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Are you
Clear About
Carbon?
Carbon is the 4th most abundant element in the universe.
All known life forms are carbon based – for example plants draw carbon dioxide (CO2) from
the atmosphere through photosynthesis. When plants die and decay, underground fossil fuels
(such as oil and coal) are formed and stored. When these fossil fuels are burnt they release
CO2 back into the atmosphere.
The word Carbon is often also used as shorthand for
carbon dioxide (CO2) or carbon dioxide equivalent (CO2e).
Are you Clear About Carbon? - Information Sheet 1 | November 2012
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the greenhouse effect, global
warming and climate change:
The Greenhouse Effect
Sun
Atmosphere
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The Greenhouse Effect compares the Earth’s atmosphere
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Solar radiation passes throught the
atmosphere.
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Net incoming solar radiation.
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Some solar radiation is reflected by the
atmosphere and the Earth’s surface.
The Greenhouse Effect, Global Warming
and Climate Change relate to theories on
the impact of increased greenhouse gases
being released in to the Earth’s atmosphere,
principally as the result of human activity.
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Solar energy is absorbed by the earth’s surface where
it is converted into heat and re-emitted as long wave
radiation back into the atmosphere.
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Some of the infared radiation is absorbed and re-emitted
by the greenhouse gas molecules. The direct effect is the
warming of the Earth’s surface and the troposphere.
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Some of the infared radiation passes through the atmosphere and is lost in space.
Are you Clear About Carbon? - Information Sheet 1 | November 2012
to that of a greenhouse. Greenhouse Gases (GHGs) in the
atmosphere absorb solar energy preventing it from escaping into
space. Although the Greenhouse Effect is a naturally occurring
process, the increase in the amount of GHGs in the atmosphere
as a result of human activity leads to the retention of more solar
energy and an increase in global temperatures.
Global Warming describes how the increase of GHGs in the
atmosphere leads to the retention of more of the sun’s energy
and the resulting increase in global temperature. Scientific
evidence supports this theory with tangible increases in global
average temperatures being recorded.
Climate Change is based on the theories of Global Warming
and the Greenhouse Effect. Due to advances in climatic
modelling, scientists have found that an overall increase in global
temperatures has an impact on weather systems, patterns and
events which may not always manifest as heat waves or higher
temperature. This highlights a much wider range of climate risks
linked to global warming, hence the term Climate Change.
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greenhouse
gases:
A GHG is any gas that is able to absorb energy in the form of heat. There
are six groups of gases that result from human activity which cause the
most concern and which are actively monitored under the Kyoto Protocol.
Carbon dioxide (CO2) is the most important GHG and accounts for 85% of
the UK’s man-made GHG emissions.
Carbon dioxide (CO2)
Methane (CH4)
CO2 is released through respiration,
decay and combustion and remains in
the atmosphere for 50-200 years. Since
the industrial revolution human activities
such as the burning of oil, gas and
coal and deforestation have increased
concentrations of CO2 in the atmosphere.
CH4 is another naturally occurring
gas, formed by microbes that break
down organic matter in oxygenpoor environments. It remains in the
atmosphere for 11-12 years and accounts
for 8% of UK man-made GHG emissions.
Globally as much as two thirds of
methane originates from man-made
sources, such as livestock (from digestive
gases and manure management),
decaying organic waste from landfills
and rice cultivation. CH4 can also be
accidentally released during drilling for
natural gas and coal.
Are you Clear About Carbon? - Information Sheet 1 | November 2012
Nitrogen dioxide (NO2)
Perfluorocarbons (PFCs)
NO2 is a toxic gas that is formed when
nitrogen and oxygen are combined at
high temperatures. It remains in the
atmosphere for between 110-120 years
and accounts for around 5% of UK manmade GHG emissions. The most important
sources of NO2 are internal combustion
engines, thermal power stations, pulp
mills, butane gas heaters or cookers, and
fertiliser use in agriculture.
PFCs are a group of gases which account
for less than 1% of UK man-made GHG
emissions. However, they are by far the
most persistent of the six GHGs and
remain in the atmosphere for up to
50,000 years. PFCs are emitted during
aluminium smelting and certain PFCs are
used in the manufacture of electronic
goods such as computers and mobile
phones.
Sulphur hexafluoride (SF6)
SF6 is an inorganic, colourless, odourless,
non-toxic and non-flammable gas
which accounts for less than 1% of UK
man-made GHG emissions. Although
only making up a small proportion of
emissions, it remains in the atmosphere
for 800-3200 years. SF6 is used for several
purposes including as a gaseous nonconducting medium in the electronics
industry, an inert gas for the casting of
magnesium and as an inert filling for
insulated glazing windows.
In addition to
these six gases,
Chlorofluorocarbons
(CFCs) and water vapour
are other important
GHGs:
Hydrofluorocarbons (HFCs)
HFCs are a group of gases which account
for 2% of UK man-made GHG emissions.
HFCs remain in the atmosphere for up
to 264 years and are commonly used
in refrigeration and air conditioning
equipment and some types of asthma
inhalers. HFCs are also emitted during
some manufacturing processes.
Control of CFCs, which are also a cause of
atmospheric ozone depletion, is covered
under the Montreal Protocol (to address
the depletion of the ozone layer) which
has been successful in phasing out the
production of these gases.
Water vapour accounts for the largest
proportion of the natural greenhouse
effect. However, human interventions are
thought to have only a marginal impact
on concentrations of water vapour.
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Measuring &
Comparing different
greenhouse gases:
GHG Multiply by the following figure
to obtain the CO2e value:
CO2 1
CH4 1
NO2 310
SF6 23,900
HFCs *
140 - 11,700
PFCs * 6,500 - 9,200
* HFCs and PFCs are groups of gases
each with different GWPs. Therefore,
the GWP values in the table are stated
as a range. HFCs and PFCs range
greatly in molecular size, generally
the larger the molecule, the more
heat it is able to absorb and therefore
the higher its GWP. The GWPs that
are used in UK National Statistics
are based on the Intergovernmental
Panel on Climate Change Second
Assessment Report.
A method has been developed to allow the
comparison of different GHGs. This methodology
uses the theory that different GHGs absorb heat
energy in the atmosphere at different rates.
The units used when GHG emissions are compared in this way to
show the carbon dioxide equivalence are: To allow this comparison the tendency of each gas to absorb heat
energy has been calculated and is referred to as a Global Warming
Potential (GWP).
For example: 1 tonne of methane (CH4) has an impact equivalent to 21 tonnes of
CO2. Therefore, 1 tonne of CH4 has a GWP of 21 tonnes of CO2e.
CO2 is the gas that other GHGs are compared to. For this purpose the
warming potential of a unit of CO2 is classified as 1. The GWP of a GHG,
for instance methane, is then expressed as the equivalent number of
units of CO2.
The following table sets out the GWP for the six GHGs monitored under
the Kyoto Protocol.
CO2e
CO2eq
CO2 Equivalent
Units of measure
CO2 equivalent (CO2e) is normally reported in tonnes.
However for very large quantities it can be expressed in multiples, for example:
• Mega Tonne – Mt (1 million tonnes)
• Giga Tonne – Gt (1 billion tonnes)
Similarly, for lower quantities smaller units of measure may be
used such as:
• Grams - g
• Kilograms - kg
Are you Clear About Carbon? - Information Sheet 1 | November 2012
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