Posted on 27 August 2013 by gpwayne

This post is part 2 of a three part series exploring the Representative Concentration Pathway (RCP) scenarios that have replaced the IPCC Special Report on Emissions Scenarios (SRES). An on-line resource of this guide is now available here: www.skepticalscience.com/rcp.php

The guide can also be downloaded as a PDF by clicking here.

Part 2: Creating new scenarios

“New sets of scenarios for climate change research are needed periodically to take into account scientific advances in understanding of the climate system as well as to incorporate updated data on recent historical emissions, climate change mitigation, and impacts, adaptation, and vulnerability”.

Source: IPCC Scenario Process For AR5

Science always seeks to improve its knowledge, its skills, its tools and methods. As scientists make new discoveries, these have to be incorporated in the scenarios they use. Computers improve at tremendous rates, so models that would have been impossible to run 20 years ago are now not only feasible, but desirable. Scientists and decision makers investigating climate change want faster results, more resolution, more detail, better validation and easier data exchange between related and disparate lines of research. These demands inform an on-going requirement to improve the scenarios.

Scenarios don’t just contain reference data; they also specify a process. The previous IPCC scenarios like SRES were run in sequence (see graphic below). This resulted in protracted development and delivery times. According to the IPCC: “Lags in the development process meant that it was often many years until climate and socioeconomic scenarios were available for use in studies of impacts, adaptation, and vulnerability”.

Not only that, but changes to prior processes in a sequential model meant going back and re-running the simulation in order to incorporate the new or changed data.

Figure 5. Approaches to the development of global scenarios: (a) previous sequential approach; (b) proposed parallel approach. Numbers indicate analytical steps (2a and 2b proceed concurrently). Arrows indicate transfers of information (solid), selection of RCPs (dashed), and integration of information and feedbacks (dotted). Source: Moss et al. (2008).

The new Representative Concentration Pathways employ a process intended to make the modelling less time-consuming, more flexible, with a reduced economic cost of computation:

“In the new process… emissions and socioeconomic scenarios are developed in parallel, building on different trajectories of radiative forcing over time…Rather than starting with detailed socio-economic storylines to generate emissions and then climate scenarios, the new process begins with a limited number of alternative pathways (trajectories over time) of radiative forcing levels (or CO2-equivalent concentrations) that are both representative of the emissions scenario literature and span a wide space of resulting greenhouse gas concentrations that lead to clearly distinguishable climate futures.

“These radiative forcing trajectories were thus termed “Representative Concentration Pathways” (RCPs). The RCPs are not associated with unique socioeconomic assumptions or emissions scenarios but can result from different combinations of economic, technological, demographic, policy, and institutional futures”.

Source: IPCC Scenario Process for AR5

Design Criteria

Four design criteria were agreed for the RCPs, as described in Moss et.al. 2008 and quoted here from van Vuuren 2011:

1. The RCPs should be based on scenarios published in the existing literature, developed independently by different modeling groups and, as a set, be ‘representative’ of the total literature, in terms of emissions and concentrations (see further in this section); At the same time, each of the RCPs should provide a plausible and internally consistent description of the future;
2. The RCPs should provide information on all components of radiative forcing that are needed as input for climate modeling and atmospheric chemistry modeling (emissions of greenhouse gases, air pollutants and land use). Moreover, they should make such information available in a geographically explicit way;
3. The RCPs should have harmonized base year assumptions for emissions and land use and allow for a smooth transition between analyses of historical and future periods;
4. The RCPs should cover the time period up to 2100, but information also needs to be made available for the centuries thereafter.

Working with the Stakeholders

Early in the process of developing new scenarios, the IPCC decided to act only as a catalyst for the process, inviting the research community to develop the scenarios. The subsequent RCP development process was led by the research community (the Integrated Assessment Model Consortium) at the request of the IPCC, but independent of them. A meeting was convened in September 2007 to discuss and agree the way forward:

“The meeting brought together over 130 participants, including users of scenarios and representatives of the principal research communities involved in scenario development and application. The representatives of the scenario user community included officials from national governments,  including many participating in the United Nations Framework Convention on Climate Change (UNFCCC), international organizations, multilateral lending institutions, and nongovernmental organizations (NGOs). The principal research communities represented at the expert meeting were the integrated assessment modeling (IAM) community; the impacts, adaptation, and vulnerability (IAV) community; and the climate modeling (CM) community. Because of this broad participation, the meeting provided an opportunity for the segments of the research community involved in scenario development and application to discuss their respective requirements and coordinate the planning process”.

Source: IPCC Expert Meeting Report

The four IAM groups responsible for the four published scenarios that were selected as “predecessors” of the RCPs, generated the basic data sets from which the final RCPs were developed. Over the following two years, a unique collaborative effort between integrated assessment modellers, climate modellers, terrestrial ecosystem modellers and emission inventory experts led to the agreement and specification of the four Representative Concentration Pathways (RCPs).

Improvements over SRES

Perhaps the most innovative aspect of the RCPs is that instead of starting with socio-economic ‘storylines’ from which emission trajectories and climate impacts are projected (the SRES methodology), RCPs each describe an emission trajectory and concentration by the year 2100, and consequent forcing. Each trajectory represents a specific synthesis drawn from the published literature. From this ‘baseline’, researchers can then test various permutations of social, technical and economic circumstances. These permutations are called ‘narratives’, equivalent to the ‘storylines’ employed in SRES.

“As stand alone products, the RCPs have limited usefulness to other research communities. First and foremost, they were selected with the sole purpose of providing data to climate models, taking into consideration the limitations in climate models differentiating levels of radiative forcing. They lack associated socioeconomic and ecological data. They were developed using idealized assumptions about policy instruments and the timing of participation by the international community.

“Therefore, there is a need to develop socioeconomic and climate impact scenarios that draw on the RCPs and associated climate change projections in the scenario process. Referencing the RCP and  climate  change projections has two potential benefits; they would facilitate comparison across research results in the CM, IAM, and IAV communities, and facilitate use of new climate modeling results in conjunction with IAV research.

“The parallel phase has several components. Within CMIP5, CM teams are using the RCPs as an input for model ensemble projections of future climate change. These projections will form the backbone of the IPCC's Working Group I assessment of future climate change in the 5th Assessment Report (AR5).   The IAM community has begun exploring new socioeconomic scenarios and producing so-called RCP replications that study the range of socioeconomic scenarios leading to the various RCP radiative forcing levels. In the meantime, IAV analyses based on existing emission scenarios (SRES) and climate projections (CMIP3) continue.

“In the integration phase, consistent climate and socioeconomic scenarios will inform IAM and IAV studies.  For example, IAV researchers can use the new scenarios to project impacts, to explore the extent to which adaptation and mitigation could reduce projected impacts, and to estimate the costs of action and inaction. Also, mitigation researchers can use the global scenarios as “boundary conditions” to assess the cost and effectiveness of local mitigation measures, such as land-use planning in cities or changes in regional energy systems.

“These scenarios need to supply quantitative and qualitative narrative descriptions of potential socioeconomic and ecosystem reference conditions that underlie challenges to mitigation and adaptation. And they have to be flexible enough to provide a framework for comparison within which regional or local studies of adaptation and vulnerability could build their own narratives. The defining  socioeconomic conditions of these scenarios have been designated Shared Socioeconomic reference Pathways (SSPs).”

Source: A framework for a new generation of socioeconomic scenarios for climate change impact, adaptation, vulnerability, and mitigation research; Arnell, Kram, Carter et.al.

For the first time, policy decisions can be tested; previous scenarios were describes as ‘no-policy’, meaning the scenarios did not respond to changes driven by political or legislative inputs, so mitigation or adaptation strategies could not be incorporated.

The principle difference in approach is that previously, SRES specified the socio-economic circumstances for each scenario, which essentially ‘locked in’ the options for socio-economic change (and led to a proliferation of SRES scenarios - 40 in total, each a slightly different variation on common socio-economic variables). Models were programmed to generate emissions and subsequent climate scenarios. The socio-economic variables of the SRES scenarios were socially and policy-proscriptive, inflexible in a way emissions and climate change outcomes were not.

By fixing the emissions trajectory and the warming, RCPs come at the problem the other way round. Socio-economic options become flexible and can be altered at will, allowing considerably more realism by incorporating political and economic flexibility at regional scales. Policy decisions on mitigation and adaptation can be tested for economic efficacy, both short and long term.  Researchers can test various socio-economic measures against the fixed rates of warming built into the RCPs, to see which combinations of mitigation or adaptation produce the most timely return on investment and the most cost-effective response.

Figure 6: Overview of the RCP development process, adapted from van Vuuren et.al. 2011

Development Aims and Products

There were five end-products expected from development process:

1. Four Representative concentration pathways (RCPs). Four RCPs…produced from IAM scenarios available in the published literature: one high pathway for which radiative forcing reaches >8.5 W/m2 by 2100 and continues to rise for some amount of time; two intermediate “stabilization pathways” in which radiative forcing is stabilized at approximately 6 W/m2 and 4.5 W/m2 after 2100; and one pathway where radiative forcing peaks at approximately 3 W/m2  before 2100 and then declines. These scenarios include time paths for emissions and concentrations of the full suite of GHGs and aerosols and chemically active gases, as well as land use/land cover…

2. RCP-based climate model ensembles and pattern scaling. Ensembles of gridded, time- dependent projections of climate change produced by multiple climate models including atmosphere–ocean  general  circulation  models  (AOGCMs),  Earth  system  models  (ESMs), Earth  system  models  of  intermediate  complexity,  and  regional  climate  models  will  be prepared for the four long-term RCPs, and high-resolution, near-term projections to 2035 for the 4.5 W/m2 stabilization RCP only.

3. New IAM scenarios. New scenarios will be developed by the IAM research community in consultation with the IAV community exploring a wide range of dimensions associated with anthropogenic climate forcing…Anticipated outputs include alternative socioeconomic driving forces, alternative technology development regimes, alternative realizations of Earth system science research, alternative stabilization scenarios including traditional “not exceeding” scenarios, “overshoot” scenarios, and representations of regionally heterogeneous mitigation policies and measures, as well as local and regional socioeconomic trends and policies…

4. Global narrative storylines. These are detailed descriptions associated with the four RCPs produced in the preparatory phase and such pathways developed as part of Product 3 by the IAM and IAV communities. These global and large-region storylines should be able to inform IAV and other researchers.

5. Integrated scenarios. RCP-based climate model ensembles and pattern scaling (Product 2) will be associated with combinations of new IAM scenario pathways (Product 3) to create combinations of ensembles. These scenarios will be available for use in new IAV assessments. In addition, IAM research will begin to incorporate IAV results, models, and feedbacks to produce comprehensively synthesized reference.

(End-product definitions excerpted from the IPCC report “Towards New Scenarios...”)

Beyond 2100 - Extended Concentration Pathways (ECP)

During the consultation phase, modelling communities made clear their interest in exploring longer-term processes. To facilitate these investigations, a single extension was developed for each RCP, extending the scenarios up to the year 2300. These data form the Extended Concentration Pathways (ECPs).

Since socio-economic factors cannot be predicted reliably over long timescales, the ECPs were developed using simple rules to extend GHG concentrations, emissions and land-use data series. The ECPs are intended only as the basis for long-term climate simulations.

A supplemental RCP called SCP6to4.5 was also developed with a peak at 2100, followed by a decline, to facilitate specific investigations into physical asymmetries and reversibility of climate, carbon cycle, and biophysical impacts systems (e.g. ecosystems, sea level rise).

Table 1: Basic rules for deriving extended concentration pathways (van Vuuren et.al. 2011)

An on-line resource of this guide is now available here: www.skepticalscience.com/rcp.php

The complete guide can also be downloaded as a PDF by clicking here.