The land or land uses takes quite a bit of abuse as the human population increases. The land takes in chemicals, and if over-cropped it can be drained of its nutrients (which is what lead to the Dust Bowl). The land's plants filter out some CO2 from the air from proudction but not as much as humans create. The land's water source changes from clean and fresh to usually more acidic or fertilized which either kills the fish or algee grows everywhere cutting off oxygen from both aquatic plants and animals.
