On Monday while discussing dark roofing materials we mentioned albedo, a word we are sure the majority of non-scientists have never heard of. That may not be the case for long. Here's the deal.
Albedo is defined as high solar reflectance, i.e. the earth's capability of bouncing the sun's rays back into the stratosphere. If a surface absorbs all light it looks black and has an albedo of 0; if it is perfectly reflective it looks white and has an albedo of 1. All surfaces, in fact all objects, have an albedo within those two numbers. A new sheet of copy paper has an albedo of 1 while a brick wall or a freshly plowed field has an albedo closer to 0 than to 1.
When the rays are absorbed or bounced, so is the accompanying heat from those rays. Thus the many chocolate colored roofs and black parking lots in populated areas create an urban heat island effect which may make a city 3° to 8° warmer then less populated nearby areas. This also impacts air quality as hotter air is dirtier air.
But there is a bigger problem. One of the biggest reflective surfaces on earth is the polar ice but we all know that this immense expanse of whiteness has been disappearing at an accelerating rate as global warming increases. Thus we may be trapped in a loop with rising temperatures reducing the earth's albedo which further raises the earth's temperature.
Oh, oh.
Sam Kornell writing in Miller-McCune on-line magazine reported recently on a study conducted by three current or former Lawrence Berkley National Laboratory scientists, Hashem Akbari, Arthur Rosenfeld, and Surabi Menon, on albedo. Akbari and his colleagues had been studying the impact of white roofs and surfaces on urban heat islands for almost twenty years but in 2004 they decided to take a look at the impact on climate change. Even they were stunned by their findings.
"'When we did the calculations, initially we couldn't believe the results,' Akbari said. 'So we re-checked the numbers in different ways.' Again the results were unambiguous: Every 100 square feet of roof area turned from a dark color to white are equivalent to offsetting the emission of one ton of heat-trapping, atmospheric CO2."
Kornell reports that the average American is responsible for about 20 tons of CO2 emissions each year - the largest carbon footprint of any country on earth. But, all of the activities that contribute to this carbon load - using our stove, driving a car, heating and cooling our homes - adds up to the atmospheric equivalent of 2,000 square feet of white roof.
Therefore, a family of four, living in a typical size American home could change to a white roof and offset at least 25 percent of the families energy use (actually a lot more as it takes no more energy to heat a house for four people than for two and most 2-year olds don't drive.)
The three scientists estimate that permanently retrofitting roofs and pavement in tropical and temperate regions of the world would offset 44 gigatons of CO2 emissions. It takes about a year and a half for the entire world to cook up 44 gigatons of CO2.
And it is a cheap cure. Akbari would like to see $3 billion of the stimulus money directed toward painting roofs and pavement and projects that it would, even after taking into account the slight increase in heating costs from losing dark roofs, result in a savings of $2 billion a year in unspent air conditioning cost.
So maybe white roofs are a cheap no-brainer though some detractors don't think so. We will next look at the materials and techniques of lightening up the earth.