Solutions for Greening Cities

Mark Altaweel

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An increasingly popular proposal for addressing climate change and threats to the natural environment is using nature-based solutions, whereby particularly available land or land that can be reclaimed becoming returned to a natural or close to natural environment.

Increasing urban green space

One recent proposal has been to use parking spaces or lots in urban settings as areas for increasing green space or vegetation cover. Applying analysis on this shows potential benefits that can help cities reach climate change goals, improve life quality, while also providing benefits to nature.

What are the benefits of increasing vegetation in cities?

Nature-based solutions are increasing in popularity because they are seen as relatively cheap, in that nature does much of the work, and these solutions provide both long-term reduction/offsetting of greenhouse gases and ways to adapt to climate change, such as increasing greenery to offset urban heat island effects.

A picture of a rain garden in between a street and a sidewalk.
Installing vegetative structures in urban areas like this rain garden can provide a vegetated and sunken area for runoff to percolate into the ground instead of becoming part of stormwater diversion. Photo: Alisha Goldstein, U.S. EPA, public domain.

Major organizations, such as the UN’s Environmental Programme, have been increasingly advocating such approaches for cities.[1] 


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Additionally, greenery provides benefits such as reducing flooding and excess runoff that often overwhelms city infrastructure. One example is Los Angeles attempting to plant 90,000 trees across the city or even Paris setting a target of 50% of its urban areas having vegetated surfaces.

These goals may sound lofty or even politically motivated; however, they might be needed given projections on temperatures in the decades to come.

Challenges to converting public spaces to vegetation

One problem cities face is retrofitting public spaces, as these spaces are contested and their use is often for a variety of conveniences or services deemed of value. Additionally, lofty goals are mostly unrealized as it requires sacrifice of existing spaces and political will to implement given goals.

Paved streets or street parking are one clear target of resurfacing in cities as these often have impermeable surfaces, which increases flooding risk, that also increase heat during heatwaves as they absorb light. These areas often constitute more than a quarter of urban spaces in many larger Western cities and pavement prevents almost any vegetation from growing, limiting the benefits that trees and shrubs provide.

A strip of planted vegetation in a parking lot with permeable pavers.
Adding vegetation to parking areas along with permeable pavers can help to offset urban heat island effect and excessive stormwater runoff. Photo: Jeanna Henry, U.S. EPA, public domain.

Converting parking spaces to parks and greenery

One potential nature-based solution for paved streets is having on-street parking moved to garages with extra capacity. Smart phone applications that provide peer-to-peer information on available parking or cities organizing centralized parking systems that can be accessed by residents can enable awareness of where available parking is.

These existing solutions can be expanded to then enable a more efficient use of street parking spaces whereby these spaces are repurposed to be used as green spaces.[2]

Looking at redundant parking spaces on streets that are located near parking garages, which would help residents to find parking, a recent study in Melbourne showed there is considerable potential to create a lot of green space even in relatively dense urban areas. A spatial model was created that looked at benefits of tree cover, interception of water from storms, and ecological connectivity and benefits for local fauna by increasing green space.

A diagram showing the change from parking to planted trees with a inset diagram of a tree showing how it would be planted in the parking space.
Converting parking spaces can benefit cities by increasing tree canopy, reducing stormwater runoff, and reducing the total amount of impermeable surfaces in a cities. Figure: “Finding space for nature in cities: the considerable potential of redundant car parking” by Croeser et al., npj Urban Sustain, 2022, CC BY 4.0.

Overall, between 31-59 hectares of green space or tree canopy can be added to the city by removing redundant on-street parking to public garages that are available, which is a considerable percentage for a city currently having about 254 hectares of tree canopy. In other words, a solution which centralizes parking to facilitate more efficient use of existing garages would help Melbourne increase its tree canopy by about 25%, helping it to significantly reach its nature-based goals addressing climate change and adaptation.

Additionally, in modeling scenarios, many fauna species would benefit because streets have the benefit of reducing ecological fragmentation, which would allow them to more freely migrate. This included species such as the Blue-banded bee and New Holland honeyeater.

For storm water capture, more than 27 tons of pollutants and 202 tons of sediment could be captured or prevented from infiltrating freshwater sources by 2027. Additional benefits would also include a significant reduction in heat in urban areas for residents, helping to offset forecasted temperature increases in the next decades.  

While the study in Melbourne shows a lot of promise, other research has also advocated that nature-based solutions will also need to ‘consult’ with nature itself to find the best solutions. In other words, treating nature as a stakeholder in decision-making or redesigning spaces might be important. This can be done by considering how plants and wildlife could benefit from specific designs as well specific environmental benefits.

Many current proposals still put human benefit at the center, which could be counterproductive in the long-term given ecological threats and decreasing urban life quality. Re-balancing cities such that they maximize natural and human stakeholder interests is advocated as a better overall strategy as this helps to assess any design proposal through the lens and benefits of these stakeholders.

Determining what tree species to plant in cities could be done with an assessment of what species might benefit the most, such as the most threatened species. Overall, it is argued, nature-based solutions risk committing similar design mistakes of the past if they fail to emphasize the importance of nature’s stakeholders.[3]

References

[1]    An article highlighting the UN’s nature-based approaches can be found here:  https://www.unep.org/resources/report/smart-sustainable-and-resilient-cities-power-nature-based-solutions.

[2]    For more on how repaving streets and urban spaces can improve climate and other environmental benefits while helping cities achieve nature-based solutions, see:  Croeser, T., Garrard, G.E., Visintin, C. et al. Finding space for nature in cities: the considerable potential of redundant car parking. npj Urban Sustain 2, 27 (2022). https://doi.org/10.1038/s42949-022-00073-x.

[3]    For more on nature-based stakeholder interest in urban and nature-based design, see: Pineda-Pinto, M., Frantzeskaki, N. & Nygaard, C.A. The potential of nature-based solutions to deliver ecologically just cities: Lessons for research and urban planning from a systematic literature review. Ambio 51, 167–182 (2022). https://doi.org/10.1007/s13280-021-01553-7.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.