Gasses with natural and significant human sources:
Methane (CH4)
Methane is the second biggest contributing greenhouse gas, and
is responsible for 20 percent of the enhanced (human caused)
greenhouse effect. It is about 23 times more powerful a greenhouse
gas than carbon dioxide, and has an atmospheric lifetime of roughly
12 years.
Sources of methane include decomposing organic waste (in nature
and in garbage dumps), and the raising of livestock. It's also
emitted during the production and transport of coal and natural
gas. Although natural sources exist, human activities are
significantly contributing to the amount of methane in the
atmosphere. Globally, atmospheric concentrations of methane have
increased by about 150 percent since 1750, and are now at higher
levels than in the last 400,000 years. Once in the atmosphere,
methane decays into carbon dioxide over a period of a few
years.
Nitrous oxide (N2O)
Nitrous oxide is 296 times more powerful a greenhouse gas than
carbon dioxide, and remains in the atmosphere for 114 years. It is
naturally emitted from oceans and soils, but human driven sources
are increasing its atmospheric concentrations.
Uses include some agricultural (mostly nitrogen fertilization)
and industrial activities, and it is created during combustion of
fossil fuels and other organic matter. Nitrous oxide also has a
variety of direct uses - including as an aerosol propellant and as
an anaesthetic (i.e. "laughing gas").
Artificial gasses with very high global warming potential:
Hydrofluorocarbons (HFCs)
HFCs make up only a small portion of greenhouse gas emissions,
but they are extremely potent greenhouse gases.
Depending on the exact type of HFC, they are up to 20,000 times
more powerful greenhouse gasses than carbon dioxide, and have
atmospheric lifetimes of up to 260 years.
Some uses of HFCs are in refrigeration (both commercial and
domestic), in air-conditioning (homes, cars, offices etc), and they
are also used as foam blowing agents, solvents, fire fighting
agents and aerosol propellants.
HFC use and production surged after they were actively promoted
as replacement refrigerants when a phase out of the ozone depleting
chlorofluorocarbons (CFCs) was mandated by the Montreal Protocol.
This is despite Greenpeace's successful Greenfreeze project, which
proved that more natural and benign alternatives are commercially
viable for refrigeration. In fact, safer alternatives exist for
almost every use of HFCs - making them a good target for emission
reductions.
More info:
June 2004:
Unilever, Coca Cola and McDonalds ditch climate-wrecking
refrigeration.
Perfluorocarbons (PFCs)
PFCs are from 5,700 to 10,000 times more powerful greenhouse
gasses (depending on the exact type) than carbon dioxide, and have
an atmospheric lifetime of up to 50,000 years. PFCs are
by-products of aluminium smelting. They are also used in
semi-conductor manufacture, and as substitutes for ozone depleting
chemicals. Emissions of PFCs are small even compared to HFCs.
However, given their potency, long lifetimes and availability of
alternatives already on the market, PFCs should be urgently phased
out.
Sulphur Hexafluoride (SF6)
Sulphur Hexafluoride is the most potent greenhouse gas evaluated
by the Intergovernmental Panel on Climate Change. It is 23,900
times more powerful a greenhouse gas than carbon dioxide, and has
an atmospheric lifetime of 3,200 years.
It has a number of uses including in Nike Air shoes, car tyres,
for electrical insulation, semiconductor manufacture, and in the
magnesium industry.
Like PFCs, the effects of Sulphur Hexafluoride to date are
fairly small. However, since it is a very persistent and potent
greenhouse gas, there is concern about its continuing build up in
the atmosphere. Given its potency, long lifetime and availability
of alternatives already on the market, Sulphur Hexafluoride should
be urgently phased out.
The European Union is currently designing legislation to control
emissions of these gases. For more information see the Greenpeace's
EU unit website.
Water and ozone:
Ozone (O3)
Ozone occurs both naturally, and from human activities. It is
present both in the upper atmosphere, where it forms the ozone
layer shielding us from harmful levels of ultraviolet solar
radiation, and in the lower atmosphere, where it is the main
component of smog.
Some people confuse the issue of ozone depletion with climate
change. In reality, they are separate but related. The man made
chemicals that destroy the ozone layer are greenhouse gases, as
are some of the chemicals that are replacing them. Also, as the
Earth's lower atmosphere warms and traps more heat, the upper
atmosphere (where the ozone layer is) becomes colder, which
facilitates the chemical reactions that damage the ozone layer.
Water vapour (H2O)
Water vapour is the most abundant greenhouse gas. The direct
effect of human activity on global water vapour concentrations is
thought to be negligible. However, water vapour is important for
climate change because of an important feedback effect. Warmer air
can hold more moisture, enhancing climate change. The exact size of
this important feedback remains to be determined by scientists.
Editor's note: Gases
are commonly compared to one another according to their Global
Warming Potential (GWP), which refers to their warming effect over
a set time compared to the same amount (by weight) of carbon
dioxide.
Comparing GWPs is useful because it takes into account both the
warming potential of each molecule of a gas, and its atmospheric
lifetime (how long it stays in the air). Carbon dioxide is the
commonly accepted point of reference (with a GWP of 1) because it
is the most significant greenhouse gas from human activities.
For simplicity, this page refers to the warming potential of
each gas relative to carbon dioxide over a 100-year period. This is
the same as its GWP with a hundred year benchmark. Thus, a kilogram
of carbon dioxide emission has a GWP of one, while a kilogram of
nitrous oxide has a GWP of 310 - which we have expressed here as
"nitrous oxide is 310 times more powerful a greenhouse gas than
carbon dioxide".
However, it is worth noting that since some gases will stay in
the atmosphere much longer than 100 years, their total greenhouse
effect over time is actually greater than expressed here.
Atmospheric lifetime = How long the gas stays in the
atmosphere.