This study investigates the changes of landfalling atmospheric rivers (ARs) over the west coast of North America in response to future warming using the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble. The consistency between CMIP5 models and four reanalysis data sets in reproducing the historical ARs indicates the capability of CMIP5 capturing the AR events. The result shows a strikingly large increase of AR days by the end of the 21^st century under Representative Concentration Pathways (RCP) 8.5 scenario, with fractional increases of ~50% to 600%, depending on the seasons and landfall locations. By applying scaling method to the water vapor based on future changes of water vapor or temperature, we identified the contributions of thermodynamics and dynamics on changes of ARs. The increases of ARs are predominantly controlled by the super-Clausius-Clapeyron rate of increase of atmospheric water vapor with warming, while changes of winds that transport moisture in the ARs, or dynamical effect, mostly counter the thermodynamical effect of increasing water vapor. The consistent negative effect of wind changes on AR days during spring and fall can be linked to the robust poleward shift of the subtropical jet in the North Pacific basin.