2rsta.royalsocietypublishing.orgP h i l .T r a n s .R .S o c .A376:20160452 ........................................................
Model Intercomparison Project (CMIP5) ensemble, forced with the RCP8.5 concentration
scenario. To provide more detailed representations of climate processes and impacts, the
spatial resolution was N216 (approx. 60 km grid length in mid-latitudes), a higher resolution
than the CMIP5 models. We used a set of impacts-relevant indices and a global land surface
model to examine the projected changes in weather extremes and their implications for
freshwater availability and vulnerability to food insecurity. Uncertainties in regional climate
responses are assessed, examining ranges of outcomes in impacts to inform risk assessments.
Despite some degree of inconsistency between components of the study due to the need to
correct for systematic biases in some aspects, the outcomes from different ensemble members
could be compared for several different indicators. The projections for weather extremes
indices and biophysical impacts quantities support expectations that the magnitude of change
is generally larger for 2°C global warming than 1.5°C. Hot extremes become even hotter, with
increases being more intense than seen in CMIP5 projections. Precipitation-related extremes
show more geographical variation with some increases and some decreases in both heavy
precipitation and drought. There are substantial regional uncertainties in hydrological impacts
at local scales due to different climate models producing different outcomes. Nevertheless,
hydrological impacts generally point towards wetter conditions on average, with increased
mean river ﬂows, longer heavy rainfall events, particularly in South and East Asia with the
most extreme projections suggesting more than a doubling of ﬂows in the Ganges at 2°C global
warming. Some areas are projected to experience shorter meteorological drought events and
less severe low ﬂows, although longer droughts and/or decreases in low ﬂows are projected
in many other areas, particularly southern Africa and South America. Flows in the Amazon
are projected to decline by up to 25%. Increases in either heavy rainfall or drought events
imply increased vulnerability to food insecurity, but if global warming is limited to 1.5°C, this
vulnerability is projected to remain smaller than at 2°C global warming in approximately 76%
of developing countries. At 2°C, four countries are projected to reach unprecedented levels of
vulnerability to food insecurity.
This article is part of the theme issue ‘The Paris Agreement: understanding the physical and
social challenges for a warming world of 1.5°C above pre-industrial levels’.
1. Introduction
The majority of climate-change impacts assessments have tended to be framed in terms of future
time horizons, e.g. impacts by the middle or end of the twenty-ﬁrst century [ 1,2]. However,
with international climate policy now largely focused on limiting warming to speciﬁc levels of
global mean temperature such as 2°C [3] or 1.5°C [4], policy-relevant climate impacts assessments
increasingly need to be framed in terms of such warming levels.
There are two major research questions concerning the impacts of climate change at 1.5°C and
2°C global warming, which are relevant to both mitigation and adaptation policy areas.
(i) How much larger are the impacts at 2°C compared to 1.5°C? This is the primary question
arising from the Paris Agreement [ 4] and is relevant to mitigation policy, informing
judgements and actions on holding the global temperature rise to ‘well below 2°C’ and
‘pursuing efforts to limit the temperature increase to 1.5°C’.
(ii) What regional climate conditions and related hydrological and ecological conditions
could occur at a particular level of global warming, such as 2°C? This is relevant to
adaptation policy and planning—exploring the possible outcomes for these levels of