The Greenhouse
Effect
natural greenhouse effect
http://www.elmhurst.edu/~chm/vchembook/globalwarmA5.html
Greenhouse Analogy: Energy from the sun
in the form of some ultraviolet and visible
light (short wavelength) passes through the
glass of the greenhouse. As the light strikes
various surfaces in the greenhouse they are
heated. These surfaces in turn re-radiate the
heat in the form of infrared radiation (long
wavelength). However, the IR radiation is
blocked from escaping by the glass. IR is not
able to pass through the glass, hence the
greenhouse air heats up fairly dramatically.
The greenhouse gases have the same property as the glass towards the IR
radiation. Think of the greenhouse gases acting as an invisible glass shield
around the earth.
gases in the atmosphere
Earth
The sun emits ultraviolet (UV) radiation
which hits the earth
These rays of light hit
the earth and reflect
back towards space in
the form of UV as well
as Infrared (IR)
radiation
gases in the atmosphere
pick up some of the heat
from the IR rays and
disperse them back into
the earth’s atmosphere
keeping the earth warm
enough to sustain life
Would you be able to feel the heat?
On a molecular level…
When certain gases in the atmosphere absorb IR Radiation
their vibrational modes are excited and vibrate, causing them
to collide with other molecules and transfer energy
When the electrons return to their ground state,
they re-emit the energy with a frequency equal
to the frequency of energy gap between the two
levels
Only certain gases do this, just like dogs can
hear frequencies that humans cannot because
their eardrum vibrates with different
wavelengths than ours
If this didn’t happen…
The climate would be an average of 60°F colder
and the earth could not sustain life as we know it.
So greenhouse gases are called
greenhouse gases because they keep
some heat in the atmosphere to
sustain life on earth as a greenhouse
does to sustain life in the greenhouse
when it is cold outside
Let’s find out which atmospheric gases
are green house gases
Greenhouse gas molecules are able to
vibrate with the absorption of heat
Natural Versus Enhanced
Climate chemists believe that humans are
producing more of the naturally occurring
greenhouse gases than the atmosphere can
naturally handle from…
Anthropogenic
The most abundant atmospheric gas, Nitrogen
molecules have a strong bond which makes it
chemically stable and non-reactive in most
circumstances. Nitrogen's simple structure is unable
to absorb either visible or infrared light. As a result,
nitrogen is not a greenhouse gas.
Oxygen is the second most abundant
atmospheric gas. Why do you think
that Oxygen is also not a greenhouse
gas?
greenhouse gases definition
Greenhouse gases are gases that:
• allow visible light and UV radiation (shortwavelength/high frequency) to pass through them
• but (because of the nature of their covalent bonds
in their molecules) absorb the infrared radiation
(longer-wavelength radiation) of the same
frequency as the one the Earth converts the energy
from the Sun into and
• reradiates this infrared radiation back to the Earth.
Greenhouse factor
• Compares ability of a greenhouse gas to absorb IR
to the same amount of carbon dioxide which has a
factor of 1.
• Example: greenhouse factor of methane is 30 which
means it absorbs as much as 30 molecules of
carbon dioxide or 1 molecule of methane absorbs
30 times more as 1 molecule of carbon dioxide.
greenhouse gas: CO2
• Sources:
– Human: burning fossil fuels and wood, forest fires,
burning waste
– Natural: respiration, decay of organic matter, natural
forest fires
• Relative effect: Most important greenhouse gas (50%
contribution) because of its great abundance (second
largest after water vapour) and the large range of
wavelengths over which it absorbs IR.
Greenhouse gases: H2O
• Sources:
–Human: combustion of hydrocarbons
–Natural: evaporation
• Relative effect: 0.1 - Least effective in trapping
radiation but is most abundant.
Greenhouse gases: CH4
• Sources:
– Human: cattle farming, rice paddies (wet soil means any
organic matter in it is decomposed without oxygen) ,
petroleum and natural gas production.
– Natural: digestive tracts of ruminants, cattle, bogs or
marshes, bacterial fermentation – when organic matter
is decomposed anaerobically, methane gas is produced.
• Relative effect: 30 - Low abundance in atmosphere but it
is more effective in absorbing infrared radiation, however,
its atmospheric lifespan in the atmosphere is short.
Greenhouse gases: N2O
• Sources:
– Human: use of nitrogen based fertilizers
– Natural: bacterial action
• Relative effect: 150 - Very effective in absorbing
radiation, fairly long atmospheric life.
Greenhouse gases: CFCs
• Sources:
– Human: refrigerators, air- conditioning, aerosols
in spraying cans, foaming agents
– Natural: none!!!!
• Relative effect: 10 000 – 25 000 - Very effective in
absorbing radiation, long atmospheric life but low
abundance.
Greenhouse gases: SF6
• Human source: electrical
insulators
• Greenhouse effect: 24 000
Some greenhouse gases are not
naturally occurring – they are
manmade
Sulfur
Hexafluoride
Carbon
tetrafluoride
Hexafluoroethane
CFC’s or Chlorofluorocarbons
Effects of particulates
• particulates scatters and reflect the incoming
sunlight (visible and UV) so that less solar radiation
enters the atmosphere;
• particulates also cause a lowering of the
temperature as they provide condensation nuclei
around which water particles condense to form
clouds reducing solar heating;
• volcanic eruptions and large forest fires greatly
increase the amount of particulates.
So why do greenhouse gases have
such a bad reputation?
terminology
• greenhouse effect = the absorbing of
some of the infra-red radiation
radiated from the Earth in the
atmosphere which is then reradiated
back to Earth; this results in…
• global warming = a gradual increase in
planet-wide temperatures
Global warming effects (1)
• World wide rise in sea levels resulting from:
– Partial melting of glaciers and polar ice caps
– Thermal expansion of water (as a result of
heating).
• Changes in crop yields: some crops will grow
better, other worse.
Global warming effects (2)
• Changes in distribution of commercial crops
• Changes in the distribution of pests and diseasecarrying organisms e.g. malaria.
• More severe weather conditions:
– floods in particular of coastal areas; more severe storms
e.g. monsoon floods in Pakistan 2010
– More severe droughts e.g. 2010 worst drought in
Amazon
Global warming: evidence
global temperatures since 1850s
http://www.ncdc.noaa.gov/oa/climate/research/anomalies/anomalies.html
•
What can the mean global temperature anomaly be used for? This product is a global-scale climate
diagnostic tool and provides a big picture overview of average global temperatures compared to a reference
value.
UV light in the atmosphere would
break thte bonds of the Chlorine in the
CFC’s and release it. Chlorine was
found to deplete the ozone.
HFC – Hydrofluorocarbons was the
answer
And Hydrofluorocarbons
Which once saved the earth!
Right now HFC’s do not contribute to
climate change as much as CO2 but…
Chemists to the rescue!
Why should I care??????