Researchers emphasize evaluation of tradeoffs in battling urban heat island
Urbanization will increase average temperatures in Arizona’s Sun Corridor and white roofs can combat that, but not without consequences for the region’s hydroclimate
TEMPE, Ariz. – A team of researchers from Arizona State University have found that warming resulting from megapolitan expansion is seasonally dependent, with greatest warming occurring during summer and least during winter. Among the most practical ways to combat urbanization-induced warming – the painting of building’s roofs white – was found to disrupt regional hydroclimate, highlighting the need for evaluation of tradeoffs associated with combating urban heat islands (UHI).
“We found that raising the reflectivity of buildings by painting their roofs white is an effective way of reducing higher average temperatures caused by urban expansion,” said Matei Georgescu, an assistant professor in ASU’s School of Geographical Sciences and Urban Planning. “However, increased reflectivity also modifies hydroclimatic processes and, in the case of the ‘Sun Corridor,’ can lead to a significant reduction of rainfall.
Our maximum Sun Corridor expansion scenario leads to a 12% reduction in rainfall, averaged across the entire state. Painting roofs white leads to an additional 4% reduction in rainfall.”
The research is presented in the paper, “Seasonal hydroclimatic impacts of Sun Corridor expansion,” published in the Sept. 7, 2012 issue of Environmental Research Letters. Georgescu, the lead author of the paper, is joined by Alex Mahalov, The Wilhoit Foundation Dean’s Distinguished Professor in the School of Mathematical and Statistical Sciences at ASU, and Mohamed Moustaoui, an associate professor in ASU’s School of Mathematical and Statistical Sciences.
The paper focuses on Arizona’s Sun Corridor, the most rapidly growing megapolitan area in the United States. Located in a semi-arid environment, the Sun Corridor is composed of four metropolitan areas: Phoenix, Tucson, Prescott and Nogales. With a population projection expected to exceed 9 million people by 2040, the rapidly expanding megapolitan offers the opportunity to identify tradeoffs focused on sustainable expansion of the built environment.
The authors utilized 2050 projections of Sun Corridor growth developed by the Maricopa Association of Governments (MAG), the regional agency for metropolitan Phoenix that provides long-range and sustainably oriented planning. They conducted continuous multi-year, multi-member, continental scale numerical experiments for several 2050 Sun Corridor growth and adaptation scenarios and compared results with a modern day Sun Corridor representation.
“For a maximum expansion scenario, we find greatest warming to occur during summer, in excess of 1 degree C (1.8 degrees F) when averaged over the entire state of Arizona. Warming remains considerable during both spring and fall seasons, approaching 0.9 C. For a minimum expansion scenario, the consistent theme of maximum warming during summer with reduced, although still significant, warming during spring and fall seasons persists,” Georgescu added.
Whereas previous research has documented the contribution of cool roofs as an effective UHI mitigation approach, this work emphasizes the need to broadly evaluate impacts by exploring consequences that extend to hydrology and rainfall.
“Truly sustainable development will have to consider impacts extending beyond average temperature,” Georgescu explained. “A crucial step in that approach is to identify potential adaptation and mitigation strategies and assess tradeoffs, to ensure that we make smart decisions with minimum damaging consequences.”
All three co-authors are affiliated with ASU’s College of Liberal Arts and Sciences and the Global Institute of Sustainability.
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Philip Bradley says September 8, 2012 at 4:05 am
“Steel roofs are a better option as they reflect solar energy outside the visible spectrum as well as the visible spectrum. Take a look at where I live, south of the river, Perth, on Google Earth. More than 2 out of 3 roofs look white, but in fact they are steel.”
Phil,
We have a brown colored steel garage door (facing east) at our place in the Sierra foothills. The combination of the brown color (which matches the cedar siding color on the building) with the rather poor insulation characteristics of the metal door led to a huge heat transfer into our garage until about 11:45 am every morning in the late spring to early fall. We found that using a UV shade (installed with about a 6″ air gap between the garage door and the shade) prevents the suns mix of wavelengths getting to the metal door which has made it possible to do some temperature sensitive activities in the garage.
What exactly happenes to to the energy in the different wavelengths of light from the sun I am not to sure about. The temperature of the shade doesn’t seem that hot when I touch it. I assume some of the wavelengths get reflected away from the shade at a many different angels. Our gravel parking/driving area doesn’t seem any hotter in front of the garage, but I have never actually measured the temperature. In any case the shade does it’s job in the spring to fall time frames and we roll it up to let the suns rays warm the garage in the winter.
Seems like a well-controlled experiment should be possible. Find one of those semi-abandoned military bases fairly close to a city. Keep good temp and precip records there for a year, and calculate deltas from the city’s readings.
Paint all the roofs on the base white, or at least all the big flat roofs. Keep the same records the next year. See if the base is now cooler or wetter than before, as delta from the city’s values.
Philip Bradley: “The person who invents a roofing material that changes from low to high albedo (reflectance) when temperatures warm past about 25C is going to make a fortune.”
It’s called a parasol, isn’t it? Though to stay with the zeitgeist one would need thermistors, Arduino boards, rare-earth magnets, and a research grant from DARPA. To keep seasonal issues in check one needs only an RTC chip. And for wind related wear issues one needs only keep about a miniature turbine to detect wind-speed and provide electrical cogeneration needs for recharging keeping oil-cell magnesium/carbon battery arrays up. For wind intermittent issues one can use a Joule Thief charger driven by an earth battery.
Certainly less absurd than Solyndra and contains a higher buzzword-to-content ratio as well. Maybe enough to get an NGO to sport funds for inflicting this on Africans as a live human test.
“””””…..polistra says:
September 8, 2012 at 5:27 pm
……………………..”””””””
Well such an experiment would reveal if / or not the base was cooler or wetter than before.
It would reveal nothing else, as it simply isn’t possible to rerun last year’s weather conditions over again this year after making alterations to the experimental apparatus; and that I’m afraid is the problem with the whole of climate “science”
In experimental science if you controllably and measurably alter an apparatus; and then submit it to an uncontrollably and unmeasurably different stimulus signal, you have the poster child for the “garbage in; garbage out” condition.
That sort of experimentation leads to conclusions similar to the one reached after cutting all four legs off a frog to see how that impairs their jumping ability; and their complete failure to jump “proves” that sans all four legs, frogs become stone deaf, and simply ignore your shouts of encouragement.
Make a plant covered roof. In summer the transpiration keeps it cool. In winter, rake off the leaves and the soil will keep you warmer 😉
Back in grandma’s day, you had beanpole shade… you plant green beans or pea plants in front of your windows, you have shade and supper.
The main problem is that even with something as simple as painting roofs a different colour, no one can agree on whether t is good or bad.
Of course, apart from the paint companies, the big beats of AGW don’t make any money out of this so don’t expect them to rush to the rescue.
On the other hand, if the research is right and that the correlation can be found to have a causative link, then it just shows that if they can mess up painting roofs, think how much worse they can mess up with all their other strategies.
OK, what happens if, following a temperature rise they manage to suppress or partially suppress the usual consequent CO2 rise?
Maybe we will then find that the CO2 rise actually is a part of another mechanism that counter-intuitively has a role in recovery from warming.
Here in the mid-Atlantic, it prb’ly saves alittle money to have a dark roof since the heating season is much longer than the cooling season. But a white roof might extend the lifetime of an asphalt-shingle roof a bit — heat degrades materials.
“The authors utilized 2050 projections of Sun Corridor growth…”
Models
Also, IMO it’s not necessarily the color of the roofing surface it’s the nature of irregular surfaces. In nature you have terrain, rocks, trees, leaves on the trees, etc. They to not absorb at the same rate nor to they reflect in the same uniform directions.
@EM Smith, the problem with planting on rooftops is that is requires much more in maintaining it. The whole idea of painting rooftops is to minimize costs and seal possible rain leakage. Putting thousands of pounds of water saturated dirt and plants is quite a risk.
Maus says:
September 8, 2012 at 5:47 pm
The level of absurdity is always quantifiable by putting the complexity of the apparatus in relation to the energy density of the energy source. By that measure, even Solyndra beats your Rube Goldberg device. Solyndra was probably only a factor of three or so worse than ordinary thin film cells, which of course was sufficient to make them perish in the marketplace.
JJ says:
September 8, 2012 at 11:14 am
“Now, models are called “experiments”.
No. A model is a hypothesis. An experiment would be what you would do to test your model. Your “numerical experiments” tell you nothing about the world. They only tell you how the model you created behaves.”
Exactly, or nearly.
The model is the theory (it is the formula).
The model run is the prediction of the theory, or the hypothesis.
Reality is the experiment to test the hypothesis.
They are confidence tricksters.
DirkH: “Solyndra was probably only a factor of three or so worse than ordinary thin film cells, which of course was sufficient to make them perish in the marketplace.”
So your statement is that failing in the marketplace is not absurd. But that succeeding in the marketplace is absurd. Like these guys: http://www.awningtime.com/sensors/sensors.html
Knock yourself out with that.
After the Ice Age for a period of about 4,000 years the Sahara had vegetation which increased heat absorption. The greater warmth increased the volume and rapidity of rising hot air, resulting in moist air being drawn in from the ocean and seas. This increased rainfall, which may be the same process as in Arizona. White roofs would reduce UHi, and thereby reduce rising hot air and the drawing in of moist air.