Global distribution of natural freshwater wetlands and rice paddies, their net primary productivity, seasonality and possible methane emissions

A global data set on the geographic distribution and seasonality of freshwater wetlands and rice paddies has been compiled, comprising information at a spatial resolution of 2.5° by latitude and 5° by longitude. Global coverage of these wetlands total 5.7×106 km2 and 1.3×106 km2, respectively. Natural wetlands have been grouped into six categories following common terminology, i.e. bog, fen, swamp, marsh, floodplain, and shallow lake. Net primary productivity (NPP) of natural wetlands is estimated to be in the range of 4–9×1015 g dry matter per year. Rice paddies have an NPP of about 1.4×1015 g y−1. Extrapolation of measured CH4 emissions in individual ecosystems lead to global methane emission estimates of 40–160 Teragram (1 Tg=1012 g) from natural wetlands and 60–140 Tg from rice paddies per year. The mean emission of 170–200 Tg may come in about equal proportions from natural wetlands and paddies. Major source regions are located in the subtropics between 20 and 30° N, the tropics between 0 and 10° S, and the temperate-boreal region between 50 and 70° N. Emissions are highly seasonal, maximizing during summer in both hemispheres. The wide range of possible CH4 emissions shows the large uncertainties associated with the extrapolation of measured flux rates to global scale. More investigations into ecophysiological principals of methane emissions is warranted to arrive at better source estimates.

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