Atmospheric methane(CH(4)) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium(1,2), after which levels increased once more(3). The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors(1,4-8). Here we show that the late-twentieth century changes in the CH(4) growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH(4) mixing and (13)C/(12)C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of (13)C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The (13)C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 +/- 18%) of the decrease in Northern Hemisphere CH(4) emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application(9) and reductions in water use(10,11).