Representative Concentration Pathway

Based on Wikipedia: Representative Concentration Pathway

The Climate Futures We're Choosing Between

Here's a number that should make you sit up straight: by the year 2300, Earth could be anywhere from roughly the same temperature as today to twelve degrees Celsius hotter. That's not a typo. The difference between those two futures isn't fate—it's the sum of decisions we make over the coming decades.

Scientists needed a way to talk about these possible futures. Not predictions, exactly, but scenarios—carefully constructed "what if" stories backed by physics and chemistry. They call them Representative Concentration Pathways, or RCPs, and understanding them is essential to making sense of any climate discussion happening today.

What Exactly Is a Concentration Pathway?

The name itself tells you something important. Notice it says "concentration," not "emissions." This is a crucial distinction.

When we burn fossil fuels, we emit greenhouse gases into the atmosphere. But not all of those emissions stay there. The ocean absorbs some. Trees and plants soak up some. The actual concentration—how much ends up accumulating in the atmosphere—depends on these complex interactions between human activity and natural systems.

Think of it like filling a bathtub. Emissions are the water flowing from the faucet. Concentration is the actual water level in the tub. But this bathtub has drains—the oceans and forests that pull carbon back out. The RCPs describe different water levels we might end up with, regardless of exactly how we got there.

The Intergovernmental Panel on Climate Change, commonly known as the IPCC, formally adopted these pathways for their research. The IPCC is the United Nations body that synthesizes climate science from researchers around the world. When you hear about scientific consensus on climate change, the IPCC reports are usually what people are referencing.

The Numbers in the Names

The original four pathways were labeled RCP2.6, RCP4.5, RCP6, and RCP8.5. Those numbers aren't arbitrary. They represent something called radiative forcing, measured in watts per square meter.

Radiative forcing sounds technical, but the concept is simple. It measures how much extra energy the Earth's atmosphere is trapping compared to pre-industrial times—specifically, the year 1750, before the Industrial Revolution began transforming human civilization. A higher number means more energy trapped, which means more warming.

So RCP8.5 means the atmosphere would be trapping 8.5 additional watts of energy per square meter by the year 2100. RCP2.6 would trap only 2.6 extra watts. That threefold difference in energy translates to dramatically different worlds.

The Full Spectrum of Futures

The IPCC has since added more pathways to fill in the gaps and explore the extremes. Here's the current lineup:

RCP1.9 represents the most ambitious pathway—limiting warming to below 1.5 degrees Celsius. This is the aspirational target of the Paris Agreement, the international climate accord signed in 2015. Achieving it would require unprecedented global cooperation and rapid transformation of energy systems.

RCP2.6 is what scientists call "very stringent." It requires carbon dioxide emissions to start declining by 2020 (a deadline we've already missed, though not by much), reaching zero by 2100. Even more striking, it requires what's called net negative emissions toward the end of the century. That means humanity would need to actively pull more carbon dioxide out of the atmosphere than we put in—about two billion tons per year. This pathway would likely keep warming below two degrees Celsius.

RCP3.4 sits between the stringent scenarios and more moderate ones. A variant of this pathway includes significant removal of greenhouse gases from the atmosphere—essentially betting on future technologies that can scrub carbon from the air at scale.

RCP4.5 is described as the intermediate scenario. Emissions peak around 2040, then decline. Some researchers argue this is actually the most realistic baseline scenario if you account for the simple fact that fossil fuels are finite. We'll eventually run out of easily accessible coal, oil, and gas, forcing a transition whether we plan for it or not.

Under RCP4.5, carbon dioxide emissions need to roughly halve between 2050 and 2100. Methane would need to level off by 2050 and decline modestly thereafter. This pathway would likely result in warming between two and three degrees Celsius by 2100, with sea levels about 35 percent higher than the most stringent pathway.

Here's a sobering detail: many plant and animal species would be unable to adapt to RCP4.5 conditions. The intermediate scenario is already too fast for much of the natural world.

RCP6 projects emissions peaking around 2080—four decades later than RCP4.5—before declining. By 2100, carbon dioxide concentrations would reach about 670 parts per million, compared to roughly 420 parts per million today and 280 parts per million before the Industrial Revolution. Global temperatures would rise about three to four degrees Celsius.

RCP7 represents a baseline outcome rather than a target anyone is aiming for. It's what might happen through inertia—not the worst case, but certainly not good.

RCP8.5 is the scenario that haunts climate scientists' nightmares. Emissions continue rising throughout the entire 21st century, never peaking, never declining. This has generally been used as the basis for worst-case climate projections.

The Debate Over the Worst Case

RCP8.5 has become controversial in recent years. When it was developed, researchers created it by extrapolating existing trends forward—heavy coal use continuing indefinitely, minimal policy intervention, population growth at the high end of projections.

Since the IPCC's Fifth Assessment Report came out in 2014, many analysts have argued that RCP8.5 overestimates how much coal we'd actually burn. Renewable energy has gotten dramatically cheaper. Coal plants are closing. Many countries have committed to phase out coal entirely.

But—and this is a significant but—there are wild cards that could push temperatures even higher than the RCP8.5 models predict. These are called carbon cycle feedbacks. As permafrost melts in the Arctic, it releases methane and carbon dioxide that's been frozen for thousands of years. Warming oceans absorb less carbon. Droughts kill forests that would otherwise soak up emissions. These feedback loops could amplify warming beyond what any scenario projects.

So while RCP8.5 might overestimate human emissions, it might underestimate natural system responses. The uncertainty cuts both ways.

Scientists still use RCP8.5 for projecting conditions through mid-century—roughly the 2040s and 2050s—because current emissions trajectories remain uncomfortably close to it. The divergence between pathways becomes more pronounced later in the century.

The Human Choices That Separate These Worlds

What actually has to happen for each pathway? The specifics are illuminating.

Take methane, the second most important greenhouse gas after carbon dioxide. Under RCP2.6, methane emissions need to fall to about half of 2020 levels. Under RCP4.5, they need to stabilize at about 75 percent of 2040 levels. The difference might sound modest on paper, but methane comes from rice paddies, cattle, natural gas leaks, and landfills. Transforming all those systems requires rethinking agriculture, energy infrastructure, and waste management simultaneously.

Sulfur dioxide follows a different logic. This pollutant primarily comes from burning coal and oil. It actually has a cooling effect—sulfur particles in the atmosphere reflect sunlight back into space. As we clean up air pollution (which kills millions of people annually through respiratory disease), we inadvertently remove this accidental cooling effect. Under RCP2.6, sulfur dioxide emissions decline to about 10 percent of 1980s levels. Under RCP4.5, they decline to about 20 percent. Either way, we lose that inadvertent parasol.

All pathways require negative emissions to some degree—actively removing carbon dioxide from the atmosphere rather than just reducing what we add. For the stringent pathways, these negative emissions reach about two billion tons per year. That's roughly the annual emissions of the entire European Union, except flowing in the opposite direction. We'd need forests, direct air capture machines, or some combination thereof working at enormous scale.

What the Numbers Mean for the Planet

By mid-century—the 2040s to 2060s—global temperatures are projected to rise 0.4 to 2.6 degrees Celsius above late 20th century averages, depending on which pathway we follow. By century's end, the range widens to 0.3 to 4.8 degrees.

Those numbers might sound abstract. Sea level makes them concrete.

By mid-century, global mean sea level is projected to rise 17 to 38 centimeters—roughly half a foot to a foot and a quarter. By 2100, the range is 26 to 82 centimeters. But the differences between pathways grow more dramatic as you look further ahead.

The IPCC also modeled what happens beyond 2100, extending the pathways to the year 2300. These extended projections reveal the true stakes of current decisions.

Under the extended RCP2.6 pathway, which assumes humanity achieves sustained net negative emissions after 2070, atmospheric carbon dioxide concentrations actually decline. By 2300, they'd return to around 360 parts per million—lower than today. Global warming in the late 23rd century would range from essentially zero to 1.2 degrees Celsius above late 20th century levels. The planet would be healing.

Under the extended RCP8.5 pathway, carbon dioxide concentrations reach approximately 2,000 parts per million by 2250. That's nearly seven times pre-industrial levels. The late 23rd century would be 3 to 12.6 degrees Celsius warmer.

Let that sink in. The difference between these pathways, projected over the next three centuries, is the difference between a planet returning toward historical norms and a planet transformed beyond recognition. Twelve degrees of warming would render much of the tropics uninhabitable for humans. Sea level rise would be measured in meters, not centimeters. The geography of civilization would be redrawn.

The Evolution of Climate Scenarios

The RCPs weren't the first attempt to model possible futures. They replaced something called the Special Report on Emissions Scenarios, published in 2000, which took a different approach. Those older scenarios started with assumptions about economic development, population growth, and technology—then calculated what emissions would result.

The RCPs work somewhat in reverse. They specify the concentration endpoint, then figure out what pathways of human activity could lead there. This makes them more flexible. Multiple combinations of population, technology, and policy could arrive at the same concentration level.

The IPCC's Sixth Assessment Report, released in 2021 and 2022, added another layer: Shared Socioeconomic Pathways, or SSPs. These describe different possible social and economic futures—worlds of sustainable development, regional rivalry, fossil-fueled growth, or inequality. The SSPs and RCPs can be combined to explore how different societal choices lead to different climate outcomes.

This increasing sophistication reflects something important. Climate change isn't just a physics problem. It's a human problem. The carbon cycle, the economy, technology, politics, and individual choices all interact in ways that simple models can't capture. The scenarios have evolved to embrace that complexity rather than pretend it doesn't exist.

The View from 2025

Where do we stand now? The honest answer is somewhere between the pathways, and it changes year by year based on policy decisions, technological developments, and economic shifts.

Renewable energy has grown faster than almost anyone predicted. Electric vehicles are reaching mass adoption in many markets. Dozens of countries have net-zero emissions targets on the books.

At the same time, global emissions continue rising in aggregate. The gap between pledges and action remains wide. Some feedback effects are already visible—Arctic ice is declining faster than models predicted, for instance.

The RCPs aren't destiny. They're tools for thinking about choices. The numbers—1.9, 2.6, 4.5, 6, 7, 8.5—represent different worlds we might inhabit. Understanding what separates them is the first step toward choosing which one we want to build.

The next few decades will determine which pathway we're actually on. By mid-century, the divergence between futures becomes increasingly difficult to reverse. The physics of the climate system has momentum. Carbon dioxide persists in the atmosphere for centuries. Decisions made now cast long shadows.

But that's precisely why the scenarios exist—not to predict an inevitable future, but to clarify what's at stake in the choices we make today.