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130 Third IMO GHG Study 2014
A useful summary and guide to the origin and formulation of the RCP scenarios is provided by Wayne (2013).
The “concentration” refers to that of CO 2 and the “pathways” are “representative” of possible outcomes of
energy, population, policy and other drivers that will ultimately determine the concentration of CO 2 in the
atmosphere. There are four main RCPs in use, detailed in Table 69.
Table 69 – Descriptions and sources of representative concentration pathways
RCP Description Source references Model
RCP2.6 (or 3PD) Peak in radiative forcing at ~3 W/m 2 before 2100
and decline
RCP4.5
RCP6.0
RCP8.5
Stabilization without overshoot pathway to
4.5 W/m 2 at stabilization after 2100
Stabilization without overshoot pathway to
6 W/m 2 at stabilization after 2100
Rising radiative forcing pathway leading to
8.5 W/m 2 in 2100.
Van Vuuren et al., 2006, 2007
Clarke et al., 2007; Wise et al., 2009
Hijoka et al., 2008
Riahi et al., 2007
IMAGE
GCAM
AIM
MESSAGE
The numbers associated with the RCPs (2.6–8.5) simply refer to resultant radiative forcing in W/m 2 by 2100.
Further technical details of the RCPs are given in Moss et al. (2010). The RCPs cover a range of ultimate
temperature projections by 2100 (i.e. global mean surface temperature increases over the pre-industrial period
from GHGs), from around 4.9 °C (RCP8.5) to 1.5 °C in the most optimistic scenario (RCP2.6 or RCP3PD, where
PD refers to peak and decline).
These RCPs are used to project shipping coal and liquid fossil fuel transport work, on the basis of a historical
correlation with global coal and oil consumption (see Section 3.2.3), using the IAM energy demand projections
of different fuel/energy types (EJ/yr). A set of GDP projections from the associated five SSP scenarios (see
Kriegler et al., 2012) was used for non-fossil-fuel transport projections (see Section 3.2.3).
The five SSPs each have different narratives (Ebi et al., 2013) and are summarized in Table 70.
Table 70 – Short narratives of shared socioeconomic pathways
SSP number and name
SSP1: Sustainability
SSP2: Middle of the road
SSP3: Fragmentation
SSP4: Inequality
SSP5: Conventional
development
Short narrative
A world making relatively good progress towards sustainability, with ongoing efforts to achieve
development goals while reducing resource intensity and fossil fuel dependency. It is an
environmentally aware world with rapid technology development and strong economic growth,
even in low-income countries.
A world that sees the trends typical of recent decades continuing, with some progress towards
achieving development goals. Dependency on fossil fuels is slowly decreasing. Development of
low-income countries proceeds unevenly.
A world that is separated into regions characterized by extreme poverty, pockets of moderate
wealth and a large number of countries struggling to maintain living standards for a rapidly
growing population.
A highly unequal world in which a relatively small, rich global elite is responsible for most
GHG emissions, while a larger, poor group that is vulnerable to the impact of climate changes
contributes little to the harmful emissions. Mitigation efforts are low and adaptation is difficult
due to ineffective institutions and the low income of the large poor population.
A world in which development is oriented towards economic growth as the solution to
social and economic problems. Rapid conventional development leads to an energy system
dominated by fossil fuels, resulting in high GHG emissions and challenges to mitigation.
This presented the problem of how to combine the RCPs with the SSPs and guidance was taken from Kriegler
et al. (2012), as follows.
In principle, several SSPs can result in the same RCP, so in theory many BAU scenarios can be developed.
However, in order to limit the number of scenarios, while still showing the variety in possible outcomes, it
was decided to combine each SSP with one RCP, under the constraint that this combination is feasible. The
SSPs are thus aligned with the RCPs on the basis of their baseline warming. Increased mitigation effort would
potentially result in less fossil fuel transport, probably somewhat lower economic growth until 2050 and
therefore probably lower transport demand and maritime emissions.