Each Urban Home is Worth 450 Mature Trees in Carbon
The carbon impact of building a home in Cambridge is the equivalent of planting approximately 450 trees. It is more than double the benefit of switching a household to an electric car. Given the positive carbon impact of building homes in transit-connected cities like Cambridge, urban infill is one of the most effective climate mitigations we can enact by far.
Household Emissions Modelling
Using national household surveys, researchers at the University of California at Berkeley developed econometric models of demand for energy, transportation, food, goods, and services that were used to derive average household carbon footprints. Using these models, we can analyze the relative differences of carbon footprint for homes in Cambridge vs. the surrounding communities where workers in Cambridge or Boston are likely to live without access to more homes in Cambridge.
According to their research, Cambridge households have an annual average carbon footprint of 37 tons of CO2/year — practically the lowest in the state outside Boston. This means that moving households to Cambridge over other nearby towns is a highly effective climate mitigation.
To estimate how effective, we can use the geographic distribution of workers in Cambridge. Approximately 48% of Cambridge workers live in other nearby towns in Massachusetts. Each home in Cambridge is one home which can help move a household working in Cambridge or Boston from a surrounding suburb to a much less carbon intensive location here in Cambridge.
Using the home location of 65,000 workers in Cambridge who live in surrounding towns in Massachusetts, we can estimate the effective carbon benefit of the typical new home in Cambridge. Accounting for the per-municipality delta in carbon emissions per household, the increased carbon emissions as a result of these 65,000 jobs is 718,448 tons of additional carbon emissions per year. For every home we create in Cambridge, we have an opportunity to mitigate 11.13 tons of additional carbon emissions per year.
Each household created in Cambridge rather than nearby towns saves more carbon emissions than eliminating all transportation-related emissions from that household, which are estimated at just 9.49 tons/household/year. Alternatively, it saves twice as many tons of CO2 as switching from a gas-powered car to a Tesla Model 3 .
Comparison to Urban Trees
Many in the community have objected to new housing due to concerns about tree removal and reduced carbon sequestration as a result. A typical hardwood tree can absorb as much as 48 pounds of carbon dioxide per year. This means that in order to provide the carbon equivalent benefit of adding a single household in Cambridge, it would be necessary to plant more than 460 trees. Protecting access to public trees is an important part of maintaining a livable city, and ensuring appropriate climate resilience is key to mitigating the direct consequences of climate change on our community. However, the carbon benefits of individual trees pale in comparison to the carbon benefits of infill housing.
According to the Urban Forestry Network, on average, one acre of new forest can sequester about 2.5 tons of carbon annually. This means that in order to provide similar climate benefits to a single new household in Cambridge, it would be necessary to plant 4.5 acres of new forest. Carbon sequestration via trees is an important tool for climate mitigation, but there is no meaningful comparison between the benefits of dense infill housing and tree planting in urban areas.
Embodied carbon and the carbon costs of construction are an important consideration in our overall carbon lifecycle. A recent study examined the carbon costs of construction of new homes under the Passive House standard. In their examination, they determined that CO2 cost of new construction to the Passive House standard had cradle-to-grave emissions estimates of 27 tons to 48 tons of carbon. Since passive house construction reduces household energy needs by 90% compared to prior generations of housing construction, the overall carbon impact of creating new housing to the passive house standard is a reduction of emissions something like 8–9 tons/year in Cambridge. Even ignoring other carbon-related benefits of infill housing, the carbon costs of construction in modern housing pay for themselves within 3–5 years. Once you include the overall carbon benefits of infill housing, new construction pays for its own carbon costs within the first year or two after it is occupied.
Infill housing is crucial to fighting our ongoing climate emergency. For the purposes of climate effectiveness, there’s no more powerful tool available in the local toolbox.
To help move forward Cambridge on creating greater opportunities for infill housing as a climate tool, please consider signing the Cambridge Missing Middle Housing petition.
 Provincetown, Dennis Port and West Dennis all have slightly smaller per-household carbon footprints, correlated with very limited vehicle miles travelled per household, but each has a population of less than 5000 people.
 Tesla estimates a per-mile reduction of 60% in emissions, while the Coolclimate project estimates transportation related emissions of 9.49 tons/household/year. Assuming 60% of those emissions can be reduced, this is 5.69 tons/household/year.