First, the natural ecological environment has been destroyed. The development and change of urban ecosystem is accompanied by the process of urbanization. On the one hand, urbanization has indeed created convenient and comfortable living conditions for human beings and met their own needs for survival, enjoyment and development. On the other hand, urbanization has reduced the absolute area of natural ecological environment, making it change and disappear qualitatively in a large area, which plays a more essential role for urban residents. The destruction of natural ecology has caused a series of urban environmental problems, such as heat island effect and lifestyle changes.
Second, land occupation and soil change. The proportion of land occupied by cities is not large, but with the expansion of urban area, the occupied area is getting bigger and bigger, and the growth rate is also accelerating. With the increase of urban building density, large-scale underground buildings, such as drainage systems, have largely prevented rainwater from infiltrating into the soil, which has caused the urban groundwater level to drop. It is impossible for the garbage in industrial cities to return to the land like the plant remains, animal remains, feces and human excrement and urine in rural areas in the past. The quantity and composition of various wastes in cities can not be used to increase the fertility of land, and it has also become a major problem for cities and society. "White garbage" is the most harmful garbage, which is difficult to decompose naturally and will further pollute groundwater and air.
Third, climate change and air pollution. The quality of urban atmospheric environment directly relates to the health and productivity of urban residents. Due to the dense population, developed industry and transportation in the city, a lot of fossil fuels are consumed, and smoke and various harmful gases are produced, which makes the pollution sources in the city too concentrated and the pollution amount is large and complicated. Coupled with the special urban climate, the pollution of urban atmospheric environment is often more complicated and serious. Haze has become a threat to people's survival.
Fourth, water shortage and water pollution. Water is the basic condition for the existence of cities, and many cities are facing the problem of water shortage. At present, the problem of urban water supply has become a particularly acute and prominent restrictive problem in the world. General natural water supply is difficult to solve, and urban fresh water resources are mainly affected by regional precipitation and surface river transit runoff. With the continuous growth of urban economy and the increase of population, people draw a lot of groundwater, which brings many adverse effects to urban water bodies and urban ecosystems. With the shortage of water resources, urban water pollution is becoming more and more serious. Industrial wastewater accounts for about 3/4 of the total urban wastewater, among which the wastewater from metal raw materials, chemical industry, paper industry and other industries is the most polluted. The main pollutant is ammonia nitrogen, followed by oxygen-consuming organic matter and volatile phenol. Domestic sewage, industrial waste residue and mining have also caused some pollution to the water body.
5. Dense population and insufficient green space. Dense population is a common phenomenon in cities, especially in some big cities and megacities. According to relevant data, the average population density of 42 big cities abroad is 79 18 people per square kilometer, among which 14 cities are higher than this value. China's 1990 was 26.2%, while 1998 rose to 30.4%. With the continuous development of China's social economy and the reform of household registration system, the urban population will further increase. It is estimated that the proportion of urban population in China will reach 58% in 2025 and 70% in 2050. The urban population is increasing, while the urban green space is gradually decreasing. The standard of urban green space per capita stipulated by the United Nations is 50 ~ 60m2, and few cities meet or exceed this standard. The per capita green space standard in China is 7 ~ 1 1 ㎡, while in 1993, the per capita green space in major cities in China is 4.2㎡, which is far from the United Nations standard. In recent years, although the area of urban green space in China has increased, the ecological benefits of various sods are not as good as those of trees because of quick success and instant benefit, which is a new problem.
Sixth, the ecological problems in villages and towns are serious. The ecological problems of villages and towns and their influence on urban ecosystem are special problems in the process of social and economic development in China. In the development of urban suburbs, there is no division between factory areas and residential areas; The construction of wastewater and waste treatment facilities was not considered when the factory was built; Pollution transfer from factories in large and medium-sized cities to township enterprises and so on.