This study investigates changes in the mixed layer depth (MLD) in the North Pacific Ocean in response to global warming and their impact on primary production by comparing outputs from 11 models of the coupled model intercomparison projects phase 3. The MLD in the 21st century decreases in most regions of the North Pacific, whereas the spatial pattern of the MLD is nearly unchanged. The overall shoaling results in part from intensified upper-ocean stratification caused by both surface warming and freshening.