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, which is set in the coupler namelist (user_nl_cpl) and is defined in years before 1950. You can also set the orbital year to track simulated model year. For details see the CESM1.x namelist documentation for the driver/coupler (e.g., http://www.cesm.ucar.edu/models/cesm1.2/cesm/doc/modelnl/nl_drv.html)
a. AD values, i.e. 1800, 1900, 1950, 1990, 2000, etc.
- should be expressed with the explicitly named year.
example: orb_iyear = 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 = (1950 - 506ka) = -504050
o 4ka BP: orb_iyear = (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.