Observational data from the past century show a weakening trend in summer monsoon over the Indian subcontinent. This is possibly attributed to the reduced land–sea contrast resulting from the Indian Ocean warming under the rapid increase of greenhouse gases in the atmosphere. In contrast, speleothem records indicate that the Indian summer monsoon was stronger during the last interglacial (LIG) warm period than it is today. Using climate model simulations, we show that orbital forcing effect during the LIG, as well as related ocean feedbacks, led to warming in the Eurasian continent and cooling in the Indian Ocean basin. This amplified the land–sea contrast in the region and intensified the Indian summer monsoon. Although the LIG is often portrayed as a potential analogue of future climate, our study shows that the Indian monsoon responded differently to the LIG warming period than it does to current climate warming.

The Spring Festival is the most important holiday in China, human activity and population mobility may contribute greatly to air quality, especially in the megacities. According to the satellite-based tropospheric nitrogen dioxide (NO) column and ground-based observational concentration of NO in the megacities from 2013 to 2018 around the Spring Festival, we found that NO concentration decreases obviously during the Spring Festival in China, particularly in the megacities. The tropospheric NO columns density around Beijing-Tianjin-Hebei region decreases about 40% than the period before the festival, that in Beijing and Tianjin decreases by 41.6% and 44.5% respectively. Similarly, the other two municipal cities Shanghai and Chongqing decrease by 43.2% and 31.8% in the urban. Consistently, the ground-based NO concentration in the four megacities decrease by 18.9% ~ 38.8%. Besides, the ground-based NO concentration also decreases by above 20% during the Spring Festival vacation in the other eight megacities in China.