Friday, March 11, 2011

Carbon dioxide in the atmosphere matters

Sunlight reaching our planet is dominantly a short-wavelength energy, which passes easily through our atmosphere to warm the surface of the planet. As the surface warms, it emits an increasing amount of infrared, or long-wavelength, energy back out into the atmosphere: the incoming short-wavelength energy has been converted into outgoing long-wavelength energy.

Molecules made up of two atoms of the same element, such as nitrogen (N2) or oxygen (O2), have no net change in their dipole moment when they vibrate and so do not absorb infrared energy, but 3-atom (or more) molecules, such as carbon dioxide (CO2), do. Consequently nitrogen and oxygen are not greenhouse gases as the infrared energy emitted from the planet surface would easily pass back out through an atmosphere of these gases.

By contrast, carbon dioxide does absorb the outgoing infrared energy, gradually accumulating more heat and increasing the temperature of the planet’s surface. The greater the concentration of carbon dioxide, the more the temperature increases.

It is well-known that the greenhouse effect of the carbon dioxide currently in our atmosphere is about 33°C. In other words, the planet surface is about 33°C warmer than it would be without the greenhouse effect. As it stands, this is enormously beneficial to us, for a planet 33°C cooler than now would not be habitable by humans.

However, the converse is also true. A planet substantially warmer than now would not be habitable by humans either. Consequently, fundamental questions for us as a species are whether our activity is actually capable of increasing the carbon dioxide concentration significantly and how much any such increase is liable to increase the temperature.