Some greenhouse gases such
as carbon dioxide occur naturally and are emitted to the atmosphere through
natural processes and human activities. Other
greenhouse gases (e.g., fluorinated gases) are created and emitted solely
through human activities. The principal greenhouse gases that enter the
atmosphere because of human activities are:
Carbon Dioxide (CO2):
Carbon dioxide enters the atmosphere through the burning of fossil fuels
(oil, natural gas, and coal), solid waste, trees and wood products, and
also as a result of other chemical reactions (e.g., manufacture of cement).
Carbon dioxide is also removed from the atmosphere (or “sequestered”)
when it is absorbed by plants as part of the biological carbon cycle.
Methane (CH4):
Methane is emitted during the production and transport of coal, natural
gas, and oil. Methane emissions also result from livestock and other
agricultural practices and by the decay of organic waste in municipal
solid waste landfills.
Nitrous
Oxide (N2O): Nitrous
oxide is emitted during agricultural and industrial activities, as
well as during combustion of fossil fuels and solid waste.
Fluorinated
Gases: Hydrofluorocarbons, perfluorocarbons, and sulfur
hexafluoride are synthetic, powerful greenhouse gases that are emitted
from a variety of industrial processes. Fluorinated gases are sometimes
used as substitutes for ozone-depleting
substances (i.e., CFCs, HCFCs, and halons). These gases are typically
emitted in smaller quantities, but because they are potent greenhouse
gases, they are sometimes referred to as High Global Warming Potential
gases (“High GWP gases”).