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1 .Listen to part of a lecture in an environmental science class.
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2 .Okay? Is everyone clear on what it means for a commercial building, like a store or an office building, to be a sustainable building?
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3 .It means the building\'s energy efficient and produces less waste and pollution than regular buildings,
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4 .Right.
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5 .And there are different ways to make buildings sustainable.
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6 .Like, well, we know that light colors reflect heat and dark colors absorb heat.
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7 .That applies to a building\'s walls or roof.
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8 .Most tall buildings have dark roofs, which absorb heat.
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9 .So in summer, this means more energy is expended on air conditioning.
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10 .So reflective roofing, especially white roofs, are becoming popular in cities, which brings me to what I want to discuss today, heat islands.
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11 .Who knows what a heat island is?
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12 .An area that\'s hotter than what\'s around,
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13 .Right, it\'s actually an environmental effect of urbanization.
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14 .The increased concentration of people in cities.
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15 .Urbanization typically leads to the building of large apartment houses, expansion of roads and sidewalks and so on.
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16 .And so there can be a loss of open spaces like fields or woods.
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17 .Okay, imagine it\'s really hot outside and you\'re barefoot.
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18 .Where would you rather walk? On pavement or grass?
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19 .Grass!
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20 .Right, pavement would burn your feet because synthetic surfaces absorb, retain and radiate, give off more heat than natural surfaces.
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21 .So imagine a city where acres of vegetation have been replaced with roads, sidewalks, buildings plus lots of vehicles, all radiating heat.
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22 .So do heat islands contribute to global warming?
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23 .Heat islands themselves don\'t increase global temperatures.
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24 .But the ways people deal with them can.
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25 .More on that in a moment.
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26 .First, let\'s further define a heat island.
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27 .It\'s a region of urban air and surface temperatures that are at least 10 degrees higher than nearby rural areas.
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28 .Most cities, or at least sections of most cities, fit this description.
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29 .Let me show you this image of the area around Atlanta, Georgia.
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30 .It\'s based on a satellite photo taken with a heat sensitive camera.
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31 .The darkest zones represented by the innermost circles in Gwinnett County, downtown Atlanta and Hartsfield International Airport.
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32 .These zones indicate the highest temperatures.
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33 .Two of these hot spots are urbanized areas.
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34 .Heat islands can occur year round, day or night.
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35 .In fact, temperature differences between urban and rural areas are generally largest at night, especially when there\'s no wind to move hot air out of the city.
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36 .Rural areas have more fields and trees and retain less heat due partly to the cooling effects of evaporation from soil and leaves.
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37 .Since rural areas absorb and retain less heat during the day, they cool down faster at night, the greatest difference between urban and rural temperatures in an area is what we call the maximum heat island effect.
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38 .That maximum effect usually occurs a few hours after sunset.
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39 .Question?
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40 .So in winter, wouldn\'t heat islands mean less energies needed to keep buildings warm inside?
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41 .Sure, especially in cold climate cities, heat that\'s absorbed through a building\'s walls can reduce heating demands in winter.
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42 .But this reduction is relatively modest, especially compared to increased energy demands in summer, when air conditioning on heat islands draws lots of energy from power plants.
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43 .Those power plants then release more heat, trapping greenhouse gasses into the atmosphere, and it\'s those greenhouse gasses that are blamed for global warming.
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44 .So there are many reasons to be concerned about heat islands, anything you do to reduce their effect, like reflective roofs.
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45 .I mean, if every building in Atlanta had a reflective roof, that dark red circle over the downtown area would probably shrink.
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46 .So wouldn\'t that help reduce global warming?
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47 .If all those buildings had reflective roofs?
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48 .Cause if heat islands shrink, they won\'t heat up the atmosphere as much.
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49 .Keep in mind that heat islands affect local scale temperature differences between urban and rural areas.
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50 .Global warming refers to a gradual rise of Earth\'s overall average surface temperature, and as I said, heat islands contribute to global warming only indirectly by increasing air conditioning demands in summer.
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51 .The heat that gets absorbed by city buildings and then radiates from them doesn\'t directly affect global temperature in any significant way.
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52 .In fact, the heat island effect has distorted some of the data in studies of long term climate trends.
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53 .That\'s because official temperature measurements are often taken in weather stations that are in or near urban areas.
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54 .So when researching evidence of global warming, you need to adjust for heat island effects.
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