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As a kind of renewable energy, the development and utilization of solar energy is valued by many countries. Sunshine duration (SD), as an important factor to measure solar energy, has also been widely discussed as relevant in terms of distribution and variation. The spatial patterns and variation trends in SD and related factors (wind speed, precipitation, relative humidity, mean temperature and elevation) over mainland China have been studied based on data from 569 meteorological stations during 1961-2016. The results indicated that annual SD decreased significantly at the rate of −40.7 h/10a over mainland China and the decline trend was the most pronounced in the 1980s. Seasonally, the decline rate in SD was the largest in summer (−16.8 h/10a), followed by winter (−9.9 h/10a), autumn (−9.5 h/10a) and spring (−4.5 h/10a), respectively. Spatially, the decline trend in SD was significantly higher in the eastern region than in the western region during 1961-2016, especially in North China. SD was positively correlated with wind speed (R = 0.76); however, it was negatively correlated with mean temperature (R = −0.60) and precipitation (R = −0.41). Moreover, altitude and population density may affect the values and variations of annual SD over mainland China. This study provides a new perspective for the reduction of SD in mainland of China. The drastic changes in SD, such as abrupt changes and sudden decreases, were closely related to volcanic eruptions. Among them, the mean mutation and sudden decrease of SD in the 1980s were due to the long-time weakening of the aerosol accumulated by multiple volcanic eruptions. After the volcanic eruptions in the early 1990s, volcanic aerosols were gradually dissipating, resulting in a small rebound in SD.

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Compound droughts and heatwaves have caused serious impacts on human activities, social economy and ecological environment. Droughts are usually caused by precipitation shortage. Recent studies found that with the continuous global warming, the impact of temperature rise on the compound events of drought and heatwave is further expanding, and it is urgent to carry out research on the temporal and spatial characteristics of compound droughts and heatwaves. The present study examined changes in compound droughts and heatwaves in Huai River Basin (HRB) of China during 1961–2017. The result shows that both frequency and duration of compound droughts and heatwaves increased from 2003–2017. Compound events appeared more often in summer. The more frequent and longer duration of compound droughts and heatwaves occurred in the north of HRB. The results highlight the increased trend of compound droughts and heatwaves and call for improved efforts on assessing the impact of compound extremes.