Impacts Of Climate Change On Rice Production In China And The Adaptation Measures

China is the world’s second largest rice（Oryza sativa L.）producer,accounting for～19%and 28%of the global rice planting area and yield,respectively.China has a population of more than 1.4 billion,and over 65%of Chinese people rely on rice as their staple food.Therefore,ensuring sufficient rice supply in China is of great importance to global food security.Climate change will not only change field management practices,but also affect the physiological process of rice plants,and even affect the macro-regulation of rice supply.However,some field management practices need to be optimized,such as the selection of planting methods and the determination of sowing dates;the response of yield and its components to temperature is uncertain;and the effects of climate change on the macro-regulation of rice supply need to be evaluated under different population growth and arable land conversion scenarios.In addition to assessing the impacts of climate change in these fields,effective adaptation measures are important to maintain sustainable rice productionTo this end,the process-based Crop Environment Resource Synthesis（CERES）-Rice model（Version 4.6）was used to conduct research in four fields.Firstly,Hubei province was divided into four zones according to the natural elevation differences and precipitation.Three rice cultivars of different growth duration were selected,including Lvhan1（LH1;short-duration）,Huanghuazhan（HHZ;mid-duration）,and Yangliangyou6（YLY6;long-duration）.Three planting methods,namely dry direct-seeded rice（DDSR）,wet direct-seeded rice（WDSR）,and transplanted-flooded rice（TFR）,were adopted.The effects of elevation and precipitation on rice planting methods were evaluated.Secondly,the same rice cultivars were used in these four zones,but the planting method was WDSR.Combining the simulation results of the CERES-Rice model with the empirical methane（CH₄）emission calculation equation proposed by the Intergovernmental Panel on Climate Change（IPCC）,the optimal sowing window that could achieve high yield and decreased CH₄ emissions were determined for the historical period（1999-2018）and two periods（2021-2050 and 2071-2100）under two shared socioeconomic pathways（SSP）2-4.5 and SSP5-8.5 emission scenarios.Thirdly,according to the climate zonation scheme,the main rice planting area in China was divided into six agro-climate regions.Based on the simulation outputs of the CERES-Rice model,the panel model was used to estimate the sensitivity of rice yield to the mean temperature（Ta）during the growing season in each agro-climate region,and the yield sensitivity was then decomposed into the temperature sensitivity of panicle nember per unit area（Pan＿no）,filled grain number per panicle（Grain＿no）,and single grain weight（Grainwt）.In addition,the sensitivity of yield and its components to growth degree days（GDD）and high temperature degree days（HDD）was estimated.Finally,the effects of climate change on rice yield p in eight southern provinces（Anhui,Hubei,Hunan,Jiangxi,Zhejiang,Fujian,Guangdong,and Guangxi）were investigated in the context of reduced planting intensity.According to the current area trajectories and official policies,the future planting area of single and double rice was predicted,and several experiments on the conversion of double rice to single rice were designed.Rice supply and demand under different scenarios were calculated to evaluate the relationship between rice supply and demand in the future.The main conclusions of this study were summarized as follows:（1）Effects of elevation and precipitation on the selection of rice planting methods.In low-elevation regions,the yields of WDSR adopting an optimal irrigation strategy were comparable to TFR yields.However,in high-elevation regions,the yields of direct-seeded rice were significantly lower than those of TFR because the emergence rate of direct-seeded rice was lower at low temperature.TFR yields were higher than those of WDSR by 300-450 kg ha^-1in high-elevation regions.Thus,direct-seeded rice can be selected in low-elevation areas,while TFR was still needed in high-elevation regions.The rice yield increased first and then decreased with the increasing elevation.The change of rice yield was affected mainly by the length of growth duration and the photosynthesis rate.In addition,rice cultivars with a long growth duration often required more irrigation,and high precipitation can reduce the irrigation amount,which was conducive to the planting of YLY6.Compared with low precipitation areas,planting YLY6 in high precipitation areas can reduce irrigation water by 16-35 mm.At high-elevation regions where crop water requirements were low,high precipitation can make the yield potential of DDSR comparable to that of WDSR.（2）Determination of sowing window with the highest yield and lowest CH₄ emissions.In the historical period,with a delayed sowing date,the rice yield gradually increased and then tended to be stable.CH₄ emissions were proportional to the length of growth duration.The delay of sowing date was accompanied by the increased temperature,which resulted in a shortened growth duration.Therefore,CH₄ emissions gradually decreased and then tended to be stable.The optimal sowing window was determined when the lowest CH₄emissions and the highest yields occurred simultaneously.Current planting date was mostly within the optimal sowing window.In the future scenario,except for P＿2 in SSP5-8.5,the potential sowing window and the highest yield window were extended.The potential sowing window can be extended by up to 50 days.The sowing date with the shortest growth period generally occurred at the end of the potential sowing window.The extended potential sowing window led to the delay of the lowest CH₄ emission window and thus the optimal sowing window,and the current sowing date was not within the optimal sowing window.In addition,during P＿2 in SSP5-8.5,the increase of extreme accumulated temperature（KDD）in most areas was more than 400℃day.The rising temperature resulted in high temperature stress,and the highest yield window may appear in the early or late stage of the potential sowing window.The highest yield window often did not overlap with the lowest CH₄ emission window.In this case,the optimal sowing window was determined only by the highest yield window.Generally,the optimal sowing window in the future was later than that in the historical period,and thus,the sowing date should be delayed.（3）Sensitivity of yield and its components to temperature.Grainwt in each agro-climate region did not change with the increased temperature,and the sensitivity of yield to temperature can only be decomposed into the temperature sensitivity of Grain＿no and Pan＿no.The sensitivity of rice yield to temperature was negative in all agro-climate regions.When temperature increased by 1℃,rice yield decreased by 0.35-6.09%,and most regions were controlled mainly by temperature sensitivity of Grain＿no.The sensitivity of Pan＿no to temperature was not consistent in different agro-climate regions,but Pan＿no was positively correlated with accumulated temperature during rice vegetative growth period in most regions.In general,the sensitivity of yield,Pan＿no,and Grain＿no to HDD was higher than that to GDD.The yield change was～0.01%for each additional GDD,but was 0.08-0.27%for each additional HDD.The sensitivity of yield and Grain＿no to HDD was negative,and Pan＿no was positively correlated with HDD.When the temperature increased by 1°C and 2°C,the increased HDD was the main reason for the decreased Grain＿no.The combined effect of GDD and HDD led to the increased Pan＿no.However,the positive effect of temperature increase on Pan＿no cannot offset the negative effect on Grain＿no,leading to a reduction in rice yield.Therefore,breeding rice cultivars with stable Grain＿no when exposed to high temperatures is very important to ensure rice yield under climate change.（4）The relationship between rice supply and demand in Southern China.The conversion of double rice to single rice appeared mainly in Anhui,Hubei,Hunan,and Jiangxi.In Zhejiang,Fujian,Guangdong,and Guangxi,the present rice cultivation area gradually reduced and was very close to the minimum rice cultivation area proposed by local governments.Under SSP2-4.5,the yield of single rice and double rice exhibited an upward trend,and only early rice passed the significance test.However,under the SSP5-8.5,the yield of single rice and late rice presented a significant downward trend,while the yield of early rice still showed a significant upward trend.This can be attributed to the lower average temperature during early rice growing season,which was closest to the optimal temperature for rice photosynthesis.The yield sum of early rice and late rice was higher than that of single rice and was more stable,so double rice was more adaptable to climate change.At the provincial level,the rice supply in Anhui,Hubei,Hunan,and Jiangxi was much larger than the demand in the experiments of cultivation area conversion designed in this study.In Guangxi,supply was close to demand in most cases;in the remaining three provinces,rice supply was less than demand,although demand was gradually decreased.In addition,except for the most populous scenario during 2081–2100,the total rice supply of the eight provinces can meet the demand and exceeded 100 Mt in most cases.Therefore,considering climate change,population growth,and cultivation area conversion,the total rice yields of the eight provinces are generally self-sufficient.The supply could be improved by adjusting the conversion rate of double rice to single rice.By assessing the impacts of climate change on field management practices,crop physiological processes,and rice supply and demand balance,this study provided a reference for the formulation of adaptions to cope with climate change,and also had important significance for rice sustainable production and food security.