Globally averaged surface air temperatures in some decades show rapid increases (accelerated warming decades, such as the climate shift in the mid-1970s) and in other decades there is no warming trend (hiatus decades, such as the first decade of the 21st century). In this paper, processes involved with accelerated warming decades are identified, and the relative roles of external forcing from increasing greenhouse gases and internally generated decadal climate variability associated with Interdecadal Pacific Oscillation (IPO) are addressed. Model results from CCSM4 show that accelerated warming decades are characterized by rapid warming of globally averaged surface air temperature and greater increases of heat content in the upper ocean layers and less heat content increase in the deep ocean, opposite to the hiatus decades. Internally generated hiatus periods of up to 15 years with zero global warming trend are present in the future climate simulations. This suggests that there is a chance the current observed hiatus could extend for several more years.