The Possible Effects of Global Warming on Cropping Systems in China I. The Possible Effects of Climate Warming on Northern Limits of Cropping Systems and Crop Yields in China

Objective: Climate change has now become a global problem. Under the background of global climate change, the mean annual surface air temperature in China had significantly increased in recent 100 yr, with the amplitude of 0.5-0.8°C. In recent 50 yr, mean annual temperature in northern China showed the maximum increase amplitude, and increasing temperature in China significantly since the 1980s has become a consensus in the context of global climate change. Precipitation in North China and the south of Northeast China decreased. How the change affected the agriculture or even the cropping system has attracted more and more attention of the government and scientists. In this study, the possible effects of climate warming on the countrywide northern limits of cropping systems, the northern limits of winter wheat and double rice, and the stable-yield northern limits of rainfed winter wheat - summer maize rotation in China from 1981 to 2007 were analyzed. And the possible changes of crop yield due to the changes of planting limits during the period from 1950s to 1981 and the period from 1981 to 2007 were compared and discussed. Methods: The meteorological data was obtained from the national meteorological database, rejecting some relocation stations during the period of observation, more than 40-yr-daily climate data involving 666 meteorological stations (not including Taiwan of China) from the station construction years (the 1950s) to 2007 was chosen in China. The recognized calculation methods of agricultural climatic indices were employed. According to the indices of climatic regionalization for cropping systems, the national northern limits of cropping systems, winter wheat and double rice, and the stable-yield northern limits of rainfed winter wheat-summer maize rotation during two periods, including the 1950s-1980 and 1981-2007, were drawn with ArcGIS software. When analyzing the variation of crop yields in the regions where cropping system limits might change, the authors preferred to compare the change of per unit area grain yield in the subject planting model after the cropping system (from single cropping system to two crops a year, or from two crops a year to three crops a year) or crop (from spring to winter wheats) had changed. We used the average statistical yield during 2000-2007, which could most represent the present climate condition, from the statistical yearbooks in each province, since the average yield in each province could represent the average condition of a certain crop locally. Results: (1) As the temperature rises, the accumulated temperature increases, the planting northern limits of both double cropping system and triple cropping system during1981-2007 moved northward in different degrees, compared to that during the 1950s-1980. As to the planting boundary of double cropping system, the region which had the maximum northward move amplitude turned to be Shaanxi, Shanxi, Hebei, and Liaoning provinces and Beijing municipality. It moves averagely 26 km northward in Shanxi and Hebei provinces. In the south of Liaoning Province, the double cropping area expands from the original small region in 40°1'-40°5'north latitude to the Suizhong- Anshan-Yingkou-Dalian zone; the demarcation line of double cropping area and triple cropping area based on the climate data in period II shows the maximum spatial displacement in Hunan, Hubei, Anhui, Jiangsu, and Zhejiang provinces. The demarcation line leaps from Hangzhou in Zhejiang Province to Dongshan Town, Wu County, in Jiangsu Province, moving around 103 km northward. In Anhui Province, the demarcation line near Chaohu and Wuhu cities moves 127 km northward, and the demarcation line in other regions moves averagely 29 km. To the east of Zhongxiang in Hubei Province, the demarcation line moves 35 km northward. Near by Yuanling in Hunan Province, the demarcation line moves 28 km northward. Without considering variety change and social economic factors, the per unit area grain yield of main planting patterns would increase about 54-106% if single cropping system was replaced by double cropping system, which turned out to be 27-58% if double cropping system was replaced by triple cropping system. (2) In Liaoning, Hebei, Shanxi, Shaanxi, Gansu, and Qinghai provinces, Inner Mongolia and Ningxia autonomous regions, China, the northern limits of winter wheat during 1981-2007 moved northward and expanded westward in different degrees, compared with those during the 1950s-1980. Taking Hebei Province as an example, the northern limits of winter wheat moved northward, and the per unit area grain yield would averagely increase about 25% in the change region if the spring wheat was replaced by winter wheat. (3) The climate warming also caused the northward move of planting northern limit of double rice in different degree; the change regions were mainly located in Zhejiang, Anhui, Hubei, and Hunan provinces. If the rice-wheat rotation pattern was replaced by double rice planting pattern, just from the perspective of heat resource, the per unit area grain yield would possibly increase 13.8, 12.2, 1.8, and 29.9% in Jiangsu, Anhui, Hubei, and Hunan provinces, respectively. (4) The stable yield northern limit of winter wheat-summer maize rotation moved southeastward in most area, which was caused by the local precipitation decrease during the recent 20 yr. Conclusions: During the past 50 years, the climate warming caused the northwards movement of the northern limits of cropping system, and the northern limits of winter wheat and double rice. The changes may increase the unit grain yield in the changing area, if only considering the effect of increased heat resource. However, the stable-yield northern limits of rainfed winter wheat–summer maize rotation moved southeastwards due to the decreasing rainfall.