> So I guess one should assume that the numbers in the file you linked to are some mole fractions which are probably the number of molecules of CH4 divided by the number of molecules of dry air found in one fixed volume.
Yes, that sounds right.
> click for details ?
Yeah, click your mouse on the picture to get more details. I try to do that with all important pictures on Azimuth.
> it looks as if the graphics doesn’t show the southern hemisphere - I could imagine that it is somewhat symmetric with respect to north and south (i.e. that it would ascend again, where the word “global methane” is.)
No, as Nathan pointed out the graph is very _asymmetrical_.
There are a lot of bogs in Siberia and Canada, which release methane as the dead vegetable matter rots - and a lot of frozen soil containing organic material (permafrost) in these regions, which rots as the soil warms and melts. But Antarctica is very different. For one thing, instead of a small ocean surrounded by land full of bogs, you've got a small continent covered with a thick layer of ice, surrounded by ocean.
You can read a bit about these issues on Azimuth here:
* [[Arctic methane deposits]]