Natural disasters have both natural and social attributes. The occurrence of natural disasters is the result of the natural environment changes of the Earth system acting on human society, which clearly means natural disasters are mainly affected by two elements: the influence of natural factors and the capacity of human society to bear or adapt to changes of the natural environment. As long as the variation of natural factors does not exceed the capacity of human society, it will not endanger the safety of human life and property. Therefore, it is very necessary to study the critical conditions of disasters.
Because the material circle of the Earth's surface is the environment which human beings depend on for survival and development, only natural events or forces occurring on the earth’s surface, such as the lithosphere, biosphere, hydrosphere, cryosphere, and atmosphere, may cause natural disasters. Therefore, we must thoroughly study the earth's surface giant system to have a correct understanding of the danger of natural disasters.
In their fight against the nature, to protect themselves, humans have built a lot of disaster prevention projects, such as coastal breakwaters, flood control facilities and so on. As a result of these constructions, the critical conditions of disasters have undergone significant changes, for example, the Three Gorges project on the Yangtze River reduces the flood in the middle reaches of the Yangtze River to once in a century. However, the constructions of these projects also bring some potential negative consequences, such as a large number of reservoir constructions. These reservoirs on the one hand can help guarantee the water safety for people, but on the other hand especially some dangerous weak reservoirs like ticking time bombs threaten people, because once rainfall exceeds their capacity, dam failures will cause severe floods which will devastate the downstream villages and destroy their infrastructures, like the “75.8” heavy rain, which caused the Banqiao Reservoir dam collapse in Henan province, killing more than 20, 000 people. What’s worse, a large number of reservoir constructions have also made northern rivers dried up, causing major changes in groundwater distribution and ecological environment. Besides, unreasonable human reclamation activities have led to soil erosion and desertification, and uncontrolled mining of mines and other unhealthy activities have caused an increase in geological disasters. These all the indisputable negative facts people’s mistaken activities bring to the nature. Therefore, the study of the critical conditions for disasters cannot be done once and for all. It must be constantly being revised based on the changes of the environment and the nature.
Natural disasters can also be divided into sudden and graded disasters according to their attributes. Sudden disasters include volcanic eruption, earthquake, landslide, debris flow, wind, hail, flood, etc. while graded disasters include drought, soil erosion, ground subsidence, desertification, rock desertification, sea level rise, etc. Besides, graded disasters can trigger sudden disasters after they have reached a certain level of risk, for example, the increase of carbon dioxide and other greenhouse gases can cause climate warming especially when the concentration of carbon dioxide and other greenhouse gases accumulates to a certain critical point, it may cause sudden changes of climate, and then that will result in major changes in the intensity and distribution of natural disasters.
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