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A detailed investigation of these factors is beyond the scope of this paper; nevertheless, this
result illustrates the important point that the nature and patterns of the climate forcing at a
particular level of global warming can play an important role in determining the patterns of
regional impacts.
5. Conclusion
The higher-resolution HadGEM3 simulations project consistent increases in temperature-related
extremes, with larger changes at 2°C compared to 1.5°C and local changes being larger than the
global annual mean. There is a higher degree of spatial variation in our projections compared
with CMIP5-based studies.
In the model projections examined here, changes relating to the water cycle are complex, both
in their geographical pattern and in the variation between different models. The length of ﬂooding
events generally increases across world in all models, but maximum rainfall can either increase or
decrease depending on locations. Global patterns of increase and decrease show some consistency
between the different GWLs, but also some local differences. Worldwide, most impacts broadly
tend to increase with global warming in most areas. For global mean changes, even when the sign
of change is uncertain, individual realizations generally show reduced impact at 1.5°C compared
with 2°C. However, this does not always hold even at the scale of major global river basins.
Vulnerability to food insecurity increases more at 2°C global warming than 1.5°C in
approximately three-quarters of countries assessed. The vulnerability increase can arise from
increases in either ﬂooding or drought. Reduced drought leads to decreased vulnerability in a
limited number of cases.
Most simulations here project a general increase in mean streamﬂow in most of the basins
examined, but with a number of notable exceptions in the tropics. While ﬂows in the Ganges are
consistently projected to increase by 30–110% at 2°C, Amazon ﬂows could either increase by 3%
or decrease by 25%. Ensemble-mean changes in river ﬂow often do not give a full impression of
the magnitude of changes that may be possible, so adaptation planning in particular should not
rely on ensemble-mean projections and instead consider a range of outcomes. The seasonal low
streamﬂows also increase in many basins, but not as many as for the mean ﬂows—many basins
see decreased low ﬂows in some or all projections.
Broadly, changes in weather extremes at 1.5°C global warming could be estimated by scaling-
back the impacts at 2°C, if this is done with individual ensemble members rather than the
ensemble mean. However, this was not always the case for impacts that depend on more complex
process or interactions between more than one climate variable, such as run-off and an indicator
of vulnerability to food insecurity.
Dataaccessibility. This article has no additional data.
Competinginterests. We declare we have no competing interests.
Funding. This research received funding from the European Union Seventh Framework Programme FP7/2007–
2013 under grant agreement no. 603864 (HELIX: ‘High-End cLimate Impacts and eXtremes’; www.
helixclimate.eu). The work of R.A.B., C.B., J.C., L.G., K.L. and K.R. was additionally supported by the Joint
UK BEIS/Defra Met Ofﬁce Hadley Centre Climate Programme (GA01101).
Acknowledgements. The authors thank Ed Pope, Jason Lowe and Dann Mitchell for advice and discussion,
Alissa Haward and Maria Pearce for project management and administration of HELIX, and two anonymous
reviewers whose comments substantially improved the paper.
References
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