"Growing rice emits methane, a potent greenhouse gas — to the tune of 25 million to 100 million metric tons of methane every year"
"When rice paddies are flooded, methane-producing bacteria thrive on the carbohydrates secreted by rice roots in the oxygen-free soils."
A new type of GMO rice seems to produce less than 10% as much methane--but GMOs are evil!
GMO rice could reduce greenhouse gas emissions, study says - LA Times
Ecky gave the statistic that cows produce 70 - 120kg of methane per year. There are 1.5 Billion cows, so 10.5 - 180 metric tons of methane--roughly as much as rice paddies.