What are the best ways for U.S. cities to combat climate change? A new study co-authored by an MIT professor indicates it will be easier for cities to reduce emissions coming from residential energy use rather than from local transportation — and this reduction will happen mostly thanks to better building practices, not greater housing density.
The study analyzes how extensively local planning policies could either complement the Obama administration’s Clean Power Plan (CPP) of 2015 or compensate for its absence. The CPP is intended to reduce greenhouse gas emissions from power plants. In early 2016, a U.S. Supreme Court ruling halted the measure’s potential enactment; the legal case is unresolved and the Trump administration has announced it intends to unwind the CPP.
“Our take-home message is that cities can do a lot at the local level with housing stock,” says David Hsu, an assistant professor in MIT’s Department of Urban Studies and Planning, and one of three co-authors of a new paper detailing the study’s findings. However, he adds, “In transportation, cities can’t make up for the loss of a national strategy.”
The researchers also found that policies with the biggest local impact vary from city to city, with faster-growing Sun Belt cities such as Houston and Phoenix having the potential to enact a bigger reduction in residential emissions than older cities such as Boston or Philadelphia, which see less change in their housing stock.
“For some cities, some policies will clearly be more effective than others,” Hsu observes.
The paper, “Intersecting Residential and Transportation CO2 Emissions,” appears online in the Journal of Planning Education and Research, with print publication forthcoming. Hsu’s co-authors are John D. Landis, a professor of city and regional planning at the University of Pennsylvania, who is the corresponding author, and Erick Guerra, an assistant professor of city and regional planning at the University of Pennsylvania.
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To conduct the study, the researchers examined economic, environmental, and demographic data from 11 major U.S. cities, then developed models projecting emissions through the year 2030, based on a series of different policy scenarios.
For instance, to analyze ways of cutting emissions from residential energy by 2030, the researchers modeled a baseline scenario in which housing characteristics remained the same. They also modeled scenarios featuring a variety of changes, including the implementation of new energy-efficient construction standards, the building of more multifamily homes, and the retrofitting of homes to save energy.
Simply requiring newly built homes to be more energy efficient would reduce residential emissions by an average of 6 percent by 2030. But requiring existing homes to be retrofitted would yield a further 19 percent reduction of residential emission, on average, across the 11 cities.
Perhaps surprisingly, there was relatively less benefit from a scenario in which the number of newly built single-family homes was reduced by 25 percent by 2030 and replaced by multifamily buildings. This greater housing density “would have virtually no incremental benefit in terms of reduced residential energy use and CO2 emissions,” as the paper states.
“Shifting people to multifamily buildings is what planners have always wanted to do, but that’s actually not as effective as most advocates would have thought,” Hsu says.
The main reason for this, the researchers find, is that as new homes become more energy-efficient, the energy-use differences between larger single-family homes and homes in multifamily dwellings will shrink, thus “reducing the energy and emissions benefits of any substituting attached homes for detached ones,” as the paper states. (The study did find that in Phoenix, one of the 11 cities examined, greater density would have a notable effect on emissions.)
In any case, as Hsu notes, the impact of policies related to construction standards and retrofitting alone is significant: “You can do a lot of things at the local level to affect housing stock that are basically equivalent or even more aggressive than the Clean Power Plan.”
All told, housing accounts for about 20 percent of U.S. carbon dioxide emissions, according to the U.S. Energy Information Administration. As the researchers state in the paper, the “full suite of residential energy conservation programs” could lower total U.S. carbon dioxide emissions by nearly 12 percent compared to the “business-as-usual” projections for 2030, when coupled with the CPP, and by 9 percent even without implementation of the CPP.