Near-modern CESM1.0 | Orbital Parameterization

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.,


          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.