Workshop Report - IPCC

Annex I: Paper
Radiative forcing
Socio-economic
reference pathway
Policycontext
Figure 2.2: The policy context may still vary within a SSP. Therefore, the policy context can be defined as an
additional axis to explore in scenario analysis. Note it is not implied that the forcing level and the policy context,
nor the policy context and the SSP, are uncorrelated.
2.3.4 The climate dimension
The vertical axis in the scenario framework is defined in terms of RCPs, i.e. the level of radiative forcing. As
explained above, the choice is made for practical reasons (RCPs form the connecting element between IAM and
CM models; radiative forcing targets are commonly used across IAM model analysis). It should be noted that
climate impacts may differ strongly even within a certain radiative forcing level. Global mean temperature
change is not directly determined by forcing level alone; factors such as climate sensitivity and the pattern of
climate change play a key role. Climate impacts are usually a result of more than temperature change alone. For
example, precipitation, radiation, wind, and humidity can have important effects on society and natural systems.
Impacts also depend critically on the timing, pattern, frequency, duration, and intensity of weather events,
especially extreme events. As shown by several studies, including Arnell et al. (2011), for instance, the same
level of climate change defined in terms of change in global mean temperature may result in very different
changes in the risk of water scarcity as a function of the direction, magnitude, and pattern of changes in
precipitation and their interplay with population density. Thus, projections obtained from a single climate model
for a given RCP level might indicate changes in climate that contrast greatly from projections from another
climate model for the same forcing. Therefore, there are additional uncertainties to account for in deploying this
scenario framework. For near-term scenarios, in which pathways of concentrations vary little across different
RCPs, it is the climate model uncertainty (especially the representation of natural variability of climate) that
dominates projections of future climate. The importance for climate of the concentration pathway on which the
world has embarked, described by the RCPs, only becomes apparent when the RCPs diverge in the second half
of the 21st century.
Workshop on Socio-Economic Scenarios - 13