Trends of atmospheric methane in the southern hemisphere

Atmospheric observations spanning the past three years show that methane increased at 1.2 (±0.3)% per year at Cape Grim in Tasmania (41°S). This rate of increase can be compared to the 1.9 (±0.4)% per year observed at Cape Meares in Oregon (45°N) over the past two years. Over the corresponding period the concentration at Cape Grim increased by 1.4 (±0.4)% per year. The southern hemisphere data also suggest seasonal variations with minimum concentrations in March and maximum in September. These results are based on 26 large volume, stable air samples collected cryogenically in stainless steel flasks and 75 smaller volume air samples collected in glass flasks, all analyzed by a gas chromatograph using a flame ionization detector.