Criteria for Greenhouse Gas
Country: United States
Date: March 2009
If CO2 has an atomic weight greater then H2O how can it
possibly be a greenhouse gas?
The molecular weight is not the direct criterion for a greenhouse gas. What "defines" a
greenhouse gas is how much infrared energy (heat) the gas can absorb. The infrared
radiation range extends from the end of the visible spectrum, about 700 nm, to the
beginning of the microwave spectrum. The transition is not clear cut but let us just
say the transition is 10 cm^-1(i.e. 10 wave numbers). OOPS, we have a problem, we need
to convert wave numbers, the usual units used in the infrared region to nanometers, here
is how:10 cm^-1 = 0.1 cm (We just took the reciprocal.).
0.1 cm x 1 m/100 cm x 10^9 nm / m = 10^6 nm. So 10 cm^-1 = 10^6 nm. So for approximate
estimations, we can say the infrared region extends from one thousand to one million
The principle molecular mechanism for a greenhouse gas is to absorb energy in this range.
It does so by changing its vibrational and/or its rotational state. There are rules that
govern how these changes can occur. But there is another mechanism. Most atmospheric
gases are relatively transparent to visible light. So during the day sunlight's visible
energy (400 to 700 nm) is absorbed by the Earth's surface -- ignore clouds to keep things
simple, but it does not change the reasoning that much. When the visible light is
absorbed by the Earth it is converted into heat (i.e. into infrared radiation) --
that is what keeps us warm. But as this converted infrared energy re-radiates back
into space, it must pass through the atmosphere, the atmosphere containing these gases
that absorb the infrared radiation. So the heat energy cannot escape back into space.
The greenhouse gases act like a blanket, preventing the escape of the infrared heat
energy. This is oversimplified, but I hope it convinces you that molecular weight, all
by itself, is not the criterion for a gas to be a greenhouse gas.
Both water vapor and CO2 are greenhouse gases.
There are two things we need to consider here: (1) how molecules such as CO2 and H2O
produce the "greenhouse effect", and (2) why, if both CO2 and H2O produce this effect,
CO2 has become a focus of the greenhouse effect.
CO2 and H2O both absorb portions of infrared light (essentially radiated heat) and while
some of this absorbed light is re-emitted outward into space, a good portion of it is
emitted back to the surface of the Earth. Thus, energy/heat that would otherwise leave
the Earth is emitted back. Since both H2O and CO2 have mechanisms for absorbing infrared
light (the bonds of the molecules of CO2 and H2O can rotate, vibrate, bend to the
frequency of infrared), both can act as greenhouse gases.
CO2 has become a focus of discussion because there is a noted rise in the amount of CO2
(from somewhere in the 315 ppm in 1958 to 380 ppm in 2008). While water vapor is more or
less constant - CO2 is rising, and this is why we talk about CO2 a lot . . . but both
CO2 and H2O produce a greenhouse effect.
Greg (Roberto Gregorius)
The effectiveness of a gas as a greenhouse gas is dependent
on how well it absorbs and emits infrared radiation.
Only gases with more than one nucleus absorb infrared radiation
with any efficiency, but certain molecular structures are more
able to do so than others. Therefore, having a greater atomic weight
than water does not disqualify a gas as a greenhouse gas.
Of equal or even greater importance than molecular structure to the
effectiveness as a greenhouse gas are the abundance of the gas and
the lifetime of the gas in the atmosphere.
Water vapor is by far the most abundant, with CO2 being second most
abundant, and methane third most abundant. These are 1, 2, and 3
in importance as well. Even though water vapor has a short lifetime
of several days, it is constantly replenished by the Earth's water cycle
and so hardly changes in total amount at any time. CO2 and methane
have very long lifetimes, and so are also important, but to a lesser
degree than water vapor. There are many other greenhouse gases, such
as ozone and CFCs, but they are not very abundant and thus play a much
David R. Cook
Argonne National Laboratory
It is not the mass nor the weight of a molecule that makes it a greenhouse gas.
Greenhouse gases have just the right molecular size such that the long wave infrared
radiation emanating from Earth's surface resonates with the molecule.
Heat from the sun warms the surface of Earth. The warm surface radiates heat (long wave
radiation) back into the atmosphere. The long wave radiation interacts with molecules of
a specific size and structure; these types of molecules are known as the greenhouse
gases. The greenhouse gases (such as: CO2, CH4, CO, H2O, O3, NOx, and CFC) absorb the
heat radiated from Earth and warm the atmosphere.
You might have seen Ella Fitzgerald break a glass with her singing voice, or a video of
the Tacoma Narrows bridge collapse -- both are examples of resonance frequencies.
Matching the wavelength (infrared heat from Earth's surface) to the molecular size
(greenhouse gases), results in the molecules heating up, thus referred to as a
greenhouse gas that leads to global warming.
Professor of Geology
Department of Environmental Sciences
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Update: June 2012