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 futur that can change the amount of the sun’s energy trapped by earth (known as ‘radiative forcings’ and measured as watts per squared meter). Climate researchers adopted 4 pathways spanning a broad range of values (2.6, 4.5, 6.0, and 8.5 watts per squared meter) 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 modeling effort looking at how factors such as population, economic growth, education, urbanization and the rate of technological development determine the level of greenhouse gas emissions. 5 SSPs were adopted based on 5 socio-economic 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 modeling 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 long as it is plausible within the underlined narrative.

The upcoming IPCC report (AR6, 2021) kept the 4 scenarios of the last report (AR5, 2014) for coherence but also introduces 3 new ones: RCP 1.9, 3.4 and 7.0. RCP1.9 focuses in limiting warming to below 1.5°C, the aspirational goal of the Paris Agreement. Scenario RCP3.4 represents an intermediate pathway between RCP2.6 and RCP4.5, while RCP7.0 corresponds the medium to high-end of the range of future emissions and is an alternative baseline outcome (a “business as usual” type) that can be seen as an updated RCP8.5 scenario rather than a mitigation target.

Source carbonbrief.org
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