| Summary: | The global evolution of atmospheric CH4 has been documented by sporadic direct measurements in the atmosphere during the 1960s and 1970s and by systematic survey only since 1979. The data from this time up to 1983 indicate an increasing trend at a rate of about 1% per year (Rasmussen and Khalil, 1986; Steele et al., 1987; Blake and Rowland, 1988). The most recent measurements indicate a decrease of the global accumulation of atmospheric CH4 during the years 1991 and 1992, and a return to a 1%/yr increase afterward (Steele et al., 1992; Dlugokencky et al., 1994; Lowe et al., 1994). The analysis of infrared solar absorption spectra (Rinsland et al., 1985; Zander et al., 1989) provides additional data of global concentrations for a few specific years (1951, 1975, 1981, 1984–87) and shows a mean increase of about 30% over the past 40 years. This long-term accumulation of CH4 in the atmosphere is related to human activities, particularly from agriculture.
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