Emission Scenarios and Climate Projections

The plausible futures of greenhouse gas emissions used to drive climate models, and the range of projected warming and change they produce.

Definition

An emission scenario is an internally consistent description of a plausible future evolution of greenhouse gas and aerosol emissions, and a climate projection is the modeled response of the climate system to a chosen scenario, expressed with its associated uncertainty.

Scope

This topic covers the scenarios that describe possible future emissions and the climate projections that result from running models under them. It treats the Shared Socioeconomic Pathways and Representative Concentration Pathways, how socioeconomic storylines are translated into emissions and concentrations, the resulting projections of temperature, precipitation, and sea level, and the partition of projection uncertainty among scenario choice, model differences, and internal variability.

Core questions

• How are socioeconomic storylines translated into emission scenarios?
• What range of future warming do the scenarios produce?
• How is projection uncertainty divided among scenario, model, and variability?
• How should projections be interpreted given that scenarios are not predictions?

Key theories

Scenario framework

Future emissions are explored through a set of plausible pathways rather than predicted, so projections are conditional on the chosen scenario and span the range of human choices.

Sources of projection uncertainty

The spread in projections arises from scenario choice, differences among models, and internal variability, with their relative importance depending on the timescale and variable.

Mechanisms

Integrated assessment models translate socioeconomic storylines into trajectories of emissions and land use, which are converted to greenhouse gas and aerosol concentrations and used as forcing for climate models. Running many models under each scenario yields projections whose spread reflects scenario differences, structural model differences, and internal variability; near-term projections are dominated by variability and model spread, while late-century projections are dominated by scenario choice.

Clinical relevance

Scenarios and projections provide the quantified range of possible futures that governments and planners use to set emission targets, design adaptation measures, and assess climate risk under different policy choices.

Evidence & guidelines

The IPCC Sixth Assessment Report uses a core set of Shared Socioeconomic Pathway scenarios to project global warming from roughly 1.5 degrees Celsius under deep mitigation to over 4 degrees Celsius under very high emissions by the end of the century.

History

Scenario development progressed from the SRES scenarios of around 2000 to the Representative Concentration Pathways of the fifth assessment and then to the Shared Socioeconomic Pathways, which pair concentration pathways with explicit socioeconomic narratives in the most recent assessment.

Debates

Plausibility and use of high-emission scenarios

Whether the highest-emission pathways remain plausible as a baseline, and how scenarios should be communicated to avoid being mistaken for predictions, is actively debated.

Key figures

• Brian O'Neill
• Keywan Riahi
• Detlef van Vuuren
• Nebojsa Nakicenovic

Seminal works

• oneill2016
• ipccar6wg1

Frequently asked questions

Are climate projections predictions?

No; they are conditional on a chosen emission scenario, so they show what would happen given particular human choices rather than forecasting which future will occur.

What are the SSP scenarios?

The Shared Socioeconomic Pathways are storylines of future society, paired with emission levels, used to explore a range of plausible climate futures from strong mitigation to high emissions.