Projected Changes in Yield and Water Use Efficiency of Cold-Region Rice and the Role of CO2 Under Climate Change

Climate change is reshaping yield formation and water use in cold-region rice production through rising air temperatures, altered precipitation patterns, and increasing atmospheric CO2 concentrations. However, the responses of yield, crop evapotranspiration (ETc), and water use efficiency (WUE) to climate forcing and elevated CO2 remain insufficiently quantified for cold-region rice systems in Northeast China. This study simulated changes in rice yield, ETc and WUE during the 2030s–2090s relative to the 2000–2020 baseline period under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios at 10 agro-meteorological stations in Heilongjiang Province. Simulations were conducted using the AquaCrop model driven by CMIP6 multi-model climate data, and the contribution of elevated CO2 was quantified by comparing the rising-CO2 and fixed-CO2 treatments. The results showed that under SSP5-8.5, the maximum air temperature in the 2090s is projected to increase by 5~6 °C relative to the baseline period, while precipitation is projected to range from −10% to 20%. Compared with the fixed-CO2 treatment, rice yield under the rising-CO2 treatment is projected to increase by 18.70%. Although ETc showed an overall increasing trend, rising CO2 attenuated its increase. Under SSP5-8.5 in the 2090s, ETc increased by only 2.70% under rising-CO2 treatment, compared with 11.61% under fixed-CO2 treatment. As a result of increased yield and ETc, the WUE improved by 15.42% and 14.28% under SSP2-4.5 and SSP5-8.5, respectively, in the 2090s, whereas it remained below the baseline level under the scenarios without CO2 effects. These findings indicate that rising CO2 may enhance yield and moderate ETc increases, thereby providing useful information for regional grain-yield assessment, agricultural water-resource evaluation, and climate-change adaptation planning.