The United States' rural areas serve many important functions. To name two basic functions, they provide space for the growing of food and they provide space for the natural environment, which actually helps to keep the whole environment clean. For example, plant growth replenishes oxygen in the air we breathe, yet cities tend to have more concrete than trees. Urban sprawl reduces rural areas which means that these important functions are reduced. We have already seen signs of this, with wild animals coming into populated areas due to habitat destruction. Urban sprawl, therefore, is a significant threat to the environment.
Yes, urban sprawl is bad for the environment. There are very few "green" cities on the planet, and even those cities are hardly improving the environment. Urban sprawl is very good for human networking. But under no circumstances do cars and streets, and with that the lack of plants and animals, improve the ecology of an area. Earth would be better off if cities and sprawl stopped spreading.
Urban sprawl destroys the environment, therefore it is indeed an environmental threat. Where I live in the United States, we bought a 100 year old home in the middle of nowhere 30 years ago. We were surrounded by farmland and forest. Now the forest is gone, so are the farms, and hundreds of beige McMansions have risen in their place. And most of them are empty eysores due to the foreclosure crisis.
The ever outreaching sprawl of suburbia is using up land plots for lawns that could be used for forests. The energy required to run the proposed 3 bedroom home in the valley is more than the 3 bedroom condo in the city that already exists. Construction requires wood, which reduces more forests. The longer commutes require more fuel which means less petroleum in the world and more exhaust fumes to clog the atmosphere.
Urban sprawl is an environmental threat to most communities. There are way to many houses and large roads out in the suburbs. We should live in more compact communities. We should drive our vehicles less or maybe perhaps get rid of them all together. There is way too much pollution because of this.
If Urban areas replace Suburban areas, that is actually a good thing. A suburban house has a big lawn, and 3 cars. An Urban house has no lawn, and instead, a garden terrace, and 1 car in a public garage, as people who live in large cities don't need to walk 1 hour to go to their office; they can walk 1 block.