The abundance of N2O rose from 270 ppb in 1750 to 319 ppb in 2005 (IPCC, 2007a). N2O is now increasing in concentration at an average rate of approximately 0.25 percent per year. Total emissions (13.9 to 18.9 Tg N yr–1) are constrained by the predicted atmospheric lifetime and observed growth rate (Prather et al., 2001), with the growth rate indicating the level of anthropogenic emissions (~6 Tg N yr–1). Inventory estimates of N2O emissions have considerable uncertainties (Prather et al., 2009) as illustrated by the large range in the size of the total global source reported by the Intergovernmental Panel on Climate Change (IPCC): 8.5-27.7 Tg N yr–1 (IPCC, 2007a, Table 7.7).
CFCs are covered by the Montreal Protocol on Substances That Deplete the Ozone Layer, and they are either stabilized or decreasing in concentration. However, HFC, SF6, and PFC abundances are currently increasing (Prinn et al., 2005; Velders et al., 2005; IPCC, 2007a).
Anthropogenic emissions are the emissions of a gas resulting from human activities. Although this definition is easy to apply to fossil-fuel burning, where all emissions are anthropogenic, it becomes problematic for forestry, cropland management, and other land-use sources and sinks, where it is difficult to distinguish emissions and removals due to human influence (e.g., management practices) from those due to natural factors. For example, climate change and fertilization of plants by anthropogenic CO2 and nitrogen deposition probably affect plant growth rates all over the world, but our understanding of these effects is incomplete and will likely remain so for the foreseeable future. To address the problem, the IPCC and UNFCCC have adopted a convention of treating all emissions and removals (sinks) on land that is managed, as anthro-