Shared Socioeconomic Pathways

• 1. Introduction

  ›

  Shared Socioeconomic Pathways arrived in the IPCC Sixth Assessment Report in 2021 as five named portraits of the world at 2100. Scientists gave each one a road metaphor. "Taking the Green Road." "Middle of the Road." "A Rocky Road." "A Road Divided." "Taking the Highway." Each label is shorthand for a whole system of choices about energy, economic growth, inequality, education, and global cooperation. These are not predictions. They are structured stories, each internally consistent, built to explore different possible futures for human civilization. The question they raise is not which future will come. It is what would have to be true about the world for each of these roads to be the one humanity travels.

• 2. A Framework for Futures

  ›

  Integrated Assessment Models, known as IAMs, are the tools that translate each SSP's storyline into projected greenhouse gas emissions under different climate policies. Each SSP runs on two tracks to supply those models. The first track is qualitative: a narrative storyline that describes how the logic of the scenario connects its elements to each other. The second is quantitative, providing data on national population, urbanization, and GDP per capita.

  Ongoing work on European shared socioeconomic pathways specifically targets agricultural and food systems, combining them with Representative Concentration Pathways to produce locally specific scenarios. The first pathway in the framework, SSP1, carries the subtitle "Sustainability."

• 3. The Green Road

  ›

  "The world shifts gradually, but extensively, toward a more sustainable path." Those words open SSP1's own narrative. The key organizing principle is inclusive development that respects what the scenario calls "predicted environmental boundaries." Management of the global commons slowly improves. Investment in health and education accelerates the demographic transition, reducing population growth over the century. Economic success broadens its definition: this world measures progress by human well-being rather than material growth alone. Inequality falls both between countries and within them. Consumption shifts toward lower resource use and lower energy intensity. SSP1's narrative attributes this transformation to "an increasing commitment to achieving development goals." That commitment must sustain across the full century. SSP2, the next scenario on the spectrum, tests what the world looks like without any dramatic shift from the status quo.

• 4. Middle of the Road

  ›

  In SSP2, global and national institutions "work toward but make slow progress in achieving sustainable development goals." That phrasing from the scenario's own narrative captures its essential character. Social, economic, and technological trends do not shift markedly from historical patterns. Development and income growth proceed unevenly: some countries make relatively good progress, while others fall short of expectations. Environmental systems experience degradation, though some areas improve. Population growth is moderate and levels off in the second half of the century. Income inequality persists or improves only slowly. SSP2 does not project crisis. It projects continuation, with the same vulnerabilities the world has always carried and without sufficient momentum to resolve them. SSP3, the next pathway, describes what happens when those vulnerabilities are actively compounded rather than merely endured.

• 5. Rivalry and Division

  ›

  SSP3's driving force is resurgent nationalism. Concerns about competitiveness, security, and regional conflict push countries to focus increasingly on domestic and, at most, regional issues. Policies shift over time toward national and regional security. Nations pursue energy and food security within their own regions at the expense of broader development. Investment in education and technological development declines. Economic development is slow; consumption becomes more material-intensive. Population dynamics diverge: industrialized nations see low growth, while developing nations see high growth. A low international priority for environmental concerns leads, in SSP3's own language, to "strong environmental degradation in some regions."

  SSP4 begins from a different kind of failure: highly unequal investment in human capital. Growing disparities in economic opportunity and political power widen stratification both across and within countries. A gap opens over time between two distinct societies. One is internationally connected, contributing to knowledge- and capital-intensive sectors of the global economy. The other is a fragmented collection of lower-income, poorly educated populations working in labor-intensive, low-tech conditions. Social cohesion degrades as this gap widens. Conflict and unrest become increasingly common. SSP4's energy sector expands in two directions: toward carbon-intensive fuels including coal and unconventional oil, and toward low-carbon energy sources. Environmental policy focuses on local concerns in middle- and high-income areas. Technology development remains high within the connected economy, while the majority of the world's population works in conditions disconnected from it. Both SSP3 and SSP4 leave environmental governance outpaced by development; SSP5 asks whether faster development, sustained by fossil fuels, might eventually close that gap.

• 6. Taking the Highway

  ›

  SSP5 places its faith in competitive markets, innovation, and participatory societies as the engines of rapid technological progress. Global markets become more integrated. Strong investment flows into health, education, and institutions to build human and social capital. Global population peaks and then declines during the 21st century. At the same time, this drive for development is coupled with the exploitation of abundant fossil fuel resources. Resource- and energy-intensive lifestyles spread worldwide. The global economy grows rapidly. Local environmental problems, including air pollution, are successfully managed. SSP5 holds that humanity can manage social and ecological systems effectively, including through geo-engineering if necessary. The IPCC Sixth Assessment Report used all five scenarios to project temperature outcomes for 2100.

• 7. The Temperature Reckoning

  ›

  Each of the five temperature projections carries a label in the format SSPx-y.z. The label combines the underlying SSP number with the expected level of radiative forcing at the year 2100, measured in watts per square meter. Across all five scenarios, that radiative forcing spans 1.9-8.5 watts per square meter. The naming convention encodes the full spectrum: SSP1-1.9 at the low end and SSP5-8.5 at the high end.