On Mon, 26 Nov 2007 19:42:52 -0800 (PST), "Phil."
wrote:

snip
The reaction that removes CH4 from the atmosphere is with the hydroxyl
radical which gives an atmospheric lifetime of ~10 years, however OH
is only present at low concentrations so a sudden increase in CH4
could lead to a much longer lifetime due to a deficiency in OH.
snip

I imagine, though I cannot say if I've actually read a report on it,
that CO from incomplete combustion since humans started using fossil
fuels on a large scale has also competed for atmospheric hydroxyls,
which exist in small quantities (I would guess -OH is created by photo
disassociation of H2O or from lightning and similar discharges.) If
so, the CO competition for hydroxyl scrubbing might also contribute to
that longer lifetime of CH4.

Jon

--
Man will never be free until the last king is strangled
with the entrails of the last priest. [Denis Diderot]