DAILY SCIENCE

Hitting climate targets depends on building smaller homes and more multi-family units—not just energy efficiency
Carbon emissions from US households have been falling since 2005, but not fast enough
May 11, 2021

Greenhouse gas (GHG) emissions from U.S. households have been falling since 2005, according to a new study. But the analysis also reveals hidden patterns that could block the emissions reductions necessary to limit climate change.

Researchers from the Yale School of the Environment gathered figures from various government databases and calculated energy consumption and GHG emissions associated with heating, cooling, water heating, and other activities (such as lighting, appliances, and electronics) in U.S. households from 1990 through 2015. They used mathematical equations to model how factors such as population growth and distribution, housing types and density, the age of housing stock, the number of people per household, and energy sources (such as oil, natural gas, or electricity) affect energy use and emissions.

Annual GHG emissions from U.S. households peaked in 2005, the researchers report in Environmental Research Letters. Since then, emissions have been decreasing by about 2% per year.

One reason for this that newer homes and appliances are more energy efficient. But the main explanation is that the U.S. electric grid is decarbonizing. Less fossil fuel power and more renewable power means that the average kilowatt hour of electricity results in fewer GHG emissions than it used to.

The decrease isn’t fast enough to keep global warming within 1.5 °C – that would require emissions to fall about 7% per year.

And meanwhile, other trends are pushing things in the opposite direction, the researchers found. Population growth, more houses with air conditioning, larger homes, and fewer people per household — the latter of which result much more square footage per person in newer homes — are all driving residential energy use and emissions upwards.

Housing is thought to be one of the easier aspects of the energy system to decarbonize. The mantra that has arisen in recent years is “Electrify Everything,” meaning that we should switch most energy-consuming household systems to electric versions, all while thoroughly decarbonizing the electricity supply.

And in fact, that strategy will likely bear increasing payoff in the coming years. “With electricity now much less GHG intensive than it was 15 years ago, I expect that the GHG savings from switching to electric heating are much larger now, especially when switching to heat pumps for space/water heating,” says study team member Peter Berrill, a graduate student at Yale School of the Environment.

But the new study adds an asterisk to the “Electrify Everything” strategy. “Going forward, it is too risky to rely on lower GHG intensity electricity supply to deliver all of the necessary emission reductions in the residential sector,”  Berrill says. “This will likely be very much insufficient to putting residential emissions on track to meet climate goals.”

The study suggests that the fabric of the built environment matters, not just what goes on in individual homes. It is the first to show how the shrinking number of people per household is affecting residential energy use and GHG emissions.

“I was surprised at the extent to which reductions in household size drove increases in total energy demand and emissions,” says Berrill. “This is a demographic trend which is very likely to continue, and this brings up important questions about what types of homes we need to be building now, and for who?”

The results suggest that what really needs to change is the trend towards more floor space per person. In other words, hitting climate targets depends on building smaller single-family homes and more multi-family housing (including converting large single-family homes to multi-unit dwellings), consistent with the shrinking number of people per household. Yet another recent study by the same researchers showed that federal housing policies aren’t in line with these needs.

“We need decarbonization of electricity, but we can’t rely on that alone for the residential emission reductions,” Berrill says. “We need to also make houses more efficient (especially existing homes), and allow for smaller housing to be built/redeveloped!”

Source: Berrill P. et al.Drivers of change in US residential energy consumption and greenhouse gas emissions, 1990-2015.” Environmental Research Letters 2021.

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