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Chinese scientists find population outflow makes a city colder

Xinhua, February 6, 2016 Adjust font size:

The mass movement of people out of Beijing for the Chinese New Year (CNY) causes the temperature to drop, research shows.

During the CNY holiday, people leave big cities and return to their hometowns. According to the institute of atmospheric physics (IAP) under the Chinese Academy of Sciences, the mass migration could affect the urban climate.

The man-made urban surfaces such as buildings and roads, and the heat generated by human activity lead to higher urban temperatures, a phenomenon known as "urban heat island (UHI)".

During CNY week, the UHI effects are lessened in large cities, and increase in tourist destinations. In Beijing, the temperature during this week declines by 0.64 degrees Celsius during the day and up to 0.83 degrees Celsius at night.

"The change in temperature, though less than 1 degree Celsius, is comparable with the magnitude of global warming over the past century. " said Zhang Jingyong, lead scientist of the research.

The fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) shows that the average world temperature rose by 0.85 degrees Celsius between 1880 and 2012. Many things have changed with global warming. The sea level has risen, snow and ice cover has diminished, and extreme climate events have become more common.

"The population movement effects on urban climate is a new research topic, and the CNY holiday offers a unique opportunity to study it." Zhang said, "Our research highlights the role of population movements in urban climate, and may help deal with urban environmental problems".

Zhang said lightening the colors of roofs, or switching to white roofs and expanding green areas can help to mitigate UHI effects. Light-colored roofs reflect more solar radiation and reduce energy demand in warm seasons. Trees and grass have cooling effects through increasing evaporation and transpiration, just like sweat evaporating off skin. Low-carbon production and lifestyle can decrease heat emissions from city buildings, factories, and vehicles, and thus constitutes another effective means to mitigate the heating effects. Endi