Shown above is an Infrared Map of the Earth. Red areas represent
regions of high heat retention in the atmosphere. This is an equatorial
band because that is where the atmosphere has the most water vapor.
The earth has a natural greenhouse effect due to trace amounts of
H20 and CO2 that naturally occur. The
enhanced greenhouse effect refers to the
augmentation of these
natural gases by human activities.
What's the evidence?
The energy balance in the atmosphere is shown here:
The main components in this diagram are the following:
What Happens to the 69% of the incoming radiation that doesn't
get reflected back:
- Short wavelength (optical wavelengths) radiation from
the Sun reaches the top of the atmosphere.
- Clouds reflect 17% back into space. If the earth gets more
cloudy, as some climate models predict, more radiation will be
reflected back and less will reach the surface
- 8% is scattered backwards by air molecules:
- 6% is actually directly reflected off the surface back into
- So the total reflectivity of the earth is 31%. This is
technically known as an Albedo . Note
that during Ice Ages, the Albedo of the earth increases as
more of its surface is reflective. This, of course, exacerbates
- 19% gets absorbed directly by dust, ozone and water
vapor in the upper atmosphere. This region is called the stratosphere
and its heated by this absorbed radiation. Loss of stratospheric
ozone is causing the stratosphere to cool with time, which, of
course, greatly confuses the issue of global warming.
- 4% gets absorbed by clouds located in the troposphere. This
is the lower part of the earth's atmosphere where weather happens.
- The remaining 47% of the sunlight that is incident on top of the
earth's atmosphere reaches the surface. This is not a real
significant energy loss.
Achieving Thermal Equilibrium:
This short wavelength radiation is
absorbed by the earth which heats the earth to a finite temperature.
Since the earth wants to stay in thermal equilibrium, it must
re-radiate this energy.
The earth has an equilibrium temperature
of about 300
Kelvins . At this temperature, the wavelength of the emitted
radiation is in the infrared.
What happens to the outgoing infrared radiation?
So clearly, if human activities increase the ability for the
earth's atmosphere to absorb IR radiation, this produces a net
warming of the atmosphere over time. This is the Enhanced
- If it all went directly back into space, the earth would be
a significantly colder place than it is.
- 15% is directly radiated back by the cloud-free land surface: 6% of that is
absorbed by the atmosphere and 9% goes directly back into space
- 60% is re-radiated back into space by the net emission of the
atmosphere and the clouds. The total radiated back into space
is 69% meaning that 31% is temporarily stored as energy and emitted
- Of this 31%, 24% is used to facilitate evaporation. This heat
is later released through condensation. This process is called
- 7% is stored by the earth's crust and then re-radiated at later
times through a complicated heat exchange network of convection and
conduction. - Note, this is why heat pumps work. At a few meters
below the surface of the earth, the temperature is nearly constant
over most of the year because of this low heat flux
Concentration increases since 1750:
- Carbon Dioxide: 280 ppm 360 ppm
- Nitrous Oxides: 280 ppb 360 ppb
- Methane: 0.70 ppm - 1.70 ppm (143%)
- In principle, CFCS are very bad and will dominate the
greenhouse gases in our atmosphere if their terrestrial usage
remains high. CFC worldwide production, however, has been
significantly reduced because of concern about
The Ozone Layer
- Methane is more directly related to food production and population
growth so it could also dominate in the near future
Frozen methane is also found in the Arctic Ice Caps and will
be released due to global warming thus exacerbating the problem. This
is far more serious that people realize or has been reported.
Data showing the increasing concentration of Greenhouse Gases:
CO2 is steadily increasing at the rate of about
1 ppm per year
NOx is leveling off due to pollution abatement on
CH4 is only recently been measured. Rate of increase
was about 10 ppb per year in the late 80's and early to mid 90's.
Since 1 CH4 molecule
is worth 25 CO2 molecules, the equivalent growth rate
in terms of CO2 is 250 ppb (or 0.25 ppm)
However, in the late 90's the rate of methane addition to
the atmosphere had leveled off. The exact physical reasons
for this are unclear at the moment and how long this situation
will remain is also unclear.
If methane concentrations begin to rise again, then it will likely
dominate the enhanced greenhouse effect.
Proceed on to Global Warming