Remember that rice is the world's second-largest crop, an already significant source of methane emissions, and that methane is a more powerful, if shorter-lived, greenhouse gas than CO2:
New research published in Nature Climate Change shows that as the world warms it both increases the methane emissions from rice paddies, and decreases the crop yield of rice (something which TreeHugger has previously covered).
As for why, Science Daily sums up what the research found to be happening:
Methane in rice paddies is produced by microscopic organisms that respire CO2, like humans respire oxygen. More CO2 in the atmosphere makes rice plants grow faster, and the extra plant growth supplies soil microorganisms with extra energy, pumping up their metabolism. Increasing CO2 levels will also boost rice yields, but to a smaller extent then CH4 emissions. As a result, the amount of CH4 emitted per kilogram of rice yield will increase. Rising temperatures were found to have only small effects on CH4 emissions, but because they decrease rice yield, they also increase the amount of CH4 emitted per kilogram of rice. "Together, higher CO2 concentrations and warmer temperatures predicted for the end of this century will about double the amount of CH4 emitted per kilogram of rice produced.," explained Professor Chris van Kessel of the University of California in Davis and co-author of the study.
All this means that total methane emissions from rice production "will strongly increase," as global demand for rice increases alongside rising human population.
What can be done about it? The report says that draining rice paddies in mid-season and using different fertilizers can reduce methane emissions, while switching to more heat-tolerant varieties of rice can offset crop yield declines.
Regarding crop yield declines for rice, previously research on rice grown in Asia has shown that for every 1°C increase in minimum nighttime temperatures crop yields declined 10%.