The atmosphere consists of about 78 per cent nitrogen,
21 per cent oxygen and a range of other gases including water
vapour and CARBON DIOXIDE.
Carbon dioxide currently makes up about 0.038 per cent of the
atmosphere, and water vapour varies from a fraction of 1 per cent
to about 3 per cent.
Greenhouse gases are gases in the atmosphere that absorb and
emit infrared or heat radiation, thus trapping heat in the lower
atmosphere. Most greenhouse gases exist in the atmosphere naturally
at very small concentrations. The major atmospheric gases, nitrogen
and oxygen, which together make up 99 per cent of the
atmosphere, are not greenhouse gases.
Several gases together cause the natural greenhouse effect. One of the most
significant is ordinary water vapour. Carbon dioxide, which is a
vital natural component of the atmosphere, is also responsible for
some of the heat-trapping of the natural greenhouse effect. Without
these small quantities of naturally occurring greenhouse gases, our
planet's average temperature would be more than
30°C colder than it actually is.
In the last century or so, industrial and agricultural
activities have caused an increase in the quantity of most of these
natural greenhouse gases (apart from water vapour). In particular,
the burning of carbon-containing fuels (such as oil products, coal
and methane) has increased the concentration of carbon dioxide by
more than one third. This increase in greenhouse gas concentrations
is increasing the natural greenhouse effect, and is considered to
be responsible for most of the measured increase in global
temperature that has occurred over the last century. This is known
as the anthropogenic (human-caused) greenhouse effect, or the
enhanced greenhouse effect.
The main natural greenhouse gas is water vapour. Water vapour is
always present throughout the lower atmosphere, even if sometimes
at a very low leve. Water is constantly transferred between the
oceans, atmosphere and land in the global hydrological cycle, or
the water cycle. When condensed as liquid or ice droplets, water is
the main constituent of clouds.
Although human activities affect the water cycle, they do not
appear to have directly changed the concentration of water vapour
globally. As will become clear below, water vapour is therefore not
measured as part of
anthropogenic—human-generated—greenhouse gas emissions.
It is also worth noting that although water as a gas traps heat in
the lower atmosphere, when it is in the form of suspended droplets
(essentially clouds), it can also act to cool the surface of the
Carbon dioxide (CO2):
The other major greenhouse gas, and the one most often
discussed, is carbon dioxide. Carbon dioxide makes up a small but
growing component of the atmosphere. Its current concentration is
0.038 per cent, or 380 parts per million (ppm). Two
hundred years ago, its concentration was only about
At very small concentrations, carbon dioxide is a natural and
essential part of the atmosphere, and is required for the
photosynthesis of all plants.
Carbon dioxide enters and leaves the atmosphere from a number of
natural sources and sinks at the earth’s surface. In the
absence of human influences, the fluxes of carbon dioxide into and
out of the atmosphere were largely in balance,. The burning of
carbon-containing fuels (coal, oil and gas) has considerably
increased the concentration of the CO2 in
the atmosphere. Scientists can directly measure the concentration
of carbon dioxide in the atmosphere; they can also determine
atmospheric carbon dioxide concentrations in the past by, for
example, measuring the gas in tiny air bubbles trapped for many
thousands of years in deep layers of polar ice.
Large amounts of carbon dioxide are transferred between the
atmosphere, oceans and land vegetation in the natural global carbon
cycle. Anthropogenic emissions are adding to the amount of
CO2 in the atmosphere, and causing changes
in the amount taken up by the oceans and vegetation. The main
stores and fluxes of carbon in the global carbon cycle are
described and illustrated in sinks and
Other greenhouse gases:
There are other gases that contribute to the greenhouse effect.
Many of these are produced or augmented by human activity. Two
important examples are the gases METHANE and NITROUS OXIDE.
Methane was once called 'marsh gas' and is the main constituent
of the gas which can cause explosions in coal mines. Methane is the
principle component of the 'natural gas' that is used as a fuel
(when combusted, the carbon in methane is oxidised to produce
carbon dioxide as a waste product; however, there are also
significant fugitive or unintentional emissions of methane involved
in the extraction, transport and processing of natural gas); it is
manufactured as 'biogas' from the decomposition of organic matter;
and is also produced by the processes of digestion in ruminant
animals, including cattle and sheep.
Nitrous oxide is a powerful greenhouse gas that is produced
naturally by microbes in the soil and ocean, and is a by-product of
agricultural activity involving nitrogen fertilisers and animal
wastes. It is also given off in small quantities by the burning of
fossil fuels including oil and coal.
CHLOROFLUOROCARBONS (CFCs), HYDROCHLOROFLUOROCARBONS (HCFCs) and other carbon
compounds containing chlorine and bromine are strong greenhouse
gases. Emissions of these gases have been curtailed due to
regulation under the Montreal Protocol,
but together they have been responsible for about 12 per cent
of the enhanced greenhouse effect. Most of the contribution has
been from CFCs.
Global warming potentials:
Greenhouse gases differ in their ability to trap heat, as well
as in their concentrations in the air. Many of these gases have a
far greater warming effect than carbon dioxide for a given mass,
though their warming contribution overall is less because their
concentration in the atmosphere is much lower. The length of time
that their warming effect can persist also differs, as it depends
on how long they remain 'resident' in the atmosphere before natural
chemical or physical processes remove them.
To be able to compare the warming effects of the different
greenhouse gases, scientists have calculated the 'global warming
potential' (GWP) of each gas. The global warming potential measures
how much a particular greenhouse gas contributes to global warming.
The GWP compares the RADIATIVE
FORCING, or warming ability, of a particular gas to that
of carbon dioxide, which is used as a reference. The warming
potential of carbon dioxide is given a value of one, against which
the other gases are compared.
As explained, each gas remains for a different period of time in
the atmosphere before it is removed by natural processes. Hence the
global warming potential for each greenhouse gas is calculated over
a specific time interval; standard time intervals for these
calculations are 20 years, 100 years and 500 years.
It is appropriate to quote the length of time whenever a global
warming potential figure is quoted. The standard used to calculate
carbon dioxide equivalents (see below) is 100 years. For
example, the greenhouse gas NITROUS
OXIDE has a global warming potential of 298. This means
that one tonne of nitrous oxide in the air has the same effect as
298 tonnes of carbon dioxide over a 100-year time frame.
The global warming potential of the major greenhouse gases over
a 100-year time horizon are shown in the table below.
Global warming potential of major
||45 – 1700
||4750 – 14,400
||1.3 – 17.9
||77 – 2310
||1.4 – 270
||437 – 12,000
||740 – 50,000
||7390 – 17,700
||0.77 – 136
||59 – 14,900
Source: Intergovernmental Panel
on Climate Change, Working Group I Contribution to the Fourth
Assessment Report, Climate change 2007—the physical
science basis, Chapter 2 Changes in atmospheric constituents and
in radiative forcing, Table 2.14,
Note that some of these gases have a very large global warming
potential. This means that extremely small quantities can have
large effects in comparison to carbon dioxide. HYDROFLUOROCARBONS (HFCs) are
used in refrigeration and air conditioning; they are used as
replacements for CFCs—CHLOROFLUOROCARBONS—which are ozone-depleting
gases. PERFLUOROCARBONS were introduced as alternatives to
the ozone-depleting HFCs but have relatively high global warming
potentials. HYDROFLUOROETHERS are more recent replacements for
CFCs and HFCs; they have lower ozone-depleting properties and
generally lower global warming potentials compared with HFCs and
PFCs over a 100-year time scale, because they tend to have a
shorter lifetime in the atmosphere.
Carbon dioxide equivalents:
Much technical literature about the enhanced greenhouse effect
refers to carbon dioxide equivalents. This is a way of making it
easier to lump together the warming effects of several different
gases. A known mass of another greenhouse gas is translated to an
equivalent mass of carbon dioxide, by using its GWP. For example,
accounting', one tonne of nitrous oxide, which has a GWP of
298, will be counted as 298 tonnes of carbon dioxide
equivalents (called CO2-e).
Kyoto greenhouse gases:
The six greenhouse gases regulated under the Kyoto Protocol
- Carbon dioxide (CO2).
- Methane (CH4).
- Nitrous oxide (N2O).
- Hydrofluorocarbons (HFCs).
- Perfluorocarbons (PFCs).
- Sulphur hexafluoride (SF6).
Intergovernmental Panel on Climate Change, Working Group I
Contribution to the Fourth Assessment Report, Climate change
2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and
in radiative forcing.
Lists of gases and their global warming potentials are also on
the United Nations Environment Programme (UNEP) Global Resource
Information Database (GRID) web site based in the city of Arendal,
Norway. GRID-Arendal is at http://www.grida.no/ and the data on
GWPs is at http://www.grida.no/climate/ipcc_tar/wg1/248.htm.