A new study shows that produce from urban agriculture has six times the carbon footprint of conventional farms when emissions are linked to food alone—but the calculation changes when the social benefits of urban growing are factored into a more holistic assessment.
“Although there remains lots of work to do in better understanding how to holistically assess urban agriculture, the project represents a useful first attempt at defining best practices that support urban agriculture that’s good for the climate in addition to being good for cities,” the study’s co-lead author, Jason Hawes, a doctoral student at University of Michigan’s School for Environment and Sustainability, told The Energy Mix.
Recently published in the journal Nature Cities, the article by Hawes and colleagues compares the carbon footprint of conventionally grown food and vegetables against produce from 73 urban farms and gardens across five big cities in the United States and Europe. Unlike previous studies, which mainly focused on high-tech options like vertical indoor farms, this study emphasizes “open-air, soil-based” growing options that “comprise the bulk of food-growing spaces in cities.”
It finds that the carbon footprint of foods grown from urban agriculture (UA) is six times greater per serving than from conventional farming, with a few exceptions. Urban produce is carbon-competitive only against crops like tomatoes, which are conventionally grown in carbon-intensive greenhouses, or crops like asparagus that are usually air-freighted.
“Competitiveness depends on growing practices, both in urban and conventional settings,” the researchers explain, suggesting that urban growers could boost their carbon competitiveness by selecting crops that conventionally have a big footprint. They also find that urban farms tend to be the most climate friendly, outperforming individual growing plots and community gardens, in many cases.
The best ways for urban farmers and gardeners to boost their carbon-competitiveness is to extend infrastructure lifetimes and use urban waste as inputs, the researchers say.
But they note another factor that helps balance out UA’s high carbon footprint—one that does not usually factor into carbon measurements: the social benefits of city farms and gardens, which maximize outcomes and reduce emissions beyond what can be calculated from food servings alone.
Urban agriculture practitioners “overwhelmingly reported improved mental health, diets, and social networks,” they explain. “Because food and social benefits are co-products in UA, increasing social benefits can reduce impacts allocated to food.”
Rhonda Teitel-Payne, who works with a wide spectrum of urban growers as co-coordinator for Toronto Urban Growers, told The Mix that “there are many reasons for valuing urban farming that are not related to carbon emissions and we shouldn’t lose sight of that.”
But “the article is a good wake up call to be more focused on growing practices that do reduce the carbon footprint of urban-grown food.”
Extending Infrastructure Timelines
“Most of the climate impacts at urban farms are driven by the materials used to construct them—the infrastructure,” said Benjamin Goldstein, who co-led the study and is an assistant professor at Michigan’s School for Environment and Sustainability.
Urban farms and gardens rely on infrastructure like raised beds, composting systems, and storage sheds, all of which carry their own carbon footprint and can account for 63% of total impacts at urban sites. That means this infrastructure needs to be used for many growing seasons to “pay off’ the carbon investment.
But the development pressures of cities can make it difficult for urban farms and gardens to maintain long-term production, and their lifespans can be cut short before that carbon debt is repaid. In cities like Toronto, for example, where land is expensive and competition for space is fierce, “urban growers often end up farming on land that is only available to them for a relatively short period of time,” said Teitel-Payne.
“This is great for making good use of underutilized land, but it does mean that garden infrastructure is disassembled frequently and may end up going to waste.”
To address the issue, she recommends community land trusts and long-term lease farming projects that help sustain longer-term urban land stewardship. Some farms also anticipate movement and design themselves for relocation, like Sole Food Street Farms in Vancouver or the Bowery Project in Toronto, so that infrastructure is moved from site to site rather than discarded.
Urban Waste as Input
By reusing or upcycling materials, urban growers can reduce the emissions linked to infrastructure by as much as 52%—an amount that would make all three forms of urban agriculture carbon competitive against conventional growing if all materials were sourced from urban waste.
“According to our findings, [urban agriculture] is most climate-friendly when it serves as a hub for symbiosis of building materials, organic waste, and rainwater,” the study states. But overall, materials are only recycled opportunistically, and the recycling rates of construction and demolition waste are mostly low.
Drawing from experience with urban gardeners in Toronto, Teitel-Payne said it can be a challenge to use repurposed materials in gardens located in public spaces, as some people find the aesthetic of handmade infrastructure unappealing. She also has concerns about people using materials with toxins that can contaminate the soil.
Recycling rainwater and graywater for irrigation can also help reduce UA emissions, the study authors note. Whereas more than 50 of the sites in the study recover rainwater, only four derive the majority of their irrigation in this way, which increases emissions due to pumping, treating, and distributing water.
And another major way that reusing waste can help cut emissions is when waste systems supply inputs to build soil, such as through composting. Urban farms and gardens widely use compost derived from local food and yard waste, and are able to largely displace synthetic fertilizers used in conventional agriculture, a major source of emissions. But the researchers do note that compost itself can be a significant source of methane if it isn’t properly managed, and that methane-generating anaerobic composting was the highest-impact input for 22 of the sites in their study.
Cities can reduce emissions from anaerobic composting by centralizing compost operations and managing them professionally, or by training farmers on proper composting practices, the study authors say.
Teitel-Payne agrees that community organizations need more resources to run composting systems in more locations. “The article makes a strong case for the benefits of providing these resources,” she said.
She added that urban growers have options for improving compost management, noting that some of the community composting systems used in Toronto could help respond to the study’s concerns about methane. Compost in these systems is collected at a facility near residences and used locally, she said. The composting is also run by organizations that provide training and monitoring, “so that you don’t have the anaerobic composting that the article describes.”
Generating Social Benefits
Though emissions linked to urban agriculture are higher than conventional farms, urban growing spaces also exist as multifunctional areas that generate other social benefits.
Those outcomes were revealed during the pandemic, when many city dwellers discovered that “having access to green spaces and nature is important for mental health,” said Teitel-Payne. Not only can they help connect community members and grow social networks, but they also provide opportunities for “people who are marginalized and undervalued in other spaces,” as well as “people with disabilities, those who are vulnerably housed, and people living in poverty.”
The study authors also make note of these points, finding that after distributing UA emissions across their other functions, their carbon competitiveness increases. “Most of our urban farms and individual gardens outperform conventional agriculture when more than 90% of infrastructure impacts are allocated to non-food services,” they write.
While that number seems high, evidence from past research suggests it is attainable. “Cost–benefit analysis of a collective garden in the United Kingdom estimated that social benefits, such as improved well-being and reduced hospital admissions, accounted for 99.4% of total economic value generated onsite.”
“Because emissions allocation often follows economic value generation, growing spaces that maximize social benefits can outcompete conventional agriculture when UA benefits are considered holistically.”
Breaking down this emissions allocation can be tricky, however, because “it is extremely difficult to put a dollar value on some of the useful outcomes of urban agriculture,” said Hawes. But there may be other ways to delve into this issue more holistically, by combining life-cycle analyses with social science techniques.
“In the meantime,” he told The Mix, “it’s worth acknowledging that our carbon footprinting work is useful for identifying ways to improve urban agriculture, but that work remains to be done.”
