Sensitivity of the CH4 growth rate to changes in CH4 emissions from natural gas and coal

The observed rate of increase in atmospheric CH4 declined gradually during the late 1980s [Steele et al. 1992] and then fell dramatically by similar to 4 to 7 parts per billion by volume (ppbv) yr(-1) from 1991 to 1992 [Dlzcgokencky et al., 1994a]. Post-1993 the CH4 growth rate has recovered to pre-1991 levels. In this study, we assess the sensitivity of the CH4 growth rate to changes in CH4 emissions from the coal and gas industries over the last 15 years using a global two-dimensional (2-D) model of the atmosphere. Emissions are estimated from available production data, and in the case of natural gas, a spectrum of leakage rates is assumed. Changes in the former Soviet Union (FSU) have been assessed separately from the rest of the world (ROW). Prior to 1989, increased coal and natural gas production led to an increase in the CH4 growth rate (i.e., opposite to that which was observed). During the period 1989 to 1993 the modelled CH4 growth rate decreased by similar to 1-2 ppbv yr(-2) at northern mid-latitudes (assuming constant gas leakage rates), showing that declining emissions, primarily in the form of natural gas from the FSU, may have contributed to the general reduction in the CH4 growth rate from 1989 to 1991. However, this does not explain the large reduction which was observed between 1991 and 1992. A model run with FSU gas leakage rates decreasing by 1% yr(-1) from 5% in 1990 to 2% in 1993 shows a decrease in the CH4 growth rate of similar to 9-12 ppbv yr(-1) (or similar to 3-5 ppbv yr(-2)) over this period. This is similar in magnitude to the observed decline over the same period. This sensitivity study illustrates the large potential for significantly lowering the growth rate of atmospheric CH4 by reducing fossil fuel emissions from coal and, in particular, by reducing the leakage rates from natural gas installations.