Author William H. Frey, senior fellow for the Metropolitan Policy Program at the Brooking Institute, writes, "These trends are consistent with previous census releases for counties and metropolitan areas that point to a greater dispersion of the U.S. population as the economy and housing market pick back up, perhaps propelled by young adult millennials who may be finally departing dense urban cores as they make a delayed entrance into marriage and the housing market.

These slowdowns were especially pronounced in the biggest cities. Among the largest 22, all but two (Jacksonville, Fla. and Fort Worth, Texas) showed lower growth last year than in 2011-2012. Twelve of the cities displayed their lowest growth since 2010, including northern cities like New York City and Boston but also other pricey coastal cities like San Francisco and Sun Belt growth centers like Dallas."

Frey adds that as growth rates for larger cities have taken a dive, even booming mid-tier cities like San Antonio and Charlotte are also seeing reduced growth on the horizon. These converging trends can only mean one thing: "One consequence of big city population declines is the return of suburbanization, which was muted in the earlier years of this decade," Frey writes. 

One possibility for this change could lie in the fact that while major cities are booming economically, they are simply not building enough housing to meet demand, so young people, low-income residents, and retirees are moving to areas where housing is more plentiful and affordable. In America, that means exurbs and suburbs, where conventional, sprawl-oriented development patterns reign supreme. 

According to the California Legislative Analyst's Office, for example, the state lost nearly 5 million residents over the last decade, with the overwhelming majority heading to rambling states like Texas, Arizona, and Nevada. California's net loss of 1 million inhabitants over that time period is attributed largely to the state's sky-high housing costs. According to various reports, many of those fleeing to other states are low-income transit users, while many of those heading into the state are high income and likely, drive. Could these shifting demographics help to explain sagging transit ridership estimates in major cities?

The damage is not limited to California, however. Even New York City has seen a population decline in recent years, as retirees fly south to Florida and other states. In 2014, Florida's population surpassed New York state's count to become the third most populous state in the country behind California and Texas. Since then, Illinois, formerly the fifth most populous state, has lost ground to Pennsylvania. 

These developments come as carbon emissions surge following years of steady decline. In recent years, transportation-related carbon emissions have outpaced all other sectors of the economy, erasing hard-fought reductions in the emissions generated from electricity production. It has become increasingly obvious that while buildings themselves generate a high proportion of the carbon emitted in the United States, urban configuration and transportation design are as, if not more, polluting, overall. Unfortunately, economic and demographic changes are pushing carbon-intensive lifestyles into overdrive.

In a very real way, the urban housing crisis and the climate crisis are two sides of the same coin.