- Working Groups
To simulate changes in solar irradiance through time, the model calculates the eccentricity, obliquity and precession based on Berger et al., 1993. The model sets these parameters automatically using a variable called orb_iyear_ad, which is set in the coupler namelist (cpl.buildnml.csh) and is defined in years before 1950.
a. AD values, i.e. 1800, 1900, 1950, 1990, 2000, etc.
- should be expressed with the explicitly named year.
example: orb_iyear_ad = 1800
b. BC values, i.e. 10ka, 130ka, 506ka
- should be expressed in years relative to 1950 (1950 - <time period of interest>)
o 506ka BP: orb_iyear_ad = (1950 - 506ka) = -504050
o 4ka BP: orb_iyear_ad = (1950 – 4000) = -2050
Since we have no good estimate of the orbital variations beyond ~1Ma BP, eccentricity, obliquity, and the moving vernal equinox must be defined explicitly for pre-Quaternary experiments.