Pollution is a problem usually associated with the Industrial Age. It is caused by certain city improvements, the meltdown of a Nuclear Plant, nuclear explosions, and large city populations.
If world pollution reaches sufficiently critical levels, there is a chance that global warming will occur. It can cause forests and jungles to disappear, and grasslands to become plains, and plains to become desert. World pollution is the sum of all city pollution, nuclear explosions, and meltdowns.