Methane emission from rice cultivation: Geographic and seasonal distribution of cultivated areas and emissions

We present high‐resolution global data bases on the geographic and seasonal distribution of rice cultivation and associated methane emission. The data bases were developed by integrating extensive and eclectic information on the cultivation of rice in all 103 rice‐producing countries of the world. The geographic distribution of rice‐growing locations was developed by combining a 1° resolution land‐use data base identifying rice‐farming regimes, a 1° resolution data base of countries of the world, and country statistics on areas of annual rice harvest available from the U. N. Food and Agriculture Organization. The seasonal distribution of cultivated rice areas was derived via the integration of the data base on rice‐growing locations with information on cultivation activities and rice cropping practices for each rice‐producing country of the world; this information included seasonal rice‐cropping calendars for individual countries and statistics or estimates of the seasonal distribution of annual harvest areas in each crop cycle. Since the causes of the variability in methane fluxes from flooded rice fields have yet to be quantified, we did not attempt a new estimate of the role of rice cultivation in the global emission of methane. We evaluate, instead, the temporal and spatial distribution of emissions from a hypothetical annual source of 100 Tg methane. In 1984, 1475 × 10 9 m 2 of rice was harvested in 103 countries. Although rice cultivation extends from about 50°N to 50°S, 48% of the harvest area is confined to a narrow subtropical zone from 30°N to 20°N, while another 35% is harvested in the 10° zones directly to the north and south. Globally, about 60% of the harvested rice area is managed under a triple rice crop system, ∼15% is double cropped, and 25% is harvested from fields planted for rice once a year. In China and India, which together account for 52% of the world's harvest area, rice is harvested predominantly under a triple‐crop system. These patterns suggest that much of the potential for multiple rice cropping is currently exploited. In this analysis, methane emission is proportional to the area and duration of each harvest so that the seasonal, zonal and country patterns of annual methane emission mimic the distribution of rice‐harvest areas. Although rice is grown throughout the year, the close coupling of rice cultivation with climate results in the concentration of about 55% of the annual methane emission into four months from July through October and almost half the total emission in latitudes between 30°N and 20°N.