North-South Asymmetry of the 1900-2015 Warming

We use the statistical regression in the inference mode to analyze the hemispherical 1900-2015 warming rates and to identify significant predictors for the northern (NH) and the southern hemisphere (SH). Temperature data indicate 2000-2015 slowdown of the warming in both hemispheres compared to the 1975-2015 warming trends, from 0.24 to 0.11oC/decade for the NH and from 0.11 to 0.04oC/decade in the SH. Using the backward predictor selection rule we find that the GHGA (anthropogenic greenhouse gases and aerosols) and the AMO (Atlantic Multidecadal Oscillation) are significant predictors for the NH temperature, while the GHGA and SOL (solar irradiance) are significant predictors for the SH variability. The AMO cannot be ignored from the NH temperature predictors without a significant decrease of the multiple R2 from 0.98 to 0.89. The amplitude of the AMO contribution towards the NH temperature is ±0.2oC, while the amplitude of the SOL contribution to the SH temperature is within ±0.1oC. Within the recent warming period (1970-2015) the anthropogenic radiative forcing (GHGA) contributes about the same rate of warming to both the hemispheres: 0.13oC/decade for the NH and 0.12oC/decade to the SH. The NH has an additional contribution due to the AMO of 0.09oC/decade while the SH experienced cooling about 0.01oC/decade due to decrease of solar irradiance. The total hemispherical rate of warming determined by the regression is thus 0.22oC/dec and 0.11oC/dec for the NH and the SH respectively. Those rates are very close to the observed rates of 0.23oC/dec for the NH and 0.11oC/dec for the SH. The IPCC recommended an increase of the GHGA radiative forcing by about 1.7Wm-2 between 1970 and 2015, implies an increase in radiative forcing rate of about 0.38Wm-2/decade. Extrapolating these results linearly using our regression analysis we estimate a NH warming of about 1.3oC for CO2 doubling (radiative forcing of about 3.8Wm-2) and a corresponding warming of about 1.2oC of the SH. This leads to the NH warming of about 1.3oC at the time of CO2 doubling (radiative forcing of about 3.8Wm-2) and a corresponding warming of about 1.2oC of the SH. The AMO and SOL contributions will be a smaller fraction in the future warming as the secular human CO2 concentration grows.