GREEN CITY
Green City: a contradiction in terms?
Can our increasingly crowded cities really become sustainable havens?
By Ron van der Ploeg
fci invited Anne Loes Nillesen, Professor of Urban Design at Delft University of Technology and founding director of the urban design office Defacto, to engage critically with the concept of Green City.
I dislike beating around the bushes (and trees): visions and strategies of urban planners—those who create and implement plans for the development of cities, towns, and other urban areas and often team up with government officials, shrewd real estate developers and community members—always keep me on the qui vive. There is good reason for this.
Growing up in the countryside, I was lucky to be surrounded by never-ending green polders dotted with cows and sheep and windmills. However, I was unlucky to witness first-hand how, with a voracious appetite, surrounding towns and cities, once shimmering in the far distance, managed to eat up massive swathes of agricultural land, bringing view-wrecking suburban starter-castles and bucolic farms into grim proximity.
Rambling concretosis Adhering to the notion that any growth will have a detrimental impact on the environment, I tend to be hostile to more urbanisation, more asphalt, and more rampant concretisation at the expense of nature, no matter how sustainable or green the picture cities have of themselves may be. As a plantsman myself, I naturally encourage the use of plants and green infrastructure as an integral part of urban development. But this Green City approach can also feel like locking the stable door after the horse has bolted.
I mean, for all the purple prose, a green city, an eco-city, or a city as a sustainable haven will always sound like a contradiction in terms to me. Cities demand immense resources, generate waste, destroy landscapes and forests, swallow up wildlife habitat, and harm biodiversity and ecosystems. So, how presumptuous can a (mega) city be to consider itself a sustainability saviour? On the other hand: how presumptuous can the author of this piece be, as he himself is a city resident?
When invited to share her perspective, Anne Loes Nillesen—Professor of Urban Design at Delft University of Technology and founding director of the urban design office Defacto—says, “I perfectly understand what you mean. However, it is not urban expansion per se that comes at the cost of nature and an ecological balance, but it requires hard work to prevent that from happening. Urban development will always require additional space and make landscape elements disappear. Meanwhile, whether a polder is a natural landscape element is debatable as in essence it is a man-made feature.”
Nillesen believes you can improve agricultural land in an eco-friendly way while making it more urban. “The truth is many cultural landscapes are overcultivated and merely focused on efficiency and productivity affecting ecosystems, impacting biodiversity, soil health, and the water cycle. So, construction on such land is not always synonymous with deterioration, provided homes and buildings are designed in a way that minimises their carbon footprint and supports ecology. Ideally, we must strive to achieve densification in existing urban areas. The expansion of cities should not be taken lightly because land sustains life on Earth and supports its ecosystems.”
It’s a crowded world out there
While Nillesen and I are posing the big questions: if the presence of big cities worldwide is a fait accompli, why not make the most of it? Perhaps the answer is that we cannot eternally sigh under the weight of tonnes of steel and concrete, lament over the loss of freshly mowed meadows, and succumb to utter despair at yet another piece of nature gobbled up by cities.
The United Nations says that the world will reach a total population of 8.3 billion and is expected to grow by another two billion over the next 60 years, peaking at around 10.3 billion only in the mid-2080s.
The United Nations also estimates that by 2080, the percentage of humans in cities and towns will have doubled from roughly 40 per cent to 80 per cent, while the number of people living in cities will have quadrupled from 2.3 billion to around 9.2 billion. [source coming]
Just think of this: 9.2 billion, of whom a sizeable part are stuck in their high-rise apartments where living space is so limited that they constantly feel cramped and claustrophobic, with nothing else to do but climb the walls. It is worth considering that for most of these people, the first and only form of nature they can readily access once outside will be urban.
Anne Loes Nillesen—Professor of Urban Design at Delft University of Technology and founding director of the urban design office Defacto.
Capturing cool canopy:
Why cities find it difficult to ‘branch out’ as sustainable havens
Urban Heat
The average difference cited between city centres and surrounding areas.
As trees cool local heat-island effects and deliver a host of other benefits, Anne Loes Nillesen asks why we do not see more of them in the cityscape.
The world is growing not only more crowded, but also hotter. In the spring and summer of 2026, Europe, as well as North America and Asia, sweltered under intense heatwaves, making the concept of Green City literally a hotly debated topic.
It’s a pity: no shadow in the city
Nillesen explains that the effects of global warming are particularly noticeable in urban areas. Yet there can be a clear divide between different city districts. Nillesen notes: “Regarding the ‘feels like temperature’, some locations can definitely feel warmer than the thermometer reads, depending on various factors such as wind, direct sunlight and humidity. With temperatures higher than 30°C, the risk of heat stress significantly increases, especially in urban areas.”
In cities, omnipresent asphalt, steel and concrete trap heat and drive temperatures up, contributing to the Urban Heat Island (UHI) effect. City centres can be, on average, 4–6°C warmer than surrounding areas.
To illustrate her point, Nillesen shows the heat map of the city she works in. Unsurprisingly, inner Delft tops the heat charts. She elaborates: “This is a highly cherished but also mostly a paved and built-up area. During the daytime, this ‘hardscape’ heats up rapidly. At night, when temperatures are supposed to cool down, the retained heat is slowly released back into the air. Urban heat lingers into the night and can be dangerous, especially when coupled with high humidity.”
To illustrate her point, Nillesen shows the heat map of Delft.
The risk of high night temperatures
When people can’t escape the heat at night, heat stress can continue to build, increasing the health risks. A January 2025 study published in Nature Medicine warns that temperature-related deaths could increase by 50 per cent by the end of the century in the absence of climate mitigation policies. Nillesen notes: “The elderly are at the highest risk. In Delft, for example, the share of older people in the inner-city population is high, with high night-time temperatures significantly impacting human health.”
While showing another map revealing the city’s greenest hotspots, including the 400-hectare urban forest De Delftse Hout, Nillesen explains that Delft performs reasonably well against the World Health Organisation (WHO) proposed urban green-spaces accessibility indicator, which measures the proportion of residents living within 300 metres of a green space of at least 0.5 hectares.
“The urban periphery encompasses a number of cool and green areas. Although they are somewhat distant from the city centre, on hot days people can travel to benefit from them. Problems arise when it stays hot during the night. We cannot possibly camp en masse in De Delftse Hout.”
Agnetapark, Delft
Nillesen stresses that we cannot air-condition our way out of the problem of urban heat. But we can use cool, heat-reflective pavements, permeable pavers, light-coloured roofing, shade sails and overhangs, green walls and awnings instead. We can also opt for wooden buildings that absorb less heat.
She comments: “In Delft and in many other cities worldwide, the urban heat problem is concentrated in areas that house the largest number of monumental buildings, to which many restrictions apply. Installing a green wall on a listed building or replacing its façade can therefore be an impossible endeavour.”
We cannot air-condition our way out of the problem of urban heat.
Anne Loes Nillesen Professor of Urban Design at Delft University of Technology
Tree-lined canal, Bangkok
Planting trees
Nillesen acknowledges, “Trees and plants cool the air through evaporative processes and physical shading. One tree can cool as much as ten air conditioners.”
What’s more, trees purify the air; provide oxygen; support wildlife; boost the economic value of properties and businesses; store carbon dioxide; reduce noise; contribute to biodiversity; provide a natural setting for children to explore, play and learn; provide room for hugging and dancing—at least for those in high spirits—and sometimes serve as lasting memorials to loved ones.
Trees also make great storytelling. In Delft, for example, one of the city’s landmarks is a splendid black mulberry (Morus nigra) at the Hofje van Pauw, thought to have been planted when the courtyard was founded in 1707. At the time, Delft was home to one of the six Chambers of the United East India Company (VOC), which imported spices, porcelain (which provides inspiration for iconic Delftware in blue and white), and also trees and plants from China and India.
Anne Loes Nillesen
In her presentation Kroonlezing: bomen in de strijd tegen hitte (Crown Lecture: Trees in the Fight Against Heat), she challenges assumptions about whether trees belong in Delft’s historic landscape.
Cornelis Springer, 1877
Cornelis Springer’s painting shows the Gemeenlandshuis and Delft’s Old Church surrounded by lush greenery—evidence that trees have long formed part of the historic cityscape.
Rietveld, 1901
This 1901 photograph shows the east–west canal in central Delft, between Vrouwjuttenland and the Rijn–Schie Canal, which once sat beneath a dense, mature tree canopy.
Rietveld, 2025
This 2025 photograph shows that the same canal is now lined with much younger, narrowly formed trees, creating a markedly different streetscape.
Vrouwjuttenland, 1932
This 1932 image, with an inset of an earlier shot, shows mature trees that once created a continuous canopy along this historic canal between Verwersdijk and Vrouwenregt.
Vrouwjuttenland, 2025
This present-day view of Vrouwjuttenland shows younger and more limited tree cover along the same central Delft canal.
Quick action is needed if, in the much hotter year of 2050, we would like to benefit from their shade
Anne Loes Nillesen Professor of Urban Design at Delft University of Technology
Why do we not plant more trees?
Pondering the vast array of benefits, Nillesen asks herself why we do not see more trees in the cityscape.
For all the leafy laudations, a tree only becomes net-zero after 30 years, when carbon removal is high and all inputs required to grow the tree are offset. Most of the trees start contributing to biodiversity only after 30 years. Shade coverage is relevant sooner but naturally is at its best when a tree reaches maturity. “Urban planning prefers trees that are 10 metres tall or higher. But these are expensive, and costs tend to rise rapidly. So, quick action is needed if, in the much hotter year of 2050, we would like to benefit from their shade.”
However, there’s more to tree purchase than just the initial cost, especially when considering quality, longevity and sustainability. “Growers are very knowledgeable regarding tree growth habit, care and maintenance. So, ideally growers and landscapers sit together around the table to select the right tree for the right spot and contribute to biodiversity,” she says.
At times, the absence of trees can be linked to technical issues. Start digging a tree pit in an urban area, and the chances are you will quickly come across a network of power and broadband cables, gas and water pipes. Underground, it gets busier by the day, which is why Nillesen is a strong advocate for tree zoning rules, as these can help create the right balance between the built and green environment.
The Netherlands loosely translates as lowlands, a moniker that has become increasingly apt. In the provinces of North and South Holland, the soil comprises soft peat or clay with an artificially lowered water table to make the land suitable for agriculture or building. This situation causes land subsidence, or soil subsidence: the gradual sinking of the ground’s surface. Nillesen explains, “Many places are too wet for tree planting. With high groundwater levels, trees have hardly any space in which to root properly. In Amsterdam’s urban forest, “het Amsterdamse Bos”, many trees come off the ground, and in Delft groundwater levels are also relatively high.”
Works beside a canal in Delft city centre, where high groundwater levels and limited underground space can make tree planting difficult. Photo: RvdP.
Not everyone loves trees
It is not only in the realm of history, art and culture where not everyone is lovingly wrapping their arms around a trunk. Residents bring up issues such as bird droppings and tree sap, falling branches, possible damage to underground infrastructure, raised roots causing tripping hazards and lifting paved surfaces. Not to mention the unpleasant mess of leaves, seeds and fruit.
Granted, parks and other urban green spaces provide places to exercise, read, catch up with friends or dream away from the hustle and bustle of the city.
For women, however, this green reverie is often only half the story, as many perceive sanctuaries of greenery and tranquillity as unwelcoming and unsafe.
A series of deadly attacks on women in the Netherlands in 2025 led to more people petitioning for more safety in public spaces, not only by installing lights and security cameras, but also by felling trees and cutting back bushes.
Nillesen says: “It’s definitely an issue of importance. Creating outdoor spaces that are safe is one of the aspects of landscaping. This is why many public green spaces aren’t very impressive, mostly consisting of grass and clusters of towering trees. Perceptions of trees and other urban greenery can be complex and have changed considerably over time. In the 1980s, for example, the tendency was to trim low branches and cut back understory shrubs to reduce opportunities for drug trafficking. Today, we see that these actions can disrupt ecosystems while posing threats to biodiversity.”
“Clear sightlines are just one aspect, and this does not automatically mean tree felling.”
Anne Loes Nillesen
She continues: “Each situation requires a balance between beauty and function. It is also about offering different options for paths in urban parks, with well-lit and monitored routes combined with other alternatives.”
The question of whether trees are wonders of nature or nerve-racking features also plays out in court. We all know stories of people who have maliciously cut down their neighbour’s tree because it was blocking the light into their home or garden. When watching a court show that delves into the fiery tree-related disputes over overhanging branches and falling leaves, I realise that there will always be people who vacuum their garden!”
Katendrecht: planning for views, not shade
Further complicating the matter is that many decisions in urban planning date from the 1990s, when the climate was not as hot as it is today. In Katendrecht, a new residential development in Rotterdam, the quays have been planted with extremely slender specimens that you can hardly call trees, in order not to block the view. Nillesen says: “Today, the neighbourhood grapples with how to cool its urban heat island, which is exacerbated by the massive use of steel, concrete and glass because, architects maintain, these materials fit the district’s industrial past. The question now is how to plant back a number of large-canopy trees.”
Katendrecht, Rotterdam, where slender trees were originally planted along the quays to preserve views. Photo: RvdP.
Changing perspective
To drive change effectively, Nillesen thinks that the initial step lies in accepting new (cool) ideas and points of view, altering certain habits. “With future heatwaves in mind, I am still surprised when I hear people asking their real estate agent for a south-facing garden.”
Using a graph showing ten trees associated with insect species, Nillesen points to willow, pedunculate and sessile oak, and downy and silver birch at the top of the ranking, associated with 450, 423 and 334 species respectively; the urban designer also calls for a more ecological approach towards urban greenery.
Changing perspectives, however, is easier said than done. In cramped, narrow downtown areas, the only place to increase tree cover often is in parts of the public space currently used as parking lots. Following decades of car-centric planning, it remains a challenge to convince residents and shopkeepers about tree benefits.
Top 10 trees associated with insect species
Source: Recreated in English from material presented by Anne Loes Nillesen.
Shining examples of urban shade
New York The High Line
A 1.5-mile stretch of disused elevated railway was transformed into a public park by James Corner Field Operations, Diller Scofidio + Renfro and planting designer Piet Oudolf, using predominantly perennials and grasses.
Paris — an urban forest at the Hôtel de Ville
In June 2025, Paris opened a new urban forest on the Parvis de l’Hôtel de Ville, creating a greener public space in front of the city’s historic town hall.
Seoul Cheonggyecheon Stream
A six-kilometre stretch of highway was removed to uncover and restore the Cheonggyecheon Stream. Research found temperatures near the restored stream were almost 6°C lower than in a developed area four blocks away.
Nillesen believes cities should learn from successful urban-greening projects and replicate them wherever possible. In her view, this could help urban environments become genuinely greener—and more effectively shaded.
