When Good Gasses Go Bad: A primer on the causes of global warming

by Emily Gertz

Published October 05, 2008 @ 09:26PM PST

Our atmosphere is a mix of gasses: 21% oxygen, 78% nitrogen, and 1% other substances.  That 1% includes the "greenhouse gasses" such as carbon dioxide and water vapor, which warm the planet by absorbing infrared light.  Although these gases are getting a bad rap these days, they deserve some credit: they have made our home world a relatively comfortable place to live for eons past.

Here's how it works:

Energy from the sun comes through the atmosphere in the form of short-wave radiation: mostly as visible light and ultraviolet radiation.  About one third of the light hitting the Earth is reflected back into space.  Of the rest, the atmosphere absorbs a small portion, and the land and the oceans on the Earth's surface soak up the rest.  These warm up a bit, and then reflect the energy they've absorbed back out towards space in a degraded form: long-wave or infrared radiation.

If our atmosphere were nothing but nitrogen and oxygen, nearly all of that cozy warmth would head right back out into space, leaving the Earth's surface a cold and life-unfriendly place to be -- kind of like Mars, in fact.

But the greenhouse gases trap some of that infrared and reflect it back down to the surface.  This "blanket effect," in balance with a whole host of other factors ranging from the amount of ice covering the poles, to the flow of the massive "rivers" of alternately cool and warm water that circulate through the world's oceans, to the very existence of life itself, create the temperate conditions that have allowed life on Earth to survive and flourish.

Cut to the beginning of the Industrial Revolution, during the late 1700's. Increasing use of refined coal in Britain and the rest of Europe began the movement to mass mechanization of manufacturing and transportation; a process that would eventually lead to the invention of the internal combustion engine and electrical power generation. 

The coal we were burning -- and oil and gas we would soon start to burn -- are the fossilized remains of prehistoric animals and plants.  Since all life on earth is carbon-based, burning these petro-resources to power industrial society releases their long-buried storehouse of carbon into the atmosphere, changing the delicate balance between our blanket of greenhouse gasses and the rest of the Earth.  

Increased concentrations of carbon dioxide, an end product of petroleum combustion, mean that more of the energy radiated outwards by the land and oceans is reflected back down to the surface.  And while local conditions can vary based on topography, the current weather, and other factors, the net impact is that the Earth's surface grows warmer.

In some cases the changes are creating "positive feedback loops" that both cause and accelerate the disruption of the climate.  One example is the unprecedented reduction in Arctic sea ice -- a major regulator of global climate -- over recent summers.  Ice reflects the sun's energy back out towards space, and an ice-free ocean retains that energy and grows warmer, which leads to even greater loss of summer sea ice, and warmer temperatures on the surface of the Earth.

What do the numbers look like?  Before the Industrial Revolution, the amount of carbon dioxide in the atmosphere was under 300 parts per million.  Today it is over 380 parts per million, and the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past 100 years.  At current rates of pollution, it's estimated that by mid-century there will be well over 500 parts per million of carbon dioxide in the atmosphere.  

Image: "This highly oblique image shot over northwestern part of the African continent captures the curvature of the Earth and shows its atmosphere. You can see clouds and even the occasional thunderhead."  Source: NASA Jet Propulsion Laboratory