A - the state divided into cities
History was a driving force behind such migration, since humans tended to move to new territory when they lacked food, shelter, or other resources--meaning that most of the conflicts throughout history have been driven by the same forces that drove migration out of Africa.
The greenhouse effect is the rise in temperature that the Earth experiences because certain gases in the atmosphere (water vapor, carbon dioxide, nitrous oxide, and methane, for example) trap energy from the sun. Normal warming is mostly caused by natural gases.
Pollution to the atmosphere, and the fact that fossil fuels are only a finite resource and could very easily run out, and we wouldnt have anything left to power anything except for renewable sources of energy like solar, wind, geothermal, and hydroelectric.
