IRA FLATOW: As the song says, it’s hot, too darn hot, all around the world.

SPEAKER: This week, the Earth had its hottest day ever recorded.

CHRIS JANSING: The record for hottest day on Earth was broken again for the second day in a row.

MICHAEL STRAHAN: Earth reaching its highest temperature on record for a fourth day in a row.

IRA FLATOW: And in the US, a brutal heat wave continues in the Southwest and Puerto Rico, with temperatures reaching 110 degrees and beyond. Meteorologists expect the rest of the summer to be exceptionally hot, due to both climate change, plus the El Nino weather pattern. And long stretches of hot weather are tough on the human body, as you can probably tell, often sending people to the emergency room.

In fact, a report for the Center for American Progress predicts that in the US, there will be 235,000 emergency department visits, and 56,000 hospital admissions due to extreme heat this summer, with a price tag of roughly $1 billion. And cities often fare the worst. Buildings trap in the heat and temperatures don’t drop down at night, like they do in rural areas.

But out of sight, just below the surface, it’s also getting hotter. Something called underground climate change, with some unexpected consequences. A study released this week used sensors to track increasing temperatures underground in Chicago, and how the Earth has shifted beneath the city, due to rising temperatures.

Joining me to tell us more about his research is my guest, Dr. Alessandro Rotta Loria, Assistant Professor of Civil and Environmental Engineering at Northwestern University based in Chicago. Welcome to Science Friday.

ALESSANDRO ROTTA LORIA: Thank you so much. Thank you for having me.

IRA FLATOW: So let’s start with the basics here, shall we? What is underground climate change? What’s driving it? Fill us in on this.

ALESSANDRO ROTTA LORIA: So underground climate change is a phenomenon that oftentimes is also called subsurface urban heat islands. It characterizes most, if not all urban areas worldwide, all cities. It’s a phenomenon that originates from two causes. Localized drivers that consist of underground structures, such as building basements, parking garages, train tunnels, subway tunnels, but also district heating lines, and so on and so forth, that continuously reject heat in the ground as a result of anthropogenic activity.

And the second cause, it’s a diffuse driver that as you mentioned in the introduction, consists of what we call meteorological urban heat islands, or surface urban heat islands. The idea, is that construction materials at the surface of cities, so building envelopes, absorb heat, typically coming from solar radiation or vehicles traveling, and release that heat at night.

So typically, cities are warmer than their surrounding rural areas. And on top of that, we know that there also is global warming that is basically warming up the atmosphere. And so a portion of all of that heat over time, diffuses underground. So basically, underground climate change results from really drivers that are themselves in the ground. And also, it’s exacerbated by what happens at the surface.

IRA FLATOW: Mm-hmm. So what happens underneath these cities, these underground heat islands, as you call them, as the underground temperatures rise?

ALESSANDRO ROTTA LORIA: These subsurface islands have been classically studied from various perspectives that include an environmental science perspective, because rising temperatures in the subsurface can represent a threat for the health of subsurface ecosystems. They have been studied from a public health perspective, because temperatures in underground environments, such as trains and subway tunnels, can be so hot that people feel thermal discomfort, or they can even suffer from heat-induced diseases.

The problem is also being studied from a transportation engineering perspective, because again, as a result of these extreme temperatures, there is evidence that the rates of trains traveling underground are often at the onset of buckling, forcing them to slow down or even stop, with as you can imagine, costs, significant costs over a year.

Recently, the problem has been studied from an energy perspective, because all of this heat can be re-utilized. But surprisingly, before we initiated this research program with my group, there was no study of underground climate change from a civil engineering perspective. And so the rationale of what we did, basically consists of the fact that materials, such as soils, rocks, and concrete, deform when subjected to temperature variations.

And so the overarching question that we asked ourselves when we started this work was, what is the influence of these temperature variations in the ground, and specifically, on the ground deformations? And what are the impacts of these ground deformations on the performance of civil infrastructure?

IRA FLATOW: So you set up wireless sensors to track the temperatures beneath Chicago’s downtown, right? Just to learn– just to learn the stuff that you don’t know.

ALESSANDRO ROTTA LORIA: Correct. So, the idea was really to deploy this internet of things solution, that basically consists of wireless temperature sensors that we installed in several environments at the surface, but especially in the subsurface of the Chicago Loop district, with the main aim to measure the temperature in those environments that eventually will cause heat diffusing towards the ground.

And so with the overall aim, to characterize underground climate change for Chicago. Not only we developed that tool, but we also created a computer model, a three-dimensional computer model of the entire district, that basically reproduces virtually all, or at least most of the heat sources that are present in such a district, including again, building basements, parking garages, tunnels, and so on and so forth.

Then we use a digital tool to basically simulate and retrieve the temperature that we are observing today in the underground, and use the model not only to study what likely happened in the past very thoroughly, but also what is likely to happen in the future.

IRA FLATOW: Can you tell from the model what kind of reaction the civil engineering part of this equation has? Will the structures be affected? Will the ground shake? Or, what did you discover?

ALESSANDRO ROTTA LORIA: We were able to assess that over the years, there have been significant temperature variations in the ground, and this is supported also by data that again, we gather on site. And as a result of these temperature variations that are deformations, so the ground is deforming and soils and rocks are basically moving.

IRA FLATOW: Is that a bad thing?

ALESSANDRO ROTTA LORIA: So a priori, it’s not necessarily a bad thing. But the magnitude of the ground movements that we quantified, it’s sufficient to be potentially concerning on a case by case basis. So the idea is, that based on the results of our study, we were able to quantify vertical ground displacement exceeding 10 millimeters, so half an inch. And it is recognized in civil engineering that ground movements of this order of magnitude can be problematic for the so-called operational performance of civil infrastructure.

So, what I think it’s important to stress at this stage, is that underground climate change is not a phenomenon that is likely to threaten the safety of people. It’s not a phenomenon that will lead to the collapse or the failure of structure. But it’s a phenomenon that can affect again, on a case by case basis, the conditions of normal use, the day to day operation of buildings, the aesthetic requirements of buildings, and the durability of buildings.

IRA FLATOW: So if you’re getting a movement of a half an inch, as you say, could that not lead to structural cracks in the buildings?

ALESSANDRO ROTTA LORIA: So typically, the underground structure themselves could be prone to cracking. The idea is, that excessive ground movements can result in angular distortions of structural members. They can result in unwanted settlement, that can also cause tilting. And as we mentioned, cracking.

Now, the reason why cracking, if excessive, is a problem, or at least can become a problem, is that cracks, especially in reinforced concrete structures, facilitate the permeation of water towards the reinforcement. And so as you can imagine, as you know, eventually, that can lead to corrosion. And this is why we’re talking about the durability of buildings.

IRA FLATOW: So you’re talking about the long-term effects of this continuing over and over again, the heat over a span of years.

ALESSANDRO ROTTA LORIA: This phenomenon, so underground climate change and the associated rises in subsurface temperature, are slow. We’re talking about heating rates of, at least for The Loop, 0.5 Celsius per year. The results of our work support that these deformations are significant.

And based on this assessment, we postulate that underground climate change could have contributed to at least some of the observed operational issues that have been seen in Chicago over the last century, such as again, cracking and problems that affect the foundations. So classically, these problems were attributed to inappropriate design techniques and/or construction methods. And I fully concur on the assessment.

What I postulate, is that temperature variations associated with underground climate change could have contributed to those observed issues, without even realizing it. And so I think the key will be now to really monitor this phenomenon, not only in Chicago, but I would say in general, urban areas worldwide, to assess what are the specific types of civil structures and infrastructures that are prone to problems.

IRA FLATOW: What parts of Chicago that you measured are the hottest underground? I mean, how much hotter is it underground versus let’s say, above ground that you measured?

ALESSANDRO ROTTA LORIA: Basically, the higher temperature variations are observed where the buildings are denser, like they are closer to each other. If you– you can think about this problem as follows. Like, basically, you can imagine that around each building basement or underground parking garage there is a bubble of heat that is forming over time.

Again, slowly, but continuously. So the denser are the buildings, the more these bubble of heat will be pronounced. And so this is a characteristic that, as you can imagine, varies depending on the city that you consider. With respect to the temperature anomalies that we quantified for Chicago, they are up to 27 Fahrenheit with respect to the undisturbed ground temperature.

So the idea is, that if buildings were not there, the ground temperature, which we also can measure around Chicago, would be of the order of 52 Fahrenheit. And what we have quantified for The Loop, is that value plus up to 27 Fahrenheit.

IRA FLATOW: Wow. So that’s how much warmer it is?

ALESSANDRO ROTTA LORIA: Yes. Locally, yes. On average, the ground has warmed significantly less, because again, these bubbles of heat are relatively localized around underground structures. But locally, these temperature variations can be significant. And so locally, they can represent a problem.

IRA FLATOW: Well, that leads me to this question, because more and more buildings are using the underground for heating and cooling. I’m talking about geothermal energy, right, where you sink pipes into the ground. The fact that the ground may be getting warmer in the area where these buildings are, might that affect geothermal at all?

ALESSANDRO ROTTA LORIA: So, yeah, absolutely. So as the Department of Energy says geothermal energy is a renewable energy source that is always on. It’s actually the only one. And so geothermal technologies, such as vertical or horizontal borehole heat exchangers, are very powerful, like are powerful tools to really harness energy from the ground and transfer it to buildings to meet space heating and cooling needs, but also hot water production needs.

Now, when you are in an urban area, you not only have geothermal energy, but as we are saying, you also have this waste thermal energy. So it’s like that in a city, you would have an extra amount of heat that you could harness. And this is what, like the scientific communities used to call, the geothermal potential of cities.

And so these geothermal technologies represent one mitigation approach to really hamper the intensity of underground climate change. To me, we could call it mitigation strategy number two. Prior to that, there is what I think we could call, mitigation strategy number one, which in my opinion, is the most rational approach, consisting of the retrofit of buildings and the application of thermal insulation in underground building enclosures.

So the idea is, that we take action directly at the source of the phenomenon. So we really hamper, we minimize the amounts of waste heat that are rejected in the ground.

IRA FLATOW: If you just joined us, we’re talking about underground climate change with civil and environmental engineer, Dr. Alessandro Rotta Loria. This is Science Friday from WNYC Studios. Do you think buildings need to be retrofitted to conquer this, or just the building of new buildings?

ALESSANDRO ROTTA LORIA: That’s a great point. So, new buildings, since in most countries worldwide, they are nowadays designed and built according to energy efficiency and sustainability principles. So, new buildings will not contribute significantly, if at all, to underground climate change, because again, through their thermal insulation and optimal characteristics, they will involve very minimal amounts of heat that are rejecting the ground over time.

So, underground climate change is not a phenomenon that will really affect new cities. But it’s a phenomenon that affects, by design, older cities. We should acknowledge that most of the cities that we have around us are already there. So, cities around us are old. And so especially if you go in the older world, like in Europe, in cities like Rome, Paris, London, and so on and so forth, those are the cities that are most prone to problems. And those are the cities where underground climate change will be more intense.

IRA FLATOW: Mm-hmm. So tell me what’s next for your research. What are the other questions you’re looking to answer?

ALESSANDRO ROTTA LORIA: So, the research we have carried out with my group to date has been really exploratory. So now we have highlighted that we have in front of us a potential problem that might have already represented a problem in the past, and will likely represent a problem in the future, depending on the city considered.

I think I have two ambitions. The first one is to study not only with computer models, but especially through sensing instrumentation on site, what are the types of civil structures and infrastructures that are most sensitive to underground climate change and the deformations caused by underground climate change. So for example, I would be thrilled to be able to collaborate with the city of Chicago to see whether we could install some sensing instrumentation to monitor from now on what is happening under key buildings, for example. And so this is really ambition number one.

Ambition number two, is to develop models, so computer models, data-driven models, through which we can basically simulate the influence and impacts of underground climate change much more expediently compared to what I did with the server, computational server that we have at Northwestern.

Because a tool like that will very rarely, if not kind of never, be available in practice. So the idea for developing these tools would be to really have some means to simulate and predict the influence and impacts of underground climate change, not only for scientific and engineering purposes, but also for decision making.

The idea is, that with these tools, we could create for Chicago, for the entire city of Chicago, first of all, not only for The Loop, but for the entire city of Chicago and for any other city worldwide, maps, like heat maps, that will basically tell us where it’s hot underground. So that then we could decide what we can do about that. And we can decide whether we want to go for mitigation strategy number one or number two or something else.

IRA FLATOW: This is fascinating. I really didn’t know much about underground heat islands and living in New York, we know all about the ones above ground, but not the ones underground, Dr. Rotta Loria. Thank you for taking time to be with us today.

ALESSANDRO ROTTA LORIA: Thank you so much for this opportunity.

IRA FLATOW: Dr. Alessandro Rotta Loria, Assistant Professor of Civil and Environmental Engineering at Northwestern University based in Chicago.

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