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)