Source: Explainer: The high-emissions ‘RCP8.5’ global warming scenario
The Representative Concentration Pathways (RCPs)
Projecting future climate change involves assessing a number of different uncertainties. Some of these relate to the climate system, such as how sensitive the climate might be to increased concentrations of greenhouse gas in the atmosphere. Others involve the quantity of gases emitted, using energy system models to simulate different scenarios of future emissions.
To try and capture a range of possible future emissions, energy system modellers have used integrated assessment models (IAMs) that simulate both future energy technologies and emissions. These produce emissions scenarios that are then used by scientists to run complex climate models that simulate how the climate might change in the future. As modern climate models take an enormous amount of computing power to run, the number of future emission scenarios that can be used tends to be fairly limited.
Projecting future climate change involves assessing a number of different uncertainties. Some of these relate to the climate system, such as how sensitive the climate might be to increased concentrations of greenhouse gas in the atmosphere. Others involve the quantity of gases emitted, using energy system models to simulate different scenarios of future emissions.
To try and capture a range of possible future emissions, energy system modellers have used integrated assessment models (IAMs) that simulate both future energy technologies and emissions. These produce emissions scenarios that are then used by scientists to run complex climate models that simulate how the climate might change in the future. As modern climate models take an enormous amount of computing power to run, the number of future emission scenarios that can be used tends to be fairly limited.
Many different scenarios have been developed over recent decades of climate research. However, the ones that have principally been used to drive climate model runs – and that have largely driven discussions by policymakers and the public – include:
- Six IPCC 1992 (IS92) scenarios used in the Intergovernmental Panel on Climate Change (IPCC) second assessment report (SAR)
- Six Special Report on Emission Scenarios (SRES) used in the IPCC third (TAR) and fourth (AR4) assessment reports
- Four RCP scenarios used in the IPCC fifth assessment report (AR5)
- Nine forcing scenarios being developed for the upcoming IPCC sixth assessment report (AR6) based on the Shared Socioeconomic Pathways (SSPs).
Climate modellers have somewhat different needs in future emission scenarios than energy system modellers. While energy system modellers want to explore a range of different outcomes under different socioeconomic assumptions – such as future population and economic growth – climate modellers want outcomes that lead to distinctly different warming levels in order to effectively evaluate and compare the results.
After the publication of the IPCC AR4 in 2007, there was a widespread desire to update the old SRES scenarios – developed in the late 1990s – to better reflect current technological and socioeconomic conditions. According to a “perspective” journal paper in Nature in 2010 by Dr Richard Moss and colleagues, modellers wanted to ensure that, “nearly a decade of new economic data, information about emerging technologies, and observations of environmental factors such as land use and land cover change… be reflected in new scenarios.”
However, the IPCC AR5 was scheduled to come out in 2013, and climate modellers would need scenarios to use in their models by 2010. Given the relatively short period to generate new scenarios, researchers developed a “parallel approach”. A set of “representative concentration pathways”, or “RCPs”, was therefore created for climate modellers to use in the interim while the development of more thorough socioeconomic pathways was undertaken.
The diagram below, taken from that Nature paper, shows the proposed development timeline. Once both efforts were completed, they would be integrated together in time for the 2013 IPCC AR5.
Schematic for the proposed parallel development of RCP climate scenarios and socioeconomic emissions scenarios, along with later integration in time for the IPCC AR5. Figure 4 from Moss et al 2010. Reproduced with permission.
Rather than starting with detailed socioeconomic storylines to generate emissions and climate scenarios, as had been the case with the SRES scenarios, the energy systems modeling community decided to start by creating scenarios of future “radiative forcing” for climate modelling not associated with any particular unique socioeconomic or emissions scenario. Radiative forcing is a measure of the combined effect of greenhouse gases, aerosols, and other factors that can influence climate to trap additional heat.
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