We have previously talked about the urban heat island effect. Crowded metropolitan areas are significantly warmer than surrounding rural areas as a result of human activities. Among other things, cities fill up with cars idling in traffic and cars not only produce greenhouse gas emissions, their internal combustion engines also produce a lot of heat.
In 1970, when the first Earth Day was celebrated, there were 200 million people living in the United States. Sometime during 2015, our population will top 320 million, and the Earth seems destined to harbor at least 10 billion of us by the middle of this century.