While urban farming has significant social and environmental benefits, its carbon footprint is significantly higher than traditional farming, according to a study led by the University of Michigan. Strategic improvements in crop selection, infrastructure longevity and waste utilization are recommended to improve its sustainability.A new international study led by the University of Michigan has found that fruits and vegetables grown in urban farms and gardens have an average carbon footprint six times greater than those grown conventionally.
However, under certain conditions, a handful of urban-grown crops perform as well as or better than traditional agriculture. Tomatoes grown in soil on urban open plots have a lower carbon intensity than tomatoes grown in conventional greenhouses, while the difference in emissions between conventional and urban farming disappears on air-run crops such as asparagus.
Optimizing urban agriculture for climate changebenefit
"The exceptions our study reveals suggest that urban agriculture practitioners can reduce climate impacts by growing crops that are typically grown in greenhouses or flown in, in addition to changes in site design and management," said study co-first author Jason Hawes, a doctoral student in MIT's School of Environment and Sustainability.
"Urban agriculture provides a variety of social, nutritional and place-based environmental benefits, making it an attractive feature for future sustainable cities. This work sheds light on how to ensure urban agriculture is good for the climate as well as the people and places it serves."
Urban farming, the practice of farming within cities, is growing in popularity globally and is heralded as a way to make cities and urban food systems more sustainable. It is estimated that 20% to 30% of the global urban population engages in some form of urban agriculture.
Despite strong evidence that urban agriculture has social and nutritional benefits, its carbon footprint remains under-researched. Most previously published research has focused on high-tech, energy-intensive forms of urban agriculture, such as vertical farms and rooftop greenhouses, although the vast majority of urban farms are low-tech: growing crops in soil in open-air plots.
Urban agriculture versus traditional agriculture: a comparative study
The new University of Massachusetts-led research, published today (January 22) in the journal Nature Cities, aims to fill some knowledge gaps by comparing the carbon footprint of food produced from low-tech urban farming sites to that of conventional crops. The study, which used data from 73 urban farms and gardens in five countries, is the largest publicly available study comparing the carbon footprint of urban farming and conventional farming.
Three types of urban farming sites were analyzed: urban farms (professionally managed and focused on food production), individual gardens (small plots of land managed by individual gardeners) and collective gardens (public spaces managed by groups of gardeners).
The researchers calculated the climate-changing greenhouse gas emissions associated with farm materials and activities over the life of each farm. Emissions are expressed in kilograms of CO2 equivalent per serving and then compared to conventionally raised foods.
Food produced by urban agriculture emits an average of 0.42 kilograms of CO2e per serving, which is six times higher than the 0.07kg of CO2e per serving produced by conventional farming.
"By assessing the actual inputs and outputs of an urban farming site, we are able to assign climate change impacts to each produce," said study co-first author Benjamin Goldstein, an assistant professor in MIT's School of Environment and Sustainability. "This data set shows that urban farming has higher carbon emissions per serving of fruit or vegetable than conventional farming, with a few exceptions."
Impact of urban agricultural inputs on the environment
Joshua Newell, professor and co-director of the Center for Sustainable Systems at the University of Michigan, leads part of the project at the University of Michigan. Researchers at the University of Michigan formed an international collaboration with members from universities near where food is grown. Ten of the collaborators are co-authors of the Natural Cities study.
The researchers recruited farmers and horticulturists at urban agricultural bases in France, Germany, Poland, the United Kingdom, and the United States as citizen scientists, who used daily diaries to record the inputs and harvests of their food growing bases throughout the 2019 season.
Inputs to an urban farming site fall into three main categories: infrastructure (such as raised beds to grow food or pathways between plots), supplies (including compost, fertilizers, weed-killing fabrics and gasoline for machinery), and irrigation water.
"Much of the climate impact of urban farms comes from the materials — the infrastructure — that the farms are built with," Goldstein said. "These farms are often only in operation 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 use pesticides and chemical fertilizers to grow a single crop, resulting in larger harvests and a smaller carbon footprint than urban farms.
The researchers identified three best practices that are critical to making low-tech urban farming more carbon competitive than conventional agriculture:
Extend the life of infrastructure. Extend the life of universally accessed materials and structures such as raised beds, composting infrastructure and sheds. The environmental impact (per serving) of a raised bed over 5 years is approximately four times greater than the environmental impact of a raised bed over 20 years.
Using municipal waste as a "universally accessible" input. Conserving carbon by participating in "urban symbiosis", which involves giving a second life to waste materials (such as construction waste and demolition waste) that are not suitable for new construction but have the potential to be used for universal access services. The most famous symbiotic relationship between cities and utilities is composting. This category also includes the use of rainwater and recycled gray water for irrigation.
Produce high-level social benefits. In a survey conducted for the study, an overwhelming majority of UA farmers and gardeners reported improvements in mental health, diet and social networks. The authors of the study report said that although increasing these "non-food outputs" of Uni-President Agriculture will not reduce its carbon footprint, "from the overall interests of Uni-President Agriculture, planting space that maximizes social benefits can surpass traditional agriculture."
Compiled from /ScitechDaily