Everyone is so familiar with the cliches of resurgent older cities on the East Coast that you can probably recite them by heart: decrepit or abandoned buildings replaced by gleaming condominiums; lines coming out the door of the hot new artisanal restaurant in what used to be a boarded-up storefront; yuppies whizzing by on bicycles.

People are supposedly moving back to cities — at least the high-profile, knowledge-economy ones. But even some of the hottest older cities, despite high housing prices and strong economies, are still well below their peak populations. They are growing expensive rather than big.

That’s a boon to the real-estate industry, but not for the environment. We’re watching the demographics of cities change without getting the carbon-emissions-reducing benefits that the back-to-the-cities movement promised. Here’s why.

Urban population density matters greatly for climate change: People who live in densely populated areas burn way less carbon as they go about their lives. Older cities tend to be denser than newer ones, so getting residents back into older cities is a key environmental challenge.

To get them back, though, seems to require more than just a strong economy and attractive lifestyle. It turns out that social changes mean cities need to create far more housing units than they had in the mid-20th century if they are to keep housing relatively affordable and increase their populations to their former peaks.

Consider Washington, D.C., where the peak U.S. Census count was 802,178 residents in 1950. Given that it’s the capital of a country with twice as many people as in 1950, you wouldn’t expect Washington to have shrunk, would you? Well, it has, and it hasn’t fully bounced back. Suburban sprawl and white flight — spurred by the devastating riots of the 1960s and dramatic spikes in crime through the 1980s — caused the population to dwindle to 572,059 in 2000.

In the new millennium, D.C. has experienced a remarkable resurgence. The regional economy is strong, thanks to steady government spending and growth in related sectors such as technology, defense contracting, lobbying and advocacy, and media. An increasing fashion for urban living among millennials, and wise public investments such as expanding Metrorail lines, have brought hordes of young professionals to the city. D.C.’s population has grown as a result: In 2010 it was 601,723, and last year it was up to 658,893. If the city keeps adding 15,000 people per year for the next 10 years, it will return to its peak.

But could it grow faster? Could it end up even larger? That would be good news for the climate. D.C.’s suburbs have sprawled outward far faster than the city has grown. As Washingtonian magazine observes, “The Fastest-Growing Suburbs of Washington Are In Counties You’ve Never Heard Of.” Places like Stafford, Virginia, 42 miles from D.C., are being turned into exurbs. The resultant gas-guzzling long commutes create a lot more climate pollution.

It would also be good news for D.C.’s financially stretched residents. The high demand of young professionals for housing in D.C. has driven up prices dramatically. Zillow rates the D.C. real-estate market “very hot.” Median home sale prices rose 5.7 percent last year to $485,250.

Instead of just getting more populous and diverse, the “Chocolate City” has lost almost as many black residents as it has gained white ones. Between 2000 and 2010, The Washington Post reports, “The black population dropped by more than 39,000 over the decade, down to 301,000 of the city’s 601,700 residents. At the same time, the non-Hispanic white population skyrocketed by more than 50,000 to 209,000 residents, almost a third higher than a decade earlier.” In 2011, the city lost its black majority. According to U.S. News, “From 2000 to 2010, [D.C.] was the only place – other than states with natural gas resources and fracking – that saw its residents’ earnings increase.” This reflects, unfortunately, not improvement in conditions for D.C.’s long-time residents, but their displacement and growing inequality. The average white resident of D.C. earns more than twice as much as the average Latino resident. Average rents in D.C. are growing faster than average incomes for rich and poor alike, and even the well-heeled newcomers are feeling the pinch.

D.C. doesn’t suffer from a shrinking stock of housing. In fact, it has the most dwelling units in its history — it just has fewer people in each home. This is a byproduct of social changes, some of which are clearly positive: People feel free to marry later, have fewer children, and divorce when a marriage isn’t working, and they are less likely to live with extended families. The result is smaller households. This is especially true in cities, which have become disproportionately where childless and unmarried people live.

It’s also a reflection of mass affluence. Middle-class Americans have simply become accustomed to more private space and comfort than previous generations. Kitchens and bedrooms are bigger, bathrooms and closets are both bigger and more plentiful, and kids are less often expected to share a room. According to the blog Greater Greater Washington, “The 1950 census found 14.1 percent of the District’s 224,142 occupied housing units to be ‘overcrowded’ (with over 1 person per room). By 2011, that figure had fallen by 2/3, to 4.7 percent … In 1950, 3.2 people occupied each dwelling unit [in D.C.] In 2007-2011, the number of persons per household had fallen to 2.13.” For example, a working class two-parent, two-child household is less likely to squeeze into a one or two bedroom apartment in the city. They’ll decamp for a three-bedroom house in the suburbs, leaving that apartment to one or two unmarried young people. A typical row house that once housed a large extended family or a passel of boarders may now shelter just a childless yuppie couple.

What that means is that D.C. will need more housing units than it had in 1950 if it is to equal its population from then. And that’s how D.C.’s housing affordability crisis is also a result of policies that constrain the construction of new housing in D.C.

The most egregious in D.C. is the Height Act, a law passed by Congress limiting the height of D.C. buildings. Opening up D.C. for taller buildings would allow for more residents to live in 21st century comfort.

More than just the Height Act stands in the way of density in D.C., though. Consider 14th St, NW. Once blighted, it has boomed in recent years as the surrounding neighborhoods of Logan Circle, U Street, and Columbia Heights have experienced rapid gentrification and skyrocketing home prices. Where once empty lots and one story buildings stood, apartment buildings of around six stories with high-end ground-floor retail have sprouted. But why only six stories? The apartments in these buildings are not cheap. A one bedroom condo might go for $600,000. But look at the zoning code, which only allows buildings to be 65 or 90 feet tall through most of that corridor. (That’s roughly five to 9 floors.)  Absent those arbitrary height restrictions, developers might build taller buildings with more units for slightly lower average prices.

D.C. has other zoning restrictions that limit development of new housing units. There are minimum size requirements that may challenge the ability to build micro-units, and minimum parking requirements that drive up the cost of building. D.C. is trying to amend its zoning code to allow “mother-in-law” units to be added to single-family homes. But that, like reducing parking requirements, incurs the wrath of entitled homeowners who want to protect their parking and perceived quality of life above all else.

D.C. is typical of American cities in all these particulars, and it is more lenient than its — or any city’s — suburbs, which set vast areas aside for only detached single family housing with off-street parking, no matter how expensive they become. But more than most cities or suburbs, D.C. has extremely high housing prices that suggest a lot of unmet demand.

The wax and wane of D.C.’s population is almost identical to that of Boston. At its peak in the 1950 Census, Boston had 801,444 residents. That dropped to 562,994 in 1980. The decline has reversed and Boston’s population has slowly crept back up, standing at 645,966 in 2013. The Boston Globe reports, “During the 1960s, the city actually lost housing units, and that decade was followed by 30 years of negligible construction in houses and apartments. Families changed, too. The average household size shrank, as Bostonians had fewer kids and saw more people living alone; from 3.2 in 1960, there are now just 2.3 people per household in Boston.”

Like D.C., Boston has seen a steady loss of working class families and an increase in childless white collar workers. In the 2010 Census, D.C. tied with Atlanta for the highest proportion of single-person households, fully 44 percent. Boston, meanwhile, is the youngest city in the nation, with 35 percent of its adults being under the age of 35. As Anthony Flint, a longtime Boston Globe reporter and now a fellow at the Lincoln Institute of Land Policy, a think tank in Cambridge, Mass., puts it, referring to two of Boston’s gentrifying neighborhoods and the city’s trademark three-story buildings: “If a young professional couple moves into a triple-decker in South Boston or Jamaica Plain and turns it into a roomy single-family, the Census count drops accordingly.”

Boston, like D.C., has experienced a construction boom in neighborhoods near its downtown. And it does not share D.C.’s blanket prohibition on tall buildings. High-rises are shooting up in areas like Boston’s Back Bay and downtown. But home prices are still rising.

That’s because Boston, like D.C., has a strong economy and attractive, dense, historical housing stock. The Globe notes, “In recent years, new residents have been lured by the labor market; metropolitan Boston’s job market is stronger than in most major American cities, and stronger than just about anywhere else in New England … the Boston area’s unemployment rate is consistently below the national average.”

It’s hard to move to Boston for a job when the city’s average home sales price this year passed $600,000. Like D.C., Boston’s inner-ring, transit-accessible suburbs are also expensive. “Cost of housing is a factor in the region’s growth,” says Armando Carbonell, chair of the Lincoln Institute’s department of Planning and Urban Form. “If it were less expensive to live in the region, there would be more people living in it.”

Boston has added 20,000 housing units since the turn of the century. That’s impressive, but it’s not enough to offset the trend towards fewer people in bigger units. “The number of housing units in Boston increased by more than 8 percent between 2000 and 2010, while the population grew by 4.8 percent, so there was a larger increase in housing units than population,” notes Carbonell. “People are occupying more square feet per person.”

More housing development would do more than increase density and reduce carbon emissions. It would reduce the obscene, and growing, rent burden in these cities. In 2000, 35 percent of Washingtonians and 42 percent of Bostonians were paying more than 30 percent of their income in rent. By 2011, that share had spiked to 49 percent and 54 percent, respectively.

Changing zoning codes to allow taller buildings is hard. Local opposition can be fierce. If you already own a home in a neighborhood, constraining demand to boost prices is in your financial interest. New construction will bring nothing but temporary dust and noise followed by permanent traffic and competition for on-street parking. But neighborhood homeowners must make this sacrifice for the greater good. If nothing else, it will help their children afford to stay in the city where they raised them.

High-cost cities may have to reform not just their zoning codes but the process of approving developments, in order to reduce community veto points. They could also switch from taxing the value of buildings to taxing the value of the land it sits on. That would push owners of short buildings or empty lots to develop a taller building or sell it someone who will. As a recent op-ed in The Baltimore Sun advocating a land value tax (LVT) in that city noted, “Environmentalists favor LVT because it curbs urban sprawl and makes green-field development less attractive. It encourages development and revitalization of what’s already here.”

As other older cities attract more young people, they will face the same conundrum. In some ways, they already do. D.C.’s problem is actually a lot like that of Detroit, only on a different scale: There are two very different cities within each. In certain neighborhoods that have become trendy, high demand bids up housing prices, empty lots get filled in, and the population grows. In other neighborhoods, demand remains low and abandonment continues. D.C.’s trendy areas are much bigger than Detroit’s. But large swaths of D.C.’s eastern half remain not only impoverished but pocked with vacant properties.

The optimistic environmentalist would hope that eventually prices in the desirable corners of D.C. will push prices to the point where previously undesirable neighborhoods with empty lots get developed because there is enough demand. “In New York, places that were unimaginable, parts of Brooklyn, are now unaffordable,” says Carbonell. “Rising demand is going to get people to rethink neighborhoods, and to ask, ‘Where are there opportunities with under-used housing stock or empty land?’”

But in New York, especially Brooklyn, that rising demand has increased housing prices and displaced low-income residents. New York’s population is at an all-time high, but it still only grows about 2 percent per year, because it, too, struggles with building enough supply to keep up with demand. Any city that wants to protect its residents, and help the environment, will need to figure out how to build enough housing to turn increased demand into more inhabitants — rather than just richer ones.