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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This study looks pretty weak to me. I couldn’t understand very well the methodology, but I don’t see how they can reach that conclusion. Was it through experimental data or through modelling? In the first case, correlation is not causation. In the second, well, we have learnt already a number of things about climate models reliability, haven’t we.
New UHI solution: Install geothermal heatpumps everywhere.
Cover the roofs in heat collectors. You can incorporate them into the backs of solar photovoltaic panels.
Store the summer heat deep underground, use it for heating when needed around winter and for heating water year round.
Makes more sense than fighting the warmth by pumping CO₂ underground.
What’s the worse that could happen? In warmer sunnier places like the Sun Corridor, so much heat will get pumped underground that the deep stores will generate steam, that could drive a turbine and generate electricity?
OK, I have not read the research in question, but this figure seems out of line to me. Taking any significant geographical micro-climate type area (as in 500 square miles plus) even in an “urban” area in Arizona (which isn’t exactly = to the five boroughs of NYC) it’s hard to imagine that total roof area would be more than 25% or so to begin with at most. A lot of those roofs are already more gray than black (with a certain percentage already white I’m guessing as well), and the painted white wouldn’t stay nearly 100% reflective for very long, so I’m purely guessing that we’re talking about a reflectivity change of about 50% for that less-than-25% area — or a reflectivity change of 12.5% for the total area. Is rain production really 1/3 dependent purely upon reflectivity input into the microclimate (which is what would be needed for that “4%” final result)? Heh, and of course there’s the energy angle: the reduced rain would lead to increased air-conditioning, while the white roofs would lead to decreased air-conditioning! And white roofs might be more (or less) energy-expensive to produce/maintain.
If we were talking about the entire state of Arizona being urbanized to NYC level standards it would be something else, but given the reality of the situation I have a hard time believing any effect would be significant enough to really worry about. It *is* an interesting concept though! And if other costs were commensurate it would certainly be worth considering this sort of calculation if they were deciding about blacktop vs. whitetop road material etc.
– MJM
… or increase. Or maybe not.
Maybe they should paint the roofs green. Works for Brazil.
It is a rather good idea on paper at least to combat UHI. What’s needed though is a surface that can change light reflectivity but remain cheap. For example a photoreactive paint, where it gets brighter with more light would be preferable, but I’m not sure if such a thing would have to be invented first.
An easier, but perhaps a more expensive solution would be to install a Venitian blinds-style electronic roof covers, that turns itself on when it’s needed.
P.S. If all the tarmac in a urban environment was coloured green for example, would that have an effect on the local temperature too? Would it have the same consequences as the white roof?
Kadaka, the problem with your idea is that it does not provide cover for any capping or taxing of CO2, nor for declaring CO2 to be a pollutant so it can be “regulated”.
RTF
Roger Pielke Snr will be smiling
California naturally showed everyone The Way when director of general services in 2009 ordered “white as the default color on the state’s light-duty vehicle contracts”. Science now unsettling with unintended consequences worth calculating, since Ca. total state fleet is over 50,000 ( not all are light-duty vehicles). Seems impossible though to calculate the total surface area of Ca. commercial buildings (since 2006) with a flat roof being obliged to be white or light (reflective) shade.
What on earth is a “numerical experiment”? Is that equal to running a model several times?
From Richard T. Fowler on September 8, 2012 at 12:22 am:
It’s late, I’m exhausted, and barely have enough remaining brainwaves to stumble to my bed.
But I can assure you this would be marketed as fair and equitable, as the costs would be borne by taxes on the hated “black burners” who insist on destroying the biosphere by continuing to use fossil fuel-derived energy.
And if that term sounds “racial”, don’t worry because they won’t worry about it as they gladly promote the implication that anyone who isn’t willing to do anything possible to combat climate change must obviously be of that political persuasion who’d want to do that as well. “Drill Alaskan oil, Kill African baby” and similar protestor-sign slogans will, of course, be expected.
I’ve done Phoenix and Tucson in the summer. I don’t see another one degree making a whole lot of difference. And there is one awful lot of desert and mountains in that ‘corridor’ which will overwhelm any effort of roof-painters.
Oh No! Its the What Ifs, they are worse than we thought!!!
The following text tells us all we need to know…
“All three co-authors are affiliated with ASU’s College of Liberal Arts and Sciences and the Global Institute of Sustainability.”
What if they actually got real jobs? Did real work?
I vaguely recall that green (dark) was nearly as good as black for solar water heater panels. I had an old Chevy that was forest green, and it got too hot to touch sitting in the sun. Of course, correlation doesn’t mean causation.
More warming in summer and less in winter?
Well, who’d a thunk it.
So if everyone painted their roofs white, where would all that unrained rain go?
Friends:
Long ago the UK’s Coal Research Establishment (CRE) developed a range of roofing materials. One of these had an aluminium coating (like baking foil) for thermal reflectivity and low thermal emissivity
As anticipated, the reflectance of the coating rapidly reduced due to oxidation (the silver colour tuned to grey), but the remaining reflectance was (as had been calculated) a sufficient benefit over a tarred roof for the coating to have merit.
However, the reflectance continued to degrade as a result of dirt, bird droppings, algae, etc. accumulating on the surface. It was clear that similar accumulation would occur on tarred roofs. Hence, within about a year following installation there would be surprisingly little difference between the reflectance of a flat roof covered with a ‘traditional’ tarred roofing felt and and a flat roof with the aluminium-coated roofing material. And the ‘traditional’ roofing felt was cheaper.
So, on the basis of existing experimental data (which I helped to acquire) I think the effect of painting the roofs would be much less than the reported model study suggests.
Also, the anticipated reduction to rainfall is only indicated by use of a model. Were anybody willing to provide me with sufficient funds then I would be willing to conduct a model study which indicates the painted roofs would increase rainfall (i.e. give me the money, tell me what you want, and I can give you a model which provides what you want).
Richard
The rain will still fall somewhere, it won’t stay up there forever!
Haven’t read it, but…
If white roofs tend to lower the temp toward its “natural” level, and that reduces rainfall, does that mean that urbanization has actually increased rainfall??? Plausible, I suppose, since urban areas use more water, and a lot of it evaporates, and evaporation presumably increases with temperature.
Liberal use of the word experiment.
Let me rephrase that:
They conducted continuous multi-year, massively multiplayer, continental scale role playing games for several 2050 Sun Corridor growth and adaptation levels. After they ran out of doritos, lead researcher Matei Georgescu, an assistant professor in ASU’s School of Geographical Sciences and Urban Planning, uploaded the recorded gameplay to youtube, declaring a record.
No remaining funding could be retrieved.
Wiki has a ‘cool roof’ article:
http://en.wikipedia.org/wiki/Cool_roof
“While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today’s “cool roof” pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren’t as reflective as whites or light colors, but can still save energy over other paints.
“A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the “urban heat island effect,” they may actually increase global temperature. The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs.
” A response paper titled “Cool Roofs and Global Cooling,” by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, raised additional concerns about the validity of these findings, citing the uncertainty acknowledged by the authors, statistically insignificant numerical results, and insufficient granularity in analysis of local contributions to global feedbacks.”
They could consider planting desert adapted trees in the house shaded part of the garden. Any waste water placed in the subsoil would help sustain tree shade and evaporative cooling by the leaves.
“Trees and other plants help cool the environment, making vegetation a simple and effective way to reduce urban heat islands.”
http://www.epa.gov/hiri/mitigation/trees.htm
“Trees and vegetation lower surface and air temperatures by providing shade and through evapotranspiration. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak temperatures of unshaded materials.1 Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C).2, 3
Trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings or to shade pavement in parking lots and on streets. Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof.”
Much better don’t you think?
Check the references, this has been know for years
The easiest way to compensate for the lack of rain induced locally by these white rooftops is to hold dances on the rooftops daily. The researchers from Arizona State University should have access to a talent pool capable of teaching the student to perform the traditional rain dance.
/sarc
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.
As for ‘Sun Corridor expansion scenario leads to a 12% reduction in rainfall’, I don’t know what assumptions were made to reach this conclusion, but Roy Spencer found in the USA southwest urban temperatures decreased with urban density. He suggests and I agree, that the cause is decreased vegetation (and irrigation) and hence reduced humidity.
So the 2 rainfall reductions are due to different causes. One due to fewer plants (and irrigation0, hence reduced humidity. The other due to reduced temperatures (from white roofs) and reduced thermal convection.