There's one environmental phenomenon that humans are one hundred percent responsible for, and nobody can deny: acid rain. There was no such thing as acid rain until we started burning hazardous chemicals. It may not occur frequently, but when it does, it brings nothing but death and decay for animals and the environment.
Most acid rain is caused by a deadly combination of nitrogen oxide, sulfur dioxide, and burning coal; all industrial waste products which combine inside clouds until the clouds can bear no more weight, releasing them back to the ground as liquid. The acid rain pools up inside lakes and streams and kills any fish living there. Whole species of fish have been eliminated this way. The rain that falls on land is soaked into the ground and absorbed by plants and trees, poisoning them and killing them slowly. Acid rain takes no prisoners; it can kill quickly or kill slowly, but it always kills.