Over the past few years, momentum has been building to make cities a focus of climate change action. “Although they cover less than 2 percent of the earth’s surface, cities consume 78 percent of the world’s energy,” reports the UN, “and produce more than 60 percent of all carbon dioxide and significant amounts of other greenhouse gas emissions.”

But there’s a problem: there’s a lot we don’t know about emissions in cities, because the ability to detect emissions on local, fine-grained scales is a relatively recent development. This technology is improving steadily, and this week, a paper in PNAS reports results from a detailed analysis of Salt Lake City. Its findings add to growing evidence that dense urban populations, rather than suburban sprawl, has an important role to play in climate action.

A one-of-a-kind case study

Salt Lake City has an emissions sensor network that is ahead of the game. There have been urban CO 2 monitoring projects in Pasadena and Heidelberg (Germany) for more than 10 years, but only at a single location in each city. That makes it impossible to get a multi-faceted picture of how emissions vary across the different spaces in the city.

More sensor networks that cover multiple sites within a city are springing up, but they are relatively recent, with no data going back past a few years. But the network in Salt Lake City circumvents both of these disadvantages: it has data going back all the way to 2004 from multiple sites across the city as well as a non-urban, mountainous “control” zone.

This network allowed a team of researchers led by Logan Mitchell at the University of Utah to monitor the differences in carbon levels as they've changed over time. Mitchell and his colleagues took readings every five minutes, comparing the “excess” CO 2 within the city to the CO 2 levels found in the control zone. This painted a picture of how much higher emissions were at different locations in the city compared to a “background” level in the atmosphere.

There are non-human factors influencing the amount of CO 2 that the sensors would be detecting. On both daily and seasonal timescales, patterns of temperature and air movement would cause relatively predictable changes. To focus just on the changes brought about by human activity, the researchers had to account for these rhythms. Once they were accounted for, a clear trend emerged.

No longer rural

The data showed that the increase in emissions across the sites was dependent on population density. There was population growth throughout the Salt Lake City area, but that growth had a higher impact in rural areas. In areas with fewer than 1,000 people per square mile, new housing developments brought more people out into the sprawl, and these population increases brought big increases in emissions.

In dense urban areas with more than 5,000 people per square mile, and in existing suburbs, there had also been population growth—but the increase in emissions wasn’t as high as the population growth would suggest. Traffic, the authors note, plays a large role in this difference: “on-road emissions increased when rural areas were developed into suburban areas,” they write. But per-person on-road emissions, they explain, “decline at higher population densities.”

It might not be too surprising that building housing in previously rural areas comes with a spike in emissions, but there’s still an important insight here: the car traffic associated with new suburbs is an emissions force to be reckoned with, as are the energy requirements of standalone dwellings. If cities are looking to reduce emissions, population density is an important consideration.

Climate decisions need data

Conor Gately, who researches changes in emissions across time and space and wasn’t involved in this research, told Ars that the researchers’ success in drawing strong evidence from the Salt Lake City network was an important result. “What really excites me here,” he said, “is that this is one of the first studies to show that with a reasonably-sized observing network (5-6 sites), it is possible to accurately detect annual and decadal trends in CO 2 emissions at the urban scale.”

Gately cautioned that it may not be possible to extrapolate perfectly from Salt Lake City's suburban sprawl to other cities. “The relationships [between population density and emissions are] very complex and site-specific,” he explained. “It is unclear whether their findings can be extended to other cities.”

As cities become the focus for climate action, getting this kind of data becomes all the more important. There are “a lot of cities in the US and around the world that are making bold commitments to reduce their CO 2 emissions over the next 15-30 years,” said Gately. “But our ability to actually monitor their progress toward meeting these commitments is going to require exactly this type of observation. We should be discussing how to expand these sorts of systems in a way that best supports our climate goals.”

PNAS, 2018. DOI: 10.1073/pnas.1702393115 (About DOIs).