climate hazards. However, the boundaries among these defini-
tions can blur, and concepts of complex climate change risk
continue to evolve.
30,31
Although some definitions refer only to
hazards or vulnerability, others take a more integrated perspec-
tive on interacting human and environmental systems.
1,37
Over-
all, these approaches indicate that risk may arise from a number
of pathways created by interacting drivers, and that understand-
ing the potential for either positive or negative outcomes
46
and
their severity requires appreciation of this network of interac-
tions.
30,31,47
These interactions may include events attributed
to anthropogenic climate change, such as a false spring;
31
other
human-induced events, such as conflict;
48
preconditions of risk,
such as saturated soil, which compounds extreme rainfall to
affect flooding;
31
and the systemic vulnerability of societies
reliant on complex electricity, communication, and transporta-
tion networks.
14,30,31
Other climate assessments are also
acknowledging complex risks; for example, multi-sector risk
assessment and management in the US Fourth National Climate
Assessment,
14
risk to health from multi-exposure pathways in
the US Global Change Research Program Climate and Health
Assessment,
49
interacting risks in the UK Climate Change Risk
Assessment,
13
and globally interconnected risks in the Global
Risk Report.
15
The need for transdisciplinary approaches to
complex climate change risk has also seen the development of
Figure 2. The diversity of complex climate
change risk terminology
Terms used to describe complex climate change
risk in recent IPCC Special Reports mapped onto
the IPCC risk framework used in these IPCC
Special Reports. White text shows terms used to
describe a given determinant of risk (that is, haz-
ard, exposure, and vulnerability). Black text shows
terms used to describe complex risk. Red text
highlights terms that have been used to describe
both risk and a determinant of risk, such as
‘‘compound risk’’ and ‘‘compound hazard.’’ Note
that this visual depiction of risk terminology does
not include the role of responses to climate
change affecting risk determinants or existing
risks or in driving new risks through positive or
negative side effects of responses.
new collaborations such as the My
Climate Risk Activity of the World Climate
Research Programme
50
and Future Earth
Risk Knowledge Action Network.
51
How-
ever, there remains no common frame-
work for assessment of complex climate
change risks.
This analysis of IPCC special reports
and other recent literature highlights
three important gaps where a more holis-
tic approach to climate change risk
assessment is needed. First, interacting
climate hazards are now a key focus for
risk assessment, especially for extreme
events such as concurrent heat and
drought; indeed, the IPCC definition of
compound risk focuses on ‘‘interaction
of hazards.’’
28
However, this physical sci-
ence effort on hazards has not yet been
integrated with the multiple interactions among ecological, so-
cial, and economic drivers of exposure and vulnerability. For
instance, low-income workers are often employed outdoors
and live in poorly ventilated housing, spend a greater portion of
their income on healthcare, and lose relatively more from missing
a day of work, all making them more vulnerable and exposed to
morbidity and mortality from heat waves.
52
Although integrating
quantitative and qualitative knowledge of interactions between
physical, ecological, and social systems remains challenging,
knowledge co-production approaches to complex risk assess-
ment that use integrated risk assessment models,
53,54
story-
lines, and scenario planning can highlight interactions across
system boundaries that generate risk not evident from more con-
ventional climate impact projections.
31,55,56
Second, responses to risk are often excluded as drivers of risk
even though they play a key role in driving potential outcomes,
including inaction, and are well recognized in financial and policy
domains.
37,57
Holistic consideration of risks related to climate
change impacts involving the real and perceived risks associ-
ated with response options is necessary in risk management
and decision-making processes.
53,58
Understanding response
options as part of climate change risk better explains why deci-
sion makers sometimes do not take actions to reduce risk arising
from climate hazards, for example, given risks related to
ll
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Perspective