The climate sensitivity is an important index that measures the relationship between the increase of greenhouse gasses and the magnitude of global warming. Uncertainties in the climate change projection and climate modeling are mostly related to the climate sensitivity. The climate sensitivities of coupled climate models determine the magnitudes of the projected global warming. In this paper, the authors have thoroughly reviewed the studies of climate sensitivities, and discussed the issues related to climate feedback processes, the methods used in estimating the equilibrium climate sensitivity (ECS) and the transient climate response (TCR),including the TCR to cumulative CO_2 emissions (TCRE). After presenting a summary of the sources that affect the uncertainty of climate sensitivity, the impact of climate sensitivity on climate change projection is discussed by addressing the uncertainties in the 2°C warming issue. Challenges that call for further researches are also discussed.