In 1840, Americans lived where it rained. By 1990, there was no connection between population and precipitation.* Why? Infrastructure projects brought water to arid regions, and people moved from wet cold areas to dry warm areas. Once they got there, they found that water was also abundant and cheap. Their demand for water grew to include lifestyle uses — for lawns, swimming pools, long showers and washing the car and driveway.It's pretty obvious to most people that the damage/threat/danger from the current southwestern drought is the result of too many people living in a region with vulnerable and variable water resources (JW Powell wrote about this potential problem in 1879!). It's also obvious that climate change is going to make matters worse in the future.
In other words, water projects increased population in dry areas (extensive demand), and cheap water increased water consumption in these areas (intensive demand).
This metric means that people in the region have two choices: spend much more money to live with less water or leave the area for wetter places. I am sure that the former will happen -- one billion dollar desalination plant and crop disaster at a time -- but the latter intrigues me, as an option that will make environmental and economic sense.
So now I want to know how you would rename "the Sun Belt" that people are leaving for "the Rust Belt" that promises a better life. Ideas?
* Beeson, P. E., DeJong, D. N., and Troesken, W. (2001). Population Growth in US Counties, 1840-1990. Regional Science and Urban Economics, 31(6):669–699.