The Texas Trees Foundation, in conjunction with the Georgia Institute of Technology’s Urban Climate Lab, recently released a report on Dallas’ susceptibility to increased heat and ways to mitigate the impacts. Titled, Dallas 2017: Urban Heat Management Study, it details the current expectations surrounding heat and climate change. But while most numbers you read about climate change talk in generalities, this report drills down into the ramifications for Dallas to areas approximately six blocks square.
The reason for the drill down is simple — vague doesn’t serve anyone. In that vein, the report doesn’t evaluate the plethora of green technologies available, but instead focuses on trees/vegetation and reflective and porous roofing and roadway materials. In the past few months, CandysDirt.com has talked about green roofing and other technologies (here, here) as part of flood mitigation. This report mentions but discards these due to cost and complexity … oh, and the Texas Tree Foundation sponsored the report, so sticking with what they know and all.
In terms of heat buildup, Dallas is only out-heated by Phoenix and Louisville among major metros in the US. Why? Dallas is 386 square miles in size … 135 square miles of which are covered in rooftops, roadways and parking lots, compared with 23,464 acres of parklands. Layer onto that Dallas’ population growth that results in more roofs and roadways while at the same time knocking over old-growth trees and grasslands to accommodate more housing.
Urban Heat Islands (UHI)
Heat comes from the sun and is absorbed by the Earth at varying levels depending on the surfaces the radiation hits. It’s reflected back into space also due to the surfaces but also the makeup of the atmosphere. Greenhouse gases in the atmosphere keep some heat in (which is good or we’d be a snowball), but buildup of these gasses is what traps more heat on the planet (the problem we have now). It’s beyond the scope of this report to fix greenhouse gasses directly — their concern is increasing the reflectivity of the surfaces to push as much solar radiation (heat) away from the planet.
Cities concentrate everything from the number of Starbucks per block to the amount of heat trapped near the ground where people live. While high-rises are a good solution for concentrating more people in a small area, they also magnify the amount of surface area on that land. Think about a 1-acre parcel. Cover it with a single-story building and you have a 1-acre roof and four nine-foot walls of surface (4 walls x 9 feet x 200 feet long). The total surface area of that building is 50,760 square feet or 7,200 square feet more than the original acre of bare ground. For every story constructed, another 7,200 square feet of surface area is created. For each six stories, another acre of area is created on the original 1-acre parcel.
The problem with surfaces is their ability to reflect heat back to space (generally, lighter is better than darker for reflection). Aside from the added surface area, by concentrating high-rises in an urban core, reflectivity is magnified due to insulation. A 2×2 grid of high-rises will create more heat that takes longer to dissipate where they all meet because they’re insulating each other from the cooler areas surrounding them (the wind impeding properties of cities “helps” too). This is the concept of an Urban Heat Island. Heat is trapped and dissipates more slowly causing heat related damage to property and people (heat kills people).
But just because you’re outside downtown Dallas, don’t think you’re immune, it’s just less pronounced. In fact, heat in rural areas has increased 1.5 degrees on average nationwide from 1961 to 2010. Cities have seen the same level of increase, but they started 1.4 degrees higher to begin with. Oh, and don’t pish-posh me on 1.5 degrees. These seemingly minor increases fuel the formation of heatwaves defined as temperatures over 105 degrees are sustained for more than two days. Remember 2011? June’s average high temperature were 99, July 102, and August 106, averaging 8 degrees above normal. The same was true of overnight lows.
Trees are a cheap, effective and attractive method of containing the problem.
Trees offer shade that reduces the temperature below then. Through evaporation of water through their leaves, they reduce heat above them. Using strategic forestation, temperatures can be reduced by up to 15 degrees. Conversely, UHIs can increase temperatures by 10 degrees compared to the surrounding area. You may be thinking that if enough trees are planted, we’d reverse urban warming. Nope. In a packed urban core, you’re never going to get the forest coverage to reverse the UHI phenomenon due to the surface area of high-rises that traps and stores heat.
But as overall heat increases, every little bit helps, right?
And by employing an aggressive program of tree planting and preservation citywide, we can effect real change to our environment. Notice that I included preservation. Older trees provide more shade and evaporate more water than saplings. Neighborhoods with mature tree canopies must be incented/required to preserve these trees during any redevelopment.
Chicago has planted 500,000 trees in the past 15 years and Los Angeles announced an initiative in 2013 for 1 million trees. Even in Houston, 20,000 trees were planted since 2016. What’s Dallas doing? Exactly what you think. Which is why this report and its recommendations need to be studies and implemented by Dallas.
Trees are certainly the most effective way to cut heat buildup but you can’t plant a tree in the middle of a road … or can you? Remember reflectivity? We have a lot of asphalt roadways, parking lots and alleys. Again, in June I pointed out the benefits to using permeable concrete to allow rain to seep through roadways to mitigate flooding. Turns out lighter colored roadways made of permeable concrete also allow for evaporation and heat dissipation just like plants (not nearly as efficient, but it all adds up).
Finally, while green roofs were deemed too expensive by this report (although cities like Chicago and Frankfurt are doing quite well), reflective roofing materials were given as a less costly option. In the end all these roofing technologies cost more at installation but pay back over time in the form of lower utility bills and longer-lasting roofs.
Plant some freaking trees already. The study calls for a modest 500,000 new trees (and they even tell us where to plant them for the best results). If Houston (Houston, for gosh sakes!) can plant 20,000 a year, Dallas can surely knock that out in a weekend. Less easy is the political will to act on tree preservation, roofing, and concrete requirements for public and private new construction and repair. I don’t give city leaders much credit, but with the data and paint-by-number recommendations this report gives, perhaps.
Speaking of the city, the report frames all these things under the aegis of long-term planning. The hotter it gets, the more we need to plan for it versus react to it. Opening air-conditioned school gyms and sending ambulances willy-nilly across the city as people keel over is our current low bar on the subject.
But first, I encourage you to read the report for yourselves. You’ll download it and see that it’s 81 pages and shriek, but I promise, there are a lot of pictures and big type. It’s worth an hour of your time, especially if you work at 1500 Marilla.
Remember: High-rises, HOAs and renovation are my beat. But I also appreciate modern and historical architecture balanced against the YIMBY movement. If you’re interested in hosting a Candysdirt.com Staff Meeting event, I’m your guy. In 2016 and 2017, the National Association of Real Estate Editors has recognized my writing with two Bronze (2016, 2017) and two Silver (2016, 2017) awards. Have a story to tell or a marriage proposal to make? Shoot me an email firstname.lastname@example.org.