Green plants can absorb harmful gases and purify the air. A is correct;

B. Plants can not only conserve water, but also promote water circulation in the biosphere through transpiration. B wrong;

C, green plants are the mother of life, it can provide organisms with organic matter and oxygen, C is correct;

D, the number of plants decreases, and the content of carbon dioxide in the air increases relatively, which leads to the greenhouse effect, global warming and abnormal climate. After a long time, the balance of carbon and oxygen is broken and the survival of living things is threatened. Therefore, the photosynthesis of green plants can absorb carbon dioxide and reduce the concentration of carbon dioxide in the air. D is correct.

Therefore, choose: B.