Green Menace? Study Finds Food From Urban Agriculture Has 6x Larger Carbon Footprint

Urban Garden in Nantes, France

Urban garden in Nantes, France. Credit: Baptiste Grard

Urban agriculture, while offering significant social and environmental benefits, has a carbon footprint considerably higher than conventional farming, according to a University of Michigan-led study. Strategic improvements in crop selection, infrastructure longevity, and waste utilization are suggested to enhance its sustainability.

A new University of Michigan-led international study finds that fruits and vegetables grown in urban farms and gardens have a carbon footprint that is, on average, six times greater than conventionally grown produce.

However, a few city-grown crops equaled or outperformed conventional agriculture under certain conditions. Tomatoes grown in the soil of open-air urban plots had a lower carbon intensity than tomatoes grown in conventional greenhouses, while the emissions difference between conventional and urban agriculture vanished for air-freighted crops like asparagus.

Urban Garden in Detroit

University of Michigan researchers evaluate an urban garden in Detroit. Credit: Dave Brenner, University of Michigan

Optimizing Urban Agriculture for Climate Benefits

“The exceptions revealed by our study suggest that urban agriculture practitioners can reduce their climate impacts by cultivating crops that are typically greenhouse-grown or air-freighted, in addition to making changes in site design and management,” said study co-lead author Jason Hawes, a doctoral student at U-M’s School for Environment and Sustainability.

“Urban agriculture offers a variety of social, nutritional, and place-based environmental benefits, which make it an appealing feature of future sustainable cities. This work shines light on ways to ensure that urban agriculture benefits the climate, as well as the people and places it serves.”

Urban agriculture, the practice of farming within the confines of a city, is becoming increasingly popular worldwide and is touted as a way to make cities and urban food systems more sustainable. By some estimates, between 20% and 30% of the global urban population engages in some form of urban agriculture.

Despite strong evidence of the social and nutritional benefits of urban agriculture, its carbon footprint remains understudied. Most previously published studies have focused on high-tech, energy-intensive forms of UA—such as vertical farms and rooftop greenhouses—even though the vast majority of urban farms are decidedly low-tech: crops grown in soil on open-air plots.

Urban Collective Garden at a New York City Housing Authority Site

Urban collective garden at a New York City Housing Authority site. The garden provides educational and recreational opportunities for residents, in addition to fresh produce. Credit: Nevin Cohen

Urban vs. Conventional Agriculture: A Comparative Study

The new U-M-led study, scheduled for publication today (January 22) in the journal Nature Cities, aimed to fill some of the knowledge gaps by comparing the carbon footprints of food produced at low-tech urban agriculture sites to conventional crops. It used data from 73 urban farms and gardens in five countries and is the largest published study to compare the carbon footprints of urban and conventional agriculture.

Three types of urban agriculture sites were analyzed: urban farms (professionally managed and focused on food production), individual gardens (small plots managed by single gardeners), and collective gardens (communal spaces managed by groups of gardeners).

For each site, the researchers calculated the climate-altering greenhouse gas emissions associated with on-farm materials and activities over the lifetime of the farm. The emissions, expressed in kilograms of carbon dioxide equivalents per serving of food, were then compared to foods raised by conventional methods.

On average, food produced through urban agriculture emitted 0.42 kilograms of carbon dioxide equivalents per serving, six times higher than the 0.07 kg CO2e per serving of conventionally grown produce.

“By assessing actual inputs and outputs on urban agriculture sites, we were able to assign climate change impacts to each serving of produce,” said study co-lead author Benjamin Goldstein, assistant professor at U-M’s School for Environment and Sustainability. “This dataset reveals that urban agriculture has higher carbon emissions per serving of fruit or vegetable than conventional agriculture—with a few exceptions.”

Collective Garden in London, England

Preparing seedlings for planting at a collective garden in London, England. Urban food production spaces like this can provide numerous social and community benefits but require careful crop selection and management to cut the carbon footprints of cities. Credit: Victoria Schoen

The Environmental Impact of Urban Farming Inputs

Joshua Newell, professor and co-director of the Center for Sustainable Systems at SEAS, led the University of Michigan portion of the project. The U-M researchers formed an international team of collaborators from universities near the various food-growing sites. Ten of those collaborators are co-authors of the Nature Cities study.

Farmers and gardeners at urban agriculture sites in France, Germany, Poland, the United Kingdom, and the United States were recruited as citizen scientists and used daily diary entries to record inputs and harvests from their food-growing sites throughout the 2019 season.

Inputs to the urban agriculture sites fell into three main categories: infrastructure (such as the raised beds in which food is grown, or pathways between plots), supplies (including compost, fertilizer, weed-blocking fabric, and gasoline for machinery), and irrigation water.

“Most of the climate impacts at urban farms are driven by the materials used to construct them—the infrastructure,” Goldstein said. “These farms typically only operate for a few years or a decade, so the greenhouse gases used to produce those materials are not used effectively. Conventional agriculture, on the other hand, is very efficient and hard to compete with.”

For example, conventional farms often grow a single crop with the help of pesticides and fertilizers, resulting in larger harvests and a reduced carbon footprint when compared to urban farms, he said.

Best Practices for Sustainable Urban Farming

The researchers identified three best practices crucial to making low-tech urban agriculture more carbon-competitive with conventional agriculture:

  • Extend infrastructure lifetimes. Extend the lifetime of UA materials and structures such as raised beds, composting infrastructure, and sheds. A raised bed used for five years will have approximately four times the environmental impact, per serving of food, as a raised bed used for 20 years.
  • Use urban wastes as UA inputs. Conserve carbon by engaging in “urban symbiosis,” which includes giving a second life to used materials, such as construction debris and demolition waste, that are unsuitable for new construction but potentially useful for UA. The most well-known symbiotic relationship between cities and UA is composting. The category also includes using rainwater and recycled grey water for irrigation.
  • Generate high levels of social benefits. In a survey conducted for the study, UA farmers and gardeners overwhelmingly reported improved mental health, diet, and social networks. While increasing these “nonfood outputs” of UA does not reduce its carbon footprint, “growing spaces which maximize social benefits can outcompete conventional agriculture when UA benefits are considered holistically,” according to the study authors.

Reference: “Comparing the carbon footprints of urban and conventional agriculture” by Jason K. Hawes, Benjamin P. Goldstein, Joshua P. Newell, Erica Dorr, Silvio Caputo, Runrid Fox-Kämper, Baptiste Grard, Rositsa T. Ilieva, Agnès Fargue-Lelièvre, Lidia Poniży, Victoria Schoen, Kathrin Specht and Nevin Cohen, 22 January 2024, Nature Cities.
DOI: 10.1038/s44284-023-00023-3

Co-authors of the Nature Cities paper are from McGill University in Canada, University Paris-Saclay and the Agroecology and Environmental Research Unit in France, the University of Kent in the United Kingdom, ILS Research in Germany, City University of New York and Adam Mickiewicz University in Poland.

Support for the project was provided by the UK Economic and Social Research Council, German Federal Ministry of Education and Research, French National Research Agency, U.S. National Science Foundation, Poland’s National Science Centre, and the European Union’s Horizon 202 research and innovation program.

5 Comments on "Green Menace? Study Finds Food From Urban Agriculture Has 6x Larger Carbon Footprint"

  1. The goal is to make growing your own food illegal and completely controlling the food supply.
    You will eat what you are told and like it.

  2. “…, conventional farms often grow a single crop with the help of pesticides and fertilizers, resulting in larger harvests and a reduced carbon footprint when compared to urban farms, …”

    Is the energy required to manufacture and transport the pesticides and fertilizers taken into account?

  3. I agree with Clyde Spencer; we need much more research on this matter before we can evaluate the relative merits of both agricultural & urban farming.

    Frequent Flyer also has a point…. If this project was financed by business interests; are they seeking to control the market.

    I hold this study brings a problem to our attention; yet a lot more work needs to be done before we can accept it as GOSPEL TRUTH.

  4. Instead of using “serving” as the comparative unit, I bet that if you figured a way to compare units of nutritional density, the results would be significantly different.

    I also agree with Frequent Flyer—this really smells like a study influenced by conflict of interest.

    Not to mention the “carbon footprint” angle is a gimmick to guarantee publication. In every other way locally sourced food is better. I’m only surprised they didn’t manage to shoehorn gender in there somewhere.

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