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Keeping Cool With the Albedo Effect

The co-author of A Golden Thread: 2500 Years of Solar Architecture and Technology takes a look at how white backgrounds — be they snow, concrete or rooftops — might help bend back a little bit of global warming.
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Govindasamy Bala and his colleagues at Lawrence Livermore National Laboratory discovered that by deforesting the entire world, the amount of carbon dioxide would double by 2100. But instead of a great rise in global temperature, their models determined that cutting down all the world's trees would cause the world to cool by 0.3 degrees C (a bit over half a degree Fahrenheit).

How could doubling carbon dioxide in the atmosphere decrease the temperature? Doesn't that run counter to everything we know about global warming — that pumping more carbon dioxide into the atmosphere causes the Earth's temperature to rise?

The study brings to the fore the albedo effect, another potent contributor to the Earth's temperature that usually gets left in the shade in discussing global warming. Albedo is a measure of how much different materials reflect sunlight.

Forest leaves, for example, have an albedo of 0.11 to 0.15, depending on the species. This means that they reflect only 11 to 15 percent of the incoming sun rays back into the atmosphere and absorb the rest, eventually sending most of the sunlight intercepted back into the atmosphere as heat. These longer heat waves get trapped by our atmosphere, and therefore remain close to the Earth's surface to cause it to warm up.

Conversely, if trees in colder climates were cut down, snow would face the sun. It reflects about 80 percent of the incoming sunlight back into the sky as visible light, in wavelengths short enough to get through the Earth's atmosphere and escape into space. The remaining 20 percent would be absorbed and consequently stay as heat.

So, returning to Dr. Bala's simulated deforestation, snow reflecting sunlight would spare the region almost five times the amount of solar heat had the trees remained in the computer modeling. And that would cool the Earth, despite so much extra carbon into the air.

(And just to be clear, Dr. Bala isn't anti-tree: "Apart from their role in altering the planet's climate, forests are valuable in many other aspects," he told an interviewer for his employer. "Forests provide natural habitat to plants and animals, preserve the biodiversity, produce economically valuable timber and firewood, protect watersheds and indirectly prevent ocean acidification.")

A similar discovery was made in the Arctic. Dirty snow, scientists discovered, was responsible for much Arctic warming previously blamed on greenhouse gases. Dark soot soiling the pristine snow absorbs about 95 percent of the incoming solar radiation, while pristine snow reflects more than 15 times that amount.

The powerful effect the albedo has on global warming can be seen by assuming it covered by one substance. Sea ice, for example, with an albedo of between 0.5 and 0.7, absorbs only 30 to 50 percent of incoming sunlight. Should ice compose the entire planet, the Earth would shiver at -62 degrees F.

Deserts have an albedo between 0.37 and 0.49. If the Earth were all desert, the temperature would rise to 13 degrees Celsius. Forest canopies, as we have already seen, have much lower albedos than either deserts or sea ice. It comes to no surprise that if forests completely covered the planet, the Earth's temperature would be 75 degrees F. Oceans have the smallest albedo of the group — 0.06.

It therefore comes as no surprise that an Earth completely engulfed by sea water would be hotter than one composed completely of sea ice, desert or forests. In fact, an oceanic Earth would have a temperature of about 90 degrees F.

Taking advantage of albedo could help maintain normalized temperatures throughout the globe — even as carbon emissions rise. And it doesn't involve global clear-cutting.

Some people have suggested grandiose schemes using the albedo as their weapon. One scheme envisions sending off large white planks throughout our oceans and allowing them to float where they will. Others suggest anchoring huge artificial white-painted islands in the seas. An even bolder scheme has come up — the launch of huge white umbrellas, as large as football fields, to shade the earth.

Besides appearing far-fetched and expensive, it's tough to predict what adverse effects such large-scale interventions might bring.

A simpler solution with little possibility of unintended consequences can be found in the built environment. Urban areas can be as much as 6 to 8 degrees F hotter than nearby, more pristine surroundings.

The low reflectivity of roofs and pavements — roads and parking lots — share much of the blame for these higher temperatures. Newly laid asphalt absorbs 95 percent of incoming sunlight. As it ages, its absorption drops by about 5 percent. Newly laid concrete, in contrast, reflects between 40 to 50 percent of incoming sunlight; as it ages, its reflectivity drops by 10 to 20 percent.

Topping existing asphalt pavement with two inches of cement concrete will perform the same as newly laid concrete. Measuring the temperature at the same time of day for all three pavements side by side — new asphalt, old asphalt and concrete topped asphalt — showed temperatures of 123, 115 and 88 degrees F, respectively.

Rooftops can also heat up our Earth or keep it cool depending on color. Paint them with black acrylic and they will absorb the same amount of sunlight as new pavement. Paint them with white acrylic paint and they will reflect the same amount of sunlight as does new snow. Between the two extremes — red paint and green asphalt shingles — respectively absorb 50 percent and 80 percent of the incoming sunlight.

In the United States, there are a billion square feet of "cool roofs" — white-membrane roofs — installed annually to reflect sunlight to keep the building cooler, requiring less energy for air conditioning, the trade publication pv magazine reported in January.

The amount of heat removed from the atmosphere by painting a 10-square-meter dark roof white would cool the Earth as much as removing 630 kilograms of carbon dioxide from the atmosphere. Replacing a square meter of asphalt pavement with concrete or white topping is like removing 38 kilograms of carbon dioxide over a year.

On a global scale, with urban areas comprising about 1 percent of the Earth's land surface and roofs and paved surfaces comprising about 25 percent and 35 percent of a typical urban area, respectively, lightening their color would have the same cooling effect as keeping 44 gigitons of carbon dioxide out of the atmosphere.

By changing the albedo of roofs and pavement in the urban world, it is believed we could offset the expected heat increase due to projected growth in greenhouse emissions over the next 11 years.

Tomorrow we'll take a closer look at one particular proposal to paint roofs white in urban heat islands.

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