Um, no, this has been debated many times. People generate heat and cause global warming, through use of vehicles, air conditioners, furnaces, and other direct means of energy consumption, and indirectly through purchasing of products that are manufactured or grown or transported using energy. For the most part people in denser cities generate less heat than people in suburban areas because they drive less, and live in smaller or attached or conglomerated residences that require less energy to heat or cool. The heat they collectively generate may be concentrated in a smaller area than the same number of people living spread across exurbia, but it contributes far less to global warming on a per person basis. As well, although urbanites live farther from farmland by your reckoning, that is only because the suburbs have inefficiently reassigned (read: squandered) all the farmland nearby to low density housing, freeways and shopping malls. If we all lived in denser cities, we would all be near farmland and there would be a lot more of it.