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Representative Concentration Pathways: A Guide

September 1, 2013

This is a repost of my article in the Guardian describing a guide I’ve written for Skeptical Science about Representative Concentration Pathways (RCPs). I freely admit this isn’t the most compelling subject I’ve ever written about. Nor is the guide something that anyone might mistake for a page-turner. The subject is important, however, if you’re interested enough in climate change science to study the methods and tools scientists are using to better understand global warming, and the effect it is likely to have on the environment.

I haven’t reposted the on-line version of the guide in this blog: with generous help from John Cook and Dana Nuccitelli, Skeptical Science has published a very nice presentation of the material, so I’m just linking you to it here. (There is also a stand-alone version of the guide as a PDF, which you can download by clicking here).


When  I started to research a rebuttal to the myth that future sea level rise had been exaggerated for Skeptical Science, the newer papers referred frequently to ‘Representative Concentration Pathways’ (RCPs). RCPs have replaced Special Report on Emissions Scenarios (SRES), and are used in Working Group I: The Physical Science Basis, the first part of the forthcoming IPCC Assessment Report Five (AR5), to be released in mid-September.

Needing more information, I looked for a handy ‘executive summary’ of RCP data, science and development. There wasn’t one. By the time I’d found out what I wanted to know from the primary literature, I had realised that a sensible guide/reference would be a damn useful thing to have. I assume you’ve already guessed what I did next?

RCPs are the third generation of scenarios. The first set – IS92 – were published in 1992. In the year 2000, the second generation – SRES – were released. The latest, now in use, are the RCPs.

Like their predecessors, Representative Concentration Pathways are a set of standards used primarily by climate modellers. Research takes place in many countries, and RCPs provide a common, agreed foundation for modelling climate change. The data sets are used to initialize models so that everyone starts from a place everyone else understands, using values everyone is familiar with. RCPs reduce duplication and save money; climate modelling is an expensive business. They also provide a way to compare results and communicate findings easily across a broad spectrum of interests.

So what are these Representative Concentration Pathways? There are four: RCP8.5, RCP6, RCP4.5, and RCP2.6 (the latter also referred to as RCP3PD, where ‘PD’ stands for Peak and Decline). The numbers refer to radiative forcings (global energy imbalances), measured in watts per square metre, by the year 2100.

Forcing is one key metric of the RCPs. Another is emission rates – how fast we put more greenhouse gases into the atmosphere. The third metric is emission concentrations, measured in parts per millions for each of the greenhouse gases e.g. CO2, methane, nitrous oxide, etc.

Each RCP ‘fixes’ two values in the year 2100; how much the planet has heated up, and the concentration of greenhouse gases. Each RCP differs greatly in the rate of forcing and emissions. These different rates, or trajectories, form the ‘pathways’.  Here’s two examples: on the left are the four pathways for CO2 emissions up to the year 2100, in gigatons of carbon. On the right, the corresponding concentration (total) of CO2 over the same period, measured in parts per million.


Emissions of CO2 across the RCPs (left), and trends in concentrations of carbon dioxide (right). Grey area indicates the 98th and 90th percentiles (light/dark grey) of the literature. The dotted lines indicate four of the SRES marker scenarios. (Graphs from van Vuuren (2011).

Each RCP was developed independently by a modelling team whose previous work was a close match to the starting requirements for the new scenarios. To determine the trajectories of emissions and forcings for each RCP, the teams reviewed the existing literature and synthesized values for a wide range of scientific and socioeconomic data, like population growth, GDP, air pollution, land use and energy sources.


Energy sources by sector (van Vuuren 2011)

Why were new scenarios necessary? One reason is efficiency; the RCPs allow more flexibility (and reduced costs) in modelling processes. Possibly the most important improvement is that, unlike SRES, RCPs allow teams to test different social, legislative and other policy initiatives, and see the economic effects as well as environmental; mitigation results as well as adaptation.

Variations in socioeconomic models are called narratives (or storylines) and are expected to form a key plank of on-going investigation into the best way to address climate change at both regional and global scales. The IPCCs AR5 Working Group 2 report, “Impacts, Adaptation and Vulnerabilities”, released in March 2014, will report on new “shared socioeconomic pathways” (SSPs).

Climate models are essential and valuable tools. Despite frequent attacks made by deniers, the models have made many predictions whose chief criticism should be that they are too conservative. Even the crude early models have proved surprisingly accurate; you can read about them here.

Understanding scenarios helps us understand the models and the information they produce. The Skeptical Science Beginner’s Guide to Representative Concentration Pathways is available as an online resource, (and is also available as a PDF). The guide is split in three sections: In Part 1 we explore their historical background, explain why scenarios are necessary, and who uses them. Part 2 starts with an examination of the demand for new scenarios, and describes their development. In Part 3 we take a look at the scenarios in detail, consider the technical aspects, the differences between the four RCPs, and how they compare to earlier SRES scenarios.  The public can also use the RCP database, the central resource for the data that forms each pathway, which is free to access.

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