China’s Lunar New Year exodus cools major cities
The Lunar New Year is a week-long holiday in China when businesses close and people travel to rural areas to be with their families. With about 1.5 billion people in China, this week-long break is the largest short-term suspension in human activity on Earth. On January 25, 2023, the American Geophysical Union reported on a new study led by researchers from Nanjing University that found this mass exodus produced lower temperatures in 31 major Chinese cities.
The peer-reviewed Geophysical Research Letters published the study on October 21, 2022.
Urban heat islands
Scientists have long known that cities retain heat more than rural areas such as farmland. The term for this phenomenon is the urban heat island. It can make cities up to 7 degrees Fahrenheit (4 C) warmer than surrounding areas.
Why are cities warmer? First, because surfaces such as roads and sidewalks absorb more heat than green plants. Additionally, human activities such as transportation, air conditioning and heating, building operations and industrial facilities all emit heat. Author Zihan Liu of Nanjing University said:
This study stresses the importance of anthropogenic heat-generating activities to urban climate, and particularly the urban heat island.
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Temperatures over Lunar New Year
The scientists made their discovery by examining the differences in air temperatures next to the ground. They looked at satellite data and hourly air temperature data from ground-based observation networks spanning 2017 to 2019. What they found was that the holiday exodus reduced the average urban heat island intensity in 31 cities by 33%. This is the same as an average drop in surface air temperature of 0.63 F (0.35 C) in cities.
The effect was much less on the outskirts of cities. This told researchers that city centers are the most susceptible to big swings in heat intensity when there’s a change in human activity. Liu said:
Many people leave megacities, such as Beijing and Shanghai, to travel to small cities and rural areas for traditional family gatherings. This mass human migration from large cities results in abruptly decreased anthropogenic heat-generating activities that noticeably impact urban climate.
So the scientists found that anthropogenic – or human-caused – heating is a major contributor to warming in urban areas. This insight can be useful in mitigating urban heat and protecting vulnerable people who live in city centers during times of extreme temperatures.
An earlier study by Liu and colleagues from January 2022 found similar reductions in urban heat islands during COVID-19 lockdowns.
Bottom line: A study found that when businesses close for a week and people travel to rural areas to celebrate the Lunar New Year, temperatures in large Chinese cities drop.
Source: Urban-Rural Gradient in Urban Heat Island Variations Responsive to Large-Scale Human Activity Changes During Chinese New Year Holiday