First, there is doubt about the extent and cause of this warming. Initial evidence comes from weather stations. However, most of these are in the cities, which are warmer than rural areas. This explains much of the increase in average temperature. Even if we acknowledge that the world has warmed, this warming is probably part of a natural cycle. The earth naturally warms and cools. At the time of the dinosaurs, it was warmer than today; during the last ice age, it was much cooler. In addition, the rise in temperature over the last 200 years has not been constant.