Why does methane have a Global Warming Potential of 21?

Question

According to the Intergovernmental Panel of Climate Change 1kg of methane has the equivalent effect on the environment as 21kg of CO2.

Is this because methane absorbs more terrestrial infrared radiation? If yes, why?

Any help would most appreciated.

Answer 1

Methane also lasts longer in the atmosphere. If it's there longer, it does more damage during that time.

Answer 2

It is 21 time more of a "GreenHouse" gas than that of carbon dioxide.  Which in essence means if GreenHouse gas were glass reflecting the Sun's rays then CO2 would be clear glass and Methane would be a Two way mirror.  Letting the Sun's rays through to the Earths surface then reflecting the same rays back to the Earth's surface.

Answer 3

The amount of reflection/ bending that light goes through when passing through a given gas determines the effect it has in trapping long wave radiation. These have been measured and compared to that for CO2, so that we have a system of CO2 equivalencies for gases.

If we get large scale release of arctic methane, as a result of melting, there will be no way we can return our atmosphere to a state that would stop global warming by reducing our emissions.

Answer 4

Yes, it's because it absorbs more infrared.

It's because of the exact nature (quantum mechanics is involved here) of the bonds between carbon and hydrogen in methane.

The bonds are what absorb infrared.  The kind of act like springs and may vibrate (or the molecule may rotate) as a result of absorbing energy.

The bonds in methane are more effective at absorbing infrared.

Answer 5

First there must be some errors in there calculation.They think there are billions of cubic ft in the atmosphere ,but I can not find it. Methane is very light so it should go very high in our atmosphere. It is also explosive and the first jet to run into it will get its wings blown off. The absorption figure doesn't make sense if u can not find the methane.

Answer 6

There are three factors to consider.  

First, to be an effective greenhouse gas, a molecule has to absorb in the part of the infrared region that corresponds to the earth's thermal emission spectrum.  The earth's thermal emission approximates that of a black body at approximately 290 K.  You can calculate and plot the black body curve on a spreadsheet. I provided a link to hyperphysics to give the formula and an explanation.  The area under the curve is the total power radiated.  An absorption near the peak of the black body curve has the potential to remove a larger fraction of the radiated power than an absorption at higher or lower frequencies.  

The second factor is the absorption cross section of of the molecule as a function of frequency.  A larger the cross section means a higher probability of an absorption event.  The CO2 and CH4 spectra are given in the references.  The key features to look at are the CO2 absorption band near 670 cm-1 and the CH4 absorption band near 1310 cm-1.  The central frequency is determined by the vibrational quantum number.  Notice that both bands contain a large number of sharp peaks around the central peak which correspond to different rotational states of the molecules.  You can read off the rotational quantum number J by counting the number of peaks from the central frequency and taking into account selection rules which depend upon the molecular symmetry.  The key point to note is that the integrated area of the CH4 peaks is larger than the integrated area of the CO2  peaks (note the difference in scale on the y axis when you do this).  

The third factor to consider is the lifetime of the molecule in the atmosphere.  The half life of CH4 is 7 years, whereas the half life of CO2 is about 72 years.  CH4 is more potent at short times, but its effect decays faster.  

The quoted GW potential is calculated by averaging all three factors over 100 years.

Answer 7

Sorry, but Intergovernmental Panel of Climate Change is a bunch of people who make things up for no apparent reason except to make the States look bad.

In order for methane to have a GWP of 21, global warming has to exist.

And CO2 is a naturally occuring gas, which most scientists believe is a result of global warming, not a cause!

The same scientist were warning us of "global cooling" 20 years ago. Maybe it is a 1/2 degree hotter than it was 30 years ago, but Greenland used to be green 1000 years ago, and now it is all ice.

Who are we to say  what the right temperature is? Should we try to find out the climate at the beginning of man, and reinstate those temperatures? We are discovering new species all of the time, but everyone wants to focus on how we killed off so many things with our crazy exhaust fumes. If we rivive wooly mammoth DNA a-la Jurrasic Park, will everything be better?

Global warming is a religion for people who refuse to accept that life existed more than 100 years ago. Refusal of history is a great fuel for any cult.

Answer 8

Now check out PLAIN OLD water vapor!