Increased greenhouse-gas intensity of rice production under future atmospheric conditions

Increased atmospheric CO2 and rising temperatures are expected to affect rice yields and greenhouse-gas (GHG) emissions from rice paddies(1-4). This is important, because rice cultivation is one of the largest human-induced sources of the potent GHG methane(5) (CH4) and rice is the world's second-most produced staple crop(6). The need for meeting a growing global food demand(7) argues for assessing GHG emissions from croplands on the basis of yield rather than land area(8-10), such that efforts to reduce GHG emissions take into consideration the consequences for food production. However, it is unclear whether or how the GHG intensity (that is, yield-scaled GHG emissions) of cropping systems will be affected by future atmospheric conditions. Here we show, using meta-analysis, that increased atmospheric CO2 (ranging from 550 to 743 ppmV) and warming (ranging from +0.8 degrees C to +6 degrees C) both increase the GHG intensity of rice cultivation. Increased atmospheric CO2 increased GHG intensity by 31.4%, because CH4 emissions are stimulated more than rice yields. Warming increased GHG intensity by 11.8% per 1 degrees C, largely owing to a decrease in yield. This analysis suggests that rising CO2 and warming will approximately double the GHG intensity of rice production by the end of the twenty-first century, stressing the need for management practices that optimize rice production while reducing its GHG intensity as the climate continues to change.