24rsta.royalsocietypublishing.orgP h i l .T r a n s .R .S o c .A376:20160452 ........................................................
but also a result of the different forcings inﬂuencing the atmosphere model at the time of passing
each GWL, and the interaction with the climate sensitivity of HadGEM3. The radiative forcing
of non-CO 2 forcings has previously been highlighted as a potentially important inﬂuence on
patterns of climate change at 1.5°C and 2°C global warming [ 39]. Furthermore, despite some
differences in regional climate responses between ensemble members, there were also some
remarkable consistencies especially in the changes that might be considered inconsistent with
a warming climate, such as regions such as northern South America where heavy rainfall
(Rx5day) decreases rather increasing as might be expected under a warming climate. Again, these
consistencies point to some common forcing of all simulations.
One key factor is the different times of passing a particular GWL, because the net radiative
forcing would be different even though the same emissions and concentration scenario was used
in all simulations. A given GWL was reached at a different time in each ensemble member, so
the CO2 and aerosol concentrations vary between ensemble members; in members reaching a
GWL early, such as that driven by IPSL-CM5A-LR, the CO 2 concentration is relatively lower
than in other members, and the total aerosol concentration would be relatively higher (CO 2
concentrations are projected to increase in RCP8.5, but aerosol concentrations are projected
decline). The net radiative forcing is smaller, because in RCP8.5 the increase positive radiative
forcing from CO2 is greater than the decrease in net negative radiative forcing from aerosols.
Moreover, the physiological effect of CO2 is also smaller, meaning that the consequent reduction
in transpiration and associated additional land surface warming inﬂuence would also be expected
to be smaller.
Conversely, in members reaching the same GWL later, such as that driven by GFDL-ESM2M,
CO2 concentration is relatively higher, and aerosol concentrations are lower. So, net radiative
forcing, CO 2 physiological effects and the regional-scale radiative forcings from individual
aerosol types could, therefore, be quite different in the GFDL-driven HadGEM3 simulation when
it reaches 2°C global warming 25 years later than the IPSL-CM5A-LR-driven simulation.
The spatial pattern of changes in the different ensemble members may also play a role in
inﬂuencing the global mean changes, for example, with large changes in some regions due to
faster snow-melt or changes in cloud cover in one ensemble member leading to particular changes
in regional warming that are not seen in other ensemble members. Moreover, the individual
forcings of the different aerosol components such as sulfate and black carbon differ in sign and
spatial pattern, so the overall impact on local radiative forcing and hence regional temperature
patterns is more complex. Therefore, the global mean changes may not necessarily be expected to
relative to global mean forcings.
A further complexity in identifying precise mechanisms for regional changes is the
experimental design used here, with one atmospheric model and concentration/emissions
scenario but six different SST and SIC patterns, means that the impact of spatial heterogeneity in
radiative forcings is complex and involves a mix of effects in HadGEM3 and the original CMIP5
models. In the case of aerosols, for example, our HadGEM3 simulations are driven with RCP8.5
aerosol emissions and the aerosol concentrations are then calculated within the model itself. The
spatial distributions of aerosol optical depth and radiative forcing can, therefore, be expected
to be reasonably similar, because they arise from the same emissions scenario, although some
differences may occur due to the different regional climate-change patterns. However, the impact