Short answers, pulled from the story.
Shared Socioeconomic Pathways are five climate change scenarios that project possible socioeconomic global changes up to 2100. Defined in the IPCC Sixth Assessment Report in 2021, they provide both qualitative narratives and quantitative data on population, urbanization, and GDP per capita. They are used alongside Integrated Assessment Models to derive greenhouse gas emissions scenarios under different climate policies.
There are five Shared Socioeconomic Pathways: SSP1 (Sustainability, "Taking the Green Road"), SSP2 ("Middle of the Road"), SSP3 (Regional Rivalry, "A Rocky Road"), SSP4 (Inequality, "A Road Divided"), and SSP5 (Fossil-fueled Development, "Taking the Highway").
SSP1 describes a world that shifts gradually toward more sustainable and inclusive development, with improving management of global commons, reduced inequality within and between countries, and a shift in economic emphasis from material growth to human well-being. Consumption moves toward lower resource and energy intensity.
SSP5 projects that global population peaks and then declines during the 21st century. This occurs alongside rapid economic growth, integrated global markets, strong investment in health and education, and widespread reliance on abundant fossil fuel resources.
The IPCC Sixth Assessment Report assessed temperature outcomes for all five SSPs using a naming convention in the format SSPx-y.z. Each label combines the underlying SSP with the expected level of radiative forcing at 2100, measured in watts per square meter. The range across all five scenarios is 1.9-8.5 watts per square meter.
SSP3 is driven by resurgent nationalism, regional conflict, and a focus on domestic security that reduces international cooperation. Countries pursue energy and food self-sufficiency within their own regions, investment in education and technology declines, and a low international priority for environmental concerns leads to strong degradation in some regions.