In my quest for more reliable figures for the greenhouse gas contribution of rice, I contacted several experts. I have received answers back from two of them. Dr. Reiner Wassman, scientist at the International Rice Research Institute, writes:
“The proximate amount can be derived based on the IPCC guidelines (http://www.ipcc-nggip.iges.or.jp/public/2006gl/vol4.html):
Emission of methane: 130 kg CH4/ ha/ season (assuming 100d cultivation period)
=> 3250 kg CO2eq/ ha/ season
Emission of nitrous oxide: 0.3 kg N2O-N/ ha/ season (assuming 100 kg N fertilizer per ha)
=> 229 kg CO2eq/ ha/ season
TOTAL: 3479 kg CO2eq/ ha/ season
LOW YIELDING RICE PRODUCTION (3000 kg rice/ ha) ===> 1.16 kg CO2eq/ kg rice
HIGH YIELDING RICE PRODUCTION (5000 kg rice/ ha) ===> 0.7 kg CO2eq/ kg rice.”
So, what about that 35 pound figure the Climate Change Institute was using on their site? They write,
“The reported value for the Climate Institute is incorrect. The article on Thailand’s emissions mistakenly says Thailand emits 91.6 million tons of methane annually when citing the EPA 2006 Report on Non-CO2 Gases. The actual statistic is 91.6 million metric tons of CO2 equivalent from methane. The conversion factor from methane to CO2 is already included in the statistic. Therefore, dividing your calculated 35 pounds of CO2 equivalent per pound of rice by a CH4 to CO2 conversion factor of 25 gives a result of 1.4 pounds of CO2 equivalent per pound of rice…”
Now, one last verification: does this reconcile with the estimated global contribution of rice to global warming? World rice production is estimated at 645 million tons. EIA puts CO2 equivalence of global methane emissions at 605 million tons. USDA says, “Some studies show that up to 20 percent of global methane emissions come from flooded rice fields.” But other studies put rice’s methane emissions at 20-100 million tons, or 400 – 2,000 million tons of CO2 equivalency. This suggests that it’s not the rice methane estimates that are off, but EPA’s world methane inventory.
One study uses a low-end figure consistent with other data: 25.6 million tons of methane from rice production worldwide, which equates to 538 million tons of CO2 equivalnce. Based on rice production of 645 million tons, that’s 1.2 pounds of CO2 equivalence per pound of rice. And that’s perfectly consistent with these figures– but it also represents 89% of makes EPA’s methane inventory, making EPA’s figure look ridiculously low.
NASA, in a 1996 study, puts methane emissions for rice much higher, at 146 million tons, or around 3,000 million tons of CO2 equivalency, or 4.8 pounds per pound of rice. But I’m mostly willing to concede this as an aberration.
How does 1.2 – 1.4 pounds of CO2 stack up against other foods? That’s far less than beef’s (by my calculation) 10.75 pounds. But it’s far more than soy (0.3), eggs (0.2), chicken (0.5) or turkey (0.6), and roughly the same as cheese (1.6) or pork (1.2). Other grains, such as rice and corn, can be grown in a carbon-neutral manner, making rice all the more decadent.
To be clear, rice can be grown in a more environmentally-friendly manner by not flooding the fields throughout the cultivation cycle. But it isn’t, and for that reason it remains an environmentally expensive food that much of the world relies on for survival.