What do we know exactly about urban agriculture’s environmental impacts?

kaboompics_Green garden plants

Before we develop urban agriculture policies on a big scale, it would be wise to have detailed analysis that supports its environmental claims…

A team of researchers from the Technical University of Denmark and the Massachusetts Institute of Technology reviewed existing scientific evidence on the main forms of urban agriculture in northern climates. They found that given the great diversity of urban agriculture, scientific evidence about its environmental benefits remains patchy. It does, however, reveal that the method used to cultivate plants, the product and the location are the three components of the equation cities should take into account to develop environmentally sound urban agriculture policies.

A wide array of potential environmental benefits… but many still to be proven

The first result from the review is that, as usual, one should look beyond carbon emissions when thinking about environmental impacts. Indeed, urban agriculture has a wide array of potential environmental benefits. These are:

  • Urban symbiosis, i.e. the ability to use urban material or energy flows. Plants can assimilate organic carbon and nutrients from urban waste such as compost, waste water, sewage sludge or rainwater (provided they are not contaminated with heavy metals, or – as far as rainwater is concerned – it does not deposit airborne contaminants on plants). One could also think of greenhouses that would “harvest” excess building heat.
  • Supply chains efficiencies, i.e. making it less energy or resource intensive to produce food, either through increasing yields or through reducing transport, packaging or even food losses.
  • In-situ environmental improvements, i.e. making the urban environment better. For instance, it is now established that urban agriculture reduces the urban heat island effect, as plants absorb sunlight. Other potential benefits include improvement of soil stability and organic carbon content, or reducing the amount of water that goes into the sewer system.
  • Broader environmental benefits such as reducing the carbon footprint of food, which has been studied in details, or less studied impacts such as carbon sequestration.

When it is integrated into buildings, urban agriculture can also reduce building energy needs. This happens either in a passive way, when, for instance, plants improve insulation, or in an active way, when growing methods use excess heat from buildings.

Overall, the review shows that urban agriculture is extremely diverse. Can we really compare allotments to vertical farming? Rooftop greenhouses to raised-beds? Even if many research projects are working on the topic and will yield results in the years to come, existing scientific evidence remains patchy for the moment. Available analyses usually carry out in-depth investigation of one specific method or product, but very rarely compare different methods. It is, therefore, not possible to draw a simple conclusion regarding the environmental benefits of urban agriculture.

Cities should, however, not be discouraged by such a conclusion. Indeed, existing literature already provides insights on the three factors that influence the environmental outcome of urban agriculture: the method used to cultivate plants, the product and the location. Such factors can either help decrease the inputs needed to grow food (as the more urban agriculture can integrate residual urban resource flows, the less impact it would have) or increase output (higher yields).

Inputs and yields: the method matters…

The method used to cultivate plants has major impacts on the potential benefits of urban agriculture, even if research is still to say its final word on this. On the one hand, methods with higher yields tend to also need higher resource inputs, making it difficult to gauge whether this translates into positive results. For example, temperature controlled systems are great to produce all year-round, reduce pest invasion and protect crops from the weather, but how does that make up for the higher energy needed to heat or cool the space and to build the farm? At the moment, research is not consolidated enough to draw a conclusion.

On the other hand, low input methods, such as ground-based agriculture used in allotments, can potentially lower the carbon footprint of food (provided they do not use resource-intensive inputs), and absorb urban water and nutrient flows (such as compost). But such methods tend to have lower yields than more technological systems such as greenhouses (air-conditioned or not), or even high-technologies such as hydroponics.

In the same vein, all things being equal, systems that are able to capture urban flows (water, waste, nutrients…) will have a lower environmental impact than the ones that create an extra-burden on the city demand for resources.

… so do products and location

The products also have impacts on the environmental performance of urban agriculture. For example, cultivating multiple crops in one location can indeed yield biodiversity benefits, benefits that do not occur with mono-cropping. Cultivating high-yields products (such as tomatoes or eggplant) improves the carbon footprint of the food produced.

Location also matters. For example, climate has an impact on the ability of rooftops to reduce a building’s energy needs. One study found that in Athens, rooftops would increase winter heating load because of evapotranspiration.

Embracing the diversity of urban agriculture

Such conclusions are highly relevant for cities. First, because they show that not all urban agriculture is environmentally relevant just because it provides food for the city.

Second, in general, according to Benjamin Goldstein, the article’s lead author, cities should reflect carefully upon which technology is most suitable considering their context, their climate and their urban resources. They should ensure that urban agriculture, can, as much possible, use residual urban flows (assuming that those flows cannot be put towards other more fruitful uses).

The article also points out to a potential contradiction between environmental and social outcomes of urban agriculture. Indeed, the types of urban agriculture that have the most potential to bring community benefits, such as community gardens or allotments, are also the ones that have the lowest yields and make marginal improvements to a city’s environmental footprint. They are more likely to be operated by non-profit organisations seeking a wide array of social benefits (education, social links…) beyond food production. On the contrary, high technological systems that have higher yields are more likely to be owned by private companies with little community involvement. Urban agriculture policies should therefore seek to reach a right balance between social and environmental benefits.

 


Albane GASPARD – March 2018

NB: the author would like to thank Benjamin Goldstein for his inputs and comments.

Source: Goldstein, B., Hauschild, M., Fernández, J., & Birkved, M. (2016). “Urban versus conventional agriculture, taxonomy of resource profiles: a review”. Agronomy for sustainable development, 36(1), 9.

Picture credits: Kaboompics

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