There is a place in So Cal now called The Aspens South Coast we lived in 30 years ago. It has (had?) an incredibly dense concentration of trees, something I’ve not seen since in that region, which is of course desert.
This time of year when you opened the gate that separated the treed interior from the parking lot you felt like the air conditioning had been turned on. We have very fond memories of the place. Its only disadvantage for me was that spring caused my allergies to go crazy.
I was in a heat wave in Andalusia region in Spain and was visiting Medieval Arabic monuments and gardens. They have very interesting designs in terms of shapes, plant and tree selections and wall placements that produced very noticeable temperature differences. To say nothing of the scent of the lemon trees.
Genuine question for people in the field. My understanding is that the cooling effect of trees is primarily driven by evaporative cooling. That is, the shade effect only really exists because the plant does not shrivel up and die due to storing water. How much more effective are trees vs. big swamp coolers? Even in this article, they admit that daytime cooling of half a degree requires 3 times more water.
There is something to be said for the parts about shading the surface too though. You're unlikely to cool an entire desert a significant amount with water you bring in but if that's something that happens as part of keeping the actual surfaces in the city cooler during peak heat times on top of the air cooling effects of evaporation then the sum result is greater than the parts in terms of effect.
Of course it's Vegas, I wouldn't be surprised if we decided to make the downtown completely indoors so we could just run AC in the streets too. It's not exactly the city of practicality.
>> the downtown completely indoors so we could just run AC in the streets too.
So ... a shopping mall? Many cities do this already, linking various public indoor spaces by walkways/tunnels. Also those cities where the air outside is too cold. A few canadian universities link buildings with tunnels so students can avoid going outside.
Trees excel at harvesting water. When you water a tree, there is evaporative cooling like an artificial cooler but during the night, the dew falls back on the leaves and the ground where some of it finds its way back to the trees (possibly via an invertebrate first). Also the reflectiveness of leaves helps. Then there's the soil where layers of dead leaves, wood and others accumulate, sequester CO2 and create a sponge. Finally, by virtue of making the region cooler, rain is more likely to fall. Humans can probably engineer something better but the bar is high.
For reader clarification: accumulating carbon in soil from decaying plant matter still leaves it part of short term carbon cycling, not to be confused with geological-time carbon sequestration. As you know.
Even without any evaporative effect, the air cooling of leaves (at least bringing them to the surrounding air temperature) happens more easily than that of concrete pavement due to height and larger surface area. The concrete can easily get heated much hotter than the air at even 10-20ft.
Wrt. water consumption - Mediterranean species like say olive trees are kind of optimized for low water consumption, by for example having leaves covered with wax-like stuff decreasing evaporation.
Trees are also vertical structures. Any vertical structure will absorb some of the light, turning it into heat, then be cooled by rising air. This keeps the heat from getting to the ground, with or without evaporation. In other words, instead of the sidewalk getting hot, something 20+ feet in the air get hot. Hot air rises and the air near the ground stays cooler.
The increased water usage is tough because we're serious about water reclamation here in Vegas, but you can't reclaim water lost to evaporation, which is why there are policies (and serious fines) around excessive landscape watering. It might not be a worthwhile tradeoff, especially if there are alternate cooling methods that don't involve water loss.
Trees are great, but ultimately a pretty ridiculous idea if the goal is to create shade, even if you're not worried about water consumption. Avoiding concrete walls or overhangs is smart because you don't want the thermal mass.. but of course you can build these things out of fabric or thin metal.
The funny thing is, if you build a wall or canopy to avoid the water consumption plus literally waiting a decade for a tree to get tall.. now you're probably in violation of your HOA height restrictions, etc. Desert cities need to basically drop the idea of conforming to the typical expectations of visitors and newcomers by trying to add greenery. It's better to add shade, dig underground, build wind-catchers[1], salsabils[2]. There's tons of basic things like making sure roof surfaces are more reflective, and more strategic architectural things[3] that can be done to improve things and the techniques have been used forever
part of the cooling effect that trees produce is from photosynthesis, the percentage of light converted to plant matter can be as high as 1.5%
more will be reflected, and the shaded area will of course,be shaded, and then there is transpiration of water, which varys greatly with species.other effects will be due to the built environment, where a lot of asphalt and concrete could mostly obliterate the real effects of a few trees.
from wiki "The average rate of energy captured by global photosynthesis is approximately 130 terawatts, which is about eight times the total power consumption of human civilization"
The point of that article is that in many places the evaporative cooling is the main thing but in Vegas the water situation is such that it's more about the shade so the optimal tree is something that gives shade but doesn't need a lot of water.
Right where I am sitting now I have an LED strip above my desk and when I have my shirt off (right now) I can very definitely feel the radiant energy when it is on, so if it is really hot I either turn it off or switch it to green because the eye is most sensitive to green light. In fact, as I'm writing this, I just set the backlight on the 55-inch TV I use as a computer monitor down so I'd feel more comfortable.
Trees acts for their little thermal mass and large surface exposed to air, essentially IR radiation from the Sun can't much reach the ground or humans under trees (if they are large and dense enough) and the part of the radiation touching trees get quickly dispersed in the air (climbing the atmosphere).
The limited effect is that cities are dense and can't be made as forest so trees can't do nothing for buildings taller them them.
I recently bought some wooded land and unlike most people I know I've been extremely selective cutting down trees on it.
It's actually comfortable to be outside there. Even in the summer it's almost completely shaded. I was kind of surprised how extreme it is. I know trees make it harder to work and if you're hiring people they probably can't tolerate it but since I'm doing everything myself I don't have to clear everything and wait for it to grow back.
I think they all have something they want to do with it and just don't see how to accomplish that with the trees there. I know some people just don't like how trees feel (my mom is like this for example, something about growing up in the midwest I guess) but I think for most people it's just because hired labor and machinery can't fit between them.
I don’t think planting trees is only for cooling things down. Sometimes it’s just about helping people feel like they can go outside. In really hot places, even a bit of shade can change your mind about stepping out.
I'm sure any future endeavor to plant trees in the goddamn desert will have no negative environmental consequences at all. It's not as if the city in the goddamn desert is already in the middle of a regional water crisis as of last year or anything...
> In 2014, Southern Nevada’s gross water demand was about 205 gallons per capita per day (GPCD). In the region, single- and multi-family households account for 60 percent of water consumption—70 percent of which is used for landscaping.
Having lived in the desert, and not talking Vegas, but Nevada desert where folks bought cheap lots, dragged a single or double wide trailer into it, and started a life.
Trees were the first thing planted. Fast growing trees, placed to cast shade on the house.
After a few years, those dirt lots transformed into some very nice properties where sitting outside in the shade with the desert zephyrs rustling the leaves provide a very nice, idyllic place for conservation or reading.
There are a few of these plots outside of Crestone, CO that I've always dreamed of visiting. They truly look like oases and it must be surreal to sit in the shade and read while looking out onto the surrounding desert.
Southern Nevada uses a tiny amount of water compared to most states, about 2% of the total that’s apportioned from the CO river, and recycles about 40% of it. For indoor usage, 99% of water is recycled.
I agree growing things in the desert may be inefficient, but speaking for the CO river, that problem is in California and Arizona.
There may be counter intuitive effects in there. Plants roots creates water buffer zone underground that can capture some of rainfall and make better use of it, allowing larger growth.
MIT should deploy their desert water tech in LV[0]
Since the point of the trees has been discovered to be just shade and not evaporative cooling, they just need to figure a way to reorient their panels?
Perhaps the desert will repent its desertness and accept that sand, minimal water and a massive diurnal temperature range will somehow become amical towards good old Plane trees.
OK, let's go full mad world: a vast web of PV for power. Is there a handy massive water resource deep underground? If not then moisture in the air will need redeploying. Tall towers and probably gobs of power are indicated for that.
Trees grow in the desert. Mesquites, pines, junipers and more are all widespread in areas around Vegas. You don't need a tropical paradise to have vegetation, as the native forests of Arizona and Utah show.
There are proven methods for growing plants and trees in arid regions [0], but they have disadvantages which will become more evident as desertification expands with global warming. I agree that forcing non-native trees there is a losing battle in the long run.
If people were really serious about living in deserts in a sustainable way, they can't expect to have decorative greenery or classic architecture. A society as advanced as ours should be able to make compromises that allow modern comforts while adapting so well to their environment that the cities would look almost alien.
Not alien. Arabic, Spanish, mid-mediterranean, Puebloan, and a few more.
The architecture has existed for centuries, maybe even millenia. Some of us already live that way.
The irony is that the key thing - large thermal mass - has now become the province of only those with lots of money, or those with no money. Everyone in the middle is stuck with silly construction options for a desert climate.
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This time of year when you opened the gate that separated the treed interior from the parking lot you felt like the air conditioning had been turned on. We have very fond memories of the place. Its only disadvantage for me was that spring caused my allergies to go crazy.
Totally worth it.
I see some trees but it doesn't seem like a dense concentration.
Of course it's Vegas, I wouldn't be surprised if we decided to make the downtown completely indoors so we could just run AC in the streets too. It's not exactly the city of practicality.
I think you meant to say *on top of shade* because blocking the sun is the main effect here (the geometry makes more sense too!)
Less confusing phrase from abstract which implies that the 0.5deg evaporative cooling is almost a rounding error:
So ... a shopping mall? Many cities do this already, linking various public indoor spaces by walkways/tunnels. Also those cities where the air outside is too cold. A few canadian universities link buildings with tunnels so students can avoid going outside.
Specifically, some kind of giant succulent would be best for arid climes
https://en.wikipedia.org/wiki/Aloidendron_dichotomum
Mostly the benefit is instead of having the concrete under you absorb and emit the sun, the leaves above you do.
This dramatically reduces the heat we feel at human height.
Did I read that right? 16°C seems like an enormous effect.
Seems like trees would be a small investment to effectively get "outdoor AC-ish"?
EDIT: for those of us who are more comfortable with Freedom Units, that's like going from 104°F to 75°F!
Wrt. water consumption - Mediterranean species like say olive trees are kind of optimized for low water consumption, by for example having leaves covered with wax-like stuff decreasing evaporation.
The funny thing is, if you build a wall or canopy to avoid the water consumption plus literally waiting a decade for a tree to get tall.. now you're probably in violation of your HOA height restrictions, etc. Desert cities need to basically drop the idea of conforming to the typical expectations of visitors and newcomers by trying to add greenery. It's better to add shade, dig underground, build wind-catchers[1], salsabils[2]. There's tons of basic things like making sure roof surfaces are more reflective, and more strategic architectural things[3] that can be done to improve things and the techniques have been used forever
[1] https://en.wikipedia.org/wiki/Windcatcher [2] https://en.wikipedia.org/wiki/Salsabil_(fountain) [3] https://en.wikipedia.org/wiki/Passive_cooling
https://en.wikipedia.org/wiki/Photosynthesis
Increasing *evaporative* cooling by 0.5deg requires 3x more water, but shade alone is the *main* mechanism,it doesn't require water.
>during the day, trees provide significant shade by intercepting solar radiation, reducing mean radiant temperature (up to 16 °C)
Right where I am sitting now I have an LED strip above my desk and when I have my shirt off (right now) I can very definitely feel the radiant energy when it is on, so if it is really hot I either turn it off or switch it to green because the eye is most sensitive to green light. In fact, as I'm writing this, I just set the backlight on the 55-inch TV I use as a computer monitor down so I'd feel more comfortable.
The limited effect is that cities are dense and can't be made as forest so trees can't do nothing for buildings taller them them.
Plant trees on top!
Dead Comment
Las Vegas is embracing a simple climate solution: More trees (npr.org) 21 days ago | 143 comments https://news.ycombinator.com/item?id=44231151
Nights in Las Vegas Are Becoming Dangerously Hot (nytimes.com) 10 months ago | 1 comment https://news.ycombinator.com/item?id=41223831
It's actually comfortable to be outside there. Even in the summer it's almost completely shaded. I was kind of surprised how extreme it is. I know trees make it harder to work and if you're hiring people they probably can't tolerate it but since I'm doing everything myself I don't have to clear everything and wait for it to grow back.
https://www.knpr.org/show/knprs-state-of-nevada/2024-08-29/w...
For instance where I live east of the Cascades, in the dry part of Oregon, only like 10% of the water used goes to the cities.
https://www.centraloregonlandwatch.org/update/2021/5/5/droug...
Street trees are hugely beneficial and if you want to cut something (ha ha), you want to look at things like lawns or golf courses.
Trees in cities are not about reducing water usage by any significant amount. They are still lovely, though.
I found this quote in this 2016 PDF from the EPA:
https://www.epa.gov/sites/default/files/2017-02/documents/ws...
Trees were the first thing planted. Fast growing trees, placed to cast shade on the house.
After a few years, those dirt lots transformed into some very nice properties where sitting outside in the shade with the desert zephyrs rustling the leaves provide a very nice, idyllic place for conservation or reading.
I agree growing things in the desert may be inefficient, but speaking for the CO river, that problem is in California and Arizona.
https://www.snwa.com/water-resources/where-water-comes-from/...
https://lvgea.org/water/
Since the point of the trees has been discovered to be just shade and not evaporative cooling, they just need to figure a way to reorient their panels?
[0] only sunlight needed
https://www.thebrighterside.news/post/mits-high-tech-hydroge...
OK, let's go full mad world: a vast web of PV for power. Is there a handy massive water resource deep underground? If not then moisture in the air will need redeploying. Tall towers and probably gobs of power are indicated for that.
There was.
We've been sucking them dry for a century or more, everywhere they exist.
If people were really serious about living in deserts in a sustainable way, they can't expect to have decorative greenery or classic architecture. A society as advanced as ours should be able to make compromises that allow modern comforts while adapting so well to their environment that the cities would look almost alien.
0. https://en.wikipedia.org/wiki/Desert_greening
The architecture has existed for centuries, maybe even millenia. Some of us already live that way.
The irony is that the key thing - large thermal mass - has now become the province of only those with lots of money, or those with no money. Everyone in the middle is stuck with silly construction options for a desert climate.
"Angeles" has 3, and is already too much. So they say L.A. (2 syllables).
It's the same reason you call New York in full (not N.Y.), as it's already 2 syllables.
Deleted Comment
edit: referencing the HN title, not the paper itself