23rsta.royalsocietypublishing.orgP h i l .T r a n s .R .S o c .A376:20160452 ........................................................
IPSL-CM5A-LR GFDL-ESM2M
IPSL-CM5A-MR
0
%
5–5–10–15–20 10 15 20
ensemble mean
MIROC-ESM-CHEM ACCESS1-0
HadGEM2-ES
Figure20. Differencebetween2°Cand1.5 °Cglobalwarminginpercentagechangesinmean(top)run-offinJULESsimulations
drivenbytheensembleofHadGEM3simulations.Notethattheuseofpercentagechangesemphasizeschangesinregionswhere
thebaselinestreamflowissmall.
The largest regional differences between 2°C and 1.5°C global warming tend to be in the
regions where the local impact is largest relative to the baseline. For TXx this is generally the mid-
latitudes, whereas for TX90p it is generally the tropics. So, broadly, the impacts at 1.5°C global
warming could be estimated by scaling-back the impacts at 2°C.
These results show some similarities with those from the CMIP5 models [ 9,38], but also some
notable differences. The CMIP5 models were at lower spatial resolution than the models used
here. Although the general patterns of change in TXx are broadly similar in our study and
CMIP5, with greater warming in many continental interiors, is notable that our results show more
marked geographical variation than those from CMIP5 projections ([ 9], among others), with the
continental interior warming being more intense in our projections. In particular, our results with
HadGEM3 show more intense increases in maximum temperature in North America and Europe.
Our projections of changes in consecutive dry days (CDD) broadly consistent with those found
in a subset of the CMIP5 ensemble [ 9], although there are some differences. Our ensemble mean
suggests shorter dry spells in the central Amazon, whereas ISIMIP-indicated longer dry spells.
Also, as with the temperature indices, our results show greater geographical differentiation in the
intensity of changes.
The decrease in Rx5day in some regions in our simulations contrasts with the subset of
CMIP5 models used for the ISIMIP Fast-Track projections [ 9] which suggested an increase in
Rx5day almost everywhere where at least 66% of the model ensemble agreed on the sign of the
change, including all of northern South America. The reasons for these differences require further
investigation, but some insight into possible reasons may be gained by examining the similarities
and differences between our own individual ensemble members.
For all the CLIMPAct variables, the variations in global means between the ensemble members
were consistent at 1.5°C and 2°C. That is, the members with the largest changes at 2°C also showed
the largest changes at 1.5°C, and the same was true for the smallest changes, and the relative
proportions of changes in other ensemble members. This suggests that variations between the
ensemble members at any particular GWL were not merely a consequence of internal variability