Globally averaged CH4 in 1750 was 722 ± 25 ppb (after correction to the NOAA-2004 CH4 standard scale) (Etheridge et al., 1998; Dlugokencky et al., 2005), although human influences on the global CH4 budget may have begun thousands of years earlier than this time that is normally considered ‘pre-industrial’ (Ruddiman, 2003; Ferretti et al., 2005; Ruddiman, 2007). In 2011, the global annual mean was 1803 ± 2 ppb. Direct atmospheric measurements of CH4 of sufficient spatial coverage to calculate global annual means began in 1978 and are plotted through 2011 in Figure 2.2a. This time period is characterized by a decreasing growth rate (Figure 2.2b) from the early 1980s until 1998, stabilization from 1999 to 2006, and an increasing atmospheric burden from 2007 to 2011 (Rigby et al., 2008; Dlugokencky et al., 2009). Assuming no long-term trend in hydroxyl radical (OH) concentration, the observed decrease in CH4 growth rate from the early 1980s through 2006 indicates an approach to steady state where total global emissions have been approximately constant at ~550 Tg (CH4) yr–1. Superimposed on the long-term pattern is significant interannual variability; studies of this variability are used to improve understanding of the global CH4 budget (Chapter 6). The most likely drivers of increased atmospheric CH4 were anomalously high temperatures in the Arctic in 2007 and greater than average precipitation in the tropics during 2007 and 2008 (Dlugokencky et al., 2009; Bousquet, 2011). Observations of the difference in CH4 between zonal averages for northern and southern polar regions (53° to 90°) (Dlugokencky et al., 2009, 2011) suggest that, so far, it is unlikely that there has been a permanent measureable increase in Arctic CH4 emissions from wetlands and shallow sub-sea CH4 clathrates.
Reaction with the hydroxyl radical (OH) is the main loss process for CH4 (and for hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)), and it is the largest term in the global CH4 budget. Therefore, trends and interannual variability in OH concentration significantly impact our understanding of changes in CH4 emissions. Methyl chloroform (CH3CCl3; Section 220.127.116.11) has been used extensively to estimate globally averaged OH concentrations (e.g., Prinn et al., 2005). AR4 reported no trend in OH from 1979 to 2004, and there is no evidence from this assessment to change that conclusion for 2005 to 2011. Montzka et al. (2011a) exploited the exponential decrease and small emissions in CH3CCl3 to show that interannual variations in OH concentration from 1998 to 2007 are 2.3 ± 1.5%, which is consistent with estimates based on CH4, tetrachloroethene (C2Cl4), dichloromethane (CH2Cl2), chloromethane (CH3Cl) and bromomethane (CH3Br).