Special Report on Emissions Scenarios

Based on Wikipedia: Special Report on Emissions Scenarios

In 2010, humanity broke a record nobody wanted to set. Greenhouse gas emissions jumped six percent in a single year—blowing past even the worst-case predictions that climate scientists had published just three years earlier. The models weren't pessimistic enough.

This uncomfortable fact sits at the heart of one of the most important and controversial documents in climate science: the Special Report on Emissions Scenarios, usually called SRES. Published in 2000 by the Intergovernmental Panel on Climate Change (the IPCC, that massive international body of scientists who assess climate research), this report attempted something audacious. It tried to imagine forty different futures for human civilization—and then calculate how much carbon dioxide each future would pump into the atmosphere.

Why Scientists Need Crystal Balls

Climate models are sophisticated physics simulations. Feed them the composition of the atmosphere, and they'll calculate how energy flows through the Earth system—how much sunlight gets absorbed, how heat moves between oceans and air, where storms will form. But here's the catch: the composition of the atmosphere depends on us.

How much coal will China burn in 2050? Will India leapfrog fossil fuels straight to solar? Will population peak at nine billion or keep climbing? Will we invent fusion power, or will we keep drilling for oil until the last barrel runs dry?

Nobody knows. So climate scientists do the next best thing: they imagine multiple futures and run the models against each one. The SRES was the IPCC's attempt to systematically explore that space of possibilities.

Forty Futures, Four Families

The SRES didn't just produce one scenario or two. It produced forty distinct visions of the twenty-first century, organized into four broad families. Think of these families as archetypes—different answers to two fundamental questions about humanity's trajectory.

The first question: Will the world become more integrated or more fragmented? Will nations cooperate on global challenges, or will they retreat into regional self-reliance?

The second question: Will we prioritize economic growth above all else, or will we balance growth against environmental sustainability?

Cross these two axes, and you get the four scenario families.

The A1 World: Globalized and Growth-Focused

Imagine a world where borders matter less every decade. Trade flows freely. Technologies spread rapidly from innovators to adopters. Income gaps between rich and poor countries narrow dramatically. Population rises to nine billion by 2050, then gradually declines as prosperity reduces birth rates everywhere.

This is the A1 family. It's a world of rapid economic growth—global GDP multiplying by factors of five to twenty-five over the century. But within this family, there's a crucial fork in the road: what powers all that growth?

The A1FI variant (the FI stands for "Fossil Intensive") assumes we keep burning coal, oil, and gas. It's the high-emissions nightmare scenario. The A1T variant ("T" for technology) imagines a rapid transition to non-fossil energy sources—renewables and perhaps nuclear. And A1B splits the difference, balancing all energy sources.

Same economic trajectory. Radically different atmospheric outcomes.

The A2 World: Fragmented and Growth-Focused

Now imagine the opposite of globalization. Nations turn inward. Self-reliance becomes the governing philosophy. Regional identities strengthen while global cooperation weakens. Population keeps growing because the demographic transition—where prosperity leads to lower birth rates—happens more slowly or not at all in many regions.

This is A2. Economic development continues, but it's uneven and regionally oriented. Technologies spread slowly because there's less international exchange. And emissions? They're high. Without global cooperation or rapid technology transfer, the world stays locked into carbon-intensive development paths.

The B1 World: Globalized and Sustainable

What if we got the best of both? The B1 family imagines a world that's just as integrated as A1—rapid economic growth, population peaking at nine billion then declining, technologies spreading globally. But with a crucial difference: the economy dematerializes.

In B1, growth increasingly comes from services and information rather than manufacturing and resource extraction. Clean technologies win in the marketplace. Resource efficiency improves dramatically. Global solutions emerge for economic, social, and environmental challenges.

Some B1 scenarios project that fossil fuel consumption in 2100 would actually be lower than it was in 1990. That's not just slowing growth—that's reversing it.

The B2 World: Fragmented but Sustainable

Finally, B2 imagines local solutions to sustainability challenges. Think less "United Nations climate treaty" and more "every city and region finding its own path." Population grows, but more slowly than in A2. Economic development proceeds at an intermediate pace. Technological change is less rapid and more fragmented than in the globalized scenarios.

It's a world of grassroots environmentalism rather than top-down agreements. Emissions are moderate—higher than B1, lower than A1FI or A2.

What the Scenarios Actually Predicted

Using these forty scenarios, scientists projected what the atmosphere might look like in 2100. The range was enormous.

In 1750, before the Industrial Revolution, atmospheric carbon dioxide stood at about 280 parts per million. By 2000, it had risen to 368 parts per million. Where would it go next?

Under the six main SRES scenarios, the IPCC projected concentrations ranging from 540 to 970 parts per million by 2100—roughly double to triple the levels of 2000. But that range had uncertainty baked in. Factor in unknowns about how much carbon forests and oceans would absorb, and the full range stretched from 490 to 1,260 parts per million.

The high end of that range represents a radically different planet. Scientists estimate that carbon dioxide hasn't reached 1,000 parts per million in at least three million years, and possibly much longer. At such levels, we'd be running an experiment on Earth's climate that has no precedent in human history.

The Scenarios Meet Reality

Then the twenty-first century actually happened.

During the 1990s, carbon emissions from fossil fuels grew at about 1.1 percent per year. That was consistent with most SRES scenarios. But during the 2000s, growth accelerated to three percent annually—faster than thirty-five of the forty scenarios had projected.

And that 2010 record? The six percent jump? It exceeded even A1FI, the "worst case" scenario explicitly designed to represent high-emissions futures.

The scenarios weren't pessimistic enough. Or perhaps more precisely: the scenarios assumed rational self-interest would eventually bend the emissions curve. The world proved otherwise.

The Great MER-PPP Debate

Not everyone thought the scenarios were too optimistic. In the early 2000s, two critics—Ian Castles and David Henderson—argued they were too pessimistic, for a subtle but important reason involving how economists measure wealth across countries.

Here's the issue. A dollar buys more in India than in America. If you want to compare living standards across countries, you need to account for these differences in purchasing power. Economists call this the "purchasing power parity" approach, or PPP for short.

But the SRES scenarios used a different method: market exchange rates, or MER. This is simpler—just convert everything to dollars at prevailing exchange rates—but it makes poor countries look poorer than they really are, in terms of what people can actually buy.

Castles and Henderson argued this distorted the projections. If developing countries were actually richer than MER suggested, then they had less "catching up" to do. Less catching up meant less rapid growth. Less rapid growth meant less rapid increase in energy consumption. And less energy meant less emissions.

The IPCC fired back. The critics had a point about measuring current wealth—but they'd missed a compensating error. Using PPP instead of MER would also change calculations of energy intensity (how much energy it takes to produce a dollar of economic output). These two effects largely cancel out. The atmospheric projections wouldn't change much.

Castles and Henderson eventually acknowledged they'd been wrong about emissions being overestimated. But they maintained that the choice between MER and PPP still matters for a different reason: it changes who's responsible.

Under PPP accounting, China and India have a much smaller share of global emissions relative to their economic size. Under MER accounting, their share looks larger. This isn't just academic bookkeeping. It affects the politics of climate negotiations—who should bear the burden of reducing emissions, and who should pay for adaptation.

The Fossil Fuel Availability Debate

A different group of critics attacked the scenarios from the opposite direction. They argued that the SRES projected too much fossil fuel burning—not because growth would slow, but because we'd simply run out of coal, oil, and gas.

This criticism cut at a fundamental assumption buried in the SRES: that fossil fuels would remain available in sufficient quantities to power all these scenarios. The report leaned heavily on a 1997 study by Hans-Holger Rogner, which argued that there are enough hydrocarbon molecules in the Earth's crust to sustain production for centuries.

Maybe. But molecules in the crust aren't the same as fuel in the tank. You have to extract them, and extraction gets progressively harder and more expensive as you deplete the easy deposits. Peak oil theorists argued that production limits would constrain emissions long before reserves ran out.

The criticism became pointed. Multiple studies found that most SRES scenarios projected coal production far exceeding realistic long-term possibilities. One analyst called the projections between 2020 and 2100 "absolutely unrealistic." Another identified what they called a "return to coal hypothesis"—an assumption that the world would dramatically increase coal use that seemed disconnected from physical and logistical realities.

There's an irony here. If the critics were right, then climate change might be self-limiting: we'd run out of fossil fuels before we could completely destabilize the climate. Cold comfort, perhaps, since the damage from burning even the "limited" realistic supply could still be severe.

The Neutrality Problem

The IPCC insisted that all SRES scenarios were "neutral"—none represented good or bad futures, and none was more likely than any other. This was politically necessary. The IPCC couldn't be seen as advocating for particular policies or predicting particular outcomes.

But this neutrality created its own problems. The scenarios explicitly excluded climate-specific policies. None of them assumed the world would actually do anything about climate change. The Kyoto Protocol, signed in 1997, wasn't modeled. Future climate agreements weren't modeled. Even the most environmentally-friendly scenarios (B1 and B2) achieved their lower emissions through economic and social trends, not through deliberate climate policy.

This created a paradox. The scenarios were baseline projections—what would happen without intervention. But they were being used to justify intervention. If the high-emissions scenarios came true, the results would be catastrophic. That was precisely the argument for policy action. But if policies worked, the scenarios would be wrong.

Some researchers tried to quantify this paradox. One study compared the SRES scenarios against a "no policy" projection—what would happen if the world truly did nothing about emissions. They found that most SRES scenarios actually fell outside the likely range of their no-policy projection. The scenarios were more consistent with a world that was already making efforts to stabilize greenhouse gas concentrations.

In other words, the "neutral" scenarios weren't neutral at all. They implicitly assumed some level of climate-aware decision-making, even in the high-emissions cases.

The Scenarios That Didn't Include Disasters

There's something strange about imagining forty futures for human civilization over the next century. The SRES scenarios span enormous variation in economic growth, technological development, and international cooperation. But they all share one notable assumption: history continues smoothly.

None of the scenarios include wars. None include major financial collapses. None include pandemics (a notable absence, viewed from 2025). None include environmental catastrophes or political revolutions or any of the discontinuities that have punctuated every previous century of human history.

The IPCC explicitly noted this. The scenarios were meant to explore the impact of different development paths, not to predict specific events. But this creates another layer of uncertainty that's hard to quantify. History is lumpy. The smooth curves of the SRES scenarios assume a smoothness that history rarely delivers.

Superseded but Not Forgotten

In 2014, the IPCC's Fifth Assessment Report officially retired the SRES scenarios, replacing them with a new framework called Representative Concentration Pathways, or RCPs. The RCPs worked differently: instead of modeling from socioeconomic assumptions to emissions to concentrations, they started with target concentration levels and worked backward to the policies that might achieve them.

But the SRES scenarios had already done their work. For over a decade, they framed how scientists and policymakers thought about climate futures. They established the concept of scenario families. They embedded the recognition that climate change isn't just a physics problem—it's deeply entangled with economic development, technological change, and international cooperation.

And they established an uncomfortable pattern that would continue with the RCPs: reality kept outpacing the projections. The scenarios kept being too optimistic. The world kept emitting more than expected.

Perhaps the most important lesson of the SRES is how hard it is to imagine the future—and how important it is to try anyway. The scenarios were wrong in many specifics. But they provided a framework for thinking systematically about uncertainty, a way to ask "what if" in a rigorous and comparable way.

That's what climate models need. Not a single prediction, but a range of possibilities. Not a crystal ball, but a structured way of thinking about the choices we face and the futures they might create.

The choices we make in the next few decades will determine which scenario family we end up in—or whether we end up somewhere the scenarios never imagined at all.