Climate scenarios and what they help us explore

DOI: https://doi.org/10.58248/RR108

Why climate science needs scenarios 

Understanding the causes, risks and responses to climate change brings together many fields of scientific enquiry, from physics and engineering to economics and the social sciences. These fields use measurements, observations and models to explain how the climate is changing and to assess the role of human activities in driving it.  

When looking to the future, however, this scientific toolbox needs to be expanded. The future is not predetermined and depends on choices still to be made. Researchers use climate scenarios as tools to explore the range of worlds arising from these choices.  

The Intergovernmental Panel on Climate Change (IPCC), the United Nations body that assesses the science related to climate change, describes scenarios as “alternative images of how the future might unfold”. Scenarios are neither predictions nor forecasts. They are better understood as carefully designed, described, quantified and documented “what-if” stories, developed to answer specific scientific and policy-relevant questions. 

These stories help scientists examine how assumed societal trends, including population growth, economic development, inequalities, technological change, land use and management, as well as policy decisions, interact with one another and shape future greenhouse gas emissions and their associated climate risks. Scenarios are used across almost all areas of  forward-looking climate change analysis, including climate modelling, assessment of impacts, adaptation planning and mitigation analysis. The insights they provide can help to explore uncertainty, structure decision making, or assist risk assessment and planning. 

The main types of climate scenarios 

Climate scenarios are as varied as the questions they are designed to answer. Broadly, however, there are two main groups. 

Effects of current policies on global temperature 

The first group estimates how current policies or pledges will affect greenhouse gas emissions and global warming. These scenarios typically project technological, regulatory, demographic and socioeconomic trends over the recent past into the future and assumes that the policies that are already in place will continue.  

A related type of scenario asks what would happen if pledges to take action to address climate change (such as countries’ Nationally Determined Contributions under the Paris Agreement, or longer-term low-emissions targets) or other more aspirational targets were fully implemented and achieved.  

Possible actions to reach a specific temperature 

The second main group of scenarios explores how global warming could be limited to a specified temperature level. Because global warming is closely related to the total amount of carbon dioxide added to the atmosphere by human activities, these scenarios typically examine how countries or regions could develop socioeconomically while keeping global carbon dioxide emissions within a defined carbon budget. Under the Paris Agreement, countries have committed to holding global warming well below 2°C and pursuing efforts to limit it to 1.5°C. Many studies therefore focus on how this internationally agreed goal could be achieved.  

Within this second group, a subset of scenarios also examines how efforts to limit warming interact with sustainable development goals. These studies explore how climate action, development and environmental sustainability can be pursued together; for example, by investigating when climate action could lead to trade-offs in other areas, or – alternatively – how it could strengthen achieving other societal goals. 

Different scenarios for different questions 

Which scenarios are most relevant depends on the question being asked, or the decision they are intended to inform. Projections of where the world is heading under current policies, and scenarios of what could be achieved through stronger emissions reductions, are both useful but for different purposes. 

Scenarios for climate adaptation 

For example, decisions about how to prepare and protect people, communities and essential services (such as public health systems or agricultural supply chains) from disruption – known as climate adaptation – typically require a precautionary approach. To inform what adaptations may be needed, scenarios that project greenhouse gas emissions and global warming under current trends and policies provide understanding of which levels of warming cannot yet be ruled out, as do scenarios and data that inform worst-case outcomes. 

Scenarios for climate mitigation 

Decisions about how to reduce greenhouse gas emissions in line with specific temperature limits –known as climate mitigation – requires a different approach. In this context, the most relevant scenarios are those that explore combinations of emissions reduction strategies consistent with, for example, the international climate goals of the Paris Agreement. Comparing such Paris-aligned mitigation scenarios with projections of what current policies are expected to deliver allows researchers to identify gaps in ambition and implementation. 

Scenarios that explore deep emissions reductions also provide valuable information for adaptation. They help identify the minimum levels of climate risk that societies will face, even if mitigation efforts are highly successful. 

Application to the real world 

An important consideration is that none of the scenario types described above is intended to identify the single most likely future. The likelihood of each scenario depends heavily on policy decisions made over the coming years and decades. Scenarios allow researchers to assess the possible consequences of choices and developments, but the probability that any individual emissions path will unfold exactly as described in scenarios is very low.  

Finally, while scenarios provide useful ‘what-if’ thought experiments, their relevance to inform policy decisions should be assessed in light of which options they explored, which assumptions they made about how the world and the economy work, and the real-world feasibility of their institutional and technological developments. Understanding these assumptions are critical to determining how useful they are.  

Scenario frameworks for climate research 

Scenarios are essential tools for exploring future climate risks, but not all climate scientists are specialists in scenario development. To help the wider climate science community navigate the vast range of possible futures, the scenario-modelling community has developed common frameworks and sets of representative scenarios. These help other researchers compare and synthesise findings across studies. 

Representative Concentration Pathways 

An early set of four representative scenarios covered a range of futures from very high to low emissions. In the technical literature, these are known as RCP8.5, RCP6, RCP4.5 and RCP2.6. RCP stands for “Representative Concentration Pathway”, and the number refers to the level of radiative forcing (the mechanism by which global warming occurs), in watts per square metre, reached by the end of the century.  

These RCPs described future concentrations of greenhouse gases in the atmosphere but did not define the socioeconomic developments that might produce them, allowing those underlying pathways to be explored separately.  

Shared Socioeconomic Pathways 

The research community subsequently developed a broader scenario framework based on Shared Socioeconomic Pathways, or SSPs. These group scenarios according to their underlying socioeconomic characteristics and the challenges they pose for adaptation and mitigation. For each set of socioeconomic assumptions, researchers can then explore both the greenhouse gas emissions that would result without new policies, and the options available to reduce those emissions in line with a targeted climate outcome.  

Scenarios developed within this framework are labelled in the technical literature using combinations such as SSP2-4.5: the first part identifies the socioeconomic pathway, while the second indicates the level of radiative forcing, and hence the associated climate outcome, by the end of the century.  

Recommended sets of scenarios 

To support climate modelling and enable the comparison and synthesis of results, selected sets of these scenarios are recommended for use across the broader climate research community. 

Most recently, an updated set of recommended scenarios for climate modelling has been published. This new set is narrower than that covered by the RCPs. At the high end, the highest plausible level of emissions is now slightly lower than was projected about a decade ago because of declining costs of low-carbon energy and progress in climate policies. At the lower end, the lowest emissions scenario now implies a clear overshoot of 1.5°C of global warming as it can only model a steep decline in greenhouse gas emissions from 2025 onwards, although it returns global warming to 1.5°C by the end of the century by assuming a large scale-up of carbon dioxide removal technologies in the second half of the century.  

Tracking change through updated projections 

Individual studies often develop scenarios to address specific questions. However, there are also several scientific initiatives that regularly update scenario projections to track progress over time. These initiatives help assess where the world is heading under current policies, how projected warming changes as policies and pledges evolve, and what levels of peak warming are unavoidable. 

A key international initiative in this area is the annual Emissions Gap Report by the United Nations Environment Programme. In its 2025 edition, the report stated that current policies imply a central estimate of 2.6°C of global warming by the end of this century, with a one-in-five chance that warming exceeds 3°C and continues into the 2100s. At the most ambitious end, if all pledges made by countries are fully implemented, the report’s central estimate indicates that global warming could be kept below 2°C, although 1.5°C would likely still be exceeded.  

Meanwhile, recent scenarios suggest that the lowest level of peak warming currently supported by the literature is around 1.7°C, with this minimum rising for every additional year that global greenhouse gas emissions fail to decline. 

Acknowledgements  

Prof. Joeri Rogelj is Director of Research at the Grantham Institute and Professor of Climate Science & Policy at the Centre for Environmental Policy at Imperial College London. 

POST is grateful to the author for kindly giving their time to produce this briefing.  

Questions about this briefing should be referred to Jonathan Wentworth (post@parliament.uk), who acted as parliamentary lead for this work.