Nitrous oxide. A greenhouse gas naturally present in the atmosphere as part of the Earth's nitrogen cycle, with a variety of natural sources. Human activities such as agriculture, industrial processes, wastewater management and fossil fuel combustion from transportation and coal-fired power plants, are increasing the amount of N20. Globally, about 40 per cent of total N20 emissions come from human activities. N20 has a much higher global warming potential (GWP) than carbon dioxide.