The variability of global monsoon area (GMA), global monsoon precipitation (GMP), and global monsoon intensity (GMI) in the present climate (1979–2003) and the future warmer climate (2075–2099) under Representative Concentration Pathways 4.5 (RCP4.5) scenario was examined based on 19 Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. In the present-day simulations, the ensemble mean precipitation reproduces the observed GMA, GMP, and GMI, although the spread of individual models is large. In the RCP4.5 simulations, most (17 of 19) of the CMIP5 models project enhanced global monsoon activity, with the increases of GMA, GMP, and GMI by 1.9%, 3.2%, and 1.3%, respectively, per 1 K of surface warming. The diagnosis of a column-integrated moisture budget indicates that the increase in GMP is primarily attributed to the increases of moisture convergence and surface evaporation, whereas horizontal moisture advection has little effect. A further separation of dynamic and thermodynamic factors shows that increase of the moisture convergence comes mainly from the increase of water vapor, but is partly offset by the convergence effect. The increase of the surface evaporation is caused by the increase of sea-air specific humidity difference, while the change in surface wind speed plays a minor role. The GMP experiences a great year-to-year variation, and it is significantly negatively correlated with the Niño3.4 index averaged over a typical monsoon year (defined from May to the following April) in the pre-industrial control and present-day simulations, similar to observations. Under the RCP4.5 warming, such rainfall variability is intensified, and the relationship between monsoon and El Niño strengthens. A large proportion of intensification in the year-to-year monsoon rainfall variability arises from the land monsoon region.