Measurements of an anomalous global methane increase during 1998

Measurements of atmospheric methane from a globally distributed network of air sampling sites indicate that the globally averaged CH4 growth rate increased from an average of 3.9 ppb yr(-1) during 1995-1997 to 12.7 +/- 0.6 ppb in 1998. The global growth rate then decreased to 2.6 +/- 0.6 ppb during 1999, indicating that the large increase in 1998 was not a return to the larger growth rates observed during the late-1970s and early-1980s. The increased growth rate during 1998 corresponds to an increase in the imbalance between CH4 sources and sinks equal to similar to 24 Tg CH4, the largest perturbation observed in 16 years of measurements. We suggest that wetland and boreal biomass burning sources may have contributed to the anomaly. An adaptation of a global process-based model, which included soil-temperature and precipitation anomalies, was used to calculate emission anomalies of 11.6 Tg CH4 from wetlands north of 30 degreesN and 13 Tg CH4 for tropical wetlands during 1998 compared to average emissions calculated for 1982-1993. For 1999, calculated wetland emission anomalies were negative for high northern latitudes and the tropics, contributing to the low growth rate observed in 1999. Also 1998 was a severe fire year in boreal regions where similar to1.3 x 10(5) km(2) of forest and peat land burned releasing an estimated 5.7 Tg CH4.