Publication Type:Journal Article
Source:Bulletin of the American Meteorological Society, Volume 94, Issue 8, p.S31 - S33 (2013)
Carbon dioxide (CO2) is the dominant long-lived greenhouse gas (LLGHG) in the atmosphere. Anthropogenic CO2 emissions from fossil fuel combustion and cement production are the main sources of anthropogenic CO2. The rate of change of CO2 in the atmosphere is a function of emissions and uptake, which are in turn influenced by anthropogenic emissions and modes of climate variability, such as ENSO. After a slight decline in global CO2 emissions associated with the global financial crisis (Peters et al. 2012), global emissions from fossil fuel combustion and cement production reached a record 9.5 ± 0.5 Pg C (Pg = 1015 g carbon, roughly a billion tons carbon) in 2011 (Peters et al. 2013). A new record of 9.7 ± 0.5 Pg C is estimated for 2012 (Peters et al. 2013). Despite evidence to suggest that annual global net carbon uptake has increased over the last 50 years (Ballantyne et al. 2012), atmospheric concentrations continue to rise. In 2012 the mean global CO2 mole fraction at Earth's surface was 392.6 ppm (parts per million by mole in dry air; Fig. 2.30a). This represents an increase of 2.1 ppm over 2011 and is similar to the mean increase from 2000 to 2011 of 1.96 ± 0.36 ppm yr-1. After CO2, methane (CH4) is the most important long-lived greenhouse gas, contributing -0.5 Wm-2 direct radiative forcing. Indirect effects from production of tropospheric ozone and stratospheric water vapor add another -0.2 Wm-2 (Forster et al. 2007). Atmospheric methane is influenced by both natural and anthropogenic sources. Anthropogenic sources emit -60% of total CH4, and include agriculture (ruminants, rice), fossil fuel extraction and use, biomass burning, landfills, and waste. Natural sources include wetlands, oceans, and termites. Fossil CH4 emissions (both natural geologic fossil emissions and anthropogenic fossil fuel emissions) represent about 30% of total CH4 emissions (Lassey et al. 2007). Atmospheric CH4 has increased by about a factor of 2.5 since the pre-industrial era (1750). The rate of increase slowed from more than 10 ppb yr-1 in the 1980s to nearly zero in the early 2000s (Fig. 2.30b). The reasons for the decreased growth rate are consistent with an approach to steady state, where total global emissions and CH4 lifetime have been approximately constant (Dlugokencky et al. 1998, 2003). Global observations over this period are only consistent with a reduction in CH4 emissions [e.g., associated with oil production (Simpson et al. 2012)] if emissions from other sources have increased. Following a few years of near-zero growth, CH4 began increasing again around 2007 at a rate of about 6 ppb yr-1. The global average CH4 mole fraction in 2012 was 1808.5 ppb, which represents a 5.4 ppb increase over 2011, in line with the average growth rate since 2007. Nitrous oxide (N2O) currently has the third strongest climate forcing of the LLGHGs after CO2 and CH4. Nitrous oxide is produced naturally in soils by both the oxidation of ammonium and the denitrification of nitrate. Significant emission of N20 can occur following the application of nitrogen fertilizers (including manure) on agricultural crops (Davidson 2009). Atmospheric N2O has increased from a pre-industrial mole fraction of -270 ppb (Forster et al. 2007) to -325 ppb today. The mean global atmospheric N2O mole fraction for 2012 was 325.0 ppb, an increase of 0.8 ppb over 2011 (Fig. 2.30c). This year-on-year increase is similar to the average growth rate of 0.79 ± 0.01 ppb yr-1 observed from 2000 to 2012. Halogenated gases, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), also contribute to radiative forcing. While the atmospheric mole fractions of some CFCs, such as CFC-12 and CFC-11 are decreasing, levels of their industrial replacements, such as HCFC-22 and HFC-134a are increasing (Figs. 2.30d, 2.31). Atmospheric levels of sulfur hexafluoride (SF6), associated with electrical transmission equipment, also continue to increase. Mean global SF6 was 7.60 ppt in 2012, increasing 0.29 ppt over 2011. Global annual mean mole fractions of a number of trace gases, along with 2011-12 changes are presented in Table 2.6.Recent trends in the combined radiative effect of five major LLGHGs (CO2, CH4, N2O, CFC-11, and CFC-12) and 15 minor gases are expressed by the NOAA Annual Greenhouse Gas Index (AGGI; Hofmann et al. 2006; http://www.esrl.noaa.gov.libproxy.mit.edu/gmd/aggi/). The index represents the additional influence (over pre-industrial values) from LLGHGs in a given year relative to 1990, the Kyoto Protocol baseline year. Indirect effects (e.g., arising from ozone depletion or water vapor feedbacks) are not considered in this index. Based on preliminary, global mean data through 2012, the increases in the abundances of these gases over their pre-industrial values amounted to an additional direct radiative forcing (RF) to the atmosphere totaling 2.88 Wm-2. This compares with 2.18 Wm-2 in 1990 (Fig. 2.32), resulting in an index value of 1.32. Thus, the accumulation of LLGHGs in the atmosphere since 1990 has resulted in a 32% increase in RF. The change in the RF from LLGHGs considered in the index was 1.26% between 2011 and 2012, which is similar to the average change from 2000 to 2012 (1.24%). In fact, since 2000 only one year (2009) showed an increase of less than 1%.