In the context of global warming, the problem of urban heat islands has become one of the "urban diseases" that cannot be ignored. The team of researchers Meng Qingyan of the National Engineering Laboratory for Remote Sensing Satellite Applications of the Institute of Aerospace Information Innovation of the Chinese Academy of Sciences has recently made progress in the research of thermal infrared remote sensing. Thermal infrared remote sensing is the use of spaceborne or airborne sensors to collect and record thermal infrared information of ground objects, which can provide rapid and effective technical support for urban surface temperature monitoring. The results of the research were published online in the journal Ecological Indicators.
So, what is the "body temperature" of China's major urban agglomerations? How has it changed? What are the reasons for the change? In this end, I asked the research team members Wang Zi'an and Meng Qingyan to interpret it for the readers.
The principle of thermal infrared remote sensing is that all substances, as long as their temperature exceeds absolute zero, will continue to emit infrared energy. It is not only related to the surface state of matter, but also a function of the internal composition and temperature of matter. Thermal infrared remote sensing is the use of spaceborne or airborne sensors to collect and record this thermal infrared information of the feature, and use this thermal infrared information to identify the feature and invert surface parameters such as temperature, humidity, etc.
In order to effectively monitor the spatio-temporal changes of surface temperature in China's urban agglomerations, the research team used remote sensing data to analyze the spatio-temporal evolution characteristics of surface temperature in China's urban agglomerations from 2000 to 2015, and found some changes in urban thermal environments.
To know what is to know is to know why it is so. The urban thermal environment is a composite system, in order to find the "cause" of urban high temperature, it is necessary to start from the natural environment, social economy and other elements to explore the influence mechanism of surface temperature. Our study selected influencing factors from four aspects: urban greenness, grayscale, humidity and social economy, and selected a total of 10 surface temperature influencing factors, covering natural and socio-economic factors.
(1) Beijing-Tianjin-Hebei urban agglomeration
"Body temperature" variation characteristics
From 2000 to 2005, the areas where the vigorous development of ports or the rise in temperature in coastal areas were significantly concentrated in the southwest and east, such as the south of Handan City, the west of Shijiazhuang and the south of Tangshan City. Especially in the coastal areas of southern Tangshan, eastern Langfang and northeastern Cangzhou, the surface temperature has risen significantly, which may be related to the vigorous development of ports.
The southward migration of industrial and mining enterprises before the 2005-2010 Olympic Games brought about a significant change in the cooling surface temperature of Beijing. The surface temperature of some cities in Beijing and its surrounding areas has not increased but has declined, while the surface temperature of southern Beijing-Tianjin-Hebei cities such as Hengshui, Xingtai and Shijiazhuang has risen significantly, which may be related to the southward migration of industrial and mining enterprises before the 2008 Beijing Olympic Games.
From 2010 to 2015, the surface temperature changes showed the characteristics of north rise and south decline, of which Hengshui increased the most, and Beijing, Langfang and other places also began to rise from decline to rise. Southern cities, such as Handan and Xingtai, began to see their surface temperatures drop. Surface temperatures in downtown Shijiazhuang were high in 2000 and then gradually weakened by 2015. Xingtai and Handan are also particularly pronounced, with the range of high temperatures gradually narrowing between 2000 and 2015.
Impact factor analysis
Green space can significantly reduce surface temperatures
The surface temperature of Baoding, Xingtai and Shijiazhuang is greatly affected by the greenness factor, and the distribution results of the surface temperature show that the surface temperature of the vegetation area is significantly reduced, so green space is an acceptable mitigation strategy, but the focus of different areas of the city is different.
The contribution rate of socio-economic factors in Beijing and Shijiazhuang is relatively high, which may be related to the high economic efficiency of urban land use in Beijing, Shijiazhuang and other cities with advanced manufacturing and modern service industries as pillar industries.
The influence of humidity factor in Tianjin ranks first, which may be related to the development of the water system in Tianjin.
Over time, the greenness factor has gone through a process of rising first, then falling, and then rising again from 2000 to 2015. The rise in the later period is closely related to the contribution of ecotourism cities with high vegetation coverage, such as Qinhuangdao, Chengde and Zhangjiakou.
For the influence of road and railway factors, there is an overall upward trend. The development of transport networks can lead to the formation of new land-intensive areas, which can lead to higher surface temperatures.
Spatial distribution map of surface temperature in the Beijing-Tianjin-Hebei urban agglomeration. (Number unit in legend: Celsius)
(2) Yangtze River Delta urban agglomeration
From 2000 to 2005, the areas with the increase of surface temperature increased with the expansion of urbanization, and the areas with obvious changes in surface temperature were mainly concentrated in Shanghai and its surroundings, showing a trend of outward expansion with Shanghai as the center, such as the southern part of Nantong City, the southeastern part of Suzhou City and the northern part of Jiaxing City. However, there is a large cooling trend in the southern and northwestern parts of the Yangtze River Delta urban agglomeration, such as the southwest of Taizhou City, the central and northern parts of Chuzhou City. This shows that in the process of urbanization development of the Yangtze River Delta urban agglomeration, Shanghai and its surrounding cities have developed rapidly, and the surface temperature has increased with the expansion of urbanization.
It is worth mentioning that the surface temperature in the northern section of the Yangtze River has increased significantly, which may be related to the establishment of a high-tech park in Nanjing in 2001, which has increased the number and scale of chemical enterprises.
From 2005 to 2010, the spatial distribution characteristics of surface temperature changes in Shanghai did not increase but fell. The surface temperature of Shanghai has not increased but has declined, and Suzhou and Jiaxing cities around Shanghai have also shown a certain degree of cooling trend. The rest of the Yangtze River Delta cities except these three cities show a certain heating area.
From 2010 to 2015, the surface temperature change changed to the south-descending and north-rising surface temperature change distribution characteristics were converted to the south-descending and north-rising characteristics. Among them, Yancheng and Chuzhou in northern Jiangsu have the highest heating range, while Jinhua and Taizhou in southern Zhejiang have the highest cooling range, which may be caused by the closure and relocation of chemical polluting enterprises, while Yancheng, Chuzhou, Shanghai and other heating ranges are higher, and special attention should be paid to taking more effective measures to alleviate the urban local thermal environment caused by the relatively high proportion of secondary or tertiary industries in these cities.
Water bodies are key to affecting surface temperatures
The contribution rate of the greenness factor in Jiaxing, Ma'anshan and Zhenjiang cities was higher, indicating that the surface temperature was more affected by greening, while it had a smaller impact in Anqing, Chizhou and Yangzhou.
Humidity factors in Shanghai, Ningbo, Shaoxing and Suzhou are higher contribution rate, related studies also show that water is the key factor affecting the surface temperature, in Shanghai, water can reduce the surface temperature by 3.32 °C.
In coastal cities, such as Nantong and Yancheng, the contribution rate of socio-economic factors is relatively high, indicating that the surface temperature of coastal cities is greatly affected by social economy.
Spatial distribution map of surface temperature in urban agglomerations in the Yangtze River Delta. (Number unit in legend: Celsius)
(3) Guangdong, Hong Kong and Macao urban agglomerations
From 2000 to 2005, the warming of surface temperature mainly occurred in the northern region, and the warming trend of surface temperature mainly occurred in the northern high-latitude areas, especially in the eastern cities of Shanwei and Heyuan, which not only increased greatly, but also covered a wide range. This may be related to the rapid expansion of major urban areas in urban agglomerations and the continuous increase in areas with high surface temperatures, which has almost merged the main urban areas. The cooling trend mainly occurs in the areas where the subtropical broad-leaved forest is located, such as the Yangjiang River in the west and Dongguan in the south-central part, and the surface temperature has decreased to a certain extent.
From 2005 to 2010, the trend of surface temperature change was that the upward trend of surface temperature in the south and the decline in the north began to slow down, and the rising areas were mainly concentrated in the southwest region, such as Yangjiang City and Jiangmen City. In some parts of the northwest, including Qingyuan and Zhaoqing, there was a significant decrease in surface temperatures.
From 2010 to 2015, the reduction of deforestation slowed the warming trend The overall change in surface temperature began to change from rising to decreasing, and the surface temperature of most cities in the southwest was mainly decreasing, while some cities in the northwest, such as Qingyuan and Guangzhou, began to have an upward trend. This is due to reduced biomass burning due to forest changes. The region is characterized by a historical mass deforestation that produces large amounts of aerosols, leading to a warming and drying of the climate. However, deforestation has decreased in the last decade, which has helped to reduce aerosol emissions and slow warming trends.
The decline in urban forest cover has a great impact
Due to the need for space, cities such as Foshan, Guangzhou and Zhuhai have witnessed a huge shift in forest area to urban land, reflecting significant differences in the impact of surface temperatures.
In Zhongshan City and Zhuhai City in the Pearl River Economic Circle, the contribution rate of socio-economic factors is higher, while the contribution rate of greenness factors is lower, which may be related to the decline in urban forest coverage. The remaining forest fragments are mainly concentrated in coastal hilly areas.
Dongguan City and Shenzhen City in the Shenzhen-Guanhui Economic Circle showed a more consistent distribution law, with a higher contribution rate of humidity factors and lower socio-economic factors.
For Foshan and Guangzhou in the Guangzhou-Foshan-Zhaozhou economic circle, the contribution rate of each factor is relatively consistent.
Spatial distribution map of surface temperature in guangdong-Hong Kong-Macao urban agglomeration. (Number unit in legend: Celsius)
(4) Chengdu-Chongqing Urban Agglomeration
From 2000 to 2005, the surface temperature change of the southeast of Chongqing City showed a large degree of heating trend, mainly showing "center rise and two sides falling". There has been a large degree of warming trend in the western and southeastern parts of the Chengdu-Chongqing urban agglomeration, such as the southeast of Chongqing City and the western part of Ya'an City.
From 2005 to 2010, some cities in the north showed a certain cooling trend, and the surface temperature of the southern cities showed a warming trend, especially in Cities such as Luzhou and Yibin. Some northern cities such as Deyang city and Dazhou city show a certain cooling trend.
From 2010 to 2015, the cooling amplitude of western Chongqing showed the distribution characteristics of "southward decline and north-rising". Among them, Mianyang and Nanchong in northern Sichuan have the highest heating amplitude, which may be related to the existence of urban agglomeration effects or terrain that hinder heat dissipation, while Luzhou and western Chongqing in southern Sichuan have the largest cooling amplitude.
The contribution rate of socio-economic factors in the metropolitan area is high
The contribution rate of the greenness factor in Neijiang City, Luzhou City and Nanchong City was higher, and the influence on surface temperature was larger, while the influence on Deyang City, Chongqing City and Mianyang City was smaller.
In Chengdu, Deyang and Mianyang in the Chengdu metropolitan area, the contribution rate of socio-economic factors is higher, and the contribution rate of greenness factors is lower.
Spatial distribution map of surface temperature in Chengdu-Chongqing urban agglomeration. (Number unit in legend: Celsius)