So far, the focus on cutting the US’s carbon emissions has fallen on two obvious targets: electrical production and transportation. But to engage in the sort of deep decarbonization we’ll need to address climate change, we can’t really ignore any significant source of emissions. And the places we live are significant sources—even before the pandemic kept many of us from leaving the house, US households accounted for about 20 percent of the country’s energy-related carbon emissions.
On its own, the authors of a new analysis say, US housing would be the world’s sixth largest emitter of greenhouse gases, placing it ahead of Germany. How do we get that down in order to address climate change? To find out, some researchers from the University of Michigan did an incredibly detailed analysis of the US’s housing stock, figuring out the factors that influenced its carbon emissions. They then calculated which options might bring those emissions down to where they’d be compatible with the goals of the Paris Climate Agreement.
Emissions at home
To understand US housing’s energy use, the researchers started with average samples of the housing in each state, with the number of buildings ranging from 100,000 to 10 million. This data included information on the building’s age, how much space it enclosed, how it’s heated, and so on. Their model also incorporated details of things like power use, housing density, details of the electric grid, and so on.
The researchers started by looking at basic influences on energy use. The single largest is the climate. Places that had a larger number of days that required heating or cooling tended to see their housing using more energy. So, for example, the three most energy-intensive states, housing wise, are Maine, Vermont, and Wisconsin.
Another influence is building age. The typical US house ends up being used for an average of 40 years, and the older ones were built back before a lot of efficiency technology was common. While these can be retrofitted, it’s simply more common to incorporate it in newer builds. But that won’t be good enough, given that we already have a decade’s worth of housing that, on average, will still be with us in 2050.
When it comes to greenhouse gas emissions, however, the local electric grid and availability of fuels had a major influence, with a strong correlation between household emissions and the carbon intensity of the grid. On average, a US household is associated with 45kg of CO2 equivalent emissions each year for each square meter of floor space.
Next up, the researchers split their data by postal code (using nearly 9,000 ZIP codes) and compared that to census data in order to calculate per-capita figures. These showed that a typical person’s housing contributes a bit under three tonnes of carbon emissions each year. But there was considerable variation, with the figure ranging from under half a tonne to over 10 tonnes, depending on location.
One of the main influences here was wealth. High-income inhabitants, on average, have housing that produces 25 percent more emissions per capita than low-income individuals. That seems to be related to the fact that wealthier individuals tend to have more floor space in their housing. As had been found elsewhere, population density correlates with lower emissions per capita—partly because it means smaller houses, and partly because of the advantages of multi-unit dwellings.
Meeting goals
While the US plans to pull out of the Paris Climate Agreement, it still provides a simple way to understand goals: a 28-percent emissions reduction by 2025 and an 80-percent drop by 2050. So the researchers decided to see what might be needed to get the US’ housing stock on a path to hit those goals. The researchers looked into a number of scenarios. One involved a continuation of current trends in efficiency and grid decarbonization; another involved retrofitting homes for greater efficiency; a third involved combining aggressive retrofitting with grid decarbonization. The final scenario layered distributed solar on these aggressive measures.
The good news is that by simply following existing trends, US housing can do its part to meet the 2025 goals. That will not, however, get us to the more challenging 2050 goals. That’s primarily because many homes will continue to use fossil fuels, rather than electricity, for things like heating and cooking. This can be overcome in part by boosting the rate of heat pump systems, which use electricity to power both heating and cooling, to three times their current rate. When that’s combined with distributed solar, better grids, and efficiency retrofits, housing can potentially reach the 2050 target. But increasing the number of smaller and multi-unit dwellings would greatly increase our chances.
Of course, there will be regional differences in how things work out, which the authors consider by comparing Los Angeles to Boston. Los Angeles is already on track for the Paris 2050 goals, in part because energy demand is low there (fewer heating and cooling days) and because of California’s efforts to decarbonize its grid. Boston, by contrast, has to do all of the above: extensive retrofits, decarbonize its grid, install lots of heat pumps, and have solar panels on 40 percent of its homes. Even then, the communities within the Boston area that were projected to meet the goal were significantly more dense than they are at present.
Separate from everything that has to be done is the matter of how to pay for it. Multiunit dwellings are more likely to involve rentals. In many of these, the building’s owner won’t be paying the energy bills, and therefore doesn’t have much incentive to do the needed retrofits. The authors also note that there are 25 million households in the US where poverty is severe enough that the energy bill competes with food. Figuring out how to overcome these hurdles is a society-level challenge.
PNAS, 2020. DOI: 10.1073/pnas.1922205117 (About DOIs).
https://arstechnica.com/?p=1694223