- Working Groups
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.