Future projections were crafted in association with the formulation of societal development narratives under the Shared Socioeconomic Pathways (SSPs). The goal of SSPs is to depict a range of plausible societal futures where different technological, political and environmental trajectories are described. Within each of these storylines, a trajectory of demographic changes is generated, which then, based on an assumption of technologies, lead to likely emissions patterns to reflect that pathway. From these emission lines, a suite of most representative likely radiative forcing levels (W/m2) at the end of the 21st century are then selected to provide the input to climate models (1.9, 2.6, 4.5, 6.0 and 8.5 W/m2 of change over pre-industrial, respectively). The SSPs reflect the most advanced iteration of socioeconomic narratives offered to date. They consider societal factors such as demographics, human development, economic growth, inequality, governance, technological change and policy orientations. While most factors are given as narratives that sketch broad patterns of change globally and for large world regions a subset (population, GDP, urbanization and educational attainment) are provided as quantitative, country-specific projections. These variables were chosen based on their common use as inputs to emissions or impact models and their relationships to each other.
For a complete description of SSP Narratives and scenario development, see O’Neill et al. 2017

SSP1-1.9 holds warming to approximately 1.5°C above 1850-1900 in 2100 after slight overshoot (median) and implies net zero CO2 emissions around the middle of the century.

SSP1-2.6 stays below 2.0°C warming relative to 1850-1900 (median) with implied net zero emissions in the second half of the century.

SSP2-4.5 is approximately in line with the upper end of aggregate Nationally Determined Contribution emission levels by 2030. It represents assessed temperature projections for NDCs to be between 2.7 and 3.4°C by 2100, corresponding to the upper half of projected warming under SSP2-4.5. New or updated NDCs by the end of 2020 did not significantly change the emissions projections up to 2030, although more countries adopted 2050 net zero targets in line with SSP1-1.9 or SSP1-2.6. The SSP2-4.5 scenario deviates mildly from a ‘no-additional- climate-policy’ reference scenario, resulting in a best-estimate warming around 2.7°C by the end of the 21st century relative to 1850-1900.

SSP3-7.0 is a medium to high reference scenario resulting from no additional climate policy under the SSP3 socioeconomic development narrative. SSP3-7.0 has particularly high non-CO2 emissions, including high aerosols emissions.

SSP5-8.5 is a high reference scenario with no additional climate policy. Emission levels as high as SSP5-8.5 are not obtained by Integrated Assessment Models (IAMs) under any of the SSPs other than the fossil fueled SSP5 socioeconomic development pathway.

(IPCC AR6 WG1 Technical Summary, TS1.3, TS1.6, Cross-Chapter Box 1.4)

Extreme Precipitation Events