Just beneath the surface of our cities is a second, shadow city that makes urban life possible: the array of storm sewer tunnels under our streets, the buried powerlines transporting our electricity, the pipes instantaneously delivering drinking water to our homes from a long distant treatment facility. These massive municipal infrastructures render habitable the sprawling, post-industrial cities of the developed world by replacing the ecological services lost to the urbanization process itself. No longer able to safely draw drinking water from our own wells, we must purify it at a centralized public works facility. No longer able to infiltrate rainwater through the impervious materials of the city, we must collect and divert it to centralized wastewater treatment facilities. No longer able to repurpose our residential waste as nutrients for growing food, we must truck it to towering landfills at the city’s edge. These are the tools we have developed for managing the environment in cities, and they are of little use to us in adapting to climate change.
Developed in response to the demands of rapidly growing cities in a stable climate, our conventional set of tools for environmental management are ill suited to climate instability. Climate change is altering not only the intensity of longstanding environmental stressors to be managed — a rapidly rising frequency and volume of rainfall, as one example — but the array of stressors to which municipal governments must respond. In the coming summers, US cities will experience for the first time a combination of heat and humidity too deadly to enable basic municipal services, such as trash collection, to be safely carried out. The garbage trucks themselves will be of no use in hauling away the heat; we cannot divert it to our storm sewer systems for transport to centralized treatment facilities. In a climate changed world, it is no longer sufficient to collect and treat our urban environmental effluent — stormwater, excessive heat, planet warming emissions — we must lessen the volume of effluent to be managed.
In my recently published book, Radical Adaptation: Transforming Cities for a Climate Changed World, I propose four fundamental changes to the ways in which we manage environmental hazards in cities. In place of large centralized public works facilities, infrastructure for radical adaptation must be integrated into every parcel and neighborhood. Rather than directing public resources to the areas of greatest potential physical impact, a radical approach to climate adaptation is directed to the zones of greatest human vulnerability. Novel strategies once perceived as socially untenable will need to be embraced. And last, under a radical framework, planned retreat within cities is not the last step in a series of adaptative actions but the first. In each instance, a radical approach to climate adaptation is not a scaling up of conventional management systems but an almost complete inversion of strategies developed during the now concluded era of climate stability. We must, in short, unlearn how to plan our cities and plan them anew.
Adapting everywhere all of the time
The first plans created for industrializing cities were often plans for sewers. Having established a link between frequent outbreaks of water-borne disease and contaminated drinking water sources, an imperative to separate human and animal waste from drinking water in densely settled cities would inspire the earliest sanitary sewer plans in 19th Century Chicago, London, and New York. Initially designed to transport sewage to water bodies downstream of cities, the eventual need to treat the wastewater before release would necessitate the construction of large industrial facilities for this purpose. The retrofitting of sewage system pipes and tunnels under the city would further enable the collection and transport of stormwater away from streets — increasingly paved — which carried with them a growing problem of runoff-induced flooding. The engineered public works facility for environmental management seemingly responded to an ancient and vexing question: How to prevent the environmental contaminants of a city from working against the growth of the city itself?
With a growing reliance on engineered systems as substitutes for nature-based systems, the need for the built environment to respond to ecological constraints would be lessened. Long designed to capture or impede sunlight as a means of passive climate control, buildings could be liberated from concerns with solar orientation. Long designed to collect and store rainwater for drinking and irrigation, buildings could be designed to repel rainwater alone, as vast quantities of domestic water could be provided through industrial means. If the ancient cities of the Chinese and the Indians and the Mayans and the Romans were designed in some fashion to respond to ecological considerations everywhere, the emerging industrial cities of the Americans and Europeans would be constrained by such considerations almost nowhere, as buried and remote infrastructure would obviate the need for ecologically responsive design. But a key requirement for the operation of these engineered systems has not proved durable: a stable climate.
With these centralized management systems now regularly failing from weather events exceeding their design capacity, ever larger diameter pipes will not prove sufficient to stem the flooding. We will need as well to reduce the volume of runoff and the irradiance of heat from urban surfaces to maintain the viability of urban spaces. The most effective tools for doing so are rooted more in ecology than technology, but these approaches can work hand in hand. Through a radical approach to climate adaptation, every building, every street, and every greenspace is modified to collect and store rainwater, reflect and utilize radiant energy, and integrate vegetation as the most effective means of daily climate regulation. These approaches are not needed because they beautify the city (they do) or because they strongly promote other dimensions of human wellbeing (they do) but because nature-based approaches are demonstrably superior to our engineered substitutes. By retrofitting our cities to once again adapt everywhere, we shift from seeking to control the climate to working within its changing parameters.
The principle of least-first
The earliest efforts by large US cities to adapt to climate change demonstrate the limitations of conventional approaches to environmental management. Consider, for example, the highly ambitious campaign of New York City to add one million trees to the city’s canopy in less than a decade — a campaign undertaken, in large part, to lessen heat exposure and flood risk across the city. With about 80 percent of these trees targeted to parks and other public land across New York, it has been the greenest neighborhoods of the city that have received the most new tree canopy, to the exclusion of lower-income neighborhoods that tend to have less public greenspace. Likewise, one of the largest climate adaptation awards yet granted by a US federal agency — $140 million for flood resilience planning in New Orleans more than a decade after Hurricane Katrina — would be targeted in its entirety to the Gentilly District, encompassing neighborhoods with an average household income above the median for the city as a whole.
Those communities least well protected from climate extremes by ecological bulwarks, such as high ground and extensive vegetative cover, must be prioritized for climate adaptation investments designed to lessen flood risk, moderate heat exposures, or otherwise enhance neighborhood-wide resilience to climate-related exposures rendered more intense by the state of the local ecosystem. Those communities least physiologically adapted to climate extremes, as a product of age, compromised health, or recent migration from different climate zones, must be prioritized for governmental interventions designed to lessen climate-related exposures and improve baseline population health. I refer to this adaptive strategy as “least-first,” and it is fundamental to the idea of radical adaptation.
The least-first principle is radical in concept in that it inherently acknowledges that cities are not positioned to act everywhere equally — that public investments must be prioritized. Beyond its imperative that the most vulnerable communities be prioritized in the expenditure of public funds for climate adaptation, the least-first principle further militates against the idea of an equitable distribution of funds, wherein each planning district in a city receives investment equal in proportion to its population size. While not in conflict with the idea that cities must be redesigned across their full extent to manage climate risk, the least-first principle requires that these investments be strategically deployed to the most vulnerable communities first. For municipal governments confronting the challenge of rapidly rising climate exposures, the first question to be answered is not how to intervene but where, as the socio-spatial context must inform the strategic approach. The least-first principle is in this sense a precondition to adaptation, and it requires municipal governments to be reparative in their approach to managing climate risk.
The third principle of radical adaptation requires that municipal governments embrace a set of strategies once deemed socially untenable. As the planet approaches 1.5°C of warming above pre-industrial temperatures — a threshold effectively reached in 2023 — the global area of land experiencing drought conditions in any year is projected to be 50% greater than in recent decades. In the context of drought management, a societal discomfort with the recycling of municipal wastewater — a voluminous and valuable source of drinking water available to all large cities — can no longer be accommodated in regions experiencing a pronounced reduction in annual rainfall. The recycling of wastewater requires not only that municipal governments seek to be innovative in the technologies adopted to enable this process but that they further move beyond conventional norms in their long-established modes of operation.
The redesign of cities to retain rather than discharge rainwater, to shift away from water-intensive landscape design, and to repurpose wastewater for irrigation and drinking water is to move outside of socially sanctioned norms of urban water management, at least in the context of highly industrialized cities. Derided in an earlier era as “toilet to tap,” a characterization intended to scuttle public acceptance, the recycling of wastewater for greywater or irrigation purposes is more enticingly branded today as “showers to flowers.” Necessitated by a delayed adaptive response to the now manifest stresses of climate change, radical adaptation will require that the bounds of normal be redefined.
A recycling of urban wastewater is only one of many expansions needed in our collective thinking about urban responses to climate change. Other examples include the construction of houses designed to float during flood events (known as amphibious housing); the erection of fabric canopies over streets and other pedestrian corridors for shading; or the use of construction materials designed to melt and re-solidify in response to extreme temperatures. Each of these strategies represents a departure from our conventional approaches to environmental management in cities, and their deployment will require a rapid shift both in the design of the built environmental and in the governmental policies prohibiting their use. Nowhere is this need for revised thinking more pressing than in the context of planned retreat.
Retreat comes first
A final principle of radical adaptation is that planned retreat is the first rather than the last step in climate adaptation. While planned retreat is not an established component of urban environmental management, recently proposed frameworks for climate adaptation identify retreat as one of four core elements, and the element, at least in practice, to be pursued only once other approaches have failed. In the context of sea level rise, the first steps in climate adaptation are to protect vulnerable areas with sea walls and other infrastructure; to accommodate rising waters and other impacts by designing neighborhoods for periodic flooding; and to avoid future property damage with prohibitions on new development in flood zones. In practice, it is only once the sea walls have been overtopped, the buildings have succumbed to water damage, or development restrictions have proven politically untenable that the option of planned retreat is considered. But the benefits of retreat can only be fully realized if undertaken as the first step in climate adaptation.
Retreat must come first as it enables all other adaptive responses. Whether in the form of engineered bulwarks or restored wetlands, flood management infrastructure requires extensive areas of land for construction. Strategies of accommodation, such as the creation of floodable spaces within neighborhoods, also require land acquisition in dense urban settings. And governmental policies for avoidance are most feasibly directed to land amassed through a process of retreat, repurposing these spaces to climate adaptation in perpetuity. To be effective, planned retreat must be understood not as a process of land abandonment but as a process of land assembly. When retreat comes first, it can be leveraged as a tool to protect not only the households to be resettled from the most hazardous zones but as a means of safeguarding a larger population that remains in adjacent areas. When retreat comes last, the central aim of adaptation is unmet: to augment urban resilience in advance of the next climate event.
Concurrent with deployment of planned retreat as the leading edge of climate adaptation are two corollary imperatives: 1. Retreat must be undertaken in all cities; and 2. Retreat must give rise to amenity. No city today, in any reach of the planet, is immune from four climate-related threats: extreme heat, extreme flooding, extreme drought, and critical infrastructure failure. Each of these threats requires valuable urban land for adaptation. In some cities, coastal land is needed for flood management; in others non-coastal land is needed for collecting and storing rainwater, the expansion of tree canopy for cooling, or the siting of neighborhood micro-grids for localized energy production. In this sense, the repurposing of on-street parking for bioswales or the purchase of low-lying parcels to create floodable spaces can be understood as instances of planned retreat. No city can opt out of retreating; the only choice is to retreat by design or to retreat by disaster.
The land assembled through programs of planned retreat, often aside bodies of water or in dense urban districts, is too valuable to use for climate adaptation alone. As substantial public funds will be required for the compensation of households and businesses required to relocate out of impact zones, urban residents should expect a return on these investments both in the form of greater climate resilience and quality of life. As pioneered through the Dutch approach to flood management, lands newly acquired for a widening of natural floodplains in cities can also be put to use for new parklands, biking networks, floodable amphitheaters, and wildlife sanctuaries. The same is true for smaller segments of land repurposed for adaptation in dense urban cores. The recent movement toward parklets (New York’s “streeteries”), dedicated transit and pedestrian networks compatible with green infrastructure (Atlanta’s BeltLine), and amphibious affordable housing in place of industrial docklands (Copenhagen’s Urban Rigger project) all represent a repurposing of valuable urban space for adaptive infrastructure coupled with desired public uses of these spaces. To gain public acceptance, retreat must give rise to amenity and affordability. I refer to this entwining of climate defense and urban revitalization as adaptive urbanism.
Radical adaptation recognizes that our conventional tools for environmental management are adequate neither in scale nor design to respond to the four climate-related threats all cities now confront: extreme heat, extreme water (too much or too little), and critical infrastructure failure. While our conventional array of remediation technologies for managing waste, stormwater, and pollution in cities may have proven sufficient to address climate impacts associated with a degree or less of planetary warming, these approaches are woefully ill suited to the 2–3°C of warming to which we are now committed. To be radical in our modes of adaptation is to return to an earlier approach to city making — an approach in which climate-responsiveness is integrated into every building, every street, and the shared spaces of urban life. Our approaches to climate adaptation must again become spatially dispersive, socially responsive, informed by local ecologies, and, at times, deconstructive. This is not a new but a re-learned approach to planning our cities, and it promises far more than climate adaptation alone.