recycling

A few years back, the Internet was abuzz with the idea of vertical farms running down the sides of urban towers, with the idea that growing crops where they’re actually consumed could eliminate the carbon emissions involved with shipping plant products long distances. But lifecycle analysis of those systems, which require a lot of infrastructure and energy, suggest they’d have a hard time doing better than more traditional agriculture.

But those systems represent only a small fraction of urban agriculture as it’s practiced. Most urban farming is a mix of local cooperative gardens and small-scale farms located within cities. And a lot less is known about the carbon footprint of this sort of farming. Now, a large international collaboration has worked with a number of these farms to get a handle on their emissions in order to compare those to large-scale agriculture.

The results suggest it’s possible that urban farming can have a lower impact. But it requires choosing the right crops and a long-term commitment to sustainability.

Tracking crops

Figuring out the carbon footprint of urban farms is a challenge, because it involves tracking all the inputs, from infrastructure to fertilizers, as well as the productivity of the farm. A lot of the urban farms, however, are nonprofits, cooperatives, and/or staffed primarily by volunteers, so detailed reporting can be a challenge. To get around this, the researchers worked with a lot of individual farms in France, Germany, Poland, the UK, and US in order to get accurate accounts of materials and practices.

Data from large-scale agriculture for comparison is widely available, and it includes factors like transport of the products to consumers. The researchers used data from the same countries as the urban farms.

On average, the results aren’t good for urban agriculture. An average serving from an urban farm was associated with 0.42 kg of carbon dioxide equivalents. By contrast, traditional produce resulted in emissions of about 0.07 kg per serving—six times less.

But that average obscures a lot of nuance. Of the 73 urban farms studied, 17 outperformed traditional agriculture by this measure. And, if the single highest-emitting farm was excluded from the analysis, the median of the urban farms ended up right around that 0.7 kg per serving.

All of this suggests the details of urban farming practices make a big difference. One thing that matters is the crop. Tomatoes tend to be fairly resource-intensive to grow and need to be shipped quickly in order to be consumed while ripe. Here, urban farms came in at 0.17 kg of carbon per serving, while conventional farming emits 0.27 kg/serving.

Difference-makers

One clear thing was that the intentions of those running the farms didn’t matter much. Organizations that had a mission of reducing environmental impact, or had taken steps like installing solar panels, were no better off at keeping their emissions low.

The researchers note two practical reasons for the differences they saw. One is infrastructure, which is the single largest source of carbon emissions at small sites. These include things like buildings, raised beds, and compost handling. The best sites the researchers saw did a lot of upcycling of things like construction waste into structures like the surrounds for raised beds.

Infrastructure in urban sites is also a challenge because of the often intense pressure on land, which can mean gardens have to relocate. This can shorten the lifetime of infrastructure and increase its environmental impact.

Another major factor was the use of urban waste streams for the consumables involved with farming. Composting from urban waste essentially eliminated fertilizer use (it was only 5 percent of the rate of conventional farming). Here, practices matter a great deal, as some composting techniques allow the material to become oxygen-free, which results in the anaerobic production of methane. Rainwater use also made a difference; in one case, the carbon impact of water treatment and distribution accounted for over two-thirds of an urban farm’s emissions.

These suggest that careful planning could make urban farms effective at avoiding some of the carbon emissions of conventional agriculture. This would involve figuring out best practices for infrastructure and consumables, as well as targeting crops that can have high carbon emissions when grown on conventional farms.

But any negatives are softened by a couple of additional considerations. One is that even the worst-performing produce seen in this analysis is far better in terms of carbon emissions than eating meat. The researchers also point out that many of the cooperative gardens provide a lot of social functions—things like after-school programs or informal classes—that can be difficult to put an emissions price on. Maximizing these could definitely boost the societal value of the operations, even if it doesn’t have a clear impact on the environment.

Nature Cities, 2019. DOI: 10.1038/s44284-023-00023-3  (About DOIs).