As the nation's metro areas grow ever larger, the sprawling suburbs can come with all sorts of problems: endless commutes, growing pollution, and unwalkable neighborhoods, to name a few. Now, a pair of studies released this week suggests that America's evolving cities are also bad for the economy. Not only are mammoth, spread-out metro areas economically wasteful, but they're also hurting Americans' job prospects as work disperses out into the suburbs.

One new report finds that suburban sprawl in US cities costs the country more than $1 trillion a year. (That's huge; as context, consider that the US annual GDP is around $17 trillion.) According the London School of Economics Cities project and the Victoria Transport Policy Institute, individuals and cities in the US pay $1 trillion to $1.1 trillion in additional infrastructure, public works, driving, and health costs as a result of these massive metro areas.

The additional spending on road building, parking, and police and emergency services that come with a spread-out metro area really add up, they write — sprawl increases infrastructure and public services costs by 10 to 40 percent, the report says.

It's a huge cost to try to calculate, of course, and considering various cost estimates (for road costs and land values, etc.) that author Todd Litman includes, the $1 trillion figure is perhaps best seen as just one attempt at a ballpark estimate. For his part, Litman writes that his $1 trillion figure doesn't even include cost estimates for other problems sprawl can cause — lower social mobility and more traffic accidents, for example.

More jobs, but fewer opportunities

Yet another study released this week suggests that jobs migrating to the suburbs keep people from working, and that the problem is especially pronounced in America's poorest neighborhoods.

The number of jobs within typical commuting distance to residents of major metro areas fell by 7 percent between 2002 and 2012, according to a new study from the Brookings Institution.

In addition, the situation was even worse for poor and minority neighborhoods. While the number of jobs nearby fell by 7 percent for all large-metro-area residents, it fell by 14 percent for blacks and 17 percent for Hispanics, as well 17 percent for poor neighborhoods.

One big contributor to that trend in hard-to-reach jobs also appears to be sprawl. The distance between people and work grew partly because in large metro areas nationwide, jobs both moved to the suburbs and spread out more, Brookings explains. Between 2002 and 2012, the number of jobs in urban areas fell by nearly 2 percent, but the number in suburban areas grew by more than 4 percent. However, the density of jobs in suburban areas fell, meaning that even as the number of jobs grew, they also spread even farther apart.

This paper deals in the broader phenomenon of the distance between workers and jobs, and not sprawl, per se, but it's true that the trend of far-flung jobs also grew over this time period. Other Brookings research found that between 2000 and 2010, the share of jobs located within three miles of downtown cities declined in 91 out of the 100 largest metro areas. Meanwhile, the share of jobs located 10 or more miles away from those city centers grew in 85 out of 100 metro areas.

This has implications far beyond long commute times; it matters because living farther from jobs tends to mean a longer job search, as researchers found in a 2014 NBER working paper. It also means longer stretches of unemployment — so even as jobs came back during the recent recovery, they didn't all return in places where unemployed Americans could reach them.

The ladder gets even tougher to climb

It's not just that sprawl can help keep unemployment high and make the economy less efficient; it also may play a part in the growing, entrenched divide between the richest and poorest Americans.

In a landmark 2014 study on social mobility across US metro areas, researchers Raj Chetty, Emmanuel Saez, Nathan Hendren, and Patrick Kline found that cities with less sprawl (as measured in shorter commute times) tended to have higher social mobility — that is, in those cities, children born to poor parents were more likely to move up the income ladder. The University of Utah's Metropolitan Research Center reinforced this in a 2014 report. Reid Ewing and Shima Hamidi created an index score measuring a city's compactness. And that score had a marked relationship to how well kids fare later in life:

"For every 10 percent increase in an index score, there is a 4.1 percent increase in the probability that a child born to a family in the bottom quintile of the national income distribution reaches the top quintile of the national income distribution by age 30," they wrote.

The researchers stop short of showing a direct causal relationship between sprawl and mobility, but their research contains a few clues to how this relationship might happen. For example, they find there's more economic opportunity in compact cities, as well as lower housing and transportation costs.

All of this points to an important policy dynamic: national-level politicians may tout their social-mobility and job-creation agendas ahead of the 2016 elections, but that can only go so far, particularly as cities spread out and become more economically segregated. State- and local-level policies may be the key to reducing economic gaps — promoting better zoning, affordable housing, and public transit may be the best route to combat sprawl, and it's something state legislators and mayors may be best equipped to do.