High Temperature Risk And Its Possible Effects On Rice Yield Under Climate Change

China is the world’s largest rice producing country, in which rice acreage ranks two, and yield first. Under the background of the global warming and extreme events increasing, both intensity and frequency of the high temperature stress during rice growing season have increased in the most rice producing area of china in the recent years, which have affected rice production seriously. In this study, based on meteorological data in 1961-2010, observed rice data in 1981-2009 and future climate data under RCP2.6,RCP4.5,RCP8.5 scenarios in 2021-2050, past 50 years and next 30 years’ temporal and spatial variation of the frequency and intensity of high temperature stress during rice temperature sensitive period(booting stage to milk stage) were analyzed. Besides, empirical regression equation were built with high temperature factor and rice yield data, were used to estimate the probability of mild(L1), moderate(L2), and severe(L3) yield reduction under 3 RCP scenarios in 2021-2050, to understand the historical and near future changes of high temperature riks and its impact on yield, which may provied reasonable arrangements for rice production, and provide methods for yield loss estimation due to high temperature stress. The preliminary conclusions are as follows:(1) In 1961-2010, HSD(acumulated days of Tmax which above 35℃ and last for more than 3 days) and HDD(acumulated Tmax which above 35℃ and last for more than 3 days)during rice temperature sensitivity period in major rice producing area were 1.3d and 2.0℃·d, respectively. Regions with high values of HSD and HDD mainly located in the middle and lower reaches of Yangtze River. In the last 50 years, both HSD and HDD decreasd before 1980 s and increasd after1980 s, without significant trends overall. For the spatial changes, HDD reduced in the central part of the north Yangtze River region(I), and increased in Chongqing and Zhejiang Province. Interannual fluctuations is high in southern rice region(I,II,III), while low in Yunnan-Guizhou Pleato(IV) and Northeast China(V). Trend of HDD in each month during rice growing season varied across regions, in the south Yangtze River basin(II), HDD in July increased fastest, with the trend of 1.2℃·d/10 a, followed by August in South China, HDD increased by 0.8 ℃? d / 10 a, than June to July in the Northeast China,While in the north Yangtze River region(I) and Yunnan-Guizhou Pleato(IV), HDD decreased in in July and August.(2) In the near future(2021-2050), frequency and intensity of high temperature stress during rice sensitive period increased considerably. HSD increased 1.08,1.23 and 1.49 d and HDD increased 3.89,4.19 and 4.81 ℃? d under RCP2.6, RCP4.5 and RCP8.5 scenario, respectively. The largest increas of HSD and HDD located in the Northeast China, the most places of Yangtze River basin and the west of South China, overall with zonal distribution of northeast to southwest. The smallest increase mainly distributed in the southeast coastal areas and the Southwest China. Due to differences of heat resources in baseline, continuous days changes differently in each region under RCP scenarios. In Yangtze River basin(I, II), a growing number of high temperature events which Tmax above 35℃ sustained over 5 days and a slight decrease of events which last only 1-2 days. In South China(III), frequency of high temperature events which last for 3-5 days took the lead performance. In Northeast China,high temperatuer events lasted for 1-5 days and more than 8 days were both raised. While in Yunnan-Guizhou Pleato, high temperature events in each level reduced sequentially under the scenarios of RCP2.6, RCP4.5 and RCP8.5. In the past 50 years, rice high temperature center was located in the north of Hunan province, and move forward to the south, north and then the east in turn. While in 2021-2050, the center located in the northeast of its origional place in baseline, and moved towards south, south and then north, and directly north, respectively in the future 30 years, which showed that high temperature risk increase more in the north under RCP2.6, RCP4.5, RCP8.5 scenarios.(3) Multiple regression results showed that the most sensitive period for rice to high temperature stress was booting and flowering stage, then followed by filling stage and the whole growing period. Except for Northeast China which showed significant quadratic relationship between the yield reduction rate and SDD32, yiled loss rate in other regions all showed significant negative linear correlation with HDD.The yield reduction rates(H) and the thresholds of HDD(M) increase from south to the north, which is the largest in north Yangtze River basin(I). In the baseline, high temperature induced yield reduction is largely in mild level reduction(L1) with a risk of 4.8%, and the moderate(L2) and severe reductionlevel(L3) were 2.4% and 0.05%, respectively. In the near future, except for Yunnan-Guizhou Pleato, high temperature risk would increase in all other regions. Besides, Moderate reduction(L2) risk increase most, then followed by severe(L3) and mild(L1) reduction risk. For mild and moderate reduction(L1, L2), risk mainly increased in South China and north Yangtze River Basin(I), and for severe yield reduction(L3), risk mainly increased in Northeast China.