Atmospheric methane: recent global trends

The authors report globally averaged concentrations of atmospheric methane for every month of the past 8 years based on measurements taken at six locations ranging in latitude from within the Arctic Circle to the South Pole. This record shows that methane concentration increased at an average rate of 16.6 {plus minus} 0.4 ppbv/yr or {approximately} 1.02 {plus minus} 0.02%/yr over 8 years. This trend has not been constant according to their record but has varied between 12 {plus minus} 2 and 23 {plus minus} 2 ppbv/yr over 2-year periods after seasonal variations are removed. The causes of these interannual variations are not known. They also show that the total mass of methane in the earth's atmosphere undergoes seasonal variations, with highest levels during late fall and early winters of the northern hemisphere and lowest levels in the summers. After the main features of the record are taken into account, residual random fluctuations remain, which have a variability of only 3 ppbv or 0.2% of the mean concentrations. The variation of the trends at different times during the past decade accounts for the differences of trends reported in various studies. Uncertainties in the estimates are reported as 90% confidence limits.