RCP stands for Representative Concentration Pathway, a set of scenarios used in climate science to project how Earth’s climate could change depending on future greenhouse gas levels. Each RCP describes a different trajectory of emissions, land use, and atmospheric concentrations through the year 2100, giving scientists and policymakers a way to compare possible futures. The numbers attached to each RCP (2.6, 4.5, 6.0, and 8.5) represent the amount of extra energy trapped in the atmosphere by the end of the century, measured in watts per square meter.
What the Numbers Actually Mean
The number in each RCP refers to “radiative forcing,” which is how much additional heat energy gets trapped near Earth’s surface compared to pre-industrial times. Think of it like a dial: the higher the number, the more heat builds up. RCP 2.6 means 2.6 watts of extra energy per square meter by 2100, while RCP 8.5 means 8.5 watts per square meter. That difference translates to dramatically different temperatures, sea levels, and weather patterns.
The four RCPs were designed to span the full range of plausible futures found in published climate research. They are not predictions of what will happen. They’re “what if” scenarios that let researchers model outcomes under different assumptions about energy use, policy choices, population growth, and technology development.
The Four Scenarios at a Glance
RCP 2.6 is the most ambitious pathway. It assumes aggressive emissions cuts beginning almost immediately, with greenhouse gas concentrations peaking and then declining well before the end of the century. At the time it was first published, very few scenarios in the scientific literature reached such a low forcing level, which made it something of an aspirational benchmark. Under this pathway, global sea levels are projected to rise between 26 and 54 centimeters (roughly 10 to 21 inches) above 1986-2005 levels by 2100, with an average of about 40 centimeters.
RCP 4.5 is a stabilization scenario. It assumes that the world adopts a range of technologies and strategies to reduce emissions, bringing radiative forcing to a plateau at 4.5 watts per square meter before 2100. Emissions peak sometime around mid-century and then decline, but never as steeply as in RCP 2.6. This is sometimes described as a “moderate mitigation” future.
RCP 6.0 follows a similar arc but with a later peak. Emissions continue rising until after 2060 before stabilizing, producing more warming than RCP 4.5 but still less than the highest scenario.
RCP 8.5 is the high-end pathway, often called “business as usual.” It assumes greenhouse gas emissions keep climbing through the entire 21st century with no significant policy intervention, and atmospheric CO2 concentrations more than triple relative to pre-industrial levels. Under this scenario, sea levels are projected to rise between 45 and 82 centimeters (roughly 18 to 32 inches) by 2100, with an average of 62 centimeters. That is about 50% more sea level rise than the low pathway, and the gap widens further after 2100.
What Goes Into Each Pathway
RCPs are not simply about carbon dioxide. Each pathway includes projections for multiple greenhouse gases (including methane and nitrous oxide), aerosol particles, ozone levels, and land use changes like deforestation or agricultural expansion. These variables interact in complex ways. Aerosols, for example, can temporarily cool the climate by reflecting sunlight, while methane traps heat far more effectively than CO2 per molecule but breaks down faster. The land use projections vary widely across the four RCPs, reflecting very different assumptions about food systems, urbanization, and forest management.
Climate modeling teams around the world use these inputs to run simulations, projecting temperature, precipitation, ice melt, and ocean behavior decades into the future. Because every team uses the same set of pathways, their results can be compared directly, which is what makes RCPs so useful as a shared framework.
Who Created Them and Why
The RCPs were developed through a cooperative process led by the Integrated Assessment Modeling Consortium, involving researchers from many disciplines and institutions worldwide. They were designed for the fifth round of the Coupled Model Intercomparison Project (CMIP5), which fed into the IPCC’s Fifth Assessment Report published in 2013-2014. Before RCPs, climate scientists used an older set of scenarios called SRES (Special Report on Emissions Scenarios). RCP 8.5 is nearly identical to the old high-end SRES scenario known as A1FI.
One important distinction: the term “concentration pathway” was chosen deliberately. RCPs are not complete packages of socioeconomic storylines. They describe atmospheric concentrations and their resulting energy imbalance, but they don’t lock in a single narrative about how the world gets there. Multiple combinations of policy, technology, and economic development could produce the same concentration pathway.
RCPs vs. SSPs: The Newer Framework
If you’ve read recent climate reports, you may have seen terms like SSP1-2.6 or SSP5-8.5. These are Shared Socioeconomic Pathways, the updated scenario framework used in the IPCC’s Sixth Assessment Report (AR6, published 2021-2022). SSPs combine the radiative forcing levels from the original RCPs with detailed socioeconomic narratives describing population, economic growth, inequality, and governance.
The naming convention makes the connection clear. SSP2-4.5, for instance, pairs socioeconomic pathway 2 (a “middle of the road” development storyline) with the same 4.5 watts per square meter forcing target from RCP 4.5. The AR6 core scenarios include SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, expanding the range slightly in both directions. RCPs remain relevant because decades of published research used them, and the forcing levels carry directly into the newer framework.
Why RCPs Matter in Practice
City planners designing flood barriers, insurance companies modeling coastal risk, farmers adapting crop choices, and governments setting emissions targets all rely on projections built from these pathways. When a report says a city faces a certain probability of extreme heat days by 2050, that projection is tied to a specific RCP or SSP. The choice of pathway shapes whether the projected outcome looks manageable or catastrophic.
The spread between pathways also illustrates what is at stake in policy decisions. The difference between RCP 2.6 and RCP 8.5 for sea level rise alone is roughly a foot at the high end. For temperature, it is the difference between holding warming near 1.5-2°C and pushing past 4°C. These are not abstract numbers: they translate into real differences in crop yields, water availability, wildfire frequency, and the habitability of low-lying regions. The RCP framework exists to make those differences visible and quantifiable.