5 Urban Climate Mitigation Myths

9 minutes read.
Last modified: February 2022

Before we dive into this, let’s get something straight: in the right circumstances all of the urban climate mitigation methods that are discussed below are effective and valid ways to create pleasant, liveable cities that are fit for the hotter conditions that are already arriving with the global climate change. However, they are not - or not in isolation at least - the silver bullets that they are often made out to be. Sometimes this is due to over-simplified explanations in the media and poor understandig of atmospheric physics but sometimes also due to dubious interest of parties that are making good money selling one solution or another.

We are fully aware that not everyone will be happy with what we are saying, but as conscientious engineers we feel it’s our duty to employ methods that are grounded in sound science and atmospheric physics, even if we have to swim against the current sometimes. With an individual experience in complex science & research of more than 20 years in our team and 10 years of careful work to build the to-date most advanced urban heat island simulation model, we are confident in our assessment. Science is not a popularity contest, but it works.

Myth 1: Trees will cool down the air

Don’t get us wrong, we love trees and plants and not just for a single reason. Some of us even do their own farming! We are well aware that trees in the city have lots of positive effects from pure aesthetics over fostering biodiversity, psychological effects to community-effects and more. However sometimes urban trees are presented as the one and only silver bullet solution that will magically create pleasant urban microclimate in the light of the global heat catastrophe. Unfortunately this is not correct - here is why:

Little tree on a sidewalk (CC BY-NC-ND 2.0, https://www.flickr.com/photos/therefore/86988892).
Little tree on a sidewalk.

Evaporation, shade and air velocity

The three main mechanisms how trees can create cooler microclimate at pedestrian level in summer, are evaporative cooling, shade and increased air velocity. We dive deeper into the reasons for that in our article about urban human thermoregulation, but let’s briefly summarize: Shade prevents the energy contained in direct solar radiation from reaching the surface of our skin. This reduces the heat-load on the body and makes us feel less hot, or at least more able to cope with heat. Evaporation Like humans, trees suffer when it’s hot and also like us, they ‘sweat’ to cool down - the scientific term for that is (evapo)transpiration. The third mechanism is air velocity: when air is moving faster over our skin, it is easier to cool down by sweating. The crowns of trees force the wind to flow around them to an extent. This results in slightly higher air velocity under trees - so sitting under a tree is a bit like sitting in front of a fan - but silent :)

All great stuff, you might think, what’s the issue?

Time & Cost

These effects, that we cover in detail in our article about courtyard greening, need the right conditions to work: First of all, trees need to be big and ideally also numerous. Only a big tree has enough leaf area to evaporate so much water, that the effect becomes noticeable. Only a big tree has a crown big enough to increase wind velocity underneath it. And only a big tree can create enough shade to provide refuge from the sun. There also needs to be enough water for the tree to survive and thrive and also the wind has to be right: Too little and you can even get the opposite effect (see the courtyard article linked above), too much - for example on top of high-rise building - and the tree cannot grow or might present a danger.

To get big trees you need time and or money! Until a single tree is fully grown and shows its effect on the microclimate it might take 10, 15 or even 20 years. We also need soil and especially soil of a depth that actuall let’s trees grow to appreciable size. Real estate development projects will often show huge trees on artistic renders while in reality, 15 years might pass until this will show tangible effects for citizens.

In many urban development projects there will be conditions that allow all this: deep soil, enough water, time or money to have big trees, proper wind conditions, enough space for the crowns and enough area to plant more than one tree. But in many urban settings one or more of these factors will prevent the effective use of trees as climate mitigation measure.

In these cases it makes much more sense to look at alternatives like technical shading, wind engineering etc. rather than sticking with a ‘solution’ that might never really materialize.

Mirage effect over highway. (CC BY 2.0, https://www.flickr.com/photos/13907834@N00/5994891327)
Mirage effect over highway.

Myth 2: Cool air from the countryside

It’s a really relatable thought - after all the air is cooler outside of the city - so let’s just make sure that there are not too many big buildings in the way so that the cool air from the countryside can cool us down, right?

We wish it would be so easy. Air over forest and grassland is in an energetic balance that manifest itself as the lower air temperature (see this infograph about the relative influence of these factors) and air in the city is likewise in a energetic balance that manifests as a higher temperature. At pedestrian level, this balance changes really fast within meters - that is easy to see in highway mirages: despite grass and trees surrounding a road, the air above the road is so hot, that it produces the well-known mirage effect. Altered surfaces (light colors to boost albedo, soil and grass for insulation) will mitigate this, but every city has more - and more sealed - surface than the countryside.

There is actually a way to use air ‘from the country’ to cool down the city - or more precisely - to make people feel cooler and it’s not too far from the original idea either, but uses a different mechanism. While we cannot hope to design cities that somehow funnel cold air into the city center, we can very well design cities in ways that permit the air to flow faster in the main direction of summerly winds. This increased air velocity enables people to sweat more effectively and helps to mitigate conditions that are potentially dangerous for humans.

We did participate in a number of projects where careful interaction of landscaping, architecture, zoning and urban planning allow good ventilation in summer especially in areas that are designed for summer outdoor use (playgrounds, recreation areas etc.). One caveat is, that increased air velocity can be dangerous by itself especially around high-rise buildings, so this approach needs careful consideration of risks and benefits.

A city fountain.
Small urban fountain.

Myth 3: Fountains, ponds and the sponge city

Evaporative cooling from fountains, ponds and other ways of direct water evaporation can deliver a lot of cooling energy theoretically and it’s a really good idea to use available water that is not needed otherwise for just that purpose. So why do we include it in our list? Because of its inherent mass- and energy limitation and atmospheric dilution effects.

Coupled with big trees, as we mentioned above, retained rain water can slowly evaporate which is a great idea if the soil is deep enough. However simple, stagnant basins of water, with little depth and surface have limited impact. A bit better are fountains and spraying devices that create much larger surfaces from which water can evaporate. These devices do indeed cool down the air.

To reach maximum impact however, there is a delicate balance to find: too much air movement, and the cooling effect will quickly dilute and dissipate, too little air movement and the humidity will rise, which will cause an increase in the felt temperature, because it inhibits evaporative cooling from sweating (details in our article about human thermoregulation).

So in general direct water surfaces and spraying will be effective in semi-enclosed spaces, but will quickly show either a bad cost/benefit ratio (in open spaces) or cause an increase of felt temperature (in enclosed spaces). As we mentioned in the introduction to this article: there is a time and place for this technique, but it’s not a silver bullet either.

Urban housing with supposedly green wall.
Green architecture in practice.

Myth 4: Green roofs and walls

Many of the limits discussed above are true for green roofs and walls. They need water, are sensitive to too little or too much wind, can be expensive to build and maintain etc. However, in the right circumstance they can absolutely make a lot of sense. Contrary to popular belief however, they do make much more sense for the building they cover, than the open space surrounding it.

The reasons for that are shade and insulation. Green walls and roofs form an additional layer around buildings that prevent the wall of the building from becoming hot, which in turn helps to cool the building. There is much less cooling effect on the outdoor environment, because the leaf area is often relatively small for both green roofs and walls and also because the cooling effect does not happen at street level were the public can profit from it (though inhabitants will).

For roofs it is very expensive to build soil pockets deep enough to allow bigger trees to grow and due to higher wind velocities it might be outright dangerous. In our opinion it is much more cost effective to use a combination of technical shading (for accessible areas) and low-maintenance plants / photovoltaics for roof areas that are not accessible.

Attempt of green wall of sustainable building project.
Green wall of an "air condition for a city".

The images here are from a project that touts itself as a landmark project to mitigate urban heat island effects, going so far to claim that it does not only cool down itself, but even the surrounding city by as much as 3°C air temperature, which is ludicrous in itself (it would be easy then to solve the problem of increasingly hot cities or even climate change itself!). From the images, taken a year after completion, it is clear that there is a very wide gap between the planning aspiration and reality.

Myth 5: Let’s just stay inside and use air conditioning

Global warming is increasingly threatening to make entire regions uninhabitable for humans, due to high wet bulb temperatures, that prevent humans from cooling down and lead to heat stroke and ultimately death, regardless of the physical health of the individual. So is it an option to just stay inside?

Unfortunately not. First of all there are plenty of professions that simply require humans to work out in the open - from construction work over garbage collection to police work. There already is a quickly growing industry offering powered cooling vests for people that need to work outdoors in hot conditions. Our modern urban society cannot function without the outdoors even if it is just to ensure the electricity flow into our buildings to operate the air condition. So short of building outright underground cave cities, this is not a realistic option.

And there is the second caveat, the very use of air condition results in waste heat. Waste heat that heats up the environment even more. We explain this in more detail in our article about climate mitigation architecture. The gist of it is, that we must take care to build our cities for passive cooling. The very geometry of the city must be optimized to take on as little heat as possible in human-accessible areas otherwise we will work ourself into a spiral of using more and more energy to keep the indoors cool, while heating up the outdoors even more.

Summary

We’d like to emphasize again that all of the presented ‘myths’ do make sense for urban climate mitigation if they are used in the right context. However, if these landscape architecture mitigation tools are not used correctly, they are costly and ineffective at best and dangerous and counterproductive at worst.

We do live in a complex world and in order to carve out liveable, enjoyable urban environments in this age of global warming, we are well advised to embrace this complexity, do our best to understand it and use the best available tools in the right places.

At Rheologic we have developed the tools that model these complex interactions of urban landscape, solar radiation, air flow and energy balances to be able to give our clients a robust assessment of the various trade-offs of different design options. Our models are a validated, high-resolutions approach, that can capture the urban environment with all the details that are necessary for a true-to-life representation of the actual conditions. Moreover, we have created low-barrier tools to discuss results in interdisciplinary teams - or event include citizens and tenants in the design process - using 3D models that can be interacted with right in the browser; take a look at the demo page to try it out yourself.

If you’d like to know what we can do for you, simply send us a message and let’s talk!.

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