<p>The newest iteration of the Canadian Earth System Model (CanESM5.0.3) has an Effective Climate Sensitivity (ECS) of 5.65 kelvin, which is a 54 % increase relative to the model's previous version (CanESM2 – 3.67 K), and the highest sensitivity of all current models participating in the sixth phase of the coupled model inter-comparison project (CMIP6). Here, we explore the underlying causes behind CanESM5's increased ECS via comparison of forcing and feedbacks between CanESM2 and CanESM5. We find only modest differences in radiative forcing as a response to CO<sub>2</sub> between model versions. Through the use of cloud area fraction output and radiative kernels, we find that more positive shortwave cloud feedbacks – particularly with regards to low clouds across the equatorial pacific, as well as sub/extratropical free troposphere cloud optical depth – are the dominant contributors to CanESM5's increased climate sensitivity. Additional simulations with prescribed sea surface temperatures reveal that the spatial pattern of surface temperature change explains the pattern of change in low cloud fraction, but does not fully explain the increased ECS in CanESM5. The results from CanESM5 are consistent with increased ECS in several other CMIP6 models, which has been primarily attributed to changes in shortwave cloud feedbacks.</p>