Human activities result in emissions of four principal greenhouse
gases: carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O) and the halocarbons (a group of gases containing fluorine,
chlorine and bromine). These gases accumulate in the atmosphere,
causing concentrations to increase with time. Significant increases
in all of these gases have occurred in the industrial era (see Figure
1). All of these increases are attributable to human activities.
• Carbon dioxide has increased from fossil fuel use in transportation,
building heating and cooling and the manufacture of
cement and other goods. Deforestation releases CO2 and reduces
its uptake by plants. Carbon dioxide is also released in
natural processes such as the decay of plant matter.
• Methane has increased as a result of human activities related
to agriculture, natural gas distribution and landfills. Methane
is also released from natural processes that occur, for example,
in wetlands. Methane concentrations are not currently increasing
in the atmosphere because growth rates decreased over the
last two decades.
• Nitrous oxide is also emitted by human activities such as fertilizer
use and fossil fuel burning. Natural processes in soils and
the oceans also release N2O.
• Halocarbon gas concentrations have increased primarily due
to human activities. Natural processes are also a small source.
Principal halocarbons include the chlorofluorocarbons (e.g.,
CFC-11 and CFC-12), which were used extensively as refrigeration
agents and in other industrial processes before their
presence in the atmosphere was found to cause stratospheric
ozone depletion. The abundance of chlorofluorocarbon gases is
decreasing as a result of international regulations designed to
protect the ozone layer.
• Ozone is a greenhouse gas that is continually produced and
destroyed in the atmosphere by chemical reactions. In the troposphere,
human activities have increased ozone through the
release of gases such as carbon monoxide, hydrocarbons and
nitrogen oxide, which chemically react to produce ozone. As
mentioned above, halocarbons released by human activities
destroy ozone in the stratosphere and have caused the ozone
hole over Antarctica.
• Water vapour is the most abundant and important greenhouse
gas in the atmosphere. However, human activities have only
a small direct influence on the amount of atmospheric water
vapour. Indirectly, humans have the potential to affect
water vapour substantially by changing climate. For example,
a warmer atmosphere contains more water vapour. Human
activities also influence water vapour through CH4 emissions,
because CH4 undergoes chemical destruction in the stratosphere,
producing a small amount of water vapour.
• Aerosols are small particles present in the atmosphere with
widely varying size, concentration and chemical composition.
Some aerosols are emitted directly into the atmosphere while
others are formed from emitted compounds. Aerosols contain
both naturally occurring compounds and those emitted as a result
of human activities. Fossil fuel and biomass burning have
increased aerosols containing sulphur compounds, organic
compounds and black carbon (soot). Human activities such as
surface mining and industrial processes
have increased dust in the atmosphere.
Natural aerosols include mineral dust released
from the surface, sea salt aerosols,
biogenic emissions from the land and
oceans and sulphate and dust aerosols
produced by volcanic eruptions.