With more people, these people will need a place to live, a place to get electricity from, a place to get everyday products from, and more. All of these places require space. These might be new cities and new industries. In order to have a new city, you need to clear out space. This will make the environmental quality worse. People also need to get around, they need cars, the more cars there are the more pollution will be added, degrading the environmental quaility. One other problem is these people need electricity, this means more power plants, need to be made. These usually are fossil fuels and will add pollution, and other toxic fumes into the atmosphere.