Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983--1991 was (11.6¿0.2) ppbv yr-1, but the increase in 1992 was only (1.8¿1.6) ppbv. In the Southern Hemisphere, the average increase (1983--1991) was (11.1¿0.2) ppbv yr-1, and the 1992 increase was (7.7¿1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities. ¿ American Geophysical Union 1994