There is sure a lack in the oxygen production, and there is another negative effect – expansion of deserts (desertification) preventing reforestation. Advanced urbanization of the current era causes the growing (and already huge) production of pollution. The garbage heaps are full so the new ones are established every day. They contaminate the enviroment and water with the toxic chemicals.
In conclusion, there is only one thing to say: We must stop destroying the nature, because we are destroying ourselves. What will be breathed if there are no forests producing oxygen and what will be eaten if there are no animals and no plants?.