# Modeling the Impact of Atmospheric Warming on Staple Crop Growth in China in the 1960s and 2000s
## 4 Discussion  
Climate warming had been perceived to shorten the duration of crop growing period provided that crops had constant thermal requirements. In multiple cropping systems, such  
Figure 4: Relationship between the change in \(\mathrm{T_{O}}\) days and aboveground biomass (AGB). as wheat/maize, wheat/rice and rice/rice rotations, the crops were planted and harvested subsequently in a year-long period. The crop phenology changes were restricted not only by climate warming but also by the crop rotation systems at a specific location. In single cropping systems (Hailun and Sanjiang of the present study), warming has the effect of potentially prolonging the growing duration of crops [6]. In this study, our attention was on crops' physiological responses to temperature changes in their vegetative and reproductive stages. The fixed crop phenology assumption was only for simplicity of the accumulated temperature calculation when we faced complicated crop phenology changes at different locations. We were aware that taking into consideration the phenology changes would make the calculation of the accumulated temperature more accurate. However, the possible changes of the planting date of the first crop (e.g., wheat in the wheat/maize rotation) and the harvesting date of the second crop (e.g., maize in the wheat/maize rotation) occurred on dates of low temperatures. Incorporation of the phenology changes makes no obvious differences in the accumulated temperatures. Compared to previous research results, it can be found that the response to climate warming of the three staple crops was qualitatively consistent with them [39; 40]. Taking the rice in Northeast China for example, climate warming contributed 23.2%-28.8% to the increase in rice yield in Heilongjiang province [39]. The yield of winter wheat in Henan province from 1961 to 1981 was also significantly correlated with climate warming, and in 1991-2000 and 2001-2007, the increment of yield due to temperature increase accounted for 15.6%-20.7% of the actual increment [40]. Crop adaptability to local temperatures in the two periods resulted in regional differences in crop photosynthetic productivity. Relative to the 1960s, the temperature distribution of crops growing periods in each rotation area shifted towards high-temperature regions in the 2000s (Figure 2), but significant differences in the adaptability of different crops to environmental temperature changes were found. Climate warming was generally believed to reduce yields when they exceeded an optimum level, but they can increase crops yields in cool regions where temperatures were well below the optimum level [1; 2; 36]. The temperatures in South China have been high in the past. Further increase in temperature should lead to more high temperature days of above 35 \({}^{\circ}\)C during the growth period, placing more high temperature stress on rice growth [41]. Thus, the positive effect of climate warming on crops in this region was not obvious. Temperatures in winter wheat planting areas of North and Northeast China increased significantly as well as the number of days with optimum temperatures. As temperature increases mainly occurred in the winter and spring [42] and the winter wheat growth period crossed the two seasons with the most obvious climate warming, the observed increase in temperature before heading contributed significantly to the AGB increase for winter wheat (Table 4). Our simulations did not consider adaptive adjustments and crop variety changes, which may potentially introduce uncertainties when assessing impacts of climate warming. In terms of adaptation to global warming, for example, changing sowing dates or cultivars has been suggested by several studies to compensate for negative impacts of increasing temperatures [43]. Over recent decades, there have been many changes in agronomic activities, including the use of renewed cultivars [44; 45], optimized fertilizers [46], expanded irrigation [47], and prolonged growing periods [48].