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Are you interested in the economic valuation of grey vs green infrastructure solutions?
Our debate today works with the article titled Is coastal climate adaptation infrastructure worth public investment? Evidence from stated preference economic valuation and cost-benefit analysis from 2025, by Haoluan Wang, published in the Journal of Environmental Management.
This is a great preparation to our next interview with Louis de Jaeger in episode 412 talking about the need for regenerative nature based solutions instead of degenerative infrastructure.
Since we are investigating the future of cities, I thought it would be interesting to see how green and grey adaptation infrastructure compare not just in solving the problems, but financially. This article investigates the economic viability of coastal climate adaptation by comparing green vs grey infrastructure.
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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. This conversation was produced and generated with Notebook LM as two hosts dissecting the whole research.
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Speaker 1: We are standing metaphorically at the edge of South Florida. To our left is the Atlantic Ocean. Rising increments of an inch every year to our right are millions of people, billions in real estate and a vibrant economy in Miami-Dade, Broward and Palm Beach counties. The water is coming. So the question is no longer if we defend this coastline, but how do we double down on the 20th century strategy of pouring concrete the gray degenerative infrastructure, or do we pivot to the data back toward generative approach, the green infrastructure.
Speaker 2: It is the single most expensive engineering challenge of our time, and today we’re grounding this discussion in the cold hard numbers of a 2025 study by Halo w Wang, published in the Journal of Environmental Management. The paper is titled Is Coastal Climate Adaptation Infrastructure Worth Public Investment? It’s a deep dive into cost benefit analyses and crucially what the public is actually willing to pay to survive.
Speaker 1: My position today is that the era of the concrete monolith is frankly, over. The data and weighing study is explicit regenerative green infrastructure, specifically coastal dunes and mangroves, offers economic returns that just dwarf traditional methods. We’re seeing benefit cost ratios that are double, sometimes triple that of sea walls. We are looking at a solution that doesn’t just block water. It adds value to the ecosystem and the economy. The regenerative view isn’t just environmentalism, it’s fiscal responsibility,
Speaker 2: and I’m here to represent the pragmatic view. I’m looking at the engineering reality because while the green numbers look fantastic in a spreadsheet, the study itself confirms that gray infrastructure, sea walls, and shoreline armoring remains a necessary, a durable and an economically viable baseline. We have physical constraints in densities like Miami. You can’t just plant a mangrove forest in front of a skyscraper. I’m here to argue that we cannot simply plant our way out of a crisis that requires immediate hard protection.
Speaker 1: Okay, let’s get straight into those numbers, because fiscal responsibility is a term often thrown around to justify the cheapest initial build, but that’s not what we’re seeing here at all. Wang’s study uses a metric called the benefit cost ratio, or BCR. For the uninitiated, this is a simple efficiency metric for every single dollar of public money we spend. How many dollars of value does society get back
Speaker 2: if the ratio is 1.0? You break even. Anything above that is a win.
Speaker 1: Now look at the 50 year lifespan analysis in the study. For coastal dune sand structures stabilized by vegetation, the ratio is 5.64. For mangroves, it’s 4.7. For every dollar spent on a dune, the public gets nearly $6 back in flood protection and ecosystem services. Now, compare that to your grey infrastructure. Sea walls over the same 50 year period achieve a ratio of just 2.02,
Speaker 2: which is still positive. It’s not a loss,
Speaker 1: it is positive. Sure, but it’s profoundly inefficient. Why would a government choose an investment that returns $2 when they could choose one that returns nearly six? The study also highlights that the public is willing to pay significantly more for the green options, about $63 more per year for dunes compared to sea walls. The thesis is clear green infrastructure is the superior economic choice. It just maximizes public utility.
Speaker 2: I have to push back on the idea that a 2.02 ratio makes seawalls a bad investment and almost any other sector of public policy, transportation, education. If I told you I could double your money, you would sign the check immediately. The study explicitly states, quote, all coastal climate adaptation projects examined in this study are worthy of public investment. So we aren’t choosing between a winner and a loser here. We’re choosing between two winners with very different risk profiles,
Speaker 1: but we have limited capital worthy isn’t good enough when optimal is on the table.
Speaker 2: Optimal relies on the assumption that you can build the green option everywhere. The study notes that acceptance rates for seawalls remain high at 62%. The public isn’t stupid. They recognize necessity, and the tech specifically discusses physical constraints. Green infrastructure requires a massive spatial footprint To get that high BCR from a dune, you need width. You need a slope. If you’re in downtown Miami where the property line ends at the water, you have a hard edge. You cannot retroactively squeeze a dune system between a condo tower and the ocean in those high density contexts. The sea wall provides immediate, robust protection where nature simply cannot fit.
Speaker 1: Space is a constraint. I grant you that, but let’s look at the actual cost of construction because the discrepancy is staggering. The argument for grey infrastructure usually is that concrete is solid and therefore worth it, but Wang’s data breaks down the construction costs per mile. To build one mile of protective dunes, it costs roughly $2 million. To build one mile of seawall, you’re looking at nearly 8000007.95 million to be precise.
Speaker 2: Concrete is a commodity. It’s expensive,
Speaker 1: it’s four times the price, and when you calculate the net benefits, that’s the total value minus the cost. Dunes generate $0.89 billion and mangroves 0.85 billion over that 50 year period, sea walls generate 0.38 billion. That’s less than half the value. By clinging to gray infrastructure, we are leaving hundreds of millions of dollars of societal value on the table. You’re paying a premium price for a product that delivers half the result.
Speaker 2: You’re looking exclusively at the construction price tag. I’m looking at the operational reality. The study points out that seawalls provide robust protection. There is a predictability to grey infrastructure. We have standard engineering manuals for concrete. We know how it degrades. We know how to patch it. When you deal with living infrastructure like mangroves, the study notes, they require specific environmental conditions to thrive.
Speaker 1: The maintenance costs in the study actually favour the green solutions as well.
Speaker 2: On paper perhaps, but look at the mangrove cost again. Construction is $5.8 million per mile. That’s creeping very close to the cost of the sea wall. It’s not the bargain that the dunes are. And unlike a sea wall, which works the day, the contractor leaves the site, a mangrove swamp is a complex biological system. If the salinity of the water changes or if a disease moves through the vegetation, your investment dies. A seawall doesn’t die. It just sits there. The value of grey infrastructure is its reliability. It is a known quantity for a risk averse city manager.
Speaker 1: I think you’re conflating reliability with inertia. he reason the net benefits are so much higher for green infrastructure isn’t just because construction is cheaper. It’s because the public gets so much more out of it. This brings us to the willingness to pay or WTP data in the study. This is crucial for understanding what the people of South Florida actually want.
Speaker 2: The WTP numbers are fascinating, I’ll admit. It measures how much a resident would theoretically increase their annual tax bill to support a specific project.
Speaker 1: Exactly. And the residents have a distinct preference. They hate the status quo, doing nothing, has a massive negative utility, but they are willing to pay a premium for green. Why? Because of co-benefits. When you build a sea wall, you get flood protection. That’s it. When you restore a dune or mangrove forest, you get flood protection, but you also get carbon sequestration. You get nurseries for fish stocks, and importantly, you get recreation. The study mentions beach culture. South Florida’s economy is built on tourism. A sea wall protects the hotel, but it kills the beach in front of it. A dune system protects the hotel and creates the beach Gray infrastructure is what economists call an impure public good. It protects the private homeowner, but it offers nothing to the community.
Speaker 2: I would frame that differently. You’re assuming that this willingness to pay is a purely rational assessment of all these lovely co-benefits, but the study reveals some significant heterogeneity, which is just a fancy way of saying not everyone agrees, and for different reasons.
Speaker 1: The acceptance rates are consistently higher for green, though,
Speaker 2: on average. Yes, but look at the risk profiles. The data shows that risk averse individuals. The people who are most terrified of flooding are actually less willing to pay for these projects.
Speaker 1: That seems counterintuitive. Wouldn’t the scared people pay the most?
Speaker 2: You would think so, but it suggests a scepticism about climate adaptation generally, or perhaps a paralysis. If I believe the city is going to drown anyway, why would I vote to raise my taxes? Furthermore, the study links preference to familiarity residents who know what a dune is, support it. The data shows a racial disparity Here. Black residents are statistically less likely to be familiar with these coastal adaptation strategies. So if we base our policy strictly on willingness to pay, we might be designing a coastline that caters to the preferences of white affluent residents who are familiar with these concepts while ignoring the needs of those who aren’t.
Speaker 1: That is a serious concern, but I interpret that data differently. The study also highlights that those living in the a hundred year floodplain, the people directly in the cross airs are the most willing to pay the signal from the vulnerable is that they want action.
Speaker 2: They want action undeniably, but do they want a mangrove or do they want a dry living room?
Speaker 1: The data says they’re willing to pay more for the mangroves, and this touches on the equity issue you raised. Green infrastructure can actually address equity if it’s implemented correctly. By restoring the ecosystem, you’re providing flood protection to these vulnerable floodplain populations while simultaneously improving their local environment, cleaning the air, providing cooling effects. In urban heat islands, grey infrastructure just creates a concrete barrier that often severs the community from the water.
Speaker 2: That sounds idyllic, but we have to talk about climate gentrification. The study explicitly warns about this. The study references research showing property values rise by roughly 10% within five years near these adaptation projects. Now think about the mechanism there. If you replace a crumbling sea wall with a beautiful regenerative dune system, a park, and a wildlife habitat, you are making that coastline incredibly attractive. You are driving up property values.
Speaker 1: So your argument for great infrastructure is that it’s unpleasant enough to prevent gentrification. That seems like a race to the bottom. Let’s keep our city ugly so the rent stays low.
Speaker 2: My argument is about coastal lock-in and moral hazard, which the author Hulu and Wang discusses by building any infrastructure, but especially these highly attractive green projects. We encourage people to stay in risky areas. We are signalling it’s safe to live here. Look, we built a beautiful dune. This incurs substantial social costs. Later when the sea level rise inevitably exceeds the capacity of that dune. Are we just setting people up for a harder fall 50 years from now?
Speaker 1: That is a risk with any adaptation. We can’t abandon the coast, so we must manage the risk. And this brings us to the dimension of time, which I think is the strongest argument for the regenerative approach. The study looked at different lifespans, 10, 30 and 50 years,
Speaker 2: and the longer the view, the better the green looks.
Speaker 1: The benefit cost ratio increases as the project lifespan gets longer For dunes, it goes from a ratio of roughly 2.6 at 10 years to 5.64 at 50 years. This is the definition of regenerative. Think about a sea wall from the moment the concrete sets. It is dying. Salt water, corrosion, sping, where the concrete flakes off cracking. It is a depreciating asset, a mangrove forest, or a dune system if managed correctly, grows. It establishes deeper roots. It accretes more sediment. It actually keeps pace with sea level rise to a certain degree. It is an appreciating asset. Why would we invest public money in something guaranteed to fall apart when we can invest in something that gets stronger?
Speaker 2: Because we might not make it to year 50 without the concrete. The study includes a crucial caveat, time to become effective. It’s the elephant in the room. A mangrove ceiling is not a flood barrier. It’s a stick in the mud. It takes years, sometimes decades to establish a root system dense enough to attenuate storm surge significantly. A dune needs vegetation to stabilize the sand, or it just washes away in a winter storm. If a category four hurricane hits Miami next October, the future value of that mangrove forest is irrelevant. A sea wall works. Now, immediate risk reduction is a utility that the spreadsheet might undervalue, but the resident in the floodplain desperately needs.
Speaker 1: We cannot plan our infrastructure based solely on the panic of next year we’re building for the next century. If we only build for immediate gratification, we lock ourselves into that grey cycle of build, degrade, repair.
Speaker 2: We must do both. That is the pragmatic conclusion. The study suggests that dismissing grey infrastructure entirely is dangerous for high density areas. There are places where there is literally no room for a marsh. You have a road, a sidewalk in the ocean, in those spots, grey is the only geometry that works. The study frames this as a choice, but in reality, the physical limitations often dictate the solution.
Speaker 1: I agree that physical constraints exist, but the study shows that where we can choose, which is much of the coastline, the choice is obvious. Even the hybrid approaches mentioned combining green and grey. Often rely on the green component to drive the benefit cost ratio up. The grey is the cost centre. The green is the value generator.
Speaker 2: I would argue the grey is the safety anchor and the green is the value add. But let’s look at the financial mechanism. The advocate for green often assumes the money is just sitting there. The study raises the point that while WTP is high, capturing that value is hard. These are impure goods. How do you tax a tourist for the view of a dune? How do you tax a commercial fisherman for the nursery habitat of a mangrove? A seawall assessment on a property tax bill is straightforward. Financing the complexity of green infrastructure requires a level of government agility that is often lacking.
Speaker 1: The study explicitly suggests redirecting tax revenues specifically to coastal adaptation projects because the public is willing to pay. If you line item coastal dune restoration on a tax bill, the data suggests people will pay it more willingly than sea wall construction. The political capital is with the green solution. People vote for beaches. They don’t rally for concrete slabs.
Speaker 2: The political capital is there. Yes, but as a sceptic, I worry about the shiny object syndrome. We vote for the beautiful dunes, but do we vote for the ugly, expensive maintenance dredging required to keep them there? The study assumes maintenance costs, but in reality, biological systems can fail in complex ways that concrete does not. A blight hits the mangroves. A king tide washes away the dune entirely
Speaker 1: and concrete crumbles. We’re comparing risks here, but the risk of the grey path is that we lock ourselves into a rigid lifeless coast that costs a fortune to maintain and offers zero co-benefits. The risk of the green path is that we have to learn to manage ecosystems. I would rather take the risk that requires us to be better stewards of the land than the risk that tries to fight the ocean with the wall.
Speaker 2: That is a philosophical stance and a respectable one, but looking at the summary of the data, while green is economically attractive, the sea wall remains a valid positive ROI public investment. A 2.02 ratio is nothing to sneeze at. We have to be careful not to let the perfect be the enemy of the good. In some places we will need the wall,
Speaker 1: but in most places we should build the dune. The data from South Florida is clear. Green infrastructure provides double the benefit cost ratio. It aligns with what the public wants, not just for safety, but for lifestyle, ecology, and the future of the region.
Speaker 2: I think we can agree on one thing that the study makes abundantly clear. The status quo option, doing nothing is the only option with negative utility.
Speaker 1: Absolutely. The status quo is a disaster waiting to happen.
Speaker 2: Whether we pour concrete or plant mangroves, we have to do something. The Do Nothing approach is the most expensive one of all,
Speaker 1: and that is the ultimate takeaway. The water is rising and we have to adapt. The data suggests that adaptation should be as green as possible, but as you say, it must be effective.
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