A team of sci­en­tists this week reopened the debate over sub­urbs vs. city cen­ters, with new research show­ing that car­bon diox­ide emis­sions increased as sub­ur­ban areas devel­oped to the south­west of Salt Lake City in the past decade — while com­pa­ra­ble pop­u­la­tion growth in the cen­ter of the city did not have the same effect.

It’s the lat­est evi­dence high­light­ing the envi­ron­men­tal con­se­quences of sub­ur­ban expan­sion, often accom­pa­nied by more miles dri­ven by cars and larger free-​​standing homes that require more energy for heat­ing and cool­ing. As cities become a cen­tral focal point for action on cli­mate change, the ways in which they man­age their growth will be a key question.

Yet at the same time, the research itself shows how con­tentious and com­pli­cated this debate is likely to be. The south­west­ern Salt Lake Val­ley region in ques­tion, for instance, con­tains a note­wor­thy com­mu­nity called Day­break that was itself built around energy-​​efficient homes and walk­a­bil­ity and her­alded for its design by the Urban Land Insti­tute — pre­cisely what cli­mate advo­cates would seem.

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“Of course, if you put more peo­ple where there weren’t peo­ple before, you’re going to have more emis­sions,” said Christo­pher Jones, a researcher at the Uni­ver­sity of Cal­i­for­nia at Berke­ley who heads up the Cool­Cli­mate Net­work, which stud­ies the car­bon foot­prints of dif­fer­ent U.S. com­mu­ni­ties, and was not involved in the new study.

“The ques­tion is putting them in one type of devel­op­ment, com­pared with some­where else.”

The new research was based on the longest-​​existing net­work of car­bon diox­ide sen­sors in a city and its sur­round­ings. The ear­li­est sen­sor was estab­lished in 2001 at the Uni­ver­sity of Utah and sub­se­quent sen­sors were then installed in other parts of the city, the nearby Wasatch moun­tains (rep­re­sent­ing a nonur­ban or “con­trol” envi­ron­ment), and in a rural area to the south­west of the city — which pro­ceeded to show fast pop­u­la­tion growth.

Emis­sions growth then followed.

It’s urban expan­sion. You’re tak­ing the land sur­face to a sub­ur­ban devel­op­ment where there’s cars and houses and some indus­try, and CO2 emis­sions go up,” said Logan Mitchell, a post­doc at the Uni­ver­sity of Utah and lead author of the research that appeared in the Pro­ceed­ings of the National Acad­emy of Sci­ences. “In some ways it’s not so sur­pris­ing, but it’s neat to have mea­sured it and see that response in the atmosphere.”

The research, which was pub­lished Mon­day, was co-​​authored by 13 other sci­en­tists at the Uni­ver­sity of Utah, the National Cen­ter for Atmos­pheric Research in Boul­der, Colo., and sev­eral other U.S. universities.

Over the same time period when pop­u­la­tions and emis­sions grew in the south­west part of the Salt Lake val­ley, the urban cen­ter of Salt Lake City also added about 10,000 peo­ple. But here, the study found that car­bon diox­ide emis­sions — which were already con­sid­er­ably above the lev­els observed in a nonur­ban set­ting — did not grow fur­ther in response.

Much of the rea­son prob­a­bly has to do with cars, said Mitchell — urban pop­u­la­tion growth doesn’t nec­es­sar­ily mean more cars on the road, given the fact that peo­ple are mov­ing close to the city cen­ter, and have pub­lic trans­port, walk­ing and bik­ing options.

Sub­ur­ban growth also tends to result in larger homes that require more energy to heat, and if that is pro­vided by nat­ural gas burn­ing at the site of the home, then this, too, would lead to more emis­sions, Mitchell explained. (Cities’ and sub­urbs’ green­house gas emis­sions do not all occur within their bound­aries, since the elec­tric­ity con­sumed within those bound­aries is often pro­vided by power plants located far­ther away.)

But Berkeley’s Jones, who was not involved in the study, pointed out that in per­cent­age terms, the pop­u­la­tion growth in the cen­ter of Salt Lake City would be much smaller than the growth in the sub­ur­ban area, which was pre­vi­ously rural. So he said you would expect to see smaller car­bon diox­ide changes in the urban core of the city.

The study argues that what hap­pened in Utah may be hap­pen­ing all over the United States — the find­ings, the authors say, “may be rep­re­sen­ta­tive of urban tran­si­tions in many U.S. cities that have sta­bi­liz­ing emis­sions in their urban cores, and expand­ing sub­ur­ban growth.”

But in this case, the area being stud­ied itself com­pli­cates the nar­ra­tive. The south­west­ern Salt Lake Val­ley actu­ally includes at least one newer com­mu­nity that focuses heav­ily on sus­tain­abil­ity and avoid­ing driving.

The devel­op­ment in ques­tion occurred when Ken­necott Land, a sub­sidiary of the min­ing giant Rio Tinto, built Day­break, which is about 22 miles from the cen­ter of Salt Lake City. It’s a cen­trally planned com­mu­nity that took envi­ron­men­tal and energy-​​use con­sid­er­a­tions into account, includ­ing pre­serv­ing walk­a­bil­ity and ensur­ing that the homes would be EnergyStar-​​certified.

The com­mu­nity adver­tises the green design of its homes. In fact, a 2014 story in the Salt Lake Tri­bune noted how the com­mu­nity is actu­ally “built around an intri­cate mas­ter plan with the cen­tral, sprawl-​​busting goal of reduc­ing reliance on driving.”

That’s not to say peo­ple don’t drive to work, said Cameron Jack­son, the mar­ket­ing direc­tor for Day­break Com­mu­ni­ties. “Unfor­tu­nately peo­ple do have to com­mute,” he said.

The sen­sor in ques­tion is not located directly in Day­break. Instead, it mea­sures the air across a broad expanse of the south­west Salt Lake Val­ley, which would include Day­break but also a num­ber of other communities.

I’m not nec­es­sar­ily sur­prised by the fact that, you know, a study’s been done show­ing that, 14 years ago, there was very lit­tle CO2 here, and now there is,” Jack­son said. “Because you add peo­ple to a place and that is sort of an auto­matic out­come. How­ever, our community’s been designed since its very incep­tion to address this very thing.”