With the development of modern industry, the carbon dioxide emitted by factories and automobiles is increasing. This is the main reason for the greenhouse effect on the earth at present. Therefore, carbon dioxide is often regarded as "waste" and even a dangerous "enemy". In fact, carbon dioxide has made great contributions to human beings and other creatures.

Carbon dioxide is an essential nutrient for photosynthesis and growth of green plants. Chlorophyll in leaves can change wonderfully under sunlight, transforming sulfur dioxide absorbed by leaves and water transported by roots into sugar, starch and oxygen (photosynthesis), and CO2+6H2O → C6H 12O6+6O2. Experiments show that the increase of CO2 concentration in the air can improve crop yield, so CO2 is called "gas fertilizer".

Carbon dioxide can store grains, fruits and vegetables. Due to the inhibition of hypoxia and carbon dioxide itself, food stored with carbon dioxide can effectively prevent the growth of bacteria, molds and insects in food, avoid deterioration and the generation of peroxide harmful to health, and preserve the original flavor and nutritional components of food. Carbon dioxide will not cause drug residues in grains and air pollution.

Solid carbon dioxide is usually called dry ice. It is not only commonly used for the freezing and preservation of meat and other foods, but also for artificial rainfall. When the air contains a lot of water vapor, but it cannot be condensed into raindrops due to the lack of condensation nuclei, dry ice is scattered into the air by airplanes. Due to the rapid gasification and heat absorption of dry ice, the surrounding temperature suddenly drops to form many small ice crystals, and water vapor can condense into water droplets and rain.

Carbon dioxide is denser than air, so it can neither burn nor support combustion, and can be used to put out fires. However, some highly active metals, such as magnesium bars, can continue to burn after being ignited in carbon dioxide: 2 mg+CO2 → 2 MgO+C, so it is conditional that we usually say that carbon dioxide cannot support combustion.

The colorless and odorless gas carbon dioxide has a strange temper. If its concentration in the air exceeds 5%, it will stimulate people's respiratory central nervous system, double people's breathing ability and make people feel uncomfortable. Once inhaled with high concentration of carbon dioxide, some people will feel dizzy, palpitation, sudden delirium, fainting and coma. If not rescued from the scene in time, brain edema and pulmonary edema may occur. He died of respiratory and circulatory failure. Carbon dioxide may accumulate in caves, dry wells and cellars, and people will suffocate by mistake. Therefore, people should conduct an open flame test before going into a deep well, because carbon dioxide does not support combustion. If the open flame goes out, it means that there may be carbon dioxide gas.

In recent decades, due to the sharp increase in energy consumption by human beings, forests have been destroyed and the content of CO2 in the atmosphere has been rising. Like the glass in the greenhouse, CO2 does not affect the radiation of the sun to the earth's surface, but it can block the infrared radiation reflected from the ground back to the high altitude. This is like covering the earth with a layer of heat insulation film, which raises the surface temperature of the earth and produces the "greenhouse effect". Besides CO2, there are ozone (O3), CH4, chlorofluorocarbons (including more than 20 compounds, mainly CCl3F and CCl2F2) and so on. Global warming will have a great impact on human life. The rise of the earth's temperature will lead to a large number of glaciers melting in the cold zone and the poles, and the sea level will rise, thus flooding the low-lying coastal cities and river deltas. Rising temperature will make arid tropical areas more arid, accelerate the process of land desertification and natural disasters, and even lead to the accelerated extinction of many species.

In order to protect the earth on which human beings depend, human beings should work together to prevent the greenhouse effect from being further enhanced. We can take the following measures: save energy and reduce the use of fossil fuels such as coal, oil and natural gas; Make more use of solar energy, wind energy and geothermal energy; Restrict and gradually stop the production and use of chlorofluorocarbons; Vigorously plant trees and prohibit deforestation.