The Sixth Assessment Report (AR6) introduced the concept of ‘Shared Socio-economic Pathways (SSPs), a set of five narrative scenarios that describe a different socio-economic future, based on varying assumptions about economic growth, technological development, population growth, urbanization, and other factors (Table 14).

SSP	Pathway
SSP1	Sustainability Characterized by a world making significant efforts towards sustainability, with a focus on inclusive development, reducing inequality, and environmental protection. Low challenges to mitigation and adaptation.
SSP2	Middle of the Road Represents a world that follows a path of socio-economic development similar to historical patterns. It assumes moderate population growth, economic development, and technological change. Medium challenges to mitigation and adaptation.
SSP3	Regional Rivalry Describes a fragmented world with a resurgence of nationalism, regional conflicts, and a focus on self-reliance. Economic growth is uneven, and global cooperation is limited, leading to high challenges to both mitigation and adaptation.
SSP4	Inequality Characterized by increasing inequalities both within and between countries. High-income groups invest in advanced technologies and adaptation, while a large portion of the global population faces barriers to development. High challenges to adaptation, low challenges to mitigation for the wealthier countries.
SSP5	Fossil-fueled Development Describes a world with rapid economic growth driven by fossil fuel use and technological advancement. Environmental concerns are secondary, leading to high challenges to mitigation but low challenges to adaptation due to economic growth.

The SSPs are conceptually similar to the SRES storylines used in the IPCC’s Third and Fourth Assessment Reports, but the former reflect advancements in scenario development and climate research.

In AR6 SSPs are used in conjunction with various RCP levels, which helps to understand better the interactions between various elements of socio-economic developments and climate change. The combinations are denoted by symbols, such as  SSP126, SSP245, … SSP585.  Thus, SSP585 is socio-economic scenario SSP5 leading to a radiative forcing of 8.5 W/m² by the year 2100, etc.

As of mid-2024, the world appears to be on a trajectory resembling the SSP245 pathway. This scenario, often considered a “middle-of-the-road” scenario, assumes that current policies will continue with modest improvements. In this pathway, global carbon emissions are expected to plateau around mid-century, with a slow decline afterward. The SSP245 scenario is often compared to the A1B scenario from the Fourth Assessment Report due to similarities in their underlying assumptions and projected outcomes.

More details on Shared Socio-economic Pathway SSP245.

Evaluating the SSPs

Compared to the SRES scenarios, the SSPs offer a more sophisticated and flexible approach to socio-economic scenario development. By combining different SSPs with multiple RCPs, they enable the exploration of a broader range of possible futures. Key advantages include:
• Enhanced analysis of socio-economic challenges for mitigation and adaptation: unlike the SRES scenarios, the SSPs explicitly address both mitigation (reducing emissions) and adaptation (responding to climate impacts).
• Advanced Integrated Assessment Models (IAMs): the SSPs leverage IAMs that provide greater regional detail, improved representation of technology and policy options, and detailed sectoral analyses, including land use, energy systems, and economic dynamics.

Despite these improvements, the SSP framework has notable weaknesses:
• Limited local and heterogeneity representation: the SSPs focus primarily on global and large-scale regional trends, often failing to capture critical local dynamics and social heterogeneity.
• Insufficient exploration of socio-economic discontinuities: potential disruptions, such as political or economic shocks, large-scale transformations (e.g., rapid transitions to sustainability or societal collapse), or feedback loops between climate impacts and socio-economic systems, remain underexplored. For example, severe climate events disrupting economic growth are not fully integrated into the scenarios.
• Complexity and limited accessibility: the academic nature of the SSPs can make them difficult to translate into actionable insights for policymakers and planners.

Given these challenges, it is clear that the SSPs, while a significant improvement over their predecessors, are not the final word in scenario development for climate change assessment. Future frameworks will likely build on their strengths while addressing their limitations.