The greenhouse effect

Step inside a glass building on a sunny day - you can feel the Sun's heat, more so than outside the glass enclosure. Why?

Because the glass windows transmit light, the interior is warmed. It's hard for this heat to escape because the glass walls prevent convection - that is, exchange of the warm air within for outside (cooler) air.

The Earth's atmosphere also behaves as a greenhouse, but the mechanism is a bit different.

Selective absorption of light

Infrared light is what you perceive as heat from a fire. Infrared light has an energy corresponding to the vibration of molecules. Vibrating molecules collide with one another, jostling each other about in Brownian motion.

Visible light, on the other hand, is of an incorrect energy to induce vibration in molecules directly. It causes excitation of the electrons in the molecule. However, molecules excited by visible light eventually transfer their energy into molecular vibration and translational motion.

Increased temperature/ absolute zero

What does all this have to do with the Earth's climate? The atmosphere contains gases, particularly water vapour and carbon dioxide, which absorb infrared light but transmit visible light. The atmosphere is transparent to visible light but opaque to some infrared colours (frequencies) of light.

At absolute zero (0K,  -273 deg C) the motion of molecules, atoms and so on ceases - no Brownian motion at all is present. Above absolute zero molecules move, and, as the temperature is raised they move faster. When molecules move, they emit infrared light. Above absolute zero, matter emits infrared light. This is the heat you feel from the fire: the rapidly moving molecules in the hot fire releasing infrared light.

The surface of the Earth absorbs sunlight. This is converted to molecular motion (heat). This energy is radiated back to space as infrared light but greenhouse gases in the atmosphere absorb it, preventing this energy from leaving the Earth.

So, if it wasn't for the presence of water and carbon dioxide in our atmosphere, the Earth would be much colder - below freezing, in fact.

Terestrial Greenhouse Effect

US EPA comments on global warming

Bad Greenhouse

infrared map of Earth

global change master directory

Using infrared light to analyze molecules

infrared spectroscopy

Related topics@MacEwan

Chem 102 Experiment M (Spectroscopy)

Organic Chemistry (161/163/261/263)