First of all, the oxygen concentration on the earth did not decrease from the Carboniferous period, but suddenly soared from the Carboniferous period. It should be the Permian after Carboniferous, and the oxygen concentration began to decrease gradually. Because during the Carboniferous period, the plants on the earth flourished day by day, and the oxygen they produced reached an unprecedented height.

The Carboniferous is famous for its "era of giant insects". Indeed, in the Carboniferous, the size of insects reached the peak in the history of biological evolution, with scorpions more than half a meter long, dragonflies with wings and so on. The origin of these giant insects is closely related to the oxygen concentration in Carboniferous. Generally speaking, the oxygen concentration is from low to high and then from high to low. So what caused the change of oxygen concentration in Carboniferous? Let's talk about this briefly. The analysis of radioisotopes at different levels of the earth shows that the oxygen content in any period does not change overnight, but has a long changing process, and so does the oxygen content in Carboniferous.

Although "folding orogeny" (a kind of crustal movement) stopped, the Hercynian movement (crustal rise and fall) did not stop, leaving a large area of exposed land. By the end of Devonian, ferns had become quite large on land.

The Carboniferous climate was particularly warm and humid, which was conducive to plant growth, and the coverage area of pteridophytes on land was further expanded. According to the geological strata of the Early Carboniferous, scientists found that ferns covered almost 95% of the earth at that time. With the large coverage of plants and the sunlight, the photosynthesis of plants began to absorb carbon dioxide and release oxygen. At this time, the proportion of various gases in the atmosphere began to change sharply. By the middle Carboniferous, due to the influence of plant photosynthesis, the proportion of oxygen in the atmosphere reached 35%.

In the Middle Carboniferous, there were vegetation, suitable environment and wingless insects. Later, insects gradually evolved wings. Because of the high oxygen content at that time, there were almost no natural enemies of insects on the land, and their bodies reached the peak in history, forming the famous "giant insect age" The decline of oxygen content at the end of Carboniferous is closely related to a major event, namely the Carboniferous extinction. This extinction occurred about 305 million years ago and was directly related to the collapse of the Carboniferous rainforest.