Natural emissions code consistency

[GlenPetersCICERO]

There are potentially a few inconsistencies in the code.

• methane_lifetime: The 1751 should be the conc_CH4 in year 2000, but it is hardwired
• methane_lifetime: The reference year is hardwired to 2000, not sure if that is meant
• tropospheric_ozone_forcing: The concentration is hardwired at 1832 for the reference year
• tropospheric_ozone_forcing: Uses the variable "ref_year", default of 2010, which is different approach to methane_lifetime, even though the equation is conceptually the same
• methane_lifetime: The code is constructed so that in the year 2000 the perturbation and atmospheric lifetimes equal, not sure if this is correct. It means the perturbation lifetime is around 9 years and not around 12 years (as expected).
• Any changes made above would need to be propagated into the Natural Emissions code, as the two are meant to be similar
• The methane adjustments are based on IPCC TAR, which is rather old. Not sure if updated?

[GlenPetersCICERO]

The Wigley factor in the code, 0.238, in methane_lifetime is the original implementation in the CICERO SCM, back in 1999. See Fuglestvedt and Berntsen 1999 (CICERO SCM documentation)