Nature: Increased atmospheric CO2 and rising temperatures are expected to affect rice yields and greenhouse-gas (GHG) emissions from rice paddies1, 2, 3, 4. This is important, because rice cultivation is one of the largest human-induced sources of the potent GHG methane5 (CH4) and rice is the world’s second-most produced staple crop6. The need for meeting a growing global food demand7 argues for assessing GHG emissions from croplands on the basis of yield rather than land area8, 9, 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 °C to +6 °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 °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.
Criteria for study inclusion
To be included in our data set, studies had to meet specific criteria. The atmospheric CO2 concentration for ambient and elevated treatments in CO2 enrichment studies had to be between 350-450 ppmV and 450-800 ppmV, respectively. The increase in seasonal average air temperature in warming experiments had to be equal to or less than 6°C. This cut-off point was based on the maximum expected increase in zonal mean temperature on land between now and the end of the twenty-first century1.
For warming experiment with more than two temperature levels, we included all comparisons between treatments for which the difference in temperature was equal to or less than 6 °C. For both the CO2 enrichment studies and the warming studies, means and sample sizes had to be reported for both t he control and the experimental treatments. We included studies involving experiments in pots (i.e., any container with dimensions <1 m) or in the field. We only considered studies in which soils in the control and experimental group had the same treatment history. One study was discarded for this reason. We only considered studies in which the warming and/or CO2 treatments were maintained for the entire growing season (i.e., from planting to harvest). More>>
