A greenhouse gas is a gas, either naturally occurring or
man-made, that absorbs heat in the atmosphere. Some greenhouse
gases occur naturally in the atmosphere, while others result
from
human activities. Naturally occuring greenhouse gases include
water vapor, carbon dioxide, methane, nitrous oxide, and ozone.
Certain human activities, however, add to the levels of most of
these naturally occurring gases:
- Carbon dioxide is released to the atmosphere when solid waste,
fossil fuels (oil, natural gas, and coal), and wood and wood
products are burned.
- Methane is emitted during the production and transport of
coal,
natural gas, and oil. Methane emissions also result from the
decomposition of organic wastes in municipal solid waste
landfills, and the raising of livestock.
- Nitrous oxide is emitted during agricultural and industrial
activities, as well as during combustion of solid waste and
fossil fuels.
Very powerful greenhouse gases that are not naturally occurring
include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
sulfur hexafluoride (SF6), which are generated in a variety of
industrial processes.
Each greenhouse gas differs in its ability to absorb heat in the
atmosphere. HFCs and PFCs are the most heat-absorbent. Methane
traps over 21 times more heat per molecule than carbon dioxide,
and nitrous oxide absorbs 270 times more heat per molecule than
carbon dioxide. Often, estimates of greenhouse gas emissions are
presented in units of millions of metric tons of carbon
equivalents (MMTCE), which weights each gas by its GWP value, or
Global Warming Potential.