Understanding Internal Climate Variability and the Upper Limit of 2°C Increase of Global Mean Surface Temperature

Paris Agreement in 2015 chooses the upper limit of 2˚C increase of global mean surface temperature after the pre-industrial climate. Therefore, it is important to understand which factors are associated with an increasing rate of global surface temperature in a warming climate. To examine this, we analyze the Community Earth System Model Large Ensemble (CESM-LE) dataset under Representative Concentration Pathway (RCP) 8.5 (30-ensemble member) and 4.5 scenarios (15-ensemble member), respectively. Obviously, the period when the ensemble mean surface temperature reaches to 2˚C (i.e., 2˚C-target period) is faster in the RCP8.5 than that in the RCP4.5. In addition, it is found that the trend of surface temperature simulated in the RCP8.5 is larger in the Northern Eurasia than that in the RCP4.5 in the 2˚C-target period, however, it is larger in the RCP4.5 in the Arctic than that in the RCP8.5. We further analyze how internal climate variability simulated in the RCP8.5 and RCP4.5, which is defined as the major variability of surface temperature deviated from the ensemble mean surface temperature, is associated with such a difference.