Can painting our roofs white offset tonnes of CO2 emissions??
Roofs and pavements together comprise a majority of the surfaces found in urban areas. The most common type of roofing used in North America today is Asphalt Shingle. Asphalt is also one of the most common types of pavement, along with concrete. It is generally very dark and quite unreflective, and hence has a low albedo, ranging between 0.05 and 0.2[note]VanGeen, M. (2002, 10 9). Retrieved 08 13, 2010, from Lehigh Cement Group: http://www.lehighcement.com[/note]
What exactly does Albedo mean?
Albedo refers to the fraction of incoming radiation that gets reflected off a surface. For our purposes, it is the fraction of incoming solar radiation (light and heat) that is reflected off roofs and pavements. It ranges on a scale from 0 to 1, with 1 being 100% reflective and a 0 albedo surface being unreflective. Black surfaces absorb most of the visible light incident upon them, and thus have low albedo. White, or lighter surfaces have a higher albedo and reflect most of the incident visible light. One must not forget that about half of “sunlight” is in the near infrared (NIR), and it is possible for surface that reflect visible light to absorb NIR, or vice versa.
How does Albedo work to offset CO2 emissions?
Carbon dioxide emissions into the Earth’s atmosphere contribute to the greenhouse effect – greenhouse gases allow solar radiation to pass through into the Earth but trap long wavelength infrared that is radiated back from the Earth’s surface. It thus has a positive radiative forcing– it allows more radiation to be absorbed by the Earth than is re-emitted.
Raising the surface albedo, on the other hand, has a negative radiative forcing, enabling more radiation to escape out of the Earth’s atmosphere than is absorbed. Recent calculations have shown that every extra tonne of CO2 put in the atmosphere warms the Earth at a rate of about 1 kW, whereas raising the surface albedo by 0.01 has a radiative forcing of -1.27 Wm-2 of surface area. Raising the surface albedo thus has the ability of offset the CO2 emissions.
How is this calculated?
We can use a simple formula method to estimate the change in intensity, ∆I, Wm-2 of solar radiation entering the Earth, due to a change in albedo. To do this, we begin with a few known constants: the solar constant (amount of incoming radiation from the sun, per unit area ), S = 1370Wm-2, the mean intensity, Iin = 240 Wm-2 and the average albedo, A = 0.3.
The first step is to calculate the mean intensity at the bottom of the atmosphere:
Mean at Bottom of Atmosphere = $\dfrac{S}{4}(1-A) = \bar{I}_{in}\tag{1}$
Using the values of S and A above, the above formula will give you = 240 Wm-2 = Iin
Next, the formula is re-arranged to give the change in intensity per unit change in albedo:
$\dfrac{\delta\bar{I}_{in}}{\delta A}=-\dfrac{S}{4}=-342.5\textnormal{Wm}^{-2}\tag{2}$
$\textnormal{So if }\Delta A=0.01\textnormal{ then}\Delta\bar{I}_{in}=-3.425\textnormal{Wm}^{-2}\tag{3}$
This is factor of 2-3 higher than that obtained by a sophisticated climate model, but it gives us the right idea.
What effect does raising the albedo of roofs and pavements have?
Raising the albedo of roofs and pavements are predicted to have a beneficial effect on the environment; increasing the reflectance of a roof by just 0.25 could offset 64 kg CO2 per 1m2 of roof area. So we would need just 16m2 of cool roof area to offset 1tonne of emitted CO2. If the roof of an average Vancouver house has a surface area of approximately 1200ft2 or 111m2, raising its albedo by 0.25 would offset almost 7 tonnes of CO2 emitted per year. Note: this is a ONE TIME benefit, and should be compared to the typical emissions of a single family dwelling in Vancouver of several tonnes PER YEAR.
Similarly, increasing the reflectivity of pavements by 0.15 would offset 38kg CO2 per 1m2 of pavement area. So just or 26m2 of cool paved area would offset 1tonne of emitted CO2 . Raising the albedo of just one block in Vancouver would offset 38 tonnes kg of CO2 emitted. This means that globally, we have the potential to offset 44 Gt of CO2: 24 Gt from using cool roofs and 20 Gt from making our pavements cool pavements. Considering that annual anthropogenic CO2 emissions are approximately 25Gt, raising the albedo would offset nearly 2 years of CO2 emissions.
Because of their ability to reflect a lot of the sun`s heat, cool roofs and pavements also help decrease the Earth`s surface temperature – 0.003 average increase in albedo are predicted to reduce the Earth’s surface temperature by 0.01o Celsius. Over the next sixty years, this would reduce the predicted global increase in temperature by 0.6o, lowering it from a 3o rise in temperature to a 2.4o rise in temperature[note]Science Daily. (2010, 07 20). Cool Roofs Can Offset Carbon Dioxide Emissions and Mitigate Global Warming, Study Finds. Retrieved 08 13, 2010, from Science Daily: http://www.sciencedaily.com/releases/2010/07/100719162945.htm[/note].
In summer, our homes would keep cooler and our energy consumption to cool the house would be much lower. In places where energy is obtained from the burning of fossil fuels e.g. Alberta, this lower energy demand would mean that less fossil fuels were burned and consequently, CO2 emissions from this burning would also be reduced.
The United States Department of Energy installed more than two million square feet of cool and white roofs at over locations across the country where most power went into cooling rather than heating, and found that their energy savings were, on average 500,000 dollars a year. That is an incredible 7,500,000 dollars over the next ten years[note]Energy, U. D. (2010, 07 19). NNSA Commitment to Energy Efficiency: Promoting Cool Roof Technologies. Retrieved 08 13, 2010, from National Nuclear Security Administration: http://www.nnsa.energy.gov/mediaroom/factsheets/coolroof[/note]!
How can the albedos of roofs and pavements be raised?
The simplest way to raise the albedo of a roof is to change the colour from the conventional dark red, brown or black to a white roof. Just this simple change raises the albedo by 0.4, and would offset 100 kg of emitted CO2 per 1m2 of roof! As for pavements, basic steps like using white cement mixed with the regular darker version, or incorporating light coloured aggregates into the asphalt would raise the albedo and greatly reduce CO2 emissions.
Most of the values used in this article are taken from a scientific paper available here [note]Menon, S., Akbari, H., Mahanama, S., Sednev, I., & Ronnen, L. (2010). Radiative forcing and temperature respinse to changes in urban albedos and associated CO2 offsets. Environmental Research Letters [/note]
