100 ppm higher than in the pre-industrial period. Annual anthropogenic emissions of CO2 are between 9 billion and 10 billion metric tons of carbon (Gt C yr–1), increased at 1-2 percent per year over the last three decades of the twentieth century, and 3.4 percent per year from 2000 to 2008, and are projected to decline in 2009 by almost 3 percent due to the weak economy (Canadell et al., 2007; Le Quéré et al., 2009). Approximately half of the annual increase expected
Measurement Units for Greenhouse Gases in the Atmosphere
The concentration of a gas, which is defined as the number of molecules per volume, will vary with altitude and weather systems as the density of the air changes, even if there are no sources or sinks. When a parcel of air rises and expands at lower pressure, the concentrations of all species decrease by the same factor. What is conserved is the mole fraction, the relative abundance of each. When water evaporates or condenses, which adds or removes an extra gaseous component, the mole fraction of all other components will decrease or increase, respectively, by the same proportion. Thus, the property that reflects additions and removals of a trace component is its mole fraction in dry air, which changes only when there are sources or sinks. The dry air mole fraction of CO2 is expressed as parts per million. A mole fraction of 385 ppm means that, on average, in every 1 million molecules of dry air there are 385 CO2 molecules. The mole fraction of methane is typically expressed in parts per billion, and that of the HFCs, PFCs, and CFCs in parts per trillion.
from these emissions (4-5 Gt C yr–1) accumulates in the atmosphere and the rest is taken up by carbon reservoirs in the oceans and on land (see discussions below and Figure 1.3). Because 1 ppm of CO2 in the atmosphere equals 2.12 Gt C, the atmospheric growth rate currently averages ~2 ppm per year.
The abundance of methane in the atmosphere today is much higher than in the millennium before the industrial era (1,774 parts per billion [ppb] in 2005 versus approximately 700 ppb; see IPCC, 2007a). For the 1970s and 1980s, the growth rate of methane was about 1 percent per year; the rate slowed dramatically in the 1990s and dropped to nearly zero from 2000, but began to grow again in 2007 (Rigby et al., 2008b).3 Several reasons for this anomalous growth pattern have been proposed, but no clear explanation is available (Dlugokencky et al., 2001). The literature on emissions of methane is summarized in Table 7.6 of IPCC (2007a). Estimates for anthropogenic sources range from 264 to 428 Tg CH4 yr–1 (1 Tg CH4 equals one million metric tons of methane) and for natural sources from 145 to 260 Tg CH4 yr–1, although total emissions are more tightly constrained (493 to 667 Tg CH4 yr–1).