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Adapting cities to global warming

Summary

 

The Moroccan context
 
The urban heat island and perceived temperature
Solution n°1: water evaporation
Solution no. 2: plant evapotranspiration
Solution 3: Shadows in the city
Solution No. 4: Direct and harness the wind
Solution no. 5: colors and materials
Urgent need to adopt a "new" climate urbanism

 

FIVE SOLUTIONS FOR REFRESHING MOROCCAN CITIES

 

The Moroccan context

According to United Nations projections, the world's population should reach around 8.5 billion by 2030, with an estimated urbanization rate of 60%. Demographic dynamics in Morocco confirm this upward global trend, and are projected to reach 43.6 million inhabitants in 2050 versus 33.8 in 2014, i.e. +22% in 36 years. This rise is accompanied by an increase in the urban population, with 65% of the world's population living in cities in 2030, and an estimated 75% in 2050.[1].

Such growth, including in Morocco, raises significant challenges in terms of energy bills, infrastructure, habitability and urban climate.

In Morocco, the climate in recent years has been particularly threatened by dry heat waves and heatwaves that have become recurrent with climate change: 48° degrees in Marrakech in July 2012, a record broken in July 2023 with 49.6°. Or more than 50°C in Agadir the same year, or 48°C in Beni Mellal in 2024, a heatwave that claimed around twenty lives.

Climate change is multi-faceted, and is being felt in many different ways, often as a deterioration of the past or a pessimistic projection: as indicated by studies by Dennis Meadows, James Hansen, the IPCC, or Don't look up ... Fortunately, solutions and adaptation alternatives also exist in large numbers: drawing inspiration from vernacular and climatic practices or technological advances.

The aim of this article is to explore five bioclimatic solutions or levers for adapting cities to heat waves. The term bioclimatism is often used in sustainable development literature. It's about designing to take advantage of the conditions of a site and its environment (sun, wind, etc.), rather than enduring them and compensating for them with substantial energy efforts. In contemporary practice, bioclimatic and technological approaches are complementary. But bioclimatism, in its acclimatization and sobriety, should be predominant!

 

The urban heat island and perceived temperature

The Urban Heat Island (UHI) is defined as a difference in air or surface temperature observed between an urban and a peri-urban or rural environment, often at night for the air and during the day for the surface.[2]. Indeed, the city itself is a source of temperature increase, over and above the global greenhouse effect, through its design and activity. Two main categories of local overheating can be identified. The first are due to the production and retention of heat, and are mainly of a diurnal nature (cars, density, albedo-shade); the second are linked to the limited possibilities for cooling, and occur mainly at night (cool winds, nocturnal irradiation, evaporation, etc.).[3].

The consequences of heat islands always depend on the geographical and climatic context. In cities such as Milan, Toulouse, Casablanca or Marrakech, they can lead to discomfort in summer, higher mortality rates, additional costs and higher consumption of cold energy (air conditioning in particular), etc...[4]

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Fig. 1: Diagram - principle of the urban heat island (Source Nechfate I.Sakout)

Temperatures in the city can be up to 10 degrees higher than in the countryside, and this phenomenon is particularly noticeable at night due to blocked cool winds, or that during the day, temperature differences can reach 8 degrees between a localized heat island (pavement and dense buildings) and a cool island (park) 500m apart.[5].

We need to distinguish between the lowering of air or surface temperatures in spaces, and the comfort felt by urban users at a given moment. These two notions are often confused, as the boundary is sometimes very fine.

When we talk about thermal comfort, we're talking about perceived temperature. There are many indicators of thermal comfort outdoors. For example, the UTCI (Universal thermal comfort index) is one of the most exhaustive and widely accepted by the engineering and air-conditioning community for quantifying a person's thermal comfort. Air temperature, directly linked to UHI, is one of the parameters taken into account by UTCI.

As mentioned above, air temperature is a component of thermal comfort. But the two concepts don't always show the same trend. For example, the color of an outdoor floor can influence air temperature or radiant temperature by radiation or reflection: if the floor is dark, it can overheat in summer and considerably increase air temperature, whereas if it is light, it doesn't overheat but reflects the sun onto the person walking on it, and it's the latter who overheats. Consequently, with a light-colored floor exposed to the sun, the air temperature is lower, but the risk of heat stress is greater due to radiation, in other words: a dark floor will heat up more, and faster, than a light-colored floor.

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Fig. 2: Diagram - hygrothermal comfort parameters (Source: Nechfate I.Sakout)

Solution n°1: water evaporation

When water evaporates or condenses, it absorbs or releases calories into the surrounding air, thus cooling or heating it. Evaporating water to cool the ambient air is an old idea used in buildings such as riads and traditional houses in medinas, which we can experience by approaching a coastline, for example.

The psychometric diagram (see diagram below) is a well-known tool in thermal studies for assessing air temperature, absolute humidity and relative humidity.[6]and the enthalpy contained in the air. On the diagram below, we can see that by evaporating 4g of water in 1 kg of air (equivalent to 0.9 m3 of air at these temperatures), we can reduce the temperature of this air mass by 10°C. Note that in the example shown, we start from air likely to be found in several Moroccan cities (such as Marrakech), i.e. 35°C and 28% relative humidity, to arrive at air at 25°C and 70% relative humidity. Stopping at evaporation equivalent to 50% relative humidity (optimum humidity comfort), the air temperature can drop to 28°C.

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Fig. 3: Psychometric diagram and adiabatic cooling (Source: Nechfate I.Sakout)

Ponds or fountains in Moroccan riads are a perfect example of this principle: the air circulating in the patio is cooled by the water before entering the surrounding rooms. Other examples, such as the Iranian badguirs, can be cited on a building scale: these are wind towers often linked to underground water basins. On an urban scale, this solution can be implemented by the punctual or spread presence of water to create islands of coolness, or combined with a wind strategy near a coastline or oasis.

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Fig. 4: Diagram - adiabatic urban cooling (Source Nechfate I.Sakout)

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Fig. 5: Lalla Hasna Park, Marrakech Morocco (Google source)

However, water is an increasingly scarce resource! We must therefore take care to preserve it while avoiding evaporation. Water could then be circulated underground, for example, and appear when necessary to avoid unnecessary expenditure. In hot, dry climates like Marrakech's, such a solution can reduce local temperatures by up to -10° at the height of the day (demonstrable on the psychometric diagram, with a margin of -30% to avoid too much humidity in the air).

Solution no. 2: plant evapotranspiration

To return to "bio" prefixes, bio-assistance is a field that tends to take advantage of ecosystem services, i.e. the services that living organisms (in our case, plants) can provide, such as water filtration, air purification, cooling or shading.[7]...

To cool themselves, plants use transpiration and evaporation: the body releases water, which can either accelerate heat exchange by conduction with the air (water is also a highly conductive material), or evaporates on contact with the skin (or leaf) to cool it by phase change, as described above. In the case of a tree, the stomata on the leaf surface dilate, allowing water to escape and evaporate (more so if humidity is low), cooling the outside air.

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Fig. 6: arsat Moulay Abdessalam, Marrakech Morocco (Source Google)

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Fig. 7: Diagram - evapotranspiration (Source: Nechfate I.Sakout)

The use of a regularly watered flowerbed can have the same effect, and can be all the more beneficial for the comfort felt, as the phenomenon takes place right on the ground. Note that the use of evaporation methods will naturally increase the humidity of the ambient air, so this technique is very effective in dry environments such as Marrakech or Fez, but can have a negligible effect in an already humid environment such as Casablanca or Agadir.

Among the most representative examples of traditional Moroccan design are the arsats, emblematic gardens of Arab-Andalusian landscaping. These arsats were generally located in or around the medinas, and were once places of peace and relaxation for the inhabitants of the medinas. Centuries ago, particular attention was already paid to local climates; indeed, the majority of arsat plants were (and still are) rain-fed and edible, such as olive, fig, date or mulberry trees... Such techniques are once again desirable for the future.

Solution 3: Shadows in the city

In a context where the sky is very clear in summer, direct solar radiation contributes more to the thermal discomfort of city dwellers and to the overheating of cities: getting into the shade becomes a necessity.

As previously mentioned, UTCI is an indicator that simultaneously takes into account several parameters involved in an individual's sensation of comfort: air temperature, average radiant temperature (sun and surfaces), humidity, wind speed .... This indicator is expressed in °C felt, but our reading on the graphs below corresponds to the comfort categories identified.

The graphs below show the hygrothermal comfort categories for the city of Marrakech. Each of the two graphs illustrates a situation described by the icons on the left: top in the shade with no wind, bottom exposed to the sun with wind. This shows that in Marrakesh, a city with little wind and strong sun, shade is preferable in summer and mid-season, and that wind alone is not enough to avoid thermal stress (although it can be effective if combined with other strategies).

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Fig. 8: Annual graphs of UTCI comfort categories in Marrakech (Source Nechfate I.Sakout)

There are many ways to create shade, but they must always be studied in relation to the path of the sun, the local climate, the availability of water for cooling, and the desired use: an optimal design creates shade if it's hot, and welcomes solar gain if it's cold. To take the example of ancient Moroccan medinas, the proximity of buildings created a dense, self-shaded fabric. These shady paths are complemented by light-filtering covers in certain alleyways, or by trees in patios. Note that the patio tree is often a citrus tree, with deciduous foliage that shades in summer and welcomes the sun in winter.

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Fig. 9: Shading diagrams - building, tree, courtyard (Source Nechfate I.Sakout)

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Fig. 10: Medina of Fez, Morocco (Source: Google)

Solution No. 4: Direct and harness the wind

In bioclimatic urban design, the term "grid" is often used. It's a structure that contributes to making the city: the built fabric, the green fabric, the blue fabric, the aeraulic fabric. The latter exists in the voids where air can circulate, so the morphology of urban fabrics is directly linked to the wind effects that occur there. On an urban scale, an optimal aeraulic grid is rarely rectilinear, but rather branched and irregular, and can be amplified from time to time according to the desired prevailing winds.[8]. Depending on its speed, the wind can play a role in the temperature felt by users (1m/s is equivalent to -3° felt according to the Woods ventilation guide, or -5° combined with humidity depending on Givoni), and also on lowering air temperatures by dissipating accumulated heat through accelerated convection.

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Fig. 11: Street profile and vortex formation (source: H. Wu, 1994)

The climatic design of such an urban fabric must be based on an analysis of the site's weather conditions, in synergy with the planned uses. One example is the Zenata new town, designed by architects Reichen & Robert and Atelier Franck Boutté, who specialize in environmental design. Studies carried out on this project, which is currently being finalized, have shown that the aeraulic grid enables air temperatures to be lowered by 2 to 3 degrees during annual outdoor heat peaks, and also enables the ventilation strategy to be continued throughout the building simply by opening windows. The diagram below illustrates the main objective of the design: to make the most of the desired strong winds, in this case north-westerly (left) and northerly (right), which are cooled because they come from the coast. Combined strategies are also possible: an aeraulic grid that works with a hydraulic and/or green grid...

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Fig. 12: Airflow study for the town of Zenata (source Atelier Franck Boutté)

It's also worth noting that, even on hot summer days, winds can be a source of thermal discomfort if they carry air warmer than ambient temperature (or skin temperature at 32°C). Or if wind speed or acceleration are too high, regardless of temperature or thermal stress: the Center Scientifique et Technique du Bâtiment sets a discomfort threshold at 3.6 m/s, and proposes speed ranges adapted to different uses. For example, in an urban fabric such as the medina of Essaouira, where wind speeds can be significant (even disturbing), it can be said that the city is partly shaped by the wind and protects itself from it with its walls and discontinuous alleyways; or ancient cities located in the Moroccan desert or on sandy sites protect themselves from sandstorms.

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Fig. 13: Wind comfort ranges in % annual weather (source CSTB)

 

Solution no. 5: colors and materials

The impact of exterior materials on thermal comfort is twofold. On the one hand, the impact on radiant temperature, i.e. reflection and radiation, the influential properties are albedo and emissivity. On the other hand, the impact on air temperatureThe influential property is the effusivity of the material, which is directly related to its thermal inertia.

Albedo (between 0 and 1) is directly related to color (or hue), it measures the ability of an opaque surface to reflect or absorb electromagnetic energy (sunlight, for example). It can be characterized simply by three parameters: reflection coefficient, absorption coefficient and roughness. For example, beige sand would have an albedo of around 0.3, while snow would have an albedo of around 0.9; this means that 30% or 90% of the energy intercepted is reflected, and the rest is absorbed, contributing to the heating of the surface and the material, and then to infrared radiation. Concerning emissivityIt corresponds to the material's capacity to radiate heat, especially infrared heat. It depends on its color and gloss: a matt black surface emits more than a polished beige surface.

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Fig. 14: Radiance properties of a surface (Source: Nechfate I.Sakout)

L'thermal inertia is a more complex concept, characterizing the material rather than the surface. It's the material's resistance to changes in temperature. In other words, it depends on the material's ability to store heat and release it slowly. It is directly linked to properties such as heat capacity, diffusivity and effusivity. For example, concrete and earth have high inertia, while wood, glass and lightweight synthetic insulation have low inertia. The thermal capacity defines the amount of heat the material can contain; the diffusivity corresponds to the speed at which calories move through matter, and theeffusivity is the capacity to exchange energy with the outside by conduction (by touch) or convection (by air movement). For the formulas below : λ = thermal conductivity (in W/m.K) ; ρ = material density (kg/m3).

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Fig. 15: Inertial thermal properties of a material (Source: Nechfate I.Sakout)

In terms of albedos or inertia, there's no key solution or perfect recipe either. Materials and colors must be designed in synergy with the overall comfort strategy; the project for the forecourt of Notre-Dame de Paris is a fine example. Since, for heritage reasons, the parvis cannot be shaded, and its floor must be made of light-colored stone (like the cathedral), the project is initially constrained by a high albedo exposed to the sun in summer. To offset this risk of thermal stress, the area around the forecourt is generously planted, but a special device is also used to keep the ground cool at critical moments: a closed loop of water from the Seine can circulate under the slabs, just a few centimetres away from passers-by. Since stone has a good calorific capacity, with medium effusivity and high emissivity, it takes advantage of the coolness of the water and its evaporation.

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Fig. 16: UTCI comfort study for the Notre-Dame de Paris forecourt (source Atelier Franck Boutté)

A final physical phenomenon, more often used in the building industry, is hygroscopicity of materials. A hygroscopic material is capable of absorbing and releasing retain moisture of the environment, depending on relative humidity conditions. In other words, it naturally attracts water present in the air in vapour form, and can then release it if ambient humidity falls. The sorption curve defines the amount of moisture the material can hold as a function of the relative humidity of the air. This helps to regulate ambient humidity naturally, but it can also lead to the formation of a "sorption curve". condensation (on the surface or in the pores) and then a drying which can directly influence material and air temperature, for example phase change. Wood, for example, raw clay or lime are hygroscopic materials, whereas concrete, glass or plastic are not.

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Fig. 17: Moulay Idriss Zerhoun, Morocco (Google source)

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Fig. 18: Shibām, Yemen (Google source)

The photos above show two cities built of compacted mud (an abundant material in Morocco), six thousand kilometers apart, with exposed roofs and walls covered in light-colored lime plaster. On the left, the 8th-century ancient city of Moulay Idriss Zerhoun overlooks the Saïs plain near Meknes. The narrow streets provide protection from the harsh mountain sun, and the albedo effect of the buildings prevents them from overheating. On the right, the town of Shibām in Yemen, founded in the 3rd century and enlarged in the 16th century. The city's main feature is the height of its buildings, which are made entirely of low-emissivity, hygroscopic clay, whose density regulates hygrothermal conditions, creating a microclimate conducive to comfort.

 

Urgent need to adopt a "new" climate urbanism

Integrating sustainable and resilient solutions into city design is a necessary response to growing climate challenges, particularly in Morocco, where extreme heat waves are becoming increasingly frequent. Strategies such as optimized water management, the use of plants to promote evapotranspiration, intelligent design to capture air currents, and the use of materials with high thermal inertia, not only reduce urban temperatures, but also create more pleasant and resilient living spaces. What makes these solutions all the more relevant is their low-tech nature and their deep roots in local resources. In addition to being accessible and sometimes inexpensive, these practices are universal and timeless, as evidenced by the many historical examples of Moroccan and Mediterranean cities that were already using ingenious systems to cool the air, manage water resources and encourage wind circulation. Today, it is imperative to bring these ancient skills up to date and adapt them to contemporary needs. By reintroducing these bioclimatic features, we can build cities that are more resilient in the face of climatic hazards, while preserving their cultural and environmental identity. This approach is a real bridge between past, present and future, demonstrating that simple solutions, based on a detailed knowledge of the territory and local ecosystems, are often the most effective and sustainable in adapting to and mitigating climate change.

  1. Regional and provincial population projections, Haut-Commissariat au Plan du Maroc, 2017.

  2. Dr. L. Ghazouani (researcher at UMP6) questions the definition of UHI in her recent research : Combining Satellite Data and Spatial Analysis to Assess the UHI Amplitude and Structure within Urban Areas: The Case of Moroccan Cities .

  3. Natural history of architecture, P.Rahm 2017.

  4. https://www.nature.com/articles/s41467-023-43135-z

  5. https://www.24heures.ca/2021/08/26/vague-de-chaleur-quelle-difference-de-temperature-entre-un-ilot-de-chaleur-et-un-ilot-de-fraicheur

  6. https://meteofrance.com/actualites-et-dossiers/comprendre-la-meteo/quest-ce-que-lhumidite

  7. MESH2CENPC doctoral thesis, Matteo Migliari, 2023.

  8. https://dumas.ccsd.cnrs.fr/dumas-01281685v1/document

 

 Article by Ismail Sakout

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