Another possible way to reduce global warming comes not from reduction of greenhouse gases in the atmosphere, but rather changing the albedo (reflectivity) of the earth. A change as small as increasing the net albedo by 0.5% would offset global warming.

The basis of this is quite easy to see and understand. Black T-shirts are warmer than white ones. White houses need less cooling than houses in darker colors. Los Angeles is 5 degrees Fahrenheit warmer than the surrounding areas, mostly due to dark roofs and asphalt. At high noon, the sun delivers to each square mile the equivalent of a billion watt electrical plant.

The urban heat island effect is common. White roofs, concrete pavement and shade trees could cool the city, cutting air conditioning costs by 18%. Cooler roads also lessen tire erosion.

Approximately 1% of the United States is covered with constructed objects, mostly paving. It is possible that we may already have enough land to start working on it. By adding sand or glass to ordinary asphalt (known as glassphalt) doubles its reflectivity.

The biggest obstacle is the oceans, which cover 70% of the surface area of the earth and absorb more light because they are darker than land.

There are three places where albedo can be increased:

  • Surface
  • Space
  • Atmosphere

Surface
As discussed above, increasing the reflectivity of the surface of the earth by using white roofs, concrete or glassphalt rather than asphalt is one way to increase the albedo of the surface of the earth.

Space
A far more costly way to increase the albedo is to move into space and place a massive orbiting white screen in the sky. At about 2000 kilometers on a side. Cost approximations start at $120 billion to put them up. Taking it down isn't cheap either. A side effect of this is that the night sky would permanently light-polluted, much to the disdain of astronomers and lovers.

Dust at this altitude doesn't work - it drifts away, pushed by light pressure. Which leads us to...

Atmosphere
Volcanos spew tons of dust into the upper atmosphere causing red sunsets and cooler temperatures for months to come, and can remain aloft for several years. The dust in the atmosphere reflects more sunlight than they trap in the infrared.

Even better than dust are droplets of sulfuric acid. These droplets reflect light more effectively than dust. Sulfate aerosols also form cloud forming droplets further increasing albedo.

Concerns about particulate levels from the EPA. Some lung disorders have been blamed on dust. However, high altitude dust would come down as rain, rather than stuff to make us cough.

The cheapest way to put anything into the upper atmosphere is not to fly it up, but shoot it up. Large naval guns fired straight up can put a one ton shell 20 kilometers high where it would then explode and spread as dust. This costs 1% of the solar shade.

There is another high altitude dust source: jet fuel. A change to the fuel mixture in the engine to burn rich can leave a ribbon of fog for up to three months which will spread and become invisible to the human eye. These motes would come down mostly as rain. Running rich would increase airline costs by only a few percent, adding a small change ticket prices. The post combustion fog from jet fuel wouldn't change sunsets.

In the three days following September 11, 2001 planes where grounded. Climatic date was gathered under clear skies with no jet contrails. The results recently came in about the climatic effects of the airline grounding. It turns out, that the daytime highs were as much as 5 degrees higher and nighttime lows were three degrees lower. The contrails act as a blanket buffering the atmosphere - preventing light to get to the ground during the day and preventing the heat from escaping by night.


http://www.wired.com/news/technology/0,1282,52512,00.html

Carl Sagan mentioned another theory on albedo, in which the increase in inclement weather caused by global warming increases the cloud cover naturally, acting as negative feedback for free. The drawback, of course, is that we may be stuck with the bad weather.

And we can't really depend on this, since it's still just a theory. A competing theory is that the melting of the polar ice caps due to the greenhouse effect will act as a positive feedback, and accellerate global warming, since we'll be both decreasing the amount of white reflective surface on the earth and increasing the amount of hydrocarbons in the atmosphere (there are many trapped in the ice).

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