If you’ve ever crossed a hot car park in summer, you know about albedo. Dark colours absorb heat, while lighter colours reflect it. That’s one of the principles behind the idea that a melting arctic may be a self-reinforcing feedback loop: relective ice melts and is replaced by dark water, which holds heat better and melts the remain ice even faster.
We can also use this principle to our advantage however, by increasing albedo in places where we have a little more control of the environment than we do in the Arctic – cities. Cities are prone to the urban heat island effect, being warmer than surrounding areas by 1-3C. They lack the green spaces that serve to cool the air, are full of activity and machinery, and they’re covered in acres of dark and baking tarmac.
It’s those man-made surfaces that offer us an opportunity to increase reflectivity. Using whiter concrete could reduce the heat absorption of roads and car parks, lowering the temperature in the immediate vicinity. Roofs constitute about 20-25% of urban areas, and pavements about 40%, so it’s a lot of space to work with and it could be a useful adaptation tool as cities warm. Painting the roofs of buildings white offers a double reward. It lowers external temperatures, but it also cools the building by up to 30% and reduces carbon emissions from air conditioning.
There are a number of projects at work painting roofs white. NYC cool roofs encourages New Yorkers to paint their roofs as part of the city’s plan to reduce its carbon emissions by 30% by 2030. According to their estimate, every 1,000 square feet of roof painted cuts one tonne of CO2.
The Earthbound Report 
I prefer the Living Roof concept. http://en.wikipedia.org/wiki/Living_roof
Yes, I prefer living roofs too. It turns the space into something productive. But they’re a little harder to implement than a coat of white paint.
I suddenly wonder if anyone every included the heat trapped by solar panels and collectors into the CO2 balance equations of solar energy installations. The approach will also reduce the urban heat island effect in winter, and thus more heating will be required. Actually Oleson and Feddema of the University of Kansas also came to the conclusion that globally this approach would have no noticeable effect – the effects would mainly be local. Greening has a stronger effect – as long as the care for green areas and roofs is done in a “green way”. Traditional lawn care produces more CO2 equivalent (fertilizer, lawn owers) than it has a positive effect (quoth Spiegel Online, referencing unnamed University of California researchers). http://www.spiegel.de/wissenschaft/natur/0,1518,675259,00.html.
I am living near the Rhein-Ruhrgebiet, once the largest industrial area of Europe – if not the world. Once approx. 13 Million people, most of them working in coal and steel. When I was born in the late 60s and throughout much of the 70s it was dirty, polluted, grey stinking, almost no nature. Depending on where you were you had to drive 2 hours to find a larger forest. The change is unbelievable. The region is all green, the cities are full of trees and forest patches, and from above there seem to be more forests than urban areas, while the rivers, lakes and canals are teeming with fish. The air is clean and former coal mines and steel plants turned into parks, residential areas with lakes and canals or cultural and sport event locations. I would have to look up the data, but I presume that this areas ecological and CO2 footprints dramatically improved over the last 3 decades. Anyway: I assume that many small measures that alone have “almost no noticeable effect” will, in combination, have an effect after all.
“Traditional lawn care produces more CO2 equivalent (fertilizer, lawn owers) than it has a positive effect.”
I belive the idea is to use plants native to the local area. Not to just put a lawn on the roof. The photo’s from 2000 here show how nature reclaimed the unused areas with no human intervention. http://inhabitat.com/new-yorks-high-line-park-in-the-sky-opens-today/ The park was an after thought though it looks like they are letting the plants grow wild.
every 1,000 square feet of roof painted cuts one tonne of CO2
I assume that’s per year.
Any stats on the carbon footprint of the paint?
I presume that’s per year, yes. Not sure what the footprint of paint is, but as long as you don’t have to repaint very often, I don’t suppose it would be insurmountable. You could probably paint once and then top it up with whitewash.
I guess the whole white roof thing works best in places where its hot all year round, otherwise it might work in reverse in winter.
Yes, it is certainly the case that many cultures in warmer areas use a lot of white or light colours in their building materials and external surfaces. It does make sense and certainly makes a difference to the micro-climate inside the building (which in turn affects energy needs). I’m not at all opposed to this, just curious about the stats and how warm/cool a climate needs to be before it becomes counterproductive.
A Reuters piece with some interesting stats: “In New York City alone, 12% of all surfaces are rooftops. It’s estimated that implementing a white roof program in 11 metropolitan cities could save the United States 7 gigawatts in energy usage. That’s the equivalent of turning off 14 power plants, and a cost savings of $750 million per year.”
Other than your passing comment “otherwise it might work in reverse in winter” you make no mention of the effect in winter. The physics is simple – a white surface radiates considerably less heat than a dark one, so a building with a white roof would require less heat in winter than one with a dark roof to keep the inside comfortable, another benefit of a white roof. Would the local council require planning permission for the change of colour, though, and if so, would they allow it?
Hmm, I can imagine the reaction if I painted my own roof white, being in the middle of a terrace. I don’t think it would go down very well, and it would in all fairness look horrendous. I guess it’s going to make the biggest difference on big roofs, warehouses and factories and so on, and they’re easier to get away with too.
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