To set the greenhouse gas forcings, you must first understand the namelist variables associated with them. See CAM Namelist Variables for a complete list of CAM namelist variables.
Controls treatment of prescribed co2, ch4, n2o, cfc11, cfc12 volume mixing ratios. May be set to 'FIXED' or 'RAMPED' or 'RAMP_CO2_ONLY'.
FIXED => volume mixing ratios are fixed and have either default or namelist input values.
RAMPED => volume mixing ratios are time interpolated from the dataset specified by bndtvghg.
RAMP_CO2_ONLY => only co2 mixing ratios are ramped at a rate determined by the variables ramp_co2_annual_rate, ramp_co2_cap, and ramp_co2_start_ymd.
Full pathname of time-variant boundary dataset for greenhouse gas surface values.
If scenario_ghg is set to "RAMPED" then the greenhouse gas surface values are interpolated between the annual average values read from the file specified by bndtvghg. In that case, the value of this variable (> 0) fixes the year of the lower bounding value (i.e., the value for calendar day 1.0) used in the interpolation. For example, if rampyear_ghg = 1950, then the GHG surface values will be the result of interpolating between the values for 1950 and 1951 from the dataset. Default: 0
Edit user_nl_cam and add the following lines at the end of the file. The following assumes that "my_inputdata_path" is identical to $DIN_LOC_ROOT.
scenario_ghg = 'RAMPED' bndtvghg = 'my_inputdata_path/atm/cam/ggas/ghg_hist_1765-2005_c091218.nc' rampyear_ghg = 2000