What is a greenhouse gas?
Infrared Radiation is passed through a molecule and if it can pass
through without interacting with the molecule then it is not
considered a greenhouse gas.
Oxygen and Nitrogen, the two major gases in our atmosphere have
no interaction with the IR radiation.
Water vapour however, does absorb some IR radiation, therefore
there is an interaction, making this a greenhouse gas.
The greenhouse gases absorb long wave radiation (IR radiation) but
allow for the passage of long wave radiation (eg. UV Light), which
does not interact with the gases.
The IR radiation absorbed by the greenhouse gases cause vibrations
in the bonds, then the energy is ‘re-radiated’ back to Earth.
(Just FYI, the IR radiation comes from the Earth, whereas UV light are
emitted from the Sun)
Absorption of Infrared Radiation by
Water vapour (H₂O) is the most active molecule in the absorption of
Infrared radiation and thus in heating the atmosphere. It accounts
for about 55% of IR absorption.
Carbon Dioxide (CO₂) accounts for about 18%
Methane (CH₄) for 6%
Ozone (O₃) for 5%
Natural Sources of GHGs
Incomplete oxidation of methane in the atmosphere. (The methane
originates from aerobic decomposition)
Bacterial action in the soil produces NO₂
Anthropogenic Sources of GHGs
Industrial plants that produce smoke/soot from the combustion of
The incomplete combustion of fossil fuels. (Forms CO)
High temperature combustion engines produce NO which is then
oxidised to 2NO₂
Despite looking at the relative effects on heating due to the
absorption of infrared radiation, water vapour is not the biggest
contributor to global warming.
This is because the water cycle remains fairly stable and consistent
meaning that the amount of water vapour in the atmosphere at
any one time is pretty consistent.
However the concentrations of Carbon Dioxide and Methane are
not so stable and are increasing in amounts thus absorbing more IR
and causing further heating of the planet.
Temperatures have clearly picked up following the industrial
revolution and have escalated as technologies advance
and the population increases.
In 2007 GHG levels were at 430ppm, but before the
Industrial Revolution that number was just 280ppm.
To cope with the increasing demand for energy, fossil fuels
have and are being burnt more and more, releasing growing
amounts of Carbon Dioxide into the atmosphere and heating
up the Earth as a result.
The increase in heat due to excess Carbon Dioxide in the
atmosphere causes the melting of permafrost, especially in places
such as Siberia. The frozen ground contains trapped Methane which
is released as this ice melts (at an accelerated rate) altering the
balance of Methane in the atmosphere causing further heating.
In just a single year (2004-5) NASA reported a 14% decline in
permanent sea ice.
In turn, the more sea that is exposed, the less energy is
reflected, and instead absorbed by the darker coloured oceans
resulting in even further heating of the oceans and earth.
The speed of human induced climate change is much faster than it
has naturally. Since the last ice age (10,000 years ago) and today
there has been an overall rise of 5 degrees Celsius, however at
current rates the increase is 1.4 degrees per 100 years.
Anthropogenic changes are happening much faster than natural
rates seem to have ever done, making it extremely difficult for
organisms to adjust, and the consequences could be extreme.
Just of 3 degrees has massive impacts on our well tuned ecosystems
and could result in the loss of 20-50% of species.