Aim Recent changes in crop yields have implications for future global food security, which are likely to be affected by climate change. We developed a spatially explicit global dataset of historical yields for maize, soybean, rice and wheat to explore the historical changes in mean, year-to-year variation and annual rate of change in yields for the period 1982-2006. Location This study was conducted at the global scale. MethodsWe modelled historical and spatial patterns of yields at a grid size of 1.125°by combining global agricultural datasets related to the crop calendar and harvested area in 2000, country yield statistics and satellite-derived net primary production. Modelled yields were compared with other global datasets of yields in 2000 (M3-Crops and MapSPAM) and subnational yield statistics for 23 major crop-producing countries. Historical changes in modelled yields were then examined.Results Modelled yields explained 45-81% of the spatial variation of yields in 2000 from M3-Crops and MapSPAM, with root-mean-square errors of 0.5-1.8 t ha −1 . Most correlation coefficients between modelled yield time series and subnational yield statistics for the period 1982-2006 in major crop-producing regions were greater than 0.8. Our analysis corroborated the incidence of reported yield stagnations and collapses and showed that low and mid latitudes in the Southern Hemisphere (0-40°S) experienced significantly increased year-to-year variation in maize, rice and wheat yields in 1994-2006 compared with that in 1982-93. Main conclusions Our analyses revealed increased instability of yields across a broad region of the Southern Hemisphere, where many developing countries are located. Such changes are likely to be related to recent yield stagnation and collapses. Although our understanding of the impacts of recent climate change, particularly the incidence of climate extremes, on crop yields remains limited, our dataset offers opportunities to close parts of this knowledge gap.

The central Tibetan Plateau contains numerous lakes, the largest and deepest of which is Nam Co. Heat and water budgets over Nam Co were estimated throughout the year. Latent and sensible heat fluxes from the lake are very small from spring to summer and larger from fall to early-winter. Although the land surface over the Tibetan Plateau supplies heat energy to the atmosphere during the premonsoon and monsoon periods, a deep lake such as Nam Co releases a large amount of heat in the post-monsoon period. Annual evaporation from the lake was 658 mm, much larger than the annual rainfall of 415 mm and also more than two times the annual evaporation from the land surface. The diurnal variation of surface air temperature difference over the lake showed a reverse phase of that over land in the monsoon period, a condition that likely affects the convective activity over the lake. IntroductionThe Tibetan Plateau is an outstanding topographic feature in the middle of the Eurasian continent that influences global and regional climate. From spring to summer, the Tibetan Plateau acts as a strong and elevated heat source for atmosphere and plays significant roles in forming the Asian summer monsoon (Luo and Yanai 1984;Yanai et al. 1992;Li and Yanai 1996). In contrast, during winter, the plateau acts as a heat sink (Yanai et al. 1992;Li and Yanai 1996). The heat and water circulations over the Tibetan Plateau are also important for the water resources in the Asian monsoon region, as most of the region's largest rivers (e.g., the Mekong, Brahmaputra, Chang Jiang, and Yellow River) originate from the Tibetan Plateau.Xu and Haginoya (2001) and Xu et al. (2005a) estimated heat and water budgets of the land soil surface over the Tibetan Plateau using routine meteorological data. Their results demonstrated that sensible heat flux (H) supplied from the land surface to the atmosphere increases in the spring pre-monsoon season, whereas latent heat flux (lE) becomes larger in the monsoon period. The annual Bowen ratio (H/lE) is lower in the eastern area of the Tibetan Plateau. These results are also consistent with those of atmospheric heat and moisture budgets (e.g., Luo and Yanai 1984;Yanai et al. 1992;Li and Yanai 1996;Ueda et al. 2003).The Tibetan Plateau contains numerous lakes covering a total area of 44,993.3 km 2 , including more than 1091 lakes with individual areas greater than 1.0 km 2 (Xu et al. 2009). These lakes may influence not only the heat and water circulations over the Tibetan Plateau but also the water resources of Asian monsoon areas. Xu et al. (2009) recently found that the amount of evaporation and its long-term change over Yamdrok Yumtso (one of the three largest lakes of the central Tibetan Plateau) are quite different from those over the land surface during the monsoon period. Despite the importance of the existence of numerous lakes on the Tibetan Plateau, we have little information on their heat and water budgets. Large heat storage by a lake can influence the diurnal variation of heat and water budg...

We evaluated the effect of projected increasing temperatures due to climate change on the yield and quality of rice, as well as the effectiveness of shifting the transplanting date as an adaptation measure, throughout Japan. As an indicator of rice quality, we adopted the heat stress index HD_m26, which is related to the decreased percentage of first grade rice due to high temperature, calculated as the cumulative temperature within 20 days after the heading date. We used a process-based rice growth model to assess the effect. We implemented the model for the period 1981 -2100, and shifted the transplanting dates at 7-day intervals from -70 to +70 days from the standard transplanting date. The estimated yield was categorized into three classes with different degrees of quality degradation risk according to values of HD_ m26. Relative to the current transplanting date, nationwide total production was estimated to increase slightly in most climate change scenarios, although the proportion of production with quality degradation risk may increase with the rise in temperature. It may be possible to avoid this increased risk while maintaining total production by selecting an optimum transplanting date in consideration of both yield and quality. However, a large decrease in yield was found in some areas, suggesting that the current rice producing regions in Japan would become separated into suitable and unsuitable areas as temperatures increase.

Projected global warming is expected to increase the occurrence of heat-induced spikelet sterility (HISS) of rice (Oryza sativa L.). Previous chamber experiments have shown that HISS can occur where temperature at flowering time exceeds the threshold temperature of around 35 . The occurrence of HISS is, however, difficult to predict because the thermal conditions of rice canopy can be different from the air temperature under field condition. To cope with this, we developed a simple micrometeorology model focusing canopy and panicle temperatures; IM PACT was applied to the meteorology dataset based on ANEMOS in order to analyse the Tp at Kanto and Tokai regions of extremely hot summer in 2007. There was a great gap in spatial distributions between the Tp and the daily maximum air temperature which is commonly used as a measure of HISS, because the difference of meteorology, especially relative humidity, among areas altered the panicle-air temperature difference. This strongly suggests that we must refer to the Tp instead of the air temperature in daily maximum, as a measure variable for HISS. The IM 2 PACT is a powerful tool to elucidate the Tp in the climate change impact study to bridge between the responses of crop susceptible to heat and the meteorological data.

Rain-fed lowlands are major agricultural ecosystems used for rice production in Northeast Thailand. Developing a tool to assess the effects of variable water conditions on the regional scale yield is also important to predict the effects of climate change on food supply. To estimate regional yields, we need a simple but accurate measure of the crop calendar (i.e., the distribution of planting dates), which has a strong influence on grain yield. In this article, we modeled the dependence of the crop calendar on rainfall patterns based on a survey of the region's farmers as a part of an effort to provide a stronger basis for regional yield estimates. Our survey, conducted in 11 provinces for 2 years, confirmed the existence of large windows for sowing and transplanting versus narrow windows for heading and harvesting for rain-fed lowland rice culture in all the provinces. Variable water, soil, and toposequential conditions in the paddy fields were responsible for the large sowing and transplanting windows, whereas the use of photoperiod-sensitive varieties explained the narrow windows for heading and harvesting. The crop calendar was well expressed as a function of cumulative precipitation from June onward. When the crop calendar model was combined with a simple phenologybased model that uses growing degree-days adjusted by a day-length factor, we could estimate the rice crop calendar under rain-fed lowland conditions with acceptable accuracy. The model described in this article will be combined with a crop growth model to improve regional yield estimates for rain-fed lowland rice.