Future atmospheric conditions increase the greenhouse-gas intensity of rice cultivation K.J. van Groenigen*,†, C. van Kessel ‡, B.A. Hungate* Discussion Introduction TRINITY COLLEGE DUBLIN  We extracted results for CH 4 emissions and rice yield from CO 2 enrichment and warming studies, conducted in the field or indoors. For studies in which CH 4 emissions and rice yield were both reported, we also calculated yield- scaled CH 4 emissions.  Data were analyzed using the natural log of the response ratio (ln R ) as the effect size. Warming effects were normalized for the degree of temperature increase. The ln R values were weighted by replication. Bootstrapping was used to calculate 95% confidence intervals on mean effect size estimates. Average effect sizes were back-transformed and reported as % change to ease interpretation.  Rising atmospheric CO 2 and global warming are expected to affect rice yields and greenhouse gas (GHG) emissions from rice paddies 1,2.  Rice cultivation is a major source of the potent GHG methane 3 (CH 4 ) and rice is the world’s second-most produced staple crop 4.  Growing global food demand argues for assessing GHG emissions from croplands on the basis of yield rather than land area 5.  Heterogeneity in CH 4 fluxes makes it difficult to determine their global response to elevated CO 2 and warming from individual studies.  We used meta-analysis to summarize the effect of elevated CO 2 and warming on CH 4 emissions, rice yield and yield-scaled CH 4 emissions. Materials and methods © by M. Schneider contact: Acknowledgements Funding was provided by the NSF (DEB ), the DOE Office of Science (BER) through the Western Regional Center of NICCR, and the Irish Research Council, co-funded by Marie Curie Actions under FP7. The correlation between control temperature and treatment effects on yield and yield-scaled CH 4 emissions in warming studies (Fig. 2) suggests that effects of warming will intensify as global temperatures continue to rise. It also corroborates model predictions that climate warming will affect rice yields in tropical regions more strongly than in temperate regions 8. Besides CH 4, nitrous oxide (N 2 O) is an important GHG emitted from rice paddies. However, N 2 O on average account for only 11% of the global warming potential of GHG emissions from rice paddies 9. This suggests that N 2 O will play a minor role in changing the future GHG intensity of rice cultivation. * Department of Biological Sciences, Northern Arizona University, Flagstaff, USA Results  Fig. 1a : Elevated CO 2 significantly increased CH 4 emissions (+42%), rice yield (+25%) and yield-scaled CH 4 emissions (+31%). b : Warming significantly decreased yield (-15% per 1°C) and increased yield-scaled CH 4 emissions (+12% per 1°C).  Fig. 2 : The negative effect of warming on rice yield ( a ) and the positive effect on yield-scaled CH 4 emissions ( b ) increased with the control temperature of warming studies.  Within studies that applied warming and CO 2 enrichment in a factorial design, we found no interactive treatment effects on either yield, CH 4 emissions, or yield- scaled CH 4 emissions (data not shown, see ref. 6 for details).  Climate models estimate that by the end of the 21 st century, land temperatures in rice growing regions will be 4°C warmer than today 7. Since we found no significant interactions between warming and CO 2 we calculated their combined effect by multiplying their average treatment effects. Using this approach, 4°C warming with CO 2 enrichment will increase CH 4 emissions by 58%, decrease rice yields by 34% and increase yield-scaled CH 4 emissions by 106%. Figure 1. Results of a meta-analysis on the response of CH 4 emissions, yield and yield-scaled CH 4 emissions from rice paddies to increased levels of atmospheric CO 2 and warming. a Effects of increased CO 2. b Effects of warming. The number in parentheses indicates the number of observations used in each meta-analysis. Error bars, 95% confidence intervals. 1. Van Groenigen KJ et al. Nature 475, (2011). 2. Peng S et al. PNAS 101, (2004). 5. Cassmann KG et al. Annu. Rev. Environ. Resour. 28, (2003). 7. Meehl GA et al. in Climate Change 2007: The Physical Science Basis ( eds Solomon S US-EPA. Global Anthropogenic non-CO 2 GHG emissions: (2006). CH 4 YieldYield-scaled CH 4 Control temperature (°C) Chinese rice paddies. Photo credit: Jim Hill, UC Davis. Control temperature (°C) (27) (184)(23)(42)(109) (13) a b Figure 2. Effects of warming on rice yield and yield-scaled CH 4 emissions vs. control temperatures in warming experiments. a, The normalized effect of warming on rice yield (ln R TN ) vs. control temperature in the warming experiment. b, The normalized effect of warming on yield-scaled CH 4 emissions (ln R TN ) vs. control temperature. Future CH 4 emissions from rice paddies will mostly increase due to rising atmospheric CO 2 and less due to warming, but both factors contribute to an increased GHG intensity of rice cultivation. Compared with other cereals, rice production systems show a large potential for reduction in CH 4 emissions through management practices 9. Moreover, adaptation efforts can reduce the negative effect of warming on rice yields 10. Future research should assess whether these mitigation and adaptation efforts interact with global change factors to influence CH 4 emissions from rice agriculture. Conclusions and future research References 6. Van Groenigen KJ et al. Nature Climate Change, doi: /nclimate1712 (2012). et al.) (Cambridge Univ. Press, Cambridge, 2007). 9. Linquist B et al. Glob. Change Biol. 135, (2012). 10. Wassman R et al. Adv. Agron. 102, (2009). 8. Easterling WE et al. in Climate Change 2007: The Physical Science Basis ( eds Parry M The FACE (Free Air Carbon Enrichment) experimental site in Shizukuishi, Japan. Temperature controlled open top chambers to study the effect of warming on rice yield. © IRRI Images et al.) (Cambridge Univ. Press, Cambridge, 2007). r 2 = 0.45 p = 0.01 r 2 = 0.30 p < 0.01 This poster is based on our recent article in Nature Climate Change 6. † Department of Botany, Trinity College, Dublin, Ireland ‡ Department of Plant Sciences, University of California, Davis, USA.