The Big Picture 

:What are Greenhouse Gases?

Although GHGs play a critical role in maintaining the planet's surface temperature, they make
up less than 1% of the atmosphere by volume. The most abundant GHG is water vapor.
Although it is responsible for most of the Earth’s natural greenhouse effect, its global
abundance is not directly influenced by human activity and so it is not considered an
anthropogenic (human-generated) GHG. The most important anthropogenic GHGs are
carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halocarbons.

Carbon dioxide is the anthropogenic GHG that is of most concern. Large natural flows of
CO2 between the atmosphere and the oceans, land plants, and soils have maintained
relatively constant atmospheric concentrations over the past ten thousand years. However,
over the past two centuries human activity has released CO2 at increasing rates. The "pioneer
effect" (deforestation in the Northern Hemisphere) was probably the first significant
contributor, but with the beginning of the Industrial Revolution in the 19th century the main
source quickly became the burning of fossil fuels such as coal, oil, and natural gas. Over the
past several decades, tropical deforestation has also become a significant source, making up
about 20% of present global emissions.

Measurements show that atmospheric CO2 concentrations have increased about 30% from
their pre-industrial level as a result of human activity, to a level higher than at any time in at
least the past 420,000 years. Carbon dioxide is removed from the atmosphere slowly by
being absorbed by the oceans and by land plants. Some portion of the effect of CO2 emitted
into the atmosphere is removed in a century or less, but a quarter to a half of the effect is
removed so slowly it may be considered essentially permanent.

Other greenhouse gases have been accumulating in the atmosphere as well. For example, the
CH4 concentration has more than doubled since the 1700s, and the N2O concentration has
risen by about 15%.

A recent study in journal Science reports that a new greenhouse gas has been discovered. Trifluoromethyl sulfur pentafluoride, or as its more commonly called "SF5 CF3" is very rare, yet has been discovered to trap heat better than all other known greenhouse gases and takes probably over a thousand years to break down. While it is believed to be of human origin, the details of this are still a mystery to scientists. Read the July 28,2000 NYTimes article.

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*Global Change                    Online Journal on climate Change and Ozone Depletion

*N.Y.Times Global Warming Archive