Monday, November 05, 2007

How to solve global warming

Anthropogenic global warming is caused by the emission of greenhouse gases such as carbon dioxide and methane. The Earth re-radiates energy from the Sun as infrared radiation, and greenhouse gases such as carbon dioxide and methane absorb infrared radiation, hence the temperature of the atmosphere will increase if the atmospheric concentration of carbon dioxide and methane increases.

Most proposed solutions to global warming suggest either a reduction in the anthropogenic emission of greenhouse gases, or various technological schemes for the removal of greenhouse gases from the atmosphere.

This, however, is not the correct way to approach the problem. A clue to the correct approach can be found by looking at another solution, which proposes increasing the reflectivity ('albedo') of the Earth's surface by making as much of it white as possible. This proposal works because incoming radiation at visible wavelengths is reflected back into space at the same visible wavelengths, thereby avoiding absorption by greenhouse gases.

I propose, then, that rather than looking at greenhouse gases such as carbon dioxide as the problem, it is the production of infrared radiation by the Earth which is the problem to be solved. If one could release a compound en masse, either into the atmosphere, or deposited upon the surface of the Earth, which absorbs infrared radiation and re-emits it at visible wavelengths, then the radiation emitted by the Earth will pass unhindered through the greenhouse gases into space.

This requires a so-called 'Anti-Stokes' material: "When a phosphor or other luminescent material emits light, in general, it emits light according to Stokes' Law, which provides that the wavelength of the fluorescent or emitted light is always greater than the wavelength of the exciting radiation...Anti-Stokes materials typically absorb infrared radiation in the range of about 700 to about 1300 nm, and emit in the visible spectrum." A variety of Anti-Stokes phosphors, based on yttrium, exist for the conversion of infrared radiation into visible radiation.

Intriguingly, lanthanum hexaboride is already being used on a trial basis in office windows to absorb all but 5% of the incident infrared radiation...

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