GLOBAL EMISSIONS OF METHANE DURING THE LAST SEVERAL CENTURIES

Direct atmospheric measurements of methane have been taken only during the last 20 years or so. For earlier times the concentrations of methane are determined from the analysis of air trapped in polar ice. Here we have combined all available data to construct globally averaged concentrations on a unified absolute calibration scale that extends back to about 1500 A.D. We use these data to calculate the global annual emissions of methane during the time between now and 500 years ago. According to our calculations the combined natural and anthropogenic methane emissions were around 180 Tg/yr during the 15th century, 200 Tg/yr in the early part of the 18th century and are now about 450 Tg/yr. This is an increase of a factor of 2.5 over the last 400 years, 80% of which has occurred since the turn of the century, and 50% has occurred over the last 40 years. The changes of atmospheric methane during the last century are dominated by changes in emissions with perhaps small effects from changes in OH. We find also that the rate of increase of atmospheric concentrations slowed between 1930 and 1945, which is also reflected in independent agricultural records of cattle populations and area under rice agriculture; both are large sources of methane that can be attributed to human activities. In recent times the trend of methane is falling rapidly, probably due mostly to a slowdown in anthropogenic emissions but also to possible increases of OH.