Fig. 12: Gregory plot (Gregory et al., 2004) from the abrupt-4xCO2 compared to the piControl simulation. For each year, the near-surface (2m) air temperature change between abrupt-4xCO2 and piControl simulation is plotted against the change in net downward radiative flux between the two simulations. The more the abrupt-4xCO2 simulation approaches the equilibrium, the smaller the difference in net downward radiative flux compared to the reference simulation becomes. To compute the initial radiative forcing, a regression is built from all data points and extrapolated to a change in near-surface air temperature of 0°C. α is the climate response parameter, indicating the strength of the climate system’s net feedback (radiative feedback divided by temperature response). To compute the equilibrium temperature difference, the regression is extrapolated to the equilibrium (difference in net shortwave radiation = 0).

5.2 Surface response

5.2.1 2 m temperature and precipitation

The evolution of the global and hemispheric mean temperature at 2 m above the surface in the piControl, historical, and scenario simulations is shown in Fig. 13. The piControl simulation shows no discernible trend in temperature, as expected. When considering the anthropogenic forcing, the historical simulations show a warming of 1.1 ± 0.1°C in 2005–2014 compared to 1891–1900 while for the observations the warming amounts to 0.9°C over the same period. Both in the observations and in the historical simulations the Northern (Southern) Hemisphere warming is 0.2°C higher (lower) than the global average. The more pronounced warming over the Northern Hemisphere compared to the Southern Hemisphere is partly due to the higher land partition in the Northern Hemisphere compared to the Southern Hemisphere.
Until the end of the 21st century, the global mean temperature rises by approximately 4°C from today under the strongest emission scenario SSP585. Over the Northern Hemisphere this warming is more pronounced and amounts to approximately 5°C; over the Southern Hemisphere the warming is limited to approximately 3°C. For the weakest emission scenario, SSP126, the global mean warming remains just below 2°C compared to pre-industrial conditions. The SSP126 scenario has been designed to keep global warming below 2°C – a condition that seems to be fulfilled in our simulations. Overall, the temperature increase in the AWI-CM simulations for both the strongest and the weakest emission scenario agrees with the CMIP5 multi-model ensemble mean (IPCC, 2014, their Fig. SPM.6a) and appears to be slightly stronger compared to the CMIP6 version of MPI-ESM - which is expected due to the slightly higher transient climate response in AWI-CM compared to MPI-ESM.
(a) (b)