On the temporal increase of tropospheric CH4

The available data on the tropospheric CH4 mixing ratio in the northern hemisphere are examined for a possible trend, with the following result: There was no or little increase between 1948 and 1965 as evidenced by IR measurements of the atmospheric CH4 column density. Between 1965 and 1975 there appears a weak increase in tropospheric CH4 at an average rate of about 0.5%/yr, as suggested by independent sets of CH4 measurements. Between 1978 and 1980 a CH4 increase of 1 to 2%/yr has been observed by several laboratories. The overall trend resulting for the past 30 years is subject to a fair amount of uncertainty.