Afforestation and greening wasteland can effectively improve the climate. The main reasons are as follows: (1) Increasing atmospheric temperature and regulating rainfall through transpiration.

(1) It is correct that plants can effectively improve the climate by increasing atmospheric temperature and adjusting rainfall through transpiration;

② Green plants release oxygen through photosynthesis, constantly consuming carbon dioxide in the atmosphere and maintaining the relative balance of carbon and oxygen in the biosphere, which is correct;

(3) Breathing is a process of decomposing organic matter and releasing energy.

④ It is correct that plants with developed roots can keep water and soil, prevent wind and fix sand, and effectively improve the climate;

Therefore, the reason for improving the environment is 124.

So choose: C.