Secondly, we are also rapidly changing the environment by adding too much waste to it, such as when cars and generators release carbon dioxide (CO2) into the air. Releasing waste is natural and necessary. Plants need CO2 to live, and one of their waste products is oxygen, which we need to live. But we are releasing so much CO2 into the air that the plants cannot possibly use it all. And there are fewer and fewer plants to use it when we cut down forests. The extra CO2 that collects in our atmosphere is a major cause of global warming.