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The Midnight Sidewalk · The Climate Translation

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If you've ever walked across a parking lot in the middle of the summer, you already understand
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part of today's episode.
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You feel the heat rising off the asphalt, the air above it shimmers, your shoes almost
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feel like they're sticking to the pavement.
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But here's the strange part.
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If you walk across the same parking lot at midnight, sometimes it's still warm, not because
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the air is hot, but because the ground is still releasing the heat it's stored all day.
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That's because asphalt and concrete absorb solar energy during the day and slowly release
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it back into the air at night.
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In other words, our cities act a little bit like giant heat batteries.
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Today we're going to translate something scientists call the Urban Heat Island effect.
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It's the reason many cities can be five, sometimes ten degrees Fahrenheit warmer than nearby
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rural areas.
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And the newest research is showing that the most dangerous part of that extra heat often
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isn't the afternoon temperature.
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It's the nighttime.
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Welcome back to the podcast, I'm Dr. Mac.
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And this is The Climate Translation.
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I often discuss the balance of incoming and outgoing solar radiation in my introductory
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science classes, and it's common for students to make a connection and ask, why is the city
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hotter than the countryside right next to it?
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And the answer comes down to three pieces of physics, reflection, evaporation, and
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heat storage, let's start with reflection.
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Scientists use the term albedo.
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Albedo, measured on a scale from zero to one, simply describes how much sunlight a surface
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reflects back into space.
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A bright surface has a high albedo, something closer to one.
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It reflects a lot of the incoming sunlight.
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A dark surface has a low albedo, something closer to zero.
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It absorbs most of that energy.
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Fresh snow reflects about 80 to 90 percent of sunlight.
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That's why snow-covered landscapes can feel blindingly bright.
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Grass and vegetation reflect less, maybe 20 to 25 percent.
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But asphalt, asphalt reflects only 5 to 10 percent of the sunlight that hits it.
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That means the other 90 percent becomes stored heat.
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So the first reason cities run hot is simple.
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We've covered the ground in materials that absorb energy instead of reflecting it.
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But let's talk about the second piece of physics, evaporation.
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In a forest, sunlight doesn't just heat the ground.
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A lot of that energy goes into moving water through plants.
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Trees pull water from the soil and release it through their leaves in a process called
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transpiration.
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Sometimes people say trees sweat, but that's technically not accurate.
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It's a good comparison, though, to help explain the cooling effect.
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When water evaporates, it absorbs heat from the surrounding air.
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That's the same reason sweat cools your skin.
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So in a forest, a significant portion of the sun's energy is used in moving water, not
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just simply heating the air.
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Cities remove much of that natural cooling system.
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When we replace vegetation with concrete, asphalt, and rooftops, we remove both the shade
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and the evaporative cooling.
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The energy that used to move water through plants now goes into raising temperatures instead.
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That's why a city park can feel dramatically cooler than the surrounding streets, even
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if you're only a block away.
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Now, the third part of physics is what makes urban heat islands especially dangerous.
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Heat storage.
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Materials like asphalt, brick, and concrete have something called high thermal mass.
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It's a fancy way of saying that they store large amounts of heat.
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During the day, those materials absorb solar energy for hours.
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Measurements show that asphalt surfaces can reach 140-160 degrees Fahrenheit on a hot afternoon,
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even when the air temperature is much lower.
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Grass the same sunlight may remain closer to 85-90 degrees Fahrenheit.
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But the real difference shows up after sunset.
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In rural areas, temperatures can drop quickly at night because soil and vegetation release
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heat relatively fast.
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Cities behave differently.
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All that stored heat in roads, buildings and sidewalks slowly leaks back into the air
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for hours after the sun goes down.
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It's like the city has been charging a battery all day, and at night the battery starts to discharge.
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Recent research over the last few years has highlighted something important about this
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process.
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The most dangerous part of the urban heat isn't the daytime peak, it's the nighttime minimum
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temperature.
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Your body can tolerate heat for a while if it has a chance to cool down afterward.
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But in many cities, nighttime temperatures stay 5-10 degrees Fahrenheit warmer than the
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surrounding rural areas.
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That means the body really doesn't get a chance to reset.
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Hospitals and public health researchers have found that extended periods of high nighttime
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temperatures are strongly linked to heat-related illness and mortality during heat waves.
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In other words, the danger just isn't the afternoon heat, it's a lack of nighttime
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recovery.
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And when you combine urban heat islands with global warming, which is already raising
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baseline temperatures, those nighttime lows start creeping higher and higher.
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That's why cities around the world are beginning to rethink how they design their streets,
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roofs, and green spaces.
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So at this point, it might be reasonable for you to ask me, hey Dr. Mac, if cities are
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getting too hot, why don't we just install more air conditioning?
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The idea sounds reasonable if you don't understand physics, but the reality is that air conditioning
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doesn't actually remove heat, it just moves heat around.
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An air conditioner works like a heat pump, it pulls heat energy out of the air inside
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a building and then dumps that heat outside through a condenser unit.
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If you've ever stood next to an outdoor unit behind a restaurant or an apartment building,
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you probably have felt this.
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The air blowing out the back of those systems can be very warm.
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Now, imagine that happening not with one building, but with thousands of buildings all at once.
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In dense cities like New York or Tokyo, millions of air conditioners are operating during
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heatwaves, each one is transferring indoor heat outdoors and that heat accumulates in the surrounding
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air.
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Recent research over the past few years has shown that waste heat from air conditioning can
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raise street-level temperatures by about two to four degrees Fahrenheit in dense urban
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neighborhoods, particularly during evening hours.
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Now, that may not sound like much, but remember what we talked about earlier.
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Night-time temperatures are a critical factor for heat stress.
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If cities stay warmer at night because of the stored heat and because buildings are
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exhausting that heat into the streets, even a couple degrees can make a difference.
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The cooling window gets smaller.
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And that leads to a feedback problem.
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The hotter the city becomes, the more people rely on air conditioning.
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The more air conditioning runs, the more heat is released outdoors, which pushes temperatures
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up further.
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Now, air conditioning is absolutely essential for public health in many places.
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During heatwaves, it saves lives.
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But it does show us something important about the urban heat island problem.
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Technology alone isn't going to fix the physics of a city.
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So, if dark streets and roofs absorb heat, why don't we paint everything white?
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And it's actually a pretty good question and a good idea.
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Increasing the albedo of buildings, making roofs and surfaces more reflective, is something
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many cities are experimenting with right now.
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It's called cool roofing.
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A reflective roof can stay 30 to 50 degrees Fahrenheit cooler than a traditional dark roof
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under the exact same sunlight.
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That means less heat entering a building and less heat being stored in the structure during
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the day.
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But reflective surfaces don't solve everything.
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Cities are complicated environments.
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Urban planners often talk about something called the urban canyon.
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Picture New York, narrow streets line with tall buildings on both sides.
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Sunlight enters that canyon and bounces between the surfaces, the windows, the concrete
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walls, and the asphalt pavement.
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Even if some surfaces can reflect heat, much of that energy can still get trapped within
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the street corridor.
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So reflection can help, but what cities really need is cooling through shade and evaporation.
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And that brings us back to something, surprisingly simple, trees.
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Because trees don't just block sunlight, they also move water through their leaves and
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release moisture into the air.
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The process talked about earlier called evapotranspiration.
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That process helps to remove heat energy from the environment.
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In some neighborhoods, increasing tree canopy by even 10 to 20 percent, can lower summer
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surface temperatures by several degrees.
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Which means the real solution isn't just better air conditioners.
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It's designing cities that can behave a little bit more like an ecosystem, and a little
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less like a parking lot.
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Urban planners sometimes talk about canopy coverage.
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That simply means the percentage of land shaded by tree leaves when you look at the city
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from above.
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And those leaves are doing several things at once.
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First, they block sunlight before it reaches the ground.
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A shaded, sidewalk can be dramatically cooler than the one sitting in full sun.
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Measurements often show surface temperatures dropping 20 to 40 degrees Fahrenheit when pavement
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is shaded by trees.
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That alone can make a huge difference for people walking through a neighborhood.
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But shade is only part of the story.
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The second mechanism is evapotranspiration, the process that absorbs heat through evaporation.
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Recent research comparing neighborhoods within the same city consistently show large differences
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in temperature depending on tree canopy percentage.
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Areas with extensive shade and vegetation are often several degrees cooler in air temperature
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during a heat wave than the nearby neighborhood dominated by pavement and rooftops.
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At a surface level, the difference can be even more dramatic.
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Again, that's why walking through a park in the middle of the summer can feel completely
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different than walking across a parking lot a few blocks away.
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But there's another important benefit that often gets overlooked.
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Trees help cities cool at night.
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Remember the asphalt battery we talked about earlier? All that stored heat has to go somewhere
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after sunset.
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Green spaces allow more of that heat to escape because they contain less thermal mass than
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concrete and asphalt.
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And they continue to move water through the air through evapotranspiration, as long as
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soil moisture is available.
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So neighborhoods with more vegetation often cool faster once the sun goes down.
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And as we mentioned, nighttime cooling is important.
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Because during heat waves, the most dangerous conditions often occur when temperatures stay
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elevated for multiple nights in a row.
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Your body needs cooler hours to recover.
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Cities around the world are starting to take the science seriously.
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Urban forestry programs, the systematic planting and maintenance of trees, are now being treated
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as public infrastructure, not just landscaping.
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Some cities are mapping their canopy coverage, neighborhood by neighborhood, to identify
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where additional trees could have the greatest cooling benefit.
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Others are experimenting with green roofs, where vegetation is planted directly on top
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of buildings, helping to reduce both rooftop temperatures and indoor cooling demand.
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And the most interesting part is, these solutions don't just reduce heat.
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They also improve air quality.
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They reduce storm water runoff, and they make neighborhoods more comfortable places to
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live.
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So when we talk about planting trees in cities, it's not just about aesthetics or carbon
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storage, it's about physics.
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Trees change how energy moves through an urban environment.
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They intercept sunlight, they move water, and they help break the cycle of heat storage
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that makes cities warmer than the countryside around them.
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In other words, if asphalt is the heat battery of the city, trees are part of the cooling
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system.
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So far in this episode we've talked about skyscrapers, dense streets, and cities like New York
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or Tokyo.
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But you might be asking if this has any connection to smaller cities and towns across the
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country and around the world.
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The answer is urban heat islands aren't just a big city phenomenon.
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They appear anywhere we concentrate buildings, pavement, and rooftops.
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Even relatively small cities or farming communities can develop measurable heat islands.
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Researchers studying mid-sized cities across the United States, have found that downtown districts
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can run five degrees Fahrenheit warmer than nearby rural areas during the summer.
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Sometimes even on calm sunny days.
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And the reason comes back to the same physics that we talked about earlier.
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As towns grow, they add more impervious surfaces: parking lots, roads, sidewalks, rooftops.
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They absorb solar energy during the day and then release it slowly at night.
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At the same time, development removes vegetation that once provided shade and the evaporative
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cooling.
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So even if the community doesn't have tall skyscrapers, it still can accumulate heat.
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You've probably experienced this all the time.
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Think about the difference between walking across a grassy fairground and walking across
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a shopping center.
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Once tolerable, the others like walking on a stovetop.
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Now scale that up across in the entire neighborhood.
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That's how a local heat island forms.
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And of course climate change is adding another layer.
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As average summer temperatures rise, the baseline heat increases everywhere, including in smaller
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cities.
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When that baseline warming condition combines with urban heat island effects, heat waves
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can become intense and last longer in developed areas than surrounding countryside.
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And it has practical consequences.
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Higher temperatures increase electricity demand, as more people rely on air conditioning.
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Infrastructure can experience greater stress.
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Roads soften, rail lines expand, and cooling demand pushes the electrical grid during peak
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hours.
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And from a public health perspective, extended heat exposure becomes more dangerous when
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the nighttime temperatures remain elevated.
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So the connection to the heartland isn't about skyscrapers, it's about how our communities
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are growing.
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If development focuses on pavement and rooftops, the heat island effect becomes stronger.
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But if communities and cities include tree canopy, parks, shaded streets, reflective building
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materials, each of these can help significantly reduce local temperatures.
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Like I mentioned before, urban planners are starting to think of tree canopy as climate
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infrastructure, because once trees mature, they provide cooling for decades, with no electricity
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and a lot less maintenance.
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In that sense, planting and protecting trees today is a long-term investment in your local
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climate.
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It's one of the simplest ways a community can reduce its heat exposure, while also improving
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its air quality, its stormwater management, and its neighborhood livability and beauty.
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So the lesson for places in suburban and rural areas isn't that you're immune to the
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urban heat, it's that you have an opportunity.
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Because many growing cities still have a chance to design their neighborhoods with the physics
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of heat in mind, before that asphalt battery starts to become too large.
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Cities store energy, dark surfaces absorb sunlight, concrete and asphalt, hold that heat,
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and release it slowly through the night.
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And when we replace shade and vegetation with pavement, we remove one of nature's most
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effective cooling systems.
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It's the urban heat island effect.
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And as the planet warms, the design of our cities are going to become increasingly important.
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The encouraging part is that this is one climate challenge where solutions are already well
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understood.
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Shade, reflective surfaces, parks and urban trees, they're not just aesthetic choices,
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they're tools that can change the local climate of your neighborhood.
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Understanding the physics helps us see those choices more clearly.
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I'm Dr. Mac and this has been The Climate Translation.
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If you have a question about the climate that you have been too afraid to ask, or if you have a differing
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opinion, I want to hear from you.
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I can use your viewpoints and future episodes.
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You can reach me at theclimatetranslation@gmail.com.
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I'll see you next time.