What’s the difference between “Representative Concentration Pathways” and “Shared Socioeconomic Pathways”?

In the 2014 Fifth Assessment Report (AR5) the IPCC introduced the Representative Concentration Pathways (RCPs). RCPs describe different levels of greenhouse gases and other factors in the atmosphere that might occur in the future that can change the amount of the sun’s energy trapped by Earth (known as ‘radiative forcings’ and measured as watts per square metre). Climate researchers adopted 4 pathways spanning a broad range of values (2.6, 4.5, 6.0, and 8.5 watts per square metre) to explore a broad range of possible futures to evaluate the corresponding range of warming and climate changes.

On the other hand, the socioeconomic narratives or “Shared Socioeconomic Pathways” (SSPs), are the results of a separate modelling effort looking at how factors such as population, economic growth, education, urbanisation and the rate of technological development determine the level of greenhouse gas emissions. 5 SSPs were adopted based on 5 socioeconomic narratives of the future:

• a world of sustainability-focused growth and equality (SSP1);
• a “middle of the road” world where trends broadly follow their historical patterns (SSP2);
• a fragmented world of “resurgent nationalism” (SSP3);
• a world of ever-increasing inequality (SSP4);
• a world of rapid and unconstrained growth in economic output and energy use (SSP5).

We refer you to a short article from carbonbrief.org for further reading on SSPs.

The two modelling exercises were designed as complementary. The RCPs set pathways for greenhouse gas concentrations and are used as input to climate simulations to evaluate the amount of warming that could occur by the end of the century. Whereas the SSPs set the stage on which reductions in emissions can (or cannot) be achieved using Integrated Assessment Models (i.e. not climate models). The latter is important to understand: each SSP can theoretically be coherent with several RCPs as long as it is plausible within the underlying narrative.

The IPCC report (AR6, 2021) kept the forcing levels of the previous report (AR5, 2014) for coherence, and added three new ones, 1.9, 3.4 and 7.0 W/m², run in CMIP6 as the scenarios SSP1-1.9, SSP4-3.4 and SSP3-7.0. SSP1-1.9 aims to limit warming to 1.5°C, the aspirational goal of the Paris Agreement. SSP4-3.4 is an intermediate pathway between SSP1-2.6 and SSP2-4.5, while SSP3-7.0 is a high-emission pathway without climate policy, between SSP2-4.5 and SSP5-8.5.

Source carbonbrief.org