2. Destruction of ozone shielding

Ozone (O3) is a derivative of oxygen. There is a trace amount of ozone in the natural atmosphere, and its concentration changes with altitude. The ozone concentration is the largest in the stratosphere (the layer of the atmosphere 20 to 25 km above the horizon). Ozone distributed in the stratosphere absorbs 99% of the high-energy ultraviolet rays in sunlight that are extremely harmful to the earth's biosphere. Measurements show that from 1978 to 1987, global ozone concentration decreased by 3.4% to 3.6% on average; an ozone hole was observed over Antarctica in 1985. Evidence suggests that the primary cause of ozone shielding damage is photochemical reactions of chlorofluorocarbons emitted into the atmosphere by human activities. The destruction of ozone shielding will definitely have a catastrophic impact on the earth's life systems and the human ecological environment.

The concentration of ozone in the atmosphere changes with altitude. The ozone concentration in the stratosphere is the highest between 20 and 25 kilometers above the ground. The concentration of chlorofluorocarbons before the industrial revolution was 0, but now it is 1×10-9. Substances containing chlorofluorocarbons and nitrogen oxides emitted by human activities will reduce ozone through photochemical reactions in the stratosphere. Since the 1970s, ozone in the Northern Hemisphere has decreased by 3% to 5.5%, causing an ozone hole. The Antarctic ozone hole is growing at a rate equivalent to the land area of ??the United States every year. Not only over the Antarctic, but also recently over the Arctic and Tibetan Plateau, the ozone layer has been thinning and even ozone holes have appeared. Ozone is expected to decrease by more than 53% by 2014. A hole in the ozone layer will increase ground ultraviolet radiation and increase the incidence of skin cancer. It will also bring about a series of impacts such as increased mortality of young fish and livestock plagues, reduced grain production, and climate change.

From a scientific perspective, these pressing environmental issues involve the interaction of various parts and layers of the earth, and the habitability of the earth as a planet. Understanding and predicting changes in the earth's environment is a serious challenge facing scientists around the world.

3. Land desertification

In order to obtain food, the area of ??cultivated land expanded from 400 million hm2 to 1.5 billion hm2 in 300 years, with cultivated land and pasture accounting for more than 30% of the land area. On the other hand, the forest area decreased sharply, from about 7.6 billion hm2 5000 years ago to 5.5 billion hm2 in 1860, to 2.6 billion hm2 in 1975, and to 2.3 billion hm2 in 1986. Currently, forests are disappearing from the earth at a rate of 11 million hm2/a. The direct result is desertification of the land. At present, desert area accounts for 10% of the total land area, and 43% of the land is facing the threat of desertification.

According to preliminary estimates by the United Nations Environment Program (IIED1987): desertification threatens 48 million hm2 of land, accounting for approximately 1/3 of the world's surface soil area, and affects the lives of at least 850 million people. In the early 1980s, of the 3.257 billion hectares of productive dry land in the world, approximately 1.986 billion hm2 suffered from desertification and severe desertification, accounting for approximately 61% of the productive dry lands. Land desertification has greatly changed the physical characteristics of the land surface, destroyed the surface radiation budget, and induced climate and environmental changes. The feedback effect of climate and environmental changes will further affect the process of land desertification, and so on, thus having a profound impact on the global environment. It can be seen that land desertification has become another major global environmental problem.

4. Destruction of the ecological environment

The population explosion and human activities have destroyed the earth's ecosystem in many ways, directly or indirectly, and many species of creatures are on the verge of extinction. Although it is not entirely clear how changes in species distribution and the extinction of specific species will affect humans, it is a sign that the habitability of the earth as a place for human life activities is facing increasingly serious problems. .

1. Increasing environmental pollution

The gases emitted into the atmosphere globally every year include 5.7 billion tons of CO2 and about 200 million tons of CH4. Emissions of harmful metals include 2 million tons of aluminum, 78,000 tons of arsenic, 11,000 tons of mercury, and 5,500 tons of cadmium, which are 20 to 300 times higher than the natural background value. The emission of SO2 is increasing the frequency and expanding the area of ??acid rain; the air quality has seriously declined, and 800 million people around the world live in air-polluted cities; the pollution of rivers, lakes and seas is becoming increasingly serious, and the lack of fresh water makes the lives of 1.2 billion people In water-scarce cities, 1.4 billion people live without wastewater treatment facilities; disease mortality caused by water pollution has become the most important hazard to human health; urban garbage, sewage, ship waste, oil and industrial pollution, radioactive waste, etc. are pouring into large quantities. Every year, 20 billion tons of pollutants enter the ocean from rivers, and about 5 million tons of garbage are thrown into the ocean. The ozone layer covering tens of thousands of square kilometers at the mouth of the sea is expanding.

2. Severe forest loss and species extinction

The world of biological diversity is experiencing a serious crisis. Research shows (IIED, 1987) that before interference from human activities, there were approximately 6 billion hm2 of forests and woodlands in the world. By 1954, the world's forest and woodland area had decreased to 4 billion hm2, of which temperate forests decreased by 32% to 33% and tropical forests decreased by 15% to 20%. In the past 30 years, the decline of the world's forests, especially tropical forests, has accelerated significantly, with an average annual reduction of 8 million hm2.

In Central America, it dropped from 115 million hm2 in 1950 to 71 million hm2 in mid-1983. African forests declined faster, from 901 million hm2 in 1950 to 690 million hm2 in 1983.

The continuous reduction of the world's forests directly leads to the loss of biological diversity and species extinction. It is estimated that there were once 500 million species on the earth, and there are still 5 to 10 million species, the overwhelming majority of which are invertebrates and plants (IIED, 1987). Some experts speculate that thousands of species are currently disappearing every year.

The sharp decline of forests and the massive reduction of biological species will have a huge impact on human society and economic development. In particular, the massive reduction in forest vegetation has greatly changed the distribution of sources and sinks of trace elements such as carbon and nitrogen, disrupting the cycle of trace elements in the earth system and forcing it to move from its original equilibrium state to a new equilibrium state. transition, thus bringing huge impacts to human society and natural ecosystems.

3. Shortage of fresh water resources

According to information provided by IIED, in 1987, of the approximately 14 billion m3 of water in the world, there were approximately 420 million m3 of fresh water, accounting for approximately 3% of the global water volume. , of which about 77.2% is cold stored in ice caps and glaciers, 22.4% is groundwater and soil water, and about 0.4% is lake, swamp and river water.

As a result of the water cycle, the global distribution of water is extremely uneven. From the perspective of crop water requirements, most of Africa, the Middle East and Central Asia, the western United States, northwestern Mexico, parts of Chile and Argentina, and Australia are all water-poor areas, where annual evaporation exceeds annual rainfall. On the other hand, since the 20th century, the world's water consumption has increased significantly, with annual water consumption increasing from approximately 400 billion m3 in 1990 to 3 trillion m3 in 1995, an increase of 6.5 times. By 2000, global freshwater consumption had reached 6 trillion m3. Currently, 43 countries and regions in the world are suffering from water shortage, accounting for 60% of the global land area. About 2 billion people are water-stressed, and 1 billion people do not have access to good drinking water.