Peatlands in the Earth's 21st century climate system

Peatlands occupy a relatively small fraction of the Earth's land area, but they are a globally important carbon store because of their high carbon density. Undisturbed peatlands are currently a weak carbon sink (similar to 0.1 Pg C y(-1)), a moderate source of methane (CH4; similar to 0.03 Pg CH4 y(-1)), and a very weak source of nitrous oxide (N2O; similar to 0.00002 Pg N2O-N y(-1)). Anthropogenic disturbance, primarily agriculture and forestry drainage (10%-20% of global peatlands), results in net CO2 emissions, reduced CH4 emissions, and increased N2O emissions. This likely changes the global peatland greenhouse gas balance to a C source (similar to 0.1 Pg C y(-1)), a 10% smaller CH4 source, and a larger (but still small) N2O source (similar to 0.0004 Pg N2O-N y(-1)). There is no strong evidence that peatlands significantly contributed to 20th century changes in the atmospheric burden of CO2, CH4, or N2O; will this picture change in the 21st century? A review of experimental and observational studies of peatland dynamics indicates that the main global change impacts on peatlands that may have significant climate impacts are (1) drainage, especially in the tropics; (2) widespread permafrost thaw; and (3) increased fire intensity and frequency as a result of drier climatic conditions and (or) drainage. Quantitative estimates of global change impacts are limited by the sparse field data (particularly in the tropics), the large variability present in existing data, uncertainties in the future trajectory of peatland use, interactive effects of individual impacts, and the unprecedented rates of climate change expected in the 21st century.