Extreme temperature of the mid-eighteenth century as compared to todays in Beijing

The extreme temperature events of the mid-eighteenth century can help understand the background climate conditions of the pre-industrial revolution period. However, studies are lacking mainly due to the scarceness of instrumental observations. Based on the observations made by the French Missionary Amiot in Beijing, a daily maximum and minimum temperature dataset during 1757–1762 was established using relationships developed with reference to modern observations of nearby stations. The first result of a comparison between the extreme temperature change of the mid-eighteenth century and the present situation (2014–2019) in central Beijing city shows: (1) There were mean annual summer days (the number of days with daily maximum temperature > 25 °C) exceeding 130 d, and tropical nights (the number of days with daily minimum temperature > 20 °C) of more than 52 d, as compared to more than 150 d and 80 d respectively at present; (2) The mean annual frost days (daily minimum temperature < 0 °C) and ice days (daily maximum temperature < 0 °C) were 126 d and 31 d, respectively, in 1757–1762, but only 110 d and 10 d, respectively, during 2014–2019; (3) The mean annual warm days (days when maximum temperature > 90th percentile) and warm nights (days when minimum temperature > 90th percentile) were 63 d and 30 d, respectively, compared to the present values of more than 65 d and 55 d, respectively; (4) The annual mean cool days (days when maximum temperature < 10th percentile) and cold nights (days when maximum temperature < 10th percentile) were approximately 52 d and 48 d, respectively, in 1757–1762, but they are fewer than 40 d and 30 d, respectively, today. Therefore, the occurrences of most warm events in the mid-eighteenth century were slightly lower than modern times, and the occurrences of most cold events were more frequent compared with those in modern times, with the decrease of cold events from 260 years ago to today more obvious than the increase of warm events. The rapid warming in Beijing in recent decades, especially in the urban center, has exerted a great influence on the change in extreme temperature indices, which may have been caused by both urbanization and global/regional warming.