Role of methane in global warming I am a young student in physic and I am doing a little presentation on how does climate change works. So I went on the wikipedia page for greenhouse gases.

I don't understant why methane is more dangerous for the climate change than carbon dioxyde because the rays that methane absorbe are the same as water vapor, and water vapor absorbe it more. So I don't understand why methane play such a role.
If someone have an answer I would be very grateful
 A: Before I give my answer, I need to comment on two points above.
First, contrary to a comment made above, methane IS building up in the atmosphere, just not at a large rate.  See figure 2 here from NOAA.
Second, contrary to the OP's statement, methane absorption bands do not significantly overlap those of water.  Go to this website and click on the "IR Windows Learning Tool".  In that window you can overlay water and CH4 and see what I mean.
I've been interested in the science behind climate change for a few years, but a simple answer to your question is not easily found, as I found out in trying to compose this response.  So - good question!
Now it is well-known that the change in radiative forcing, $\Delta F$, for CO2 is roughly logarithmic: $$\Delta F=5.35ln{\frac{C}{C_0}}$$  You can see this formula in table 1 from the NOAA link I gave above.  Here $C_0$ is the reference concentration and C is the concentration of interest.  Thus folks like to say that every doubling of CO2 in the atmosphere gives the same change in radiative forcing.  The reason behind this logarithmic behavior is not straight-forward.  Though the paper referenced here presents a specific model of the atmosphere to obtain this logarithmic dependence, it is plain the model contains such features as absorber concentration and absorption cross-section which play a role in the optical depth, $\tau$.
As seen also in table 1 from the NOAA reference, the change in radiative forcing for CH4 is more dependent on the difference in the square root of the concentration of interest relative to the reference concentration.  From what I can gather, this is a distinction without much of a difference as they are both less than linear.  It would be assumed also that, as with CO2, absorber concentration and absorption cross-section play similar roles in the mathematical form here as well.
Currently the amount of CO2 in the atmosphere is around 410 ppm, while that of CH4 is around 1.8 ppm.  Thus, relatively speaking, it takes very little change in CH4 to double its change in radiative forcing as compared to CO2.  Thus, the change in radiative forcing can climb very quickly for CH4.
The IPCC have developed the concept of "Global Warming Potential (GWP)", which tries to compare the strength of a well-mixed green house gas with that of CO2 (technically it compares a pulse into the atmosphere of the gas in question with a pulse of equal mass of CO2).  The concept is discussed technically in AR5, Chapter 8, pages 710-712.  As with many climate concepts, this one can be troublesome if you don't understand what it leaves out, and I am certainly no expert here.  The first paragraph on page 711 is good reading on this point.
The take-home point of GWP with respect to CH4 is the black curve in figure 8.29, reproduced below.  There we see the off-quoted number that over a 100 year span, the GWP of CH4 is roughly 28x that of CO2.
I know this was probably not the simple answer you were looking for, but I hope this provides some help.

