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We used the Community Earth System Model, Version 1.2 (CESM1.2) (Hurrell et al., 2013), which includes the Community Atmospheric Model Version 5 (CAM5), the Community Land Model Version 4 (CLM4), the Community Ice Code Version 4 (CICE4), and the Parallel Ocean Version 2 (POP2). The CAM5 and CLM4 have a horizontal resolution of ~1°x1° and 30 vertical levels (Neale et al., 2010). The CICE4 and POP2 have 60 vertical levels, with a longitudinal resolution of 1° (the latitudinal resolution was ~1/3° near the equator and gradually increased to ~1/2° near the poles), with 60 vertical ocean levels (Smith et al., 2010).We performed two types of idealized climate model simulations. First, we integrated a 900-year long present-day (PD) control experiment with an atmospheric CO2 concentration of 367 ppm. We then conducted a CO2 ramp-up and ramp-down experiment for 500 years, branching from the 28 different initial conditions from the PD control. The CO2 concentration is increased by 1 % yr-1 for the first 140 years to quadruple the concentration (1468 ppm). It is then decreased symmetrically for another 140 years, so that the concentration recovers to its original level (367 ppm). Finally, we integrated an additional 220 years with the original CO2 level. Note that this experimental design is quite similar to the protocol of the Carbon Dioxide Removal Model Intercomparison Project (or CDRMIP; Keller et al., 2018), except for the initially set CO2 level.