Cities are vulnerable to heatwaves. But green spaces can help more than we thought

[hauntedheadnc]

Cities are vulnerable to heatwaves. But green spaces can help more than we thought
ZME Science

It’s not just that the planet is getting hotter; cities are getting much hotter than their surroundings. In fact, urban heatwaves have become a common occurrence, affecting 1.7 billion people. In Europe alone, the 2023 heatwave killed over 61,000 people. Now, a new study by researchers in the UK shows how much green interventions can help cool down cities.

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The cooling effect of green infrastructure varies depending on multiple factors (including local climate, the species involved, etc). But the effect is pronounced. Botanical gardens are, on average, a whopping 5 degrees Celsius cooler than their surroundings. Even a street tree cools its immediate surroundings by almost 4 degrees. This is how much different types of green infrastructure can reduce local temperatures:

In general, the team found that the bigger the park (or any other green infrastructure), the bigger the benefits. Furthermore, if parks are linked together via green corridors, there’s also a compound effect.

[jmecklenborg]

Breaking news: it's cooler in the woods during the summer than outside the woods.

[Yuri]

Originally Posted by jmecklenborg

Breaking news: it's cooler in the woods during the summer than outside the woods.

I guess the point here is as heat waves and extreme heat events are becoming common places, people will look more for green spaces, tree lined streets more and ignore the downsides (falling trees due storms).

That's certainly the case here in São Paulo: the city has a pathetic low number of green spaces but several districts are full of tree lined streets. São Paulo was notorious for its milder summers (hottest month had 18oC min and 27oC max average). Now, the averages are good 3oC above and people will certainly seek for shades more often.

[hauntedheadnc]

Originally Posted by jmecklenborg

Breaking news: it's cooler in the woods during the summer than outside the woods.

You don't say? Well, I can't imagine why someone would have taken the time to study something so obvious, and come up with an actual temperature range -- seems like a dreadful waste of time to know exactly how much a street tree can affect the temperature of the surrounding area.

Now be nice, or else I'll start posting photos of the Nashville skyline and you'll end up blinded or driven insane at the very sight. Don't think I won't do it either.

[Steely Dan]

We planted a black oak sapling in the parkway in front of our building last summer.

Gonna take a while (couple decades) before it's casting any meaningful shade, but hey, you gotta start somewhere.

And once they get going, these black oaks can really go the distance.

My kids' school grounds a block south of us has three GIGANTIC black oaks from the before times. They brought in a tree dude who estimated that they're all 200-250 years old!! There wasn't even a Chicago yet when they first sprouted outta their acorns.

I hope the black oak we planted can go 200+ years.

[edale]

Of course trees provide shade and cool the urban environment, but I struggle a bit with desires to have extensive tree cover in climates that don't naturally support forests. I see this in the Los Angeles area all the time, where neighborhood groups and tree enthusiasts are always advocating for more and more trees to be planted in public right of ways, trees retained on private property, etc. Historically, Los Angeles was not a tree covered place (in the mountains and along natural waterways, yes, otherwise, no), and we don't get the year round water required to keep trees healthy throughout the year. Yet that doesn't stop people from wanting their neighborhoods to look like Atlanta.

You move to a place because it's warm and sunny 9 months a year, but you also want a lush, shady, forested environment? For some reason, this just really bothers me. Don't move to a semi-arid climate and expect to live in a shady forest. In drier climates, I think cities should try to promote shade structures, awnings, colonnades, etc. rather than create artificial forests that require irrigation.

[Yuri]

Originally Posted by edale

Of course trees provide shade and cool the urban environment, but I struggle a bit with desires to have extensive tree cover in climates that don't naturally support forests. I see this in the Los Angeles area all the time, where neighborhood groups and tree enthusiasts are always advocating for more and more trees to be planted in public right of ways, trees retained on private property, etc. Historically, Los Angeles was not a tree covered place (in the mountains and along natural waterways, yes, otherwise, no), and we don't get the year round water required to keep trees healthy throughout the year. Yet that doesn't stop people from wanting their neighborhoods to look like Atlanta.

You move to a place because it's warm and sunny 9 months a year, but you also want a lush, shady, forested environment? For some reason, this just really bothers me. Don't move to a semi-arid climate and expect to live in a shady forest. In drier climates, I think cities should try to promote shade structures, awnings, colonnades, etc. rather than create artificial forests that require irrigation.

I like this concept of turning an arid area into forest. Like Johannesburg, located on a dry savannah and looks like Atlanta from above, full of trees imported from Brazilian Atlantic Rainforest.

In any case, the city itself changes way more the natural environment than the trees that come along as consequence.

[mousquet]

Originally Posted by jmecklenborg

Breaking news: it's cooler in the woods during the summer than outside the woods.

The issue is not as trivial as you seem to mean it in a dense urban environment.
I'll show you, cause studies by urban planners are spectacular in that matter.

• Video Link

The guy with glasses in that report from July of 2022 is a teacher in urban planning and an activist to defend trees in Central Paris.

0:45 Right there, you're standing on place de l'Opéra, a square usually full of tourists.
But there was none when they shot that report, because we were on a hellish heatwave.
That square is only limestone of buildings around and asphalt on the ground. Not a single tree on the square.
So it's what they call "îlot de chaleur" (a heat spot) in case of heatwave. The activist calls it a hostile environment, while it's a prime location at the center of the CBD in the inner city.

Temperature at ground level would be greater than in the Sahara desert when Paris is hit by some heatwave.

1:55 55.9°C, or 132.6°F on that very spot in front of the old opera house.
It goes up to 57 or 58°C (134 to 136°F)

Now look at that...

2:40 The guy just only walked over 100m (100 yards) to boulevard des Capucines. There are trees on the sidewalk right there, then the temperature drops to 23.7°C (74.7°F).
It is so unreal that I still wonder whether his gear would be reliable. I mean the difference over 100m is so insane that it seems fake.
Still, I don't think the guy would lie. That would be too bad for his reputation as a teacher in a college.

4:15 This woman is a researcher who works on very local weather conditions. She says they were working on a French-Indian satellite to spot "îlots de chaleur" more accurately, to the scale of a single urban block, like it couldn't get any more accurate.

This is obviously highly strategic in this city today.

[iheartthed]

Originally Posted by edale

Of course trees provide shade and cool the urban environment, but I struggle a bit with desires to have extensive tree cover in climates that don't naturally support forests. I see this in the Los Angeles area all the time, where neighborhood groups and tree enthusiasts are always advocating for more and more trees to be planted in public right of ways, trees retained on private property, etc. Historically, Los Angeles was not a tree covered place (in the mountains and along natural waterways, yes, otherwise, no), and we don't get the year round water required to keep trees healthy throughout the year. Yet that doesn't stop people from wanting their neighborhoods to look like Atlanta.

You move to a place because it's warm and sunny 9 months a year, but you also want a lush, shady, forested environment? For some reason, this just really bothers me. Don't move to a semi-arid climate and expect to live in a shady forest. In drier climates, I think cities should try to promote shade structures, awnings, colonnades, etc. rather than create artificial forests that require irrigation.

Maybe places in Southern California should implement sun shades, which are pretty common in southern Spain:



https://www.themayor.eu/en/a/view/se...n-summer-10149

Originally Posted by Yuri

I like this concept of turning an arid area into forest. Like Johannesburg, located on a dry savannah and looks like Atlanta from above, full of trees imported from Brazilian Atlantic Rainforest.

In any case, the city itself changes way more the natural environment than the trees that come along as consequence.

It would be extremely wasteful to plant a forest in a place like Los Angeles, which doesn't get enough rain to support that type of environment.

[hauntedheadnc]

Originally Posted by edale

Of course trees provide shade and cool the urban environment, but I struggle a bit with desires to have extensive tree cover in climates that don't naturally support forests. I see this in the Los Angeles area all the time, where neighborhood groups and tree enthusiasts are always advocating for more and more trees to be planted in public right of ways, trees retained on private property, etc. Historically, Los Angeles was not a tree covered place (in the mountains and along natural waterways, yes, otherwise, no), and we don't get the year round water required to keep trees healthy throughout the year. Yet that doesn't stop people from wanting their neighborhoods to look like Atlanta.

Don't fret. There are other things Los Angeles can do to improve its environment and make it a healthier city to live in.

[JManc]

Originally Posted by Yuri

I like this concept of turning an arid area into forest. .

I don't because it disrupts the local ecosystem, increases the humidity, requires already finite resources; i.e., water, to be diverted to a bunch of trees that would otherwise not survive in such conditions.

[Yuri]

Originally Posted by iheartthed

It would be extremely wasteful to plant a forest in a place like Los Angeles, which doesn't get enough rain to support that type of environment.

I'm not advocating for people actively doing that, specially in our days. However, in places that already happened, like Johannesburg and even Los Angeles to some extent, it's done. Better to have this coverage than decrease the green areas.

[JManc]

Originally Posted by Yuri

I'm not advocating for people actively doing that, specially in our days. However, in places that already happened, like Johannesburg and even Los Angeles to some extent, it's done. Better to have this coverage than decrease the green areas.

It's not a 'one and done' thing though, those trees in LA and Johannesburg require constant watering.

[iheartthed]

Originally Posted by Yuri

I'm not advocating for people actively doing that, specially in our days. However, in places that already happened, like Johannesburg and even Los Angeles to some extent, it's done. Better to have this coverage than decrease the green areas.

And it's not necessarily a coincidence that both of those regions are going through water crises.

[craigs]

Trees make more sense in places like Los Angeles than green grass lawns. The region would be better off if people stopped watering lawns and shifted to watering shade trees instead.

[isaidso]

Tree planting to moderate the effects of heat and to improve air quality has been standard practice in many cities for quite a long time now. Some have gone further.

Toronto was the first North American city to require and govern the construction of green roofs on new developments. The Green Roof Bylaw sets out a graduated green roof requirement for new buildings that are greater than 2,000 m² in gross floor area. These measures help make the city more comfortable in summer but can improve food self sufficiency through urban farming.

A Roof Top farm was installed atop the Toronto-Dominion Centre pavilion in 2012


https://www.theglobeandmail.com/news...rticle4276138/

[Yuri]

Originally Posted by JManc

It's not a 'one and done' thing though, those trees in LA and Johannesburg require constant watering.

People don't water trees in Johannesburg afaik. And their shade cool the whole urban area and decrease evaporation.

Originally Posted by craigs

Trees make more sense in places like Los Angeles than green grass lawns. The region would be better off if people stopped watering lawns and shifted to watering shade trees instead.

Exactly. Trees are just there and extract water from deep ground. People water grass lawns, an American thing.

[isaidso]

Originally Posted by iheartthed

It would be extremely wasteful to plant a forest in a place like Los Angeles, which doesn't get enough rain to support that type of environment.

I doubt they'd be so foolish as to plant species that aren't suited to their climate. They would surely consult with professionals that specialize in urban forestry and climate mitigation. You want trees/plants that flourish without human intervention. Many of those species don't provide as much shade as an oak tree would but there are many benefits beyond shade.

[craigs]

My yard has numerous trees, including palms, pines, and fruit trees. Many of them are over 50 ft. tall, as most were planted around 1965. We don't water any of them. Ever. They have deep roots that extract all the water they need and that also stabilize the hillside so that there's no slippage when we get heavy rains in winter and during summer thunderstorms.

[sopas ej]

Reminds me of a meme I saw this morning:


Regarding Los Angeles and SoCal, people seem to think that there's no such things as aquifers and underground streams/rivers. Once tree roots hit a water table, you don't even really need to water it.

There are also native trees, of which the indigenous people knew/know how to cultivate. They were a food source after all, and if anyone knows about Los Angeles history, they would know about the towering 400 year-old sycamore tree that the native Tongva used to revere and hold meetings at, a tree that eventually met its demise in the late 1800s. The Spanish colonizers referred to the tree as El Aliso, from which the present-day Aliso Street in downtown LA gets its name.

Here are few samples of native SoCal trees, from California Native Trees.

Arctostaphylos “Sentinel” showing off its winter flowers.


Sycamore.


Quercus agrifolia.


And here's the story of El Aliso.