To simulate changes in Earth's orbit through time, the model calculates the eccentricity, obliquity and precession based on Berger et al., (J. Geophys. Res.1993). The model sets these parameters automatically using a variable called orb_year, which is set in the coupler namelist (cpl.buildnml_prestage.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_year = 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_year = (1950 - 506ka) = -504050
o 4ka BP: orb_year = (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 expressed individually for pre-Quaternary experiments. Details on how these variables are computed can be found by reviewing the code in the CCSM3 csm_share subdirectory.