3rsta.royalsocietypublishing.orgP h i l .T r a n s .R .S o c .A376:20160452 ........................................................
A combination of the above questions is also relevant—how does the range of outcomes at 2°C
compare to that at 1.5°C? This is also relevant to adaptation policy, as it can inform assessment on
whether to adapt to potential impacts at 2°C or just 1.5°C. Putting in place adaptation measures
to deal with potential impacts at 1.5°C and then increasing these to deal with 2°C later may
be more expensive and difﬁcult than adapting to potential risks at 2°C at the outset. On the
other hand, because adaptation actions may themselves have consequences, unnecessary over-
adaptation may have undesirable effects which it may be preferable to avoid or at least delay
until absolutely necessary.
Both questions require an appropriate assessment of uncertainty. There are considerable
uncertainties in projections of regional climate change, with different climate models projecting
regional climate changes that can differ in magnitude or even, in the case of precipitation
and impacts quantities strongly related to this, differ in sign [5,6]. This may have important
implications for regional impacts at speciﬁc levels of global warming. A common approach to
exploring and presenting such uncertainties is to examine the ensemble mean and the level of
consensus among the ensemble members on the sign of the change. While this can often be useful
in informing an assessment of the level of conﬁdence in future projections, it may not always be
sufﬁcient to fully inform decisions. Risk assessment approaches require consideration of a range
of possible risks, not just the most likely. This paper explores a range of regional climate states
and related impacts that occur at global warming of 2°C, and a range of differences with warming
limited to 1.5°C.
We examine the implications of our new climate projections by applying some commonly
used indices of climate extremes, and a further index quantifying relative vulnerability to food
insecurity which combines climate extremes indices with information on a range of factors
representing sensitivity and adaptability of food systems to climate hazards. We also use the
climate projections to drive a global land surface model to simulate changes in run-off as
an indicator of freshwater availability. We assess whether regional extremes are projected to
increase or decrease at 2°C global warming, and whether the consequent impact on drought and
vulnerability to food insecurity become greater or smaller. We also assess whether these changes
are reduced by limiting global warming to 1.5°C. We explore some of the uncertainties in these
projections, and, in particular, examine whether the use of ensemble-mean projections is a useful
simple guide to impacts projections or whether this can lead to a misleading impression for some
impacts. Regarding vulnerability to food insecurity, we consider the impacts of global warming
at 1.5°C and 2°C alongside socio-economic inﬂuences that affect the sensitivity to climate change.
We also consider our climate-change impacts results in comparison with other studies using older,
lower-resolution climate projections.
A large number of previous studies have assessed potential impacts of future climate change
using the 5th Coupled Model Intercomparison Project (CMIP5) ensemble or subsets of this [ 7],
and some have framed this in terms of impacts at global warming of 1.5°C and/or 2°C [ 8,9]. We
also base our study on a subset of CMIP5 projections, but use a new, higher-resolution atmosphere
model to provide greater spatial detail and improved representation of atmospheric processes.
2. Methodsandmodels
(a) Globalclimatesimulationsat1.5 °Cand2 °Cglobalwarming
There are a number of ways in which 1.5°C or 2°C global warming can be deﬁned—one could
be the long-term climate state following a stabilization of warming at that level, another could