A comparison of the IPCC’s

Special Report Emissions on

Scenarios Report (and its

associated “sequential” process)

with the Representative

Concentration Pathways (and

their associated “parallel”

process) for use in climate

impacts assessments

FHS in Geography

Candidate no: 630495

Submitted for Finals 2014

Word count: 4051

Abstract

There is significant uncertainty about future climate change and the socioeconomic trajectory

of society. As a result, the use of scenarios for exploring alternative possible futures is

necessitated and enables the investigation of some of the most challenging and important

questions facing the world today.

The aim of the study is to compare the two most recent sets of IPCC scenarios and the

development processes used to construct them. The “sequential” process used to produce the

SRES scenarios and the “parallel” process used in developing the RCPs are considered. First,

a technical summary is given of the SRES scenarios, then of the RCPs, and finally of their

respective parallel/sequential processes. After this, the scenarios are evaluated based upon the

following criteria: their efficiency, their promotion of integrative research, their policy

implications and their use of the existing climate change literature.

The main conclusion drawn is that that the IPCC’s decision to introduce the parallel process

was effective, particularly in terms of promoting efficiency, integrative research and policy

information. The current scenario development process is leading to insights into the nature of

natural and anthropogenic climate processes, which in turn are helpful in producing more

effective climate impacts assessments.

Introduction

Advances in climate science are increasing understanding of the climate system’s inherent

variability, as well as its response to human and natural forcings (IPCC, 2013). However, the

impacts of climate change will not only depend upon radiative forcing, but also upon how

societies respond in terms of their economies, technologies, lifestyles and policies (Moss et al.,

2010). There is therefore significant uncertainty about both how the climate system will

respond to these forcings, and in the decisions that society will make. As a result, the use of

scenarios for exploration of possible futures is necessitated and they enable investigation of

some of the most challenging and important questions facing mankind today (IPCC, 2013).

In climate change research, both socio-economic and emissions scenarios are used to construct

possible alternative futures. The scenarios therefore take into account both climatological

variables (such as future emissions of greenhouse gases and other air pollutants), as well as

socioeconomic variables (such as future changes in land use) (Van Vuuren et al., 2011). Once

constructed, the scenarios may be used as inputs to climate model runs. A common set of

scenarios that can be used across the scientific community can therefore greatly increase the

communication of model results (Van Vuuren et al., 2011). In addition, the cost of running

models may be reduced using scenarios. Simulation programming does not have to start from

scratch for each experiment, rather a scenario framework can streamline the process of building

and operating models (Wayne, 2013). At the final stage of the process, socio-economic and

emissions scenarios are useful in the assessment of impacts, adaptation and mitigation

possibilities (which typically occur after the climatic modelling results are available) (IPCC,

2013).

The IPCC’s first climate change scenarios were collectively named SA90. The scenario set was

published in 1990 and was shortly followed by the release of the IS92 scenarios (in 1992).

Following these, the IPCC’s next generation of scenarios – collectively named the Special

Report on Emissions Scenarios (henceforth referred to as “SRES”) – was released in the year

2000. The SRES scenarios were used in both the IPCC’s Third Assessment Report (henceforth

referred to as “TAR”) and Fourth Assessment Report (henceforth referred to as “AR4”). The

SRES scenarios have been used as a common reference point for much of the climate change

research over the past decade (Wayne, 2013). Most recently, the IPCC responded to calls for

improvements to SRES in 2007 by embarking on producing the latest iteration of the scenario

process. The Representative Concentration Pathways (henceforth referred to as “RCPs”) have

already been used by Working Group I to contribute towards the IPCC’s Fifth Assessment

Report (henceforth referred to as “AR5”). They are currently being used by Working Groups

II and III in their contributions for publication in 2014. For a more detailed timeline showing

the recent progress in scenario developments, the applications of these scenarios, and the

contexts in which they were released, see Figure 1. The main focus of this essay is on the SRES

and RCP scenarios, and how they were constructed.

Three major groups of scientists use scenarios to investigate climate change: First, climate

modelling groups study future climate change (using scenarios such as the RCPs as a baseline

from which to model). Second, in assessing the relationship between emissions and socio-

economic scenarios, Integrated Assessment Model groups combine information from diverse

fields of study (Wayne, 2013). The third group studies the impacts of climate change, adaption

possibilities and likely vulnerabilities. This group is associated with investigating regional-

scale impacts and draws upon a variety of disciplines (including as the social sciences,

economics and engineering) (Wayne, 2013). It is normally challenging to disentangle the works

of one group from another (see Figure 7) and this particularly difficult when using scenarios

produced using a “non-sequential” (or “parallel”) process. Nevertheless, this essay will attempt

Figure 1: A timeline showing developments in the releases of emissions/climate scenarios. The overall

course of model-based scenario development is shown (in the blue section), with the applications shown (in

the purple section) and the context is shown (in the green section).

Note: Of particular interest in this study are the major events of 1990 (IPCC SA90 emissions scenarios),

1992 (IPCC IS92 scenarios), 1999 (SRES) and 2009 (RCPs released).

Source: Moss et al., (2010)

to focus on the importance of scenarios for use by the third group which is responsible for

considering climate impacts assessments.

This essay aims to compare the two most recent sets of IPCC scenarios and the development

processes used to construct them. The “sequential” process used to produce the SRES scenarios

and the “parallel” process used in developing the RCPs are investigated. A technical summary

is first given of the SRES scenarios, then of the RCPs, and finally of their respective

parallel/sequential processes. After this, the scenarios are evaluated based upon the following

criteria: their efficiency, their promotion of integrative research, their policy implications and

their use of the existing climate change literature. The main conclusion drawn is that the IPCC’s

decision to introduce the parallel process was sensible, particularly in terms of promoting

efficiency, integrative research and policy information. It is also clear that there are several

downsides associated with the parallel approach and that the recently released RCPs by no

means represent a “crystal ball” (Moss et al., 2010). Overall however, there has been a

significant improvement in scenario development by using the parallel process. The current

scenario development process is leading to insights into the nature of natural and anthropogenic

climate processes, which in turn are helpful in producing more effective climate impacts

assessments.

Technical Summary: The SRES emissions scenarios, the RCPs,

and the sequential/parallel processes used to construct them

The first set of emissions scenarios considered is from the SRES report. The scenarios were

published by the IPCC in 2000 and describe the emissions scenarios which were used to make

projections of future climate change in the TAR and AR4. 40 different scenarios were

generated, each of which made different assumptions about future climatic and socio-economic

forces. Most of the scenarios included a rise in the consumption of fossil fuels and considered

overall global GDP growth to increase be a factor of 5-25 (WMO, 2013). The scenarios were

then organised into “families”, as shown in Figure 2.

Figure 2: Schematic illustration of the SRES emissions scenarios. The four qualitative storylines were used

to produce four sets of scenarios. These groups were referred to as “families” of scenarios (A1, A2, B1 and

B2). There were six scenario groups drawn from the four families. One group came from A2, B1 and B2,

whilst three groups came from the A1 family. The 3 A1 family groups characterise alternative developments

of energy technologies: A1F1 is was fossil fuel intensive case, A1FB is was balanced case, and A1T is was

predominantly non-fossil fuel case. The scenarios marked “HS” stand for harmonized scenarios (with

“harmonized” assumptions on global population, gross world product and final energy). Those marked

“OS” explore uncertainties in driving forces beyond these scenarios.

Source: IPCC (2000). Note: For further detail of what constitutes families A1, A2, B1 and B2, see Figure 3.

Four families (qualitative storylines) were developed and named A1, A2, B1 and B2. The A

scenarios refer to a future homogenous world where globalization is strong. The B scenarios

relate to an alternative path where regionalisation is strong and the world is heterogeneous (for

further detail, see Figure 3). Six further scenario groups were drawn from the families (see

Figure 2) and of these six scenario groups, three stem from the A1 family. The 3 A1 groups

(A1F1, A1T and A1B) refer to a fossil fuel-intensive case, a balanced case, and a predominantly

non-fossil fuel case respectively. Whilst three scenario groups were drawn from the A1 family,

only one scenario group was drawn from each of the other families. For further detail see Figure

3.

Family

name

Future world description Predicted

temperature

change by 2100

A1 A world of very rapid economic growth, global population that peaks

mid-century and declines thereafter, and the rapid introduction of new

and more efficient technologies. Major underlying themes include

convergence among regions, capacity building, and increased cultural

and social interactions, with a substantial reduction in regional

differences in per capita income.

1.4-6.4ºC

A2 A very heterogeneous world. The underlying theme is self-reliance and

preservation of local identities. Fertility patters across regions

converge very slowly, which results in continuously increasing global

population. Economic development is primarily regionally orientated

and per capita economic growth and technological change are more

fragmented and slower than in the other storylines.

1.1-2.9ºC

B1 A convergent world with the same global population that peaks in the

mid-century and declined thereafter, as in the A1 storyline, but with

rapid changes in economic structures towards a service and

information economy, with reductions in material intensity, and the

introduction of clean and resource-efficient technologies. The

emphasis is on global solutions to economic, social, and environmental

sustainability, including improved equity, but without additional

climate initiatives.

2.0-5.4ºC

B2 A world in which the emphasis is on local solutions to economic,

social, and environmental sustainability. It is a world with

continuously increasing global population at a rate lower than A2,

intermediate levels of economic development, and less rapid and more

diverse technological change than in the B1 and A1 storylines. While

the scenario is also orientated toward environmental protection and

social equity, it focuses on local and regional levels.

1.4-3.8ºC

Figure 3: The SRES scenario storylines and scenario families, as summarised by the WMO (2013).

Source: World Meteorological Organisation (2013).

Four RCPs were developed and each was named according to the radiative forcing it projected

by the year 2100. These pathways (named RCP8.5, RCP6, RCP4.5 and RCP2.6) differed

greatly in their rates of forcing and emissions (see Figure 4) (WMO, 2013). Each RCP was

independently developed by a modelling team whose previous work was a close match to the

starting requirements for the new scenarios (Moss et al., 2010). As with the SRES scenarios, a

wide range of socioeconomic considerations were taken into account when making projections

of variables such as population growth, GDP, and energy use. However, the IPCC decided in

2006 to limit its role to assessing and catalysing the large and growing scenario literature, rather

than researching them independently (Moss et al., 2013). As a result, the RCPs are not new

scenarios produced in a similar fashion to the SRES scenarios – they are instead consistent sets

of projections concerning only the existing literature on the components of the radiative forcing

pathways (WMO, 2013). As such, they serve as “baseline” inputs for climate modelling. In

preparation for the contributions IPCC’ Working Groups II and III climate modellers are

carrying out new experiments using the time series of emissions scenarios and concentrations

related to each of the RCPs for their publications in 2014.

Representative

Concentration

Pathway

Description

RCP 8.5 Rising radiative forcing pathway leading to 8.5 W/m2 in 2100.

RCP 6 Stabilisation without overshoot pathway to 6 W/m2 at stabilisation

after 2100.

RCP 4.5 Stabilisation without overshoot pathway to 4.5 W/m2 at stabilisation

after 2100.

RCP 3-PD2 Peak in radiative forcing at ~ 3 W/ m2 before 2100 and decline.

The traditional way in which model-based scenarios have been developed for use in climate

change research is through a “sequential” process (Moss et al., 2010). This usually involves a

step-by-step, often time-consuming passing on of information between scientific communities

from separate disciplines. This linear process is illustrated in Figure 5 and the SRES scenarios

were formed using it. The sequential, causal-chain development first involved producing

emissions scenarios based upon different socioeconomic futures. Next estimations were made

about future greenhouse gas concentrations and radiative forcings. From these projections, the

Figure 4: Overview of the Representative Concentration Pathways used in the IPCC’s 5th Assessment

Report.

Source: World Meteorological Organisation (2013).

resultant climate changes were calculated. The ensuing emissions scenarios were then used as

a baseline for mitigation and impacts research (illustrated in Figure 5) (Moss et al., 2010).

The “parallel” process of scenario development (currently employed in the IPCC’s AR5 report)

is not quite as conceptually straightforward (Van Vuuren et al., 2011). Climate change

researchers from different disciplines began the process by producing four scenarios of future

radiative forcings from the existing literature. Central to the parallel process is the idea that the

four defined radiative forcing pathways may be achieved by a range of socio-economic and

technological development scenarios (WMO., 2013). As such, the RCPs facilitate investigation

of the question “what are the ways in which the world could develop in order to reach a

particular RCP” (Moss et al., 2010). After the RCPs were developed using the literature, the

process has enabled the parallel investigation of both new socio-economic scenarios and

climate scenarios (using the RCPs as a baseline) (see Figure 6). The next step in the parallel

process is for the socio-economic scenarios and the climate scenarios to be integrated using a

variety of tools (such as pattern scaling and downscaling) (see Figure 6). This facilitates the

last stage which concerns research into mitigation options, as well as research into possible

impacts, adaptations and (Moss et al., 2010).

Figure 5: The sequential approach. The figure depicts the simple linear chain of causes and consequences

of anthropogenic climate change. The scenarios were made on the foundation of this sequence, and then

handed from one research community to the next. The process was lengthy and led to inconsistencies (Moss

et al., 2010).

Source: Moss et al., (2010)

The basic differences between the SRES emissions scenarios and RCPs have now been briefly

summarised, both in terms of their structure and in terms of the processes behind their

development. Next, the implications of the scenarios will be discussed, beginning with

investigating their efficiency.

Figure 6: The parallel process. This is the procedure that has been/is being used in the forthcoming climate

change research and impacts assessments. First, radiative forcing characteristics are identified which

supports the modelling of a wide range of future climates. From the literature a selection of RCPs are then

selected to provide the inputs of emissions, concentrations and land use/cover for climate models.

Alongside, or “parallel” with, the development of the RCP-based climate scenarios are the new

socioeconomic scenarios, developed to investigate socioeconomic uncertainties related to

adaption/mitigation. The socio-economic scenarios are then to be integrated with the climate scenarios using

a number of tools. This enables the final stage of research – new research and assessments – providing

insights into the costs, benefits, and risks of different futures.

Source: Moss et al., (2010)

Implications for Climate Impacts Assessments:

i) Efficiency

After using the sequential process to develop the SRES emissions scenarios, the IPCC had

several reasons to introduce the parallel process (WMO, 2013). For example, the need to

respond to new opportunities and information requirements was great, as well as the need to

increase the amount of collaborative research between disciplines (Moss et al., 2010).

However, the main reason for the development of the new parallel process was arguably to

avoid reproducing the inefficiencies experienced with the sequential development of the SRES

scenarios (IPCC, 2007). The linear process led to inconsistencies when projecting climate

changes, as well as delaying the availability of the produced climate scenarios for impacts

assessments (Moss et al., 2010). Work on the SRES emissions scenarios (IPCC, 2000) began

in 1997 and took 3 years to complete (see Figure 1). As a result, the first assessment of climate

modelling results was in the IPCC’s TAR in 2001. It was only in 2007 (with the publication of

the AR4) that a more complete set of SRES-related climate scenarios was assessed by the

IPCC. At this late stage, it was finally possible to consider the impacts, adaptations and

vulnerability research (Moss et al., 2010). However, due to the delayed process, multiple

generations of climate models were being used in the same report. This resulted in a number

of inconsistencies due to differences between the newer scenarios and the older ones (which

had been used in the climate impacts assessments). This lack of efficiency led to difficulties in

synthesising results, for example in assessing matters such as the costs and costs and benefits

of climate change (Moss et al., 2010). Difficulties were also experienced in considering

feedbacks from different models (Rogelj et al., 2012). The sequential process used in producing

the SRES scenarios was therefore responsible for the delayed and somewhat inconsistent

delivery of results for use in climate impacts assessments.

One of the reasons for adopting the parallel process in producing AR5 was therefore to reduce

the inefficiencies associated with the production of the SRES emissions scenarios. As the RCPs

have been set, climate modellers are able to prepare simulations in parallel with the work of

the integrated assessment modellers (who have been developing an ensemble of new

socioeconomic and emissions scenarios) (Moss et al., 2010). Because the work of these two

groups has been done in a parallel – rather than sequential – fashion, the overall time taken has

be reduced. This amount of saved time corresponds to the period which would otherwise have

been devoted to the up-front production of the emissions scenarios (Moss et al., 2010). The

parallel process has also been helpful to increase efficiency in another way. Because models

are developed parallel to one another, the issue of multiple generations of models being used

in the same report has been avoided. As a result, the parallel process used in developing the

RCPs and future emissions scenarios has allowed for a more efficient flow of information

between different research groups. This in turn means that researchers working on climate

impacts assessments may be more effective in their work and have begun working on their part

of the process sooner.

ii) Collaborative and integrative research

Another advantage of the parallel process is the amount of interdisciplinary and integrative

research which the approach promotes. For an illustration of the various types of integrative

research which have been used to produce AR5, see Figure 7. By introducing the RCPs

therefore, the IPCC was aiming to facilitate additional scientific advances by promoting greater

scientific collaboration (in addition simply aiming to reduce scenario development time). This

increase in scientific understanding includes an enhanced appreciation of the different types of

feedbacks and improved synthesis of research concerning adaption and policy options (Moss

et al., 2010).

There are a number of ways in which the parallel process has led to greater collaborative

research than the sequential process in climate impacts assessments. First, the parallel process

has increased collaboration through the coordination of narrative storylines and quantitative

vulnerability scenarios, which have been developed in parallel with emissions scenarios. As a

result, climate impacts research which corresponds directly to emissions and climate scenarios

has been encouraged (Moss et al., 2010). The approach has also increased the use of

socioeconomic storylines (which typically have been used to project future atmospheric

conditions, rather than to assess vulnerability and adaptive capacities). As a result, there has

been an increase in the level of integrative research in impacts assessments (Van Vuuren,

2011). This advantage may also have facilitated the downscaling of socioeconomic data for

consistent and comparable research in climate impacts assessments (Moss et al., 2010). Finally,

the outcomes of impacts assessments made using the RCPs as a baseline can be fed back into

both climate and integrated assessment modelling (Moss et al., 2010).

Improvement of integrated Earth system models (including integrated assessment models,

climate models and impacts models) (see Figure 7) is also expected to be seen when the

contributions of Working Groups II and III to AR5 are published (Moss et al., 2010). Whilst

Earth system models will not entirely replace the other classes of models, they will work as a

good compliment and bring the three considered types of model closer together. With the

publication of AR5, integrated Earth systems models are therefore likely to encourage new

insights into the process of climate change mitigation and into the process of forming climate

impacts assessments.

It is difficult to quantify the exact extent to which climate change research has thus far been

integrative and interdisciplinary. However, an assessment of the literature allows for the

realisation of a few key points. First, both sets of scenarios (SRES scenarios and RCPs) under

their respective sequential and parallel processes make an attempt to integrate research. This

effort is apparent from the beginning of the formation of climate scenarios, right through to the

end of the process where climate impacts assessments are made and mitigation strategies are

formulated. However, due to the nature of the parallel process, it would appear that the RCPs

and the corresponding emission scenarios have already promoted higher levels of integration

in the IPCC’s research. Indeed, 249 authors from 39 countries contributed to the work of

Figure 7: Image to show how Earth systems models and climate scenarios are developed using three broad

types of models and analytical frameworks: integrated assessment models, climate models, and models used

to assess impacts, adaptation and vulnerability. The parallel-process used in the development of the RCPs is

likely to promote this type of integration.

Source: Moss et al., (2010)

Working Group I (with 54,677 comments made worldwide) (IPCC, 2013) and the level of

worldwide scientific collaboration is already higher for the incoming contributions of Working

Groups II and III (IPCC, 2013).

iii) Policy Insights

The emissions scenarios from SRES and the RCP scenarios currently being used by the IPCC

must be compared in terms of the insights which they can give on climate policy. One major

change in creating the RCPs was that the new, lowest RCP scenario (Van Vuuren et al., 2007)

is significantly different from any of the SRES scenarios. Whereas the SRES emissions

scenarios are all “non-intervention” scenarios which increase in forcing over the 21st century

(Rogeli et al., 2012), the lowest RCP scenario’s forcing peaks in the 21st century at

approximately 3 W m-2 and from that point onwards declines. This difference is because, unlike

the RCPs, the SRES scenarios do not take climate policy into account. The SRES scenarios

were made as “baseline” scenarios which didn’t account for efforts designed to limit

greenhouse gas emissions (for example, the Kyoto Protocol). The RCPs on the other hand

include a mitigation scenario as well as a higher emissions scenario. They therefore explore

approaches to climate change mitigation in addition to the traditional “no climate policy

scenarios” (WMO, 2013) (although they are not supposed to be interpreted as exact forecast of

the future). As a result, policy decisions can be tested for the first time using the new approach

in climate impacts assessments (Wayne, 2013).

iv) The IPCC’s use of climate change literature

The IPCC decided in 2006 that it would not commission another set of emissions scenarios.

Rather, it stated that the task would be left to the wider research community and its role would

be limited to catalysing and assessing the growing scenario literature (Moss et al., 2010). This

decision has had implications for the RCPs which have not yet been discussed, making the

newer scenario development process different to the SRES scenarios. Unlike the RCPs, the

SRES scenarios were specifically commissioned by the IPCC and as a result may hold a

number of advantages. The main one is that, as IPCC commissioners of the report have “had

control” of the process from beginning to end, they could impose their overarching framework

on the scenario development process. However, in the case of the RCPs and new emissions

scenarios this is/will not be the case – they cannot be treated as a set of scenarios with an

overarching internal logic (Van Vuuren et al., 2011). Instead of being designed as a new, fully

integrated set of scenarios – similar to with SRES – the aim was to produce a set of scenarios

that was consistent with the existing and growing scenario literature (Van Vuuren et al., 2011).

Differences between the RCP scenarios therefore cannot be said to result directly from factors

such as a difference in socioeconomic conditions. Instead, the RCP variations may well be due

to the differences between the actual models used in their formulation. Given that the RCPs

were therefore not developed directly by the IPCC with an overarching framework (instead

having to rely on the research community), a certain element of caution must be used when

making climate impacts assessments. This concern is not necessarily apparent with the SRES

scenarios.

Conclusion

Before concluding, it should be reiterated that the parallel process associated with the RCPs

has not yet been completed. As such (and with the release of incoming reports from the IPCC’s

Working Groups I and II) this essay has been unable to consider many insights about the

scenario development process which will shortly be published. However, a review of the

existing literature has still enabled a comparison of the parallel process (involving the RCPs)

with the sequential process involving the SRES scenarios. Several differences in the

construction processes were identified and found to have strong implications for climate

impacts assessments.

One of the main advantages of using the parallel process for the IPCC’s AR5 was the efficiency

gained in producing emissions scenarios. Unlike the previous scenarios (which led to

inconsistencies in projecting climate changes and delayed the production scenarios for impacts

assessments) (Moss et al., 2010), the parallel approach has been significantly faster and less

inconsistency has emerged. Largely, this has been due to the introduction of parallel streams

of research, enabling speeding up of the process and collaboration leading to less duplication

of results.

Second, increased levels of integrative research used in the parallel process have proven useful.

By working in parallel, different research groups have managed to achieve higher levels of

integration in their research. This has not only sped up the process of scenario development,

but it has also helped to generate results which involved collaboration between groups of

different disciplines.

Third, the new generation of climate scenarios is enabling investigation into alternative climate

futures for policy-makers. The SRES scenarios contrast in this respect as they only considered

“no-policy” approaches. By incorporating a mitigation scenario and a higher emissions

scenario, investigation into the understanding of policy implications in climate change research

has recently been facilitated in a way that was not apparent with SRES.

Finally, the IPCC’s choice to limit its role to assessing and catalysing the existing and growing

literature on climate change (Moss et al., 2010) has had a number of implications. Whilst this

approach poses several advantages over the methodology of the SRES scenarios (including

increased efficiency and encouraging more integrative research), it also has had its

disadvantages. The main one is that, as IPCC commissioners of the report have not “had

control” of the process entirely, they have been unable to impose their overarching framework

on the scenario development process from beginning to end. Given that the RCPs were not

developed directly by the IPCC therefore (instead having to rely on the research community),

a certain element of caution must be used when making climate impacts assessments. This

concern was not necessarily apparent when using the SRES scenarios.

In conclusion, it would appear that the IPCC’s decision to use a new, parallel, approach to

develop the AR5 emissions scenarios has been an effective one. Although AR5 will have to be

completed before an entirely sound judgement can be made, it would seem at this point that

the parallel approach is achieving its aims of increasing efficiency and increasing collaboration

between research groups. There may be a small price to pay in adopting the new approach

(including the loss of control over the first stage of the process), but it would appear that the

advantages gained in flexibility far outweigh the disadvantages. Therefore, whilst the parallel

process of emissions scenario development by no means offers “crystal ball” (Moss et al.,

2010) predictions for the future, it is leading to insights into the nature of climate processes, as

well as into the potential possibilities for adaption and mitigation policy. In order for climate

impacts research to become more effective, all of the above will have to be accounted for.

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