Uncertainty in the 2℃ Warming Threshold Related to Climate Sensitivity and Climate Feedback

Climate sensitivity is an important index that measures the relationship between the increase in green-house gases and the magnitude of global warming. Uncertainties in 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 thoroughly review the literature on climate sensitivity, and discuss issues related to climate feedback processes and the methods used in estimating the equilibrium climate sensitivity and transient climate response (TCR), including the TCR to cumulative CO2 emissions. 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 2℃ warming. Challenges that call for further investigation in the research community, in particular the Chinese community, are discussed.