Food from urban agriculture has a carbon footprint six times greater than conventional products, study finds

A new international study led by the University of Michigan finds that fruits and vegetables grown on farms and urban gardens have an average carbon footprint six times greater than conventionally grown produce.

However, few urban crops matched or outperformed conventional agriculture under certain conditions. Tomatoes grown in the soil of outdoor urban plots had lower carbon intensity than tomatoes grown in conventional greenhouses, while the difference in emissions between conventional and urban farming disappeared for transported crops by air like asparagus.

“The exceptions revealed by our study suggest that urban agriculture practitioners can reduce their climate impacts by growing crops typically grown in greenhouses or flown in, in addition to making changes to site design and management ” said Jason, co-lead author of the study. Hawes, a doctoral student in UM’s School for Environment and Sustainability.

“Urban agriculture offers a variety of social, nutritional and local environmental benefits, making it an attractive part of future sustainable cities. This work highlights ways to ensure that urban agriculture benefits the climate, as well as ‘to people and places. it’s useful.’

Urban farming, the practice of farming within the confines of a city, is becoming increasingly popular around the world and is being touted as a way to make cities and urban food systems more sustainable. By some estimates, between 20 and 30 percent of the world’s 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 urban agriculture, such as vertical farms and rooftop greenhouses, even though the vast majority of urban farms are decidedly low-tech : crops grown in the ground on open-air plots.

The study, published in the journal Nature Cities, aimed to fill some knowledge gaps by comparing the carbon footprint of food produced in low-tech urban agriculture sites to that of conventional crops. It used data from 73 urban farms and gardens in five countries and is the largest published study comparing the carbon footprints of urban and conventional agriculture.

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

For each site, researchers calculated climate-changing greenhouse gas emissions associated with farm materials and activities over the life of the farm. Emissions, expressed in kilograms of carbon dioxide equivalent per serving of food, were then compared to those of foods raised using conventional methods.

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

“By assessing actual inputs and outputs at urban agriculture sites, we were able to attribute climate change impacts to each portion of produce,” said Benjamin Goldstein, co-senior author of the study and assistant professor at the UM School for Environment and Sustainability. “This dataset reveals that urban agriculture generates higher carbon emissions per serving of fruit or vegetable than conventional agriculture, with few exceptions.”

Joshua Newell, professor and co-director of the Center for Sustainable Systems at SEAS, led the University of Michigan portion of the project. UM researchers have formed an international team of collaborators from universities close to the different food production sites. Ten of these collaborators are co-authors of the Nature Cities study.

Farmers and gardeners from 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 production sites throughout the 2019 season.

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

“Most climate impacts on urban farms are due to the materials used to build them, namely infrastructure,” Goldstein said. “These farms typically only operate for a few years or a decade, so the greenhouse gases used to produce these materials are not used efficiently. Conventional agriculture, on the other hand, is very efficient and difficult to compete with. “

For example, conventional farms often grow a single crop using pesticides and fertilizers, leading to larger harvests and a reduced carbon footprint compared to urban farms, he said.

Researchers have identified three key best practices for making low-tech urban agriculture more carbon competitive compared to conventional agriculture:

• Extend the lifespan of infrastructure. Extend the life 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, than a raised bed used for 20 years.

• Use urban waste as DU inputs. Conserve carbon by engaging in “urban symbiosis”, which involves giving a second life to used materials, such as construction debris and demolition waste, which are not suitable for new construction but are potentially useful for UA. The most well-known symbiotic relationship between cities and urban agriculture is composting. This category also includes the use of rainwater and recycled gray water for irrigation.

• Generate high levels of employee benefits. In a survey conducted for the study, UA farmers and gardeners overwhelmingly reported improvements in their mental health, diet and social networks. Even if the increase in “non-food production” of urban agriculture does not reduce its carbon footprint, “growing spaces that maximize social benefits can outperform conventional agriculture when the benefits of urban agriculture are considered holistic way,” according to the authors of the study.

The paper’s co-authors come from McGill University in Canada, Université Paris-Saclay and the Agroecology and Environment Research Unit in France, the University of Kent in the UK, ILS Research in Germany, the City University of New York and the Adam Mickiewicz University. Poland.