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You can find the shownotes through this link.
Are you interested in how nature-based solutions can help the water-energy-food challenges?
Our debate today works with the article titled Nature-based solutions addressing the water-energy-food nexus: Review of theoretical concepts and urban case studies from 2022, by Pedro N. Carvalho, David Christian Finger, Fabio Masi, Giulia Cipolletta, Hasan Volkan Oral, Attila Tóth, Martin Regelsberger, and Alfonso Exposito, published in the Journal of Cleaner Production.
This is a great preparation to our next interview with Moein Nodehi in episode 478 talking about the importance of water for other urban services.
Since we are investigating the future of cities, I thought it would be interesting to see the role of nature-based solutions in addressing the complex water-energy-food connection in urban environments. This article advocates for systemic thinking and circularity to overcome technical and governance barriers in urban planning.
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Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation on the WTF4Cities website where you can find the shownotes and more information on supporting the podcast. This conversation was produced and generated with Gemini LM as two hosts dissecting the whole research.
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Speaker 1: Think about how we usually build cities. For over a century, our approach has been remarkably rigid.
Speaker 2: Very rigid.
Speaker 1: We pour millions of tons of concrete, we lay down iron pipes, bury these massive copper grids. We basically treat infrastructure as something fundamentally dead. We build a pipe exclusively for water, a wire just for power, and concrete highways exclusively to truck in food.
Speaker 2: It’s a completely linear, separated system, what urban planners often refer to as grey infrastructure. We like our municipal services to be highly categorized, physically isolated, and, well, easily measurable.
Speaker 1: Exactly. But recently, there is this serious push to change that model entirely. What if a city functioned more like a living, breathing ecosystem? We’re talking about nature-based solutions, or NBS. This is the idea of using living biology like expansive green roofs, urban wetlands, vertical farms to replace or augment those rigid concrete pipes.
Speaker 2: Which sounds incredibly appealing in theory, obviously, but when you try to actually build a living city, it forces us to confront something highly complex called the water-energy-food nexus, the WEF nexus. And managing that nexus in a dense modern city is honestly the absolute definition of muddying the waters of infrastructure.
Speaker 1: That is a great point, and we should probably define the WEF nexus for those who might not spend their days reading urban planning journals.
Speaker 2: Probably a good idea.
Speaker 1: So the nexus is simply the reality that these three core resources are inextricably linked. You need vast amounts of water to generate energy. Think of cooling towers and power plants. You need huge amounts of energy to pump, treat, and heat water for millions of people. And you need massive amounts of both water and energy to produce and transport food. You physically can’t alter one of these nodes without impacting the other two.
Speaker 2: Exactly. So if you’re proposing living natural infrastructure to replace our conventional systems, it has to be able to manage all three. You can’t just plant a tree and say you’ve solved the city’s problems.
Speaker 1: Which brings us to the core of our discussion today. We are diving into a fascinating two thousand twenty-two systematic review by Carvalho and colleagues on the theoretical concepts and urban case studies of these nature-based solutions. And the central question we’re exploring today is this: Are nature-based solutions currently a viable, systemic, and integrated approach to the urban WEF nexus? Or are they primarily localized single-purpose interventions that lack the empirical backing and scalability needed to address these complex challenges? I’ll be arguing that nature-based solutions are proven, viable, and interconnected solutions essential for urban secularity and managing planetary boundaries.
Speaker 2: And I take the position that nature-based solutions, at least as currently studied and implemented, are predominantly single-purpose interventions lacking the multipurpose quantitative data and scalability required to be true systemic WEF solutions.
Speaker 1: So let’s start by looking at the physical reality documented in the 19 global case studies from the Carvalho Review, because the numbers at the micro and meso scales are quite staggering. Look at the Brooklyn Grange rooftop farm in New York. The data shows it retains over six hundred and sixty-two cubic meters of rainwater per rainfall event while simultaneously producing thirteen point four tons of food a year.
Speaker 2: Wait, let’s slow down for a second because those are massive numbers. How exactly is a single roof holding that much water without collapsing? And how does that tie into the energy node of the nexus?
Speaker 1: Good question. A green roof isn’t just throwing some topsoil on some shingles. It uses engineered layers of substrate, essentially these highly specialized soil mediums that absorb and retain massive amounts of storm water like a sponge. That addresses the water node by taking immense pressure off the city’s sewer system during heavy rains. But because it’s a living layer, it also physically cools the building underneath it through a process called evapotranspiration.
Speaker 2: And for those unfamiliar, evapotranspiration is essentially the plants sweating, right?
Speaker 1: Precisely. The plants release water vapor into the air through their leaves, which actively cools the surrounding atmosphere. Plus that thick, wet soil layer acts as incredible physical insulation.
Speaker 2: Ah, I see.
Speaker 1: So the building drastically cuts its need for air conditioning, which directly hits the energy node. And of course, at Brooklyn Grange, they are actively growing crops in that soil, which addresses the food node. Another great example is the Treehouse condominium in Singapore. Its extensive vertical gardens are saving an estimated five hundred thousand dollars a year in combined water and energy costs. These microscale successes demonstrate undeniable functioning synergies across the nexus.
Speaker 2: I’m certainly not arguing that a localized rooftop farm doesn’t yield impressive numbers. Those localized synergies are fantastic, but I have to push back on that because microscale success simply does not translate to citywide integration. When the source material examines macroscale projects, the massive systemic interventions meant to actually replace conventional city infrastructure, the multipurpose synergy you’re talking about completely vanishes from the data.
Speaker 1: Vanishes? I think that’s overstating it a bit.
Speaker 2: Let’s look at the text. Take the Gorla Majora water park in Milan or Oslo’s Fornebu stormwater management system. These are massive, incredibly sophisticated engineered landscapes, and yes, they are highly effective for flood protection and nutrient removal. The paper notes Gorla reduces downstream peak flow by an astonishing eighty-six percent, but their food production potential, where is the energy generation data? It remains entirely unquantified and hypothetical at these larger scales. At the city level, planners are relying on these natural solutions as highly effective water management tools, period. It is premature to call them an integrated WEF nexus solution when they are fundamentally operating on a single functional dimension.
Speaker 1: I see why you think that, but let me give you a different perspective. Are we demanding a single silver bullet when we should be looking at a mosaic?
Speaker 2: A mosaic?
Speaker 1: Yeah. The source material specifically highlights the roof mosaic approach. The idea isn’t that one massive park has to do everything perfectly. This framework shows that when you apply urban agriculture on one roof, rainwater harvesting on another, and solar systems on a third, and you apply them collectively at the neighbourhood scale, it optimizes up to fifty percent of the WEF nexus demands. The systemic macro solution is actually just the aggregate of these highly efficient micro solutions working together.
Speaker 2: A mosaic is a beautiful concept. It truly is. But it shifts the conversation away from the most critical knowledge gap identified in the text. The review explicitly states that studies dealing with sustainability rarely measure these complex WEF resources together.
Speaker 1: That’s true.
Speaker 2: They almost always stop at binary combinations. We see studies on water and energy or water and food, but we simply do not have the quantitative data for the food energy overlap, let alone the comprehensive three-part nexus.
Speaker 1: I’m sorry, but I just don’t buy that. Let me tell you why. Without standardized design standards, we are flying blind.
Speaker 2: But does the lack of a standardized academic metric negate the physical reality of the benefit?
Speaker 1: It absolutely does when you’re trying to convince a city planner to zone an entire district around a mosaic. You cannot responsibly design public policy or replace critical gray infrastructure if you cannot mathematically quantify its structural and economic efficiency across all three nodes.
Speaker 2: Okay, but it is exactly like a doctor prescribing a potent new medication without knowing the proper dosage. If we don’t know the exact parameters of how the food node impacts the energy node at scale, we cannot responsibly build it.
Speaker 1: I hear your point, but I argue that the absence of a perfect multipurpose indicator doesn’t erase the physical causality happening right in front of us. Let me bring up constructed wetlands as the ultimate counterexample to your claim of single purpose utility.
Speaker 2: Okay, break that down. How does a constructed wetland actively hit all three nodes of the nexus?
Speaker 1: A constructed wetland uses specific vegetation, soils, and microbial communities to naturally treat wastewater. So immediately, that secures the water node. But while it’s treating the water, it is producing massive amounts of plant biomass. That biomass doesn’t just sit there. It can be harvested and converted into biogas or bioethanol through a process called anaerobic digestion.
Speaker 2: Where bacteria break down the organic matter in an entirely oxygen-free environment, which produces methane gas.
Speaker 1: Exactly. We are capturing that methane for fuel, which is a direct quantifiable contribution to the energy node. And on top of that, these biological systems naturally recover rich nutrients like nitrogen and phosphorus from the wastewater. Those nutrients can be harvested as fodder for animals or even natural fertilizer for human food crops. The biological mechanisms are closing the loop on all three resources simultaneously right now.
Speaker 2: That’s a compelling argument, but have you considered the technical barriers? The text explicitly notes a lack of guidelines for sizing green walls for graywater treatment.
Speaker 1: Let’s clarify graywater for a moment for the listener.
Speaker 2: Graywater is the relatively clean wastewater from your baths, sinks, washing machines. This is in contrast to blackwater, which is raw sewage. The text points out that engineers literally do not have the standardizing sizing guidelines- To safely use green walls to treat grey water. So we’re back to the medication dosage analogy. How can city planners adopt this systemically when the engineers do not even have the parameters to guarantee their effectiveness? How do you secure a basic building permit for a system you can’t quantitatively validate? We are missing the foundational math required to safely replace traditional piping.
Speaker 1: Planners are adopting them because the alternative, continuing to rely on failing rigid concrete systems, is no longer viable under intense climate pressure. And we have proof that when you push past the bureaucratic hurdles, the results are staggering. Let’s look at the Cheonggyecheon River restoration in Seoul.
Speaker 2: Ah, the famous highway teardown.
Speaker 1: Yes. They ripped out a massive elevated concrete highway that ran right through the centre of Seoul, and they restored the historic river underneath it. By renaturing that urban corridor, they created a system capable of sustaining 118 millimetre per hour rainfall events. That is massive systemic flood protection. But it didn’t just manage water, it increased local biodiversity by 639%.
Speaker 2: Wait, really? 639%? How do you even measure a biodiversity spike that huge in a dense concrete urban environment?
Speaker 1: They rigorously tracked the return of native fish, bird, and insect species that had completely vanished from the city centre for decades. The ecosystem literally came back to life, and crucially, restoring that river provided a vital local cooling effect. It physically dropped the ambient temperature of the surrounding city blocks.
Speaker 2: Which affects the energy node.
Speaker 1: Exactly. It drastically reduces the urban energy demands for air conditioning in those buildings. It proves that nature-based solutions can effectively replace massive grey infrastructure while inherently cascading overlapping benefits across the nexus.
Speaker 2: I do not deny the ecological beauty of the Cheonggyecheon project. It is a marvel of modern planning. But we have to ground this debate in the explicit realistic barriers listed in the Carvalho text. You framed these as bureaucratic hurdles, but the barriers keeping these solutions relegated to localized pilot projects are severe structural and economic realities.
Speaker 1: Such as?
Speaker 2: Such as strict structural weight limitations. You cannot just put a dense, water-logged green roof on a fifty-year-old apartment building without risking a literal structural collapse. Or let’s look at the retrofitting issue you brushed past with the grey water. To implement a closed loop water system in an existing building, you have to rip open the walls and install dual pipelines just to separate the grey water from the black water. The cost of retrofitting existing urban structures with secondary plumbing is astronomical.
Speaker 1: I hear your point about the scaling costs, and I admit the retrofitting budget for dual pipelines is a massive hurdle. But doesn’t the long-term cost of climate damage like flood mitigation energy grid failures outweigh that initial expense?
Speaker 2: In a purely logical world, perhaps. But we operate in an economic one. And crucially, the text points out that the high realization costs of these natural systems simply cannot compete when compared to politically mediated water tariffs.
Speaker 1: Meaning that municipal water is artificially cheap?
Speaker 2: Exactly. Because local governments heavily subsidize the price of water to keep it affordable, the return on investment for a building owner to install a half-million-dollar green water recycling system looks terrible on paper. The economic and technical friction currently keeps these solutions from being true systemic overhauls.
Speaker 1: Those barriers are certainly real. I won’t deny that. But I would frame them differently. They are engineering and governance hurdles, not fundamental flaws in the ecology of the solutions themselves. To summarize my position in light of our discussion, nature-based solutions are proven, viable, and interconnected solutions essential for urban circularity and managing planetary boundaries. The physical synergies of managing water, saving energy, and producing food are deeply present in the biology of green roofs, constructed wetlands, and urban farms.
Speaker 2: And my position remains that while the potential is vast, nature-based solutions, as currently studied and implemented, are predominantly single-purpose interventions. They lack the multipurpose quantitative data and scalability required to be true systemic WEF solutions. Until we overcome the exorbitant retrofitting costs and scaling challenges, relying on them systemically is premature.
Speaker 1: However, I think we both find strong convergence on one crucial point drawn directly from the Carvallo Review. We urgently need new standardized multipurpose indicators to fully map the WEF nexus.
Speaker 2: Absolutely. We have to be able to measure the entirety of what these systems do, the water, the energy, and the food together if we expect them to carry the heavy weight of our modern cities. Grappling with these measurement limitations sharpens our understanding of urban infrastructure.
Speaker 1: It really does. It challenges us to rethink not just how we build, but how we measure success. We encourage you to explore the source material to draw your own conclusions on the future of circular cities. We started by picturing traditional infrastructure as a series of rigid, isolated pipes and wires. The question now is whether we are ready to embrace the complexity of living ecosystems as the foundation of our urban world.
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What is the Future for Cities podcast?
Episode and transcript generated with Descript assistance (affiliate link).
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