Abstract Effective climate sensitivity (EffCS) is a key metric for quantifying climate change, yet its uncertainty remains large, primarily due to intermodel differences in cloud feedback. Here, we assess EffCS and climate feedback to an abrupt doubling (2xCO2) and quadrupling (4xCO2) of CO2 over 450 years in GFDL CM4.0. Both runs have comparable EffCS estimates during the first 50 years (2xCO2: 2.99 K; 4xCO2: 3.25 K) with the 4xCO2 EffCS estimate still slightly stronger for Years 51–150. Extending the analysis to 450 years increases the EffCS estimate, but more so in 2xCO2 (5.8 K) than in 4xCO2 (4.79 K). In both runs, the EffCS increases due to a weakening of the net feedback with warming, largely through shortwave cloud feedback increase. A weaker EffCS during the slow response for 4xCO2 relative to 2xCO2 is associated with a weaker surface albedo feedback due to an earlier sea‐ice loss in both hemispheres.