Shared Socioeconomic Pathways

Based on Wikipedia: Shared Socioeconomic Pathways

Five Roads to 2100: The Stories We Tell About Our Climate Future

Imagine five different versions of humanity, each making radically different choices over the next seventy-five years. In one future, we've embraced sustainability so thoroughly that our grandchildren barely recognize the fossil-fuel world we once lived in. In another, nationalism has fractured global cooperation so completely that each region fends for itself, hoarding resources while the planet warms. In yet another, we've doubled down on coal and oil, gambling that technology will bail us out at the last moment.

These aren't science fiction scenarios. They're the Shared Socioeconomic Pathways—known as the SSPs—and they form the backbone of how climate scientists, policymakers, and economists think about our collective future.

Why Scientists Need Stories

Here's a problem climate researchers face: they can model what happens to the atmosphere if we pump a certain amount of carbon dioxide into it, but they can't predict what humans will actually do. Will we build more coal plants or solar farms? Will global cooperation strengthen or collapse? Will population grow or stabilize?

The answers to these questions determine everything.

So instead of pretending to predict the future, scientists created five coherent narratives—internally consistent stories about how human society might evolve. Each pathway describes not just emissions, but the underlying social, economic, and political conditions that would produce those emissions. Think of them as five parallel universes, each plausible, each leading to dramatically different outcomes.

The Intergovernmental Panel on Climate Change—that's the IPCC, the United Nations body that synthesizes climate science for policymakers—adopted these pathways in their landmark Sixth Assessment Report in 2021. They've become the common language for discussing climate futures.

The Green Road: SSP1

The first pathway carries the formal name "Sustainability," but researchers nicknamed it "Taking the Green Road." It represents the optimistic scenario, though calling it optimistic undersells how much would have to change.

In this future, the world gradually but extensively shifts toward sustainable development. The transformation isn't sudden or revolutionary—it's a slow pivot, a steady reorientation of priorities. Educational investments accelerate. Health systems improve. The demographic transition—that's the shift from high birth and death rates to low ones that accompanies development—speeds up globally.

What's striking about SSP1 is the change in values it implies. Economic growth doesn't disappear, but it stops being the primary measure of success. Human well-being takes precedence. Consumption patterns shift toward lower material intensity. People buy less stuff, but the stuff they buy lasts longer and requires less energy to produce.

Inequality shrinks, both between countries and within them. The gap between rich and poor nations narrows. The gap between a country's wealthy elite and its working poor narrows too.

This isn't utopia. It's hard work sustained over decades. But it's the pathway where humanity manages the global commons—the atmosphere, the oceans, the forests—with something approaching wisdom.

The Middle Road: SSP2

The second pathway doesn't have a catchy subtitle. It's simply "Middle of the Road," and that plainness is the point.

SSP2 is the inertia scenario. Social, economic, and technological trends don't shift dramatically from historical patterns. Some countries make good progress on development; others fall short. Global institutions work toward sustainability goals but achieve them slowly, haltingly, with frequent setbacks.

Environmental systems continue degrading, though the rate of degradation slows somewhat. The intensity of resource and energy use declines, but not quickly enough to prevent continued warming. Population growth is moderate, peaking in the second half of the century before leveling off.

Income inequality? It persists. It might improve slightly here and there, but the fundamental disparities remain stubbornly intact.

This is the future where we muddle through. Not catastrophe, not triumph—just the slow grinding continuation of present trends. Many researchers consider some version of SSP2 the most likely outcome, which should concern us, because muddling through still means significant warming and its attendant disruptions.

The Rocky Road: SSP3

Now we enter darker territory. SSP3, "Regional Rivalry," imagines a world where nationalism resurges with a vengeance.

The pathway begins with familiar forces: concerns about competitiveness, anxiety about security, regional conflicts that never quite get resolved. Countries turn inward. International cooperation becomes increasingly difficult, then almost impossible. Policies shift toward national and regional security at the expense of global challenges like climate change.

Energy security and food security become national obsessions—but critically, countries pursue these goals within their own borders rather than through trade and cooperation. This is tremendously inefficient. A world where every nation tries to produce its own food and energy regardless of geography and comparative advantage is a poorer world.

Investment in education declines. Technological development slows. Economic growth sputters, but consumption remains material-intensive—people in rich countries still want their stuff, they just have less capacity to produce it cleanly.

Population dynamics split along the old development divide: low growth in industrialized nations, high growth in developing ones. Environmental concerns become a low international priority, leading to severe degradation in some regions.

This is the world of walls and barriers, of every nation for itself. Climate change becomes just another problem that falls through the cracks of fractured global governance.

The Divided Road: SSP4

If SSP3 imagines division between nations, SSP4 imagines division within them. This is the inequality pathway, subtitled "A Road Divided," and it may be the most disturbing of the five precisely because elements of it seem already underway.

In this future, highly unequal investments in human capital combine with widening disparities in economic opportunity and political power. A gap emerges—and then widens into a chasm—between two worlds occupying the same planet.

On one side: an internationally connected society participating in the knowledge economy. These people have access to excellent education, cutting-edge technology, global networks. They work in capital-intensive sectors and live in gleaming urban centers. For them, the future looks rather good.

On the other side: a fragmented collection of lower-income, poorly educated communities stuck in labor-intensive, low-technology work. They're disconnected from global flows of capital and information. For them, the future looks grim.

Social cohesion degrades catastrophically. Conflict and unrest become common features of daily life. The two worlds barely understand each other.

Technology develops rapidly, but only in the high-tech sector serving the global elite. The energy system diversifies—investments flow to both carbon-intensive fuels and low-carbon alternatives—but the benefits accrue unevenly. Environmental policies exist, but they focus on local concerns in wealthy areas. The poor breathe dirty air.

This is cyberpunk without the neon aesthetic. It's Elysium without the space station. It's the world where inequality metastasizes from a problem into a permanent structural feature of civilization.

The Highway: SSP5

The final pathway carries a name that sounds almost admiring: "Taking the Highway." Its subtitle is "Fossil-fueled Development," which captures the essential gamble.

SSP5 imagines a world with tremendous faith in markets, innovation, and human ingenuity. Global markets integrate further. Investments in health, education, and institutions enhance human capital. Societies become more participatory. By many measures, this sounds like progress.

But here's the catch: all this development is coupled with aggressive exploitation of fossil fuel resources. Coal, oil, natural gas—if it burns, we burn it. Energy-intensive lifestyles spread globally. The assumption is that we'll innovate our way out of any problems this creates.

The economy grows rapidly. Population peaks and then declines. Local environmental problems like urban air pollution get managed successfully—technology solves those. But the carbon keeps accumulating in the atmosphere.

SSP5 places its bet on geoengineering, on technological fixes applied at planetary scale. Maybe we'll find a way to pull carbon out of the air. Maybe we'll reflect sunlight back into space with aerosols. Maybe some intervention we haven't invented yet will save us.

This is the high-roller's pathway. If the gamble pays off, it looks brilliant—rapid development without the painful transitions SSP1 requires. If it fails, it looks catastrophic—we've pumped the atmosphere full of greenhouse gases while waiting for a rescue that never arrived.

From Stories to Numbers

The pathways are qualitative narratives, but climate modeling requires quantitative inputs. So researchers use something called Integrated Assessment Models—mathematical frameworks that simulate interactions between human systems and Earth systems—to translate each pathway into specific projections.

These models produce estimates of population, economic output, energy use, land use, and ultimately greenhouse gas emissions for each pathway. Different research groups run different models, generating a range of scenarios that share the same underlying narrative but differ in details.

The IPCC combined these socioeconomic pathways with estimates of radiative forcing—that's the energy imbalance in Earth's climate system, measured in watts per square meter—to create specific scenarios for their assessment. The naming convention combines the pathway number with the forcing level expected in 2100.

SSP1-1.9, for instance, means the sustainability pathway with 1.9 watts per square meter of forcing by century's end. SSP5-8.5 means the fossil-fuel pathway with a whopping 8.5 watts per square meter—a dramatic energy imbalance that would produce severe warming.

What's Actually Likely?

The IPCC deliberately avoided assigning probabilities to these scenarios. That's a methodological choice—they're presenting possibilities, not predictions.

But researchers outside the formal IPCC process have offered assessments. In 2020, climate scientist Zeke Hausfather and colleagues argued that SSP5-8.5—the high-emission, fossil-fuel pathway—had become highly unlikely given trends in renewable energy costs and adoption. They considered SSP3-7.0, the regional rivalry scenario, unlikely as well. SSP2-4.5, the middle-of-the-road pathway, seemed most probable to them.

Five years later, in January 2025, Hausfather revised that assessment. Current trends, he wrote, suggest warming by 2100 will likely fall between 2.9 and 3.7 degrees Celsius above pre-industrial levels. That range corresponds roughly to SSP3-7.0—the regional rivalry pathway they had previously considered unlikely.

This isn't good news. Nearly three degrees of warming would transform coastlines, agriculture, ecosystems, and human settlement patterns. Four degrees would be genuinely catastrophic for large portions of the planet's population.

The Utility of Imagined Futures

Why do these scenarios matter? Not because they predict the future—they explicitly don't. They matter because they structure our thinking about choices.

When policymakers ask what happens if we continue current trends, they're essentially asking about SSP2. When activists push for rapid decarbonization, they're arguing for SSP1. When nationalists advocate closing borders and prioritizing domestic industry, they're steering toward SSP3. When Silicon Valley techno-optimists promise innovation will solve everything, they're betting on something like SSP5.

The pathways make the implications of these choices explicit. They translate political positions into climate outcomes. They force us to confront the connection between how we organize society and what happens to the physical planet.

Perhaps most importantly, they remind us that the future isn't fixed. We're not passive observers watching a predetermined climate unfold. We're actors making decisions that will determine which pathway we travel—or whether we forge some new road not yet imagined.

The story isn't written yet. But the possible endings are becoming clearer, and some of them are endings we desperately want to avoid.

Beyond the Original Five

Climate modeling continues to evolve. Researchers are now working on downscaled versions of the SSPs—scenarios tailored to specific regions rather than the entire globe. European scientists, for instance, are developing pathways specific to agricultural and food systems across different parts of the continent.

These regional scenarios combine the socioeconomic pathways with something called Representative Concentration Pathways—that's the RCPs, an earlier framework focused specifically on greenhouse gas concentrations rather than the broader social narratives. By linking the two frameworks at regional scales, researchers hope to generate more useful guidance for local and national policymakers.

The science of imagined futures keeps getting more sophisticated. The scenarios keep getting more detailed. What remains constant is the fundamental insight: to understand where we're going, we first have to imagine where we might go. The five roads to 2100 are maps of possibility, and the route we take depends on choices we're making right now.