There are two contradictory trends here. Surprisingly, urbanization is actually a net plus for the environment. We think of cities, especially big ones, as being polluted and unhealthy because they concentrate our waste products, but that same population concentration allows us to get by with less waste and forces us to manage and minimize our wastes just to protect ourselves. As the urban population rises, our “footprint” on the rest of the land becomes much smaller. Cities are also vastly more efficient and productive than towns, and large cities are more efficient and productive than small ones, which is one reason we flock to them. So the increase in cities and the reduction in rural population is in general a big environmental gain.
However, the other demographic trend is more worrying, and that is the shift from north and east to south and west, from cold and wet to warm and dry. Who wouldn’t want to live in sunny California? The problem is that warm dry ecosystems are extremely fragile, and the explosion of population in areas like southern California, Las Vegas, Texas, and central Arizona has drained lakes, rivers, and aquifers at unsustainable rates, creating more and more demand to build more dams, canals, and pipelines and divert more and more water from ever more distant areas. Physical damage to desert landscapes can take centuries to heal, and many species are so thinly spread out that relatively small decreases in habitat or disruptions of migration paths can cause populations to collapse.
Even warm and seemingly wet areas are suffering. Environmental problems are increasing all along the Gulf Coast. Florida is the most extreme case, with the Everglades, once a vast river of grass, becoming so parched that former swamplands are drying out and becoming fire hazards and many unique species face extinction. In other areas, the increase in population has led people to build on the most vulnerable land, creating an urgent demand for levees, dikes, and other massive earthworks that destroy thousands of miles of pristine wild lands.