CLM2.1 namelist inputs are presented in sections 4.1-4.6 below. In what follows, "mode" has values of "offline", "ccsm", "cam" or "all", corresponding to offline mode, ccsm mode, cam mode, or all the modes. If a namelist variable setting is listed as required, the value must be set in the namelist in order for the model to execute successfully. If a setting is specified as required and the mode is only given as offline, then that variable must only be specified when running in offline mode. For namelist variable settings not listed as required, the code will provide default settings at initialization. In the following variable descriptions, we refer to examples presented in section 7.