Pollution is a problem usually associated with the Industrial Age. It is caused by certain city improvements, the meltdown of a Nuclear Plant, volcanic eruptions, nuclear explosions, and large city populations. Global Warming 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. Polluted areas can be cleaned up by workers, and pollution from cities can be lessened by building the Mass Transit System and the Recycling Center.