2 and 4×CO 2 experiments are 100 years long. In response to instantaneous CO 2 doubling and quadrupling, global mean surface air temperatures increase toward the end of the simulations by 2.5° and 5.1°C (fig. S1A), whereas ocean temperatures increase by 1.8° and 3.8°C. The top of the atmosphere (TOA) radiation imbalance reduces from around 1 to 0.2 W m −2 in the PD experiment (fig. S1B). Initially, there is a large radiation imbalance in response to greenhouse gas forcing, but it weakens gradually to around 1 W m −2 in the 2×CO 2 and 2 W m −2 in the 4×CO 2 experiments, indicating that the coupled system slowly approaches near-equilibrium conditions. However, we emphasize that a complete equilibration would take up to several hundred years. Global mean precipitation averaged over the last 20 years of the simulations increases by 3.8 and 7.1% for 2×CO