Atmospheric methane between 1000 AD and present: Evidence of anthropogenic emissions and climatic variability

Atmospheric methane mixing ratios from 1000 A.D. to present are measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resolution and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately determined. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N-S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr(-1) in 1981 and decreasing significantly since. We calculate an average total methane source of 250 Tg yr(-1) for 1000-1800 A.D:.,reaching near stabilization at about 560 Tg yr(-1) in the 1980s and 1990s. The isotopic ratio, delta(13) CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid-1980s. The combined CH4 and delta(13)CH(4) trends support the stabilization of the total CH4 source.