Atmospheric variability of methyl chloride during the last 300 years from an Antarctic ice core and firn air

Measurements of methyl chloride (CH 3 Cl) in Antarctic polar ice and firn air are used to describe the variability of atmospheric CH 3 Cl during the past 300 years. Firn air results from South Pole and Siple Dome suggest that the atmospheric abundance of CH 3 Cl increased by about 10% in the 50 years prior to 1990. Ice core measurements from Siple Dome provide evidence for a cyclic natural variability on the order of 10%, with a period of about 110 years in phase with the 20th century rise inferred from firn air. Thus, the CH 3 Cl increase measured in firn air may largely be a result of natural processes, which may continue to affect the atmospheric CH 3 Cl burden during the 21st century.