So far, the focus has been on reducing CO2 emissions in the United States on two obvious goals: power generation and transportation. But to achieve the profound decarbonization that we need to combat climate change, we cannot really ignore a significant source of emissions. And the places we live are significant sources – even before the pandemic stopped many of us from leaving home, US households made up about 20 percent of the country's energy-related carbon emissions.
According to the authors of a new analysis, US housing construction alone would be the sixth largest greenhouse gas emitter in the world and thus ahead of Germany. How can we fix that to address climate change? To find out, some University of Michigan researchers have done an incredibly detailed analysis of the U.S. housing stock, finding out the factors that have affected carbon emissions. They then calculated which options these emissions could reduce where they would be compatible with the goals of the Paris Agreement.
Emissions at home
To understand the energy consumption of US homes, researchers began by averaging samples of homes in each state, with the number of buildings ranging from 100,000 to 10 million. This data included information about the age of the building, how much space it had, how it was heated, and so on. Your model also included details about things like power consumption, case density, details of the power grid, and so on.
The researchers first examined fundamental influences on energy consumption. The biggest is the climate. Places with a greater number of days that required heating or cooling typically used more energy. For example, Maine, Vermont, and Wisconsin are the three most energy-intensive states in terms of housing.
Another influence is the age of the building. The typical U.S. home has been in use for an average of 40 years, and the older ones were dismantled before much efficiency technology was common. While these can be upgraded, it's just more common to incorporate them into newer builds. But that will not be good enough, since we already have a decade of living space, which on average will still be with us in 2050.
However, the local electricity network and the availability of fuels have had a major impact on greenhouse gas emissions, with a strong correlation between household emissions and the carbon intensity of the network. On average, a US household is associated with 45 kg of CO2 equivalent emissions per year.
The researchers next divided their data by zip code (using nearly 9,000 zip codes) and compared it to census data to calculate the per capita numbers. These showed that housing a typical person causes slightly less than three tons of CO2 emissions each year. However, there were significant differences, with the number ranging from less than half a ton to over 10 tons depending on the location.
One of the main influences here was wealth. On average, high-income residents have homes that generate 25 percent more emissions per capita than low-income people. This seems to have to do with the fact that wealthier people tend to have more floor space in their homes. As stated elsewhere, population density correlates with lower emissions per capita – partly because this means smaller houses, partly because the advantages of apartment buildings exist.
While the United States plans to exit the Paris Agreement, it provides an easy way to understand the goals: 28 percent emissions reduction by 2025 and 80 percent reduction by 2050. The researchers therefore decided to look into what might be needed to get the US real estate inventory on track to achieve these goals. The researchers examined a number of scenarios. One of these was the continuation of current trends in efficiency and network decarbonization. another concerned retrofitting houses for greater efficiency; A third concerned the combination of aggressive retrofitting with network decarbonization. The last scenario layered distributed solar energy on these aggressive measures.
The good news is that by simply following existing trends, US real estate can help achieve the 2025 goals. However, that doesn't bring us to the more challenging 2050 goals. This is mainly because many households continue to use fossil fuels for heating and cooking instead of electricity. This can be partially overcome by increasing the rate of heat pump systems using electricity for heating and cooling to three times their current rate. Combined with decentralized solar energy, better grids, and efficiency retrofits, the case may reach the 2050 goal. However, increasing the number of smaller apartments and apartments with several residential units would significantly increase our chances.
Of course, there will be regional differences in functionality that the authors take into account when comparing Los Angeles with Boston. Los Angeles is already on track to meet the Paris 2050 goals, partly because energy requirements are low there (fewer heating and cooling days) and California is trying to decarbonize its network. In contrast, Boston has to do all of this: extensive retrofitting, decarbonization of the power grid, installation of many heat pumps and solar panels in 40 percent of its homes. Even then, the communities in the Boston area that were supposed to reach the destination were much denser than they are now.
Separated from everything that needs to be done is the question of how to pay for it. Multi-family houses are more likely to be rented out. In many cases, the owner of the building does not pay the energy costs and therefore has no great incentive to carry out the necessary retrofits. The authors also note that there are 25 million households in the United States where poverty is so severe that energy bills compete with food. Finding how to overcome these hurdles is a challenge on a societal level.
PNAS, 2020. DOI: 10.1073 / pnas.1922205117 (About DOIs).