The Greenhouse Effect

Much has been made of the Greenhouse Effect and its responsibility in the current rise in global temperatures that we are seeing. While it is true that the greenhouse effect is in some ways responsible for global climate change, it is essential to point out that the greenhouse effect is also essential to human life on this planet. Without it, the average global temperature would plummet to -19°C. This is the same average temperature as in Alert, Nunavut, the northernmost permanently inhabited place in the world.

Instead, because of the greenhouse effect, we enjoy a relatively warm average global temperature of 14°C. But how does this work?

Virtually all of the energy that reaches the earth comes from the sun in the form of light. Most of this light is in the visible range, and is the light that we are able to see. While some of it is reflected back into space by the atmosphere, and a small portion absorbed by our atmosphere, the majority of it reaches the earth's surface, warming the land and the ocean. The land and ocean in turn, release low temperature radiation back into the atmosphere, in a form known as infra-red (IR) radiation.

Over 99% of our atmosphere is composed of oxygen, nitrogen, and argon, but for the most part these gases do not interact with IR radiation. Fortunately for us, our atmosphere also contains small quantities of water vapour, responsible for our rains, and carbon dioxide, essential to plant life on earth. Both of these gases are able to absorb IR radiation, and trap the heat being radiated from the earth's surface. As our atmosphere warms, it becomes excited and vibrates, emitting IR radiation in all directions, most of which again reaches the earths surface starting the process all over again.

It is this recycling of heat that we call the greenhouse effect, and is why we call water vapour and carbon dioxide (amongst others) greenhouse gases.