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The phrases "greenhouse gases" and "global warming" are in the same breath and most people will think of the carbon dioxide produced by burning fossil fuels such as coal and oil. But CO₂ is not the only greenhouse gas and fossil fuels are not the only source of such gases. A surprising and neglected one is the world’s ruminant livestock — cattle, sheep and so on. Ruminants play host to bacteria that digest the otherwise indigestible grass and other cellulose-rich plants those animals eat, making nutrients such as fatty acids available to the beasts the bacteria inhabit.
   But the complicated ecosystem of a ruminant’s stomach includes other creatures, too. Many are methanogens — organisms that react carbon dioxide with hydrogen made by the cellulose-digesting bugs, to create water and methane. A lot of methane. A hundred million tonnes of it a year for all the world’s domesticated ruminants, according to the United Nations’ Food and Agriculture Organization. And methane is a greenhouse gas 25 times more powerful than CO₂. Altogether, according to estimates by Andy Reisinger, of the New Zealand Agricultural Greenhouse Gas Research Centre, methane emitted from livestock is responsible for about 14% of global warming since the beginning of the Industrial Revolution.