Phoenix has allocated a lot of money to plant trees and put in shade structures across the Valley. It has an Action Plan for Trees and Built Shade that represents more than $50 million in public and private investments over the next five years.
But, how much difference can trees make? Peter Ibsen, a research ecologist with the U.S. Geological Survey, has some answers.
Ibsen was part of a new study that aimed to get some answers here. To do it, he and his team put up sensors all across the Valley that measure humidity and temperature — and they did this in seven other cities around the country. They measured how shade from trees affects temperature.
The study found that trees can make a whole lot of difference, especially in arid, desert cities like ours. Ibsen joined The Show to discuss.
Full conversation
LAUREN GILGER: The city of Phoenix has allocated a lot of money to plant trees and put in shade structures across the valley. It has an action plan for trees and built shade that represents more than $50 million in public and private investments over the next five years. But how much difference can trees make? Well, our next guest has some answers.
Peter Ibsen is a research Ecologist with the US Geological Survey and he was part of a new study that aimed to get some answers here. To do it he and his team put up sensors all across the valley that measure humidity and temperature and they did this in seven other cities around the country.
Their measurement how shade from trees affects temperature and they found trees can make a whole lot of difference, especially in arid desert cities like ours.
PETER IBSEN: I mean, we have a lot of interest in trying to figure out from the USGS side how different land covers, influence temperature, especially during heat waves. But we also want to answer this question of sort of if you expand that to like this nationwide context, how much does the regional climate sort of put its finger on the scale of how much cooling or warming these land covers provide. So that's why we, we did this across these eight cities.
I mean, it'd be great to do it across more. But we picked these eight for one reason, they capture a really good gradient of like heat and dryness from from Vegas and Phoenix being hot dry cities to Baltimore, Miami being hot and humid in the summer.
GILGER: And what you found here in comparing the cooling effects of trees in these various cities in these various different climates is kind of hopeful for Phoenix, right? Like is that the most arid cities like ours saw the biggest cooling effects from trees?
IBSEN: Yeah, yeah, I mean for one, I will say we found cooling effects in all the cities. So even in Baltimore, Miami in the humid cities across the board, trees and cities are reducing air temperature on hot days. But yes, in the hottest and driest of cities that that magnitude of cooling increases.
So for example, like if we looked at the amount of cooling, you're getting trees in Miami that could be around, you know two and a half to three degrees centigrade. Whereas in Phoenix, that maximum cooling is up to four and a half to five degrees.
GILGER: So the dry heat thing that we talk about all the time here really seems to have an effect.
IBSEN: Yeah, we we really see that that mechanism that dry heat is like if you think of a tree as a living straw. So basically, we see trees as doing, like you said, cooling from two main ways like they, they provide shade.
We all know that the USGS is not in the business of just saying it's cooler in the shade, but they also provide cooling through this thing called transpiration, which is very similar to how people sweat. Its water is evaporated from the leaf surface and that both cools the tree and the leaf. But it also sort of creates this aura of cooling the air temperature around the tree.
So on very hot dry days, the tree is very much a mechanistic straw that the dry air is sucking up the moisture from the soil through the tree. So it comes out of the leaves.
GILGER: That creates a cooling effect, that'll make it feel cooler.
IBSEN: Exactly. Yeah, you get this basically a tree acting as a swamp cooler or like I have seen in Phoenix on the times I've been there, those little misters that come out on people in restaurants.
GILGER: We love misters in Phoenix.
IBSEN: Yes, a tree is a giant mister.
GILGER: So tell us like, how far can you tell how far this kind of cooling carries? Like if there's a tree in my front yard, is it gonna cool down the whole area around my house, like people say their, their lawns do here.
IBSEN: Yeah. So we looked at this, like I mentioned, we put the sensors in the trees, but the way we measured like our scale of analysis to use our like science terms was to expand this to around 60 square meters around each sensor.
So we weren't just basically measuring that effect of the single tree where we put the sensor, but we're measuring the effect of all the land covers surrounding that tree at that sort of like buffer.
So when we're looking at that relationship of tree canopy or grass or buildings or even sidewalks to the air temperature, we're not just talking about that single point but about all the area within 60 meters surrounding that tree.
GILGER: But that's a pretty big range. Yeah, okay. So let me ask you Peter about the difference between something like a tree then and this kind of cooling effect you found there and the way it works with the moisture involved and something like a manufactured shade structure, which we also see a lot of in Phoenix and there's always a debate it seems about which might be more effective.
IBSEN: Yeah, that is a fantastic question. And, and they are both effective. So I like to think of trees are they are a tool in the tool kit that we use for heat mitigation, they do specific things.
So a shade structure like you mentioned will do a fantastic job at providing shade as will a tree with a healthy crown, it provides shade as well. The added bonus that is why we believe at the USGS.
We're seeing this increase of cooling in these dry cities is that atmospheric pressure that pulls all this water out of the tree. So you're getting the same shade benefits, but you're also getting this, like I mentioned this wider localized cooling as that water evaporates and just creates this bubble of reduced air temperature surrounding that.
So we looked at grass and turf as well and we saw cooling effects around grass and turf. But what we didn't see is that grass and turf had that increase with aridity and dryness and heat.
GILGER: So let me ask you then lastly, I mean, what do you hope your findings are used for like not just in a cities like Phoenix and Las Vegas but in all of these places?
IBSEN: Good, good question because if you, if you're not familiar with the USGS, we are an applied science agency and we don't want this just to be a, a academic paper or a scientific paper that is just read by other scientists, we would like this data to be used. So one of the ways that this can be used is for one identifying areas to swap out land covers.
So we found this cooling effect of trees. We also measured the sidewalks and buildings as well to see how much they warm.
So you could think of this not necessarily as being used as well. If we add this many trees, we get this much cooling, you could use these results to also do the math to calculate. Well, if we remove this much impervious surface or this much pavement and add this many trees, then we actually get the reduction of warming from the surfaces. Plus that addition of cooling from trees.
One other important thing to mention though and this is important for Phoenix and Las Vegas. This is all very water dependent. So we really want people to know that if you want this type of cooling, if cities and managers and planners are aiming to plant trees, these added benefits we see in the desert cities, we really see that coming because of that access to water.
So any of these trees, they, they need to be irrigated and have a enough soil moisture to provide that sort of sweating, transpiration benefit that we, that we see in the ones that we've measured.
GILGER: All right, we'll leave it there, Peter Ibsen, Research Ecologist with the US Geological Survey at the Geoscience and Environmental Change Center. Joining us to talk about this study, Peter thanks so much. This is useful.
IBSEN: Thank you so much for calling, I love to talk about this work.
