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Study finds cities can reduce CO2 more easily from residential conservation than transportation

A new study by a team from the University of Pennsylvania and MIT suggests it will be easier for cities to reduce CO2 emissions coming from residential energy use rather than from local transportation. This reduction will happen mostly thanks to better building practices, not greater housing density. The study is published in the Journal of Planning Education and Research.

The study used a series of fixed-ratio projections and scenarios to explore the potential for local residential energy conservation mandates and compact growth programs to reduce locally-based CO2 emissions in eleven representative US metropolitan areas. The modeling showed that averaged across all eleven metros, residential energy conservation mandates could reduce residential CO2 emissions in 2030 by an average of 30% over and above 2010 levels.

In terms of implementation, residential conservation standards were found to be goal-effective, cost-effective, scale-effective, and in the case of new construction standards, reasonably resistant to local political pushback. Local compact growth programs do not perform as well. If accompanied by aggressive efforts to get drivers out of their cars, compact growth programs could reduce auto-based 2030 CO2 emissions by as much as 25% over and above any emissions reductions attributable to higher fuel economy standards.

Unaccompanied by modal diversion programs, the stand-alone potential for local compact growth programs to reduce auto-based CO2 emissions falls into a more modest range of 0 to 7 percent depending on the metropolitan area. Based on past performance, local compact growth programs are also likely to have problems in terms of their goal- and scale-efficiency, and their potential to incur political pushback.

— Landis et al.

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 US 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. In transportation, cities can’t make up for the loss of a national strategy.

—David Hsu, co-author

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.

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% by 2030. But requiring existing homes to be retrofitted would yield a further 19% reduction of residential emission, on average, across the 11 cities.

There was relatively less benefit from a scenario in which the number of newly built single-family homes was reduced by 25% 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, the authors found.

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.

—David Hsu

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. (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, Hsu noted, 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% of US carbon dioxide emissions, according to the US Energy Information Administration. The researchers found that the full suite of residential energy conservation programs could lower total US. carbon dioxide emissions by nearly 12% compared to the “business-as-usual” projections for 2030, when coupled with the CPP, and by 9% even without implementation of the CPP.

On the transportation side, the researchers also modeled urban emissions growth through the year 2030. They again evaluated a baseline scenario in which current conditions essentially continue intact, as well as a handful of alternate scenarios in which total vehicle-miles traveled varies due to increased use of mass transit and changes in housing density.

The team also compared the effects of these local planning efforts to the reduction in emissions that would occur under the Obama administration’s plan to increase the fuel efficiency of the automobile fleet to 54.5 miles per gallon by the year 2027.

The study’s bottom-line finding is that, by 2030, a federal mandate increasing vehicle fuel efficiency to 54.5 miles per gallon would reduce auto-based emissions in the 11 cities by 38%, in the absence of any additional mass transit or density programs. That number would increase to 46%, on average, if the cities adopted robust transit and density policies. (The Trump administration has stated it will review and may possibly drop the existing fuel-efficiency plan.)

The results for increasing the average fuel efficiency of the US fleet are still stronger than what we can do on the planning side,” Hsu says. However, he notes, that is a relative outcome, and incremental emissions reductions from increased use of mass transit, among other things, may well be worth pursuing at the municipal level. The 11 cities analyzed in the study are Atlanta, Boston, Chicago, Cleveland, Denver, Houston, Los Angeles, Miami, Philadelphia, Phoenix, and Seattle.

Changing circumstances could alter the model’s projections—e.g., a rapid electrification of the automobile fleet. The paper also makes clear that the average effects found across the 11 cities vary considerably. Mandating that newly built homes be more energy efficient would reduce residential emissions by 10 to 13% in Houston and Phoenix, but only by 3 to 5% in slower-growing metro areas, including Boston, Chicago, Cleveland, and Philadelphia.

The amount of emissions reduction possible in any urban area also depends on existing levels. Cleveland and Denver, which both rely heavily on coal-fired power plants, have the highest rates of emissions per units of energy produced; they produce 34.3 and 32.7 kilograms of carbon dioxide emissions per million BTU, respectively.

Los Angeles, by contrast, produces only 10.7 kilograms of carbon dioxide emissions per million BTU, making its energy use about one-third as carbon-intensive as that of Cleveland and Denver. Ultimately each metropolitan area, Hsu suggests, may have to find its own path toward a clean energy future.


  • John D. Landis, David Hsu, Erick Guerra (2017) “Intersecting Residential and Transportation CO2 Emissions: Metropolitan Climate Change Programs in the Age of Trump” Journal of Planning Education and Research doi: 10.1177/0739456X17729438


Thomas Pedersen

In Phoenix and Houston it is probably correct that more CO2 would be saves by making the house (insulation + AC) more efficient, provided that the power comes from high-CO2 sources.

However, I we are to do anything serious about climate change - and many other pollutants from energy generation - (nearly) all electricity production has to be switched to zero-emission sources within a couple of decades. That is why the transportation sector needs to electrify itself.

Luckily, the power-hungry sun belt cities are also the easiest to switch to solar (and wind and nuclear), because of the concurrence of power production and demand. Also, here solar energy is fast becoming 'too cheap to meter'. Well, not quite, but prices are plummeting faster than most projected just a few years ago. Solar + cold (ice) storage for the A/C + batteries for lights, TV, etc. and you could practically go off-grid in those states (not that I recommend that).

And you see clearly that a number of BEV's in the garage with a combined capacity of 2-300 kWh would dovetail quite nicely in this scenario.

Making the A/C more efficient or installing more/better insulation will compete with the prospect of just installing more panels, with the incremental cost of adding another panel dropping daily.

As much as I am personally in favor of energy efficiency, I'm just not sure it makes business sense in many cases with cheap, clean energy.

Brent Jatko

Great points by Thomas Pedersen.

I'd add anecdotally that there are many houses in Houston that are low-efficiency because they are old homes without insulation and with a lot of air leaks, etc.

I am hoping that homeowners will choose to exploit the insurance for rebuilding after the hurricane to put in more efficient A/C and heating systems.

Some of the affected houses were fairly large (2500+ square ft.) and not new (built ca. 1965) and could save a lot of energy in a remodel job.


The building codes just be upgraded for builders to construct greener houses, even if initial cost is 5% to 10% more.

“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.”

Because the CPP was not designed to minimize carbon emissions.  The CPP, especially its criteria for "cleanliness", was designed to eliminate coal and nuclear power and designate natural gas as the winner, supplemented by whatever "renewables" were able to be profitable in the coming regime.

China is looking to supply residential (and presumably commercial) space heat with meltdown-proof "swimming pool" reactors.  Converting the electric grid from fossil to nuclear power would decarbonize it also.  The reductions achieveable this way are radically greater than 12%.

A 12% reduction should be instantly seen as grossly inadequate.  We need something between 80% and 110% (net carbon sequestration).  Any program which cannot deliver reductions on this scale should be instantly dismissed as a non-starter.

the power-hungry sun belt cities are also the easiest to switch to solar (and wind and nuclear)

If you don't have nuclear, your backup for the unreliable solar and wind is fossil.  For the foreseeable future, that means natural gas (with all its attendant methane leakage).

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