Every six to seven years, climate modelling groups around the world run a coordinated set of simulations that explore how the climate could change in the future.

These simulations form a key line of evidence for future projections used in Intergovernmental Panel on Climate Change (IPCC) assessment reports.

They are built around a set of common scenarios – or “pathways” – of future greenhouse gas emissions.

A new set of scenarios has now been published for the seventh phase of the Coupled Model Intercomparison Project (CMIP7).

These replace the “shared socioeconomic pathways” (SSPs) that drove the previous generation of climate models and featured heavily in the IPCC’s sixth assessment report (AR6).

The new scenarios are quite different from their predecessors in a number of notable ways.

Rather than being named, somewhat enigmatically, according to their “radiative forcing levels”, the new scenarios are named simply by their emissions trajectories – ranging from “low-to-negative” to “high”.

They no longer consider “no-climate-policy” baseline worlds, but instead explore the implications of current policies continuing, being strengthened, or weakening.

These new scenarios also dramatically revise high-end future emissions downward, far below the highest scenarios in prior generations, in order to reflect a world where a 21st century dominated by coal use is no longer plausible.

At the same time, they revise the lowest emissions scenarios upwards relative to those featured in the AR6, with at least some “overshoot” of the Paris Agreement’s “aspirational” target to limit global warming to 1.5C now “unavoidable”.

While modelling groups are just getting started on the full Earth-system model simulations, the emissions scenarios give a clear picture of the range of futures that will inform the IPCC’s seventh assessment cycle (AR7).

Here, Carbon Brief unpacks how the new scenarios were designed and how they differ from the SSPs published almost a decade ago.

The article also compares CO2 emissions and warming outcomes between the new scenarios and their predecessors, explores the range of future warming outcomes and examines why the high end of the scenario range has shifted markedly downward.

Finally, Carbon Brief examines the scale of carbon dioxide removal (CDR) built into the scenarios and new extensions of scenarios to 2150 and beyond.

Key highlights from Carbon Brief’s analysis of the new scenarios include:

• The seven new scenarios give a range of global warming in 2100 from 1.6C to 3.3C above pre-industrial levels – markedly narrower than the 1.5C to 4.7C range in their SSP predecessors.
• The top of the scenario range has fallen for the first time in four generations of climate modelling. The highest scenarios used in the three previous IPCC assessment cycles all produced around 4.6-4.9C of global warming in 2100, whereas CMIP7’s high scenario only reaches 3.3C and has around half the cumulative CO2 emissions.
• The new “medium” scenario that is analogous to policies in place today reaches 2.9C in 2100, crossing 2C around 2050 and 3C around 2110, with a one-in-four chance of exceeding 4C by 2150.
• The lowest scenarios have shifted modestly upwards, as a future that avoids any overshoot of 1.5C is no longer considered plausible. The very-low scenario peaks at around 1.8C mid-century before falling back close to 1.5C by 2100.
• The updated socioeconomic assumptions underpinning the new scenarios describe a more crowded and less wealthy planet than the original SSPs, with the global human population now peaking at 10.1bn people around 2080 in the medium pathway and income per person in 2100 between 10% and 25% lower.
• Every scenario that limits warming leans heavily on carbon dioxide removal, with cumulative removals by 2150 ranging from 655GtCO2 in the very-low scenario to 2,360GtCO2 in low-to-negative scenario.

A new generation of scenarios

To simulate how human activity could shape the climate of the future, climate modellers must estimate future levels of “radiative forcings” – the external drivers that cause global warming. These include atmospheric concentrations of greenhouse gases, air pollutants and land-use changes.

Given that no one knows how the future will unfold, modellers use a handful of scenarios that span a wide range of plausible outcomes.

The Scenario Model Intercomparison Project (ScenarioMIP) coordinates the development and running of emissions scenarios for climate models used in IPCC reports.

In April 2026, high-level details about the new set of scenarios for CMIP7 were published in the journal Geoscientific Model Development (GMD).

On 1 September, the underlying emissions data was released into the public domain by the ScenarioMIP team.

There are seven new CMIP7 scenarios designed to drive model simulations for AR7. The first model runs took place in spring 2026 and initial results are expected later this year.

The previous SSP scenarios were starting to show their age. Finalised in 2015-17 using historical data ending in 2015, several years projected by the SSP scenarios were already in the past by the time AR6 concluded in 2021. Meanwhile, the world had changed considerably.

(For a full guide to the SSPs, see Carbon Brief‘s 2018 explainer.)

Storylines and emissions levels

The most visible change in the new generation of scenarios is their names. Where the SSPs combined five socioeconomic “storylines” with radiative forcing targets (SSP1-2.6, SSP5-8.5, etc), the CMIP7 scenarios are named simply for the emissions trajectory that they follow.

The table below summarises the seven scenarios and the integrated assessment model (IAM) that produced each “marker” run – in other words, the specific IAM run used to generate the scenario that, in turn, will be used by CMIP7 climate models. IAMs run simulations of how the future energy system and emissions may evolve under different assumptions around socioeconomics, future technology costs and climate policy.

The table below also details how the scenario fares against a number of key metrics assessed by Carbon Brief, including CO2 emissions and warming outcomes.

(For more on Carbon Brief’s approach, see: Methodology.).

Warming values are medians (with the 5-95% range) from the 841-member FaIR ensemble used in this article (see: Methodology); the marker model assignments come from the ScenarioMIP database. Note that scenario names in the database differ from the official CMIP7 names (for example, the high-to-low scenario appears as “SSP5 – Medium-Low Emissions_a”).

Each of the new scenarios is built on a set of updated SSP storylines similar to those used in the original SSP scenarios. These include assumptions about future population, technological and economic growth, as well as potential for international cooperation that shape the resulting emissions pathways. The socioeconomic assumptions underlying these revised SSPs were updated in 2024 with new population and economic projections.

Most of the new emissions scenarios are now based on the “middle-of-the-road” SSP2 that assumes current socioeconomic trends broadly continue, with only one scenario using each of SSP1 (“sustainability”), SSP3 (“regional rivalry”) and SSP5 (“fossil-fuelled development”). None of the new scenarios uses SSP4 (“inequality”).

The solid lines in the figure below show updated global human population, GDP and GDP per capita values in CMIP7 (solid lines), compared to the original SSPs from CMIP’s sixth phase (CMIP6), shown by the dashed lines.

The change in socioeconomic assumptions is substantial. Global population was revised upward in nearly every scenario, with the updated SSP2 projecting there will be 9.9 billion people in 2100 – an increase of 1 billion people compared to the 2013-era SSP.

GDP was revised downward in the high-end growth scenarios (SSP1 and SSP5), slightly upward in SSP3 and SSP4 and was largely unchanged in SS2.

The combination of these changes means that income per person in 2100 is around 10-25% lower in most scenarios, with only SSP3 and SSP4 seeing mostly unchanged income per capita.

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In short, the socioeconomic world underlying the new scenarios is somewhat more crowded and less wealthy per person than the one the SSPs originally imagined.

Another notable change is the shift in the SSP that underlies the highest future emissions scenario.

In the original SSPs, the “very high” SSP5-8.5 scenario was based on SSP5, while the new “high” scenario in CMIP7 is based on SSP3.

The GMD study explains that this is because IAM teams that developed the scenarios found that SSP3 and SSP5 variants produced similar emissions. They judged that the “fragmented” SSP3 world – which is characterised by large challenges to adaptation – to be more relevant for exploring high-end risks.

No more ‘baseline’ scenarios and other changes

In another important change, the authors of the CMIP7 scenarios decided to eliminate “baseline” scenarios that assumed a world without any climate policy. These scenarios were previously used as a counterfactual against which to compare climate-changed worlds.

Instead, the range of future emissions scenarios starts with current policies and explores ways that they could be strengthened, weakened, or kept the same. The high scenario explores a plausible “rollback of current mitigation policies“.

The medium scenario, by contrast, extends climate policies officially implemented as of 2025, without assuming countries achieve their Paris Agreement pledges – known as nationally determined contributions (NDCs) – or net-zero targets that are not yet backed by legislation.

In their GMD paper, the authors of the CMIP7 scenarios emphasise that the medium scenario “should not be considered as a ‘most likely’ scenario”, but that it can provide a benchmark against which the effect of future policy strengthening or weakening can be measured. It is roughly analogous in its emissions levels to the old SSP2-4.5 scenario.

The new low scenario explores a world where climate policy is rapidly strengthened and warming by 2100 is limited to below 2C. This makes it analogous to the old SSP1-2.6 scenario.

The very-low scenario limits global warming to around 1.5C by 2100, similar to the old SSP1-1.9 scenario. However, it involves a greater degree of overshoot mid-century, reflecting the fact that global emissions did not begin to rapidly decline in 2020 as envisioned by SSP1-1.9. As the authors of the GMD ScenarioMIP paper point out: “At this point of time, some overshoot of the 1.5C seems unavoidable.”

In addition, there are a number of scenarios that start on one path before undertaking rapid mitigation. These high-to-low, medium-to-low and low-to-negative scenarios are intended to explore futures where mitigation is further delayed, followed by a rapid turn-around later in the century.

The scenario developers noted that there is no specific likelihood or probabilities assigned to any scenario, but rather only a judgement that all are within the realm of plausibility given where the world is today. They also said that “there might be potential futures outside the ScenarioMIP scenario range”.

Timescales and other changes

In addition to the shift away from baseline scenarios, there are three other notable design changes made in CMIP7.

First, CMIP7 models will be driven by emissions of CO2 and other greenhouse gases, rather than set atmospheric concentrations.

In every previous generation of models, the ScenarioMIP experiments required that modelling groups simulate future climate using the same set of CO2 concentrations. For CMIP7, models with an interactive carbon cycle are asked to run in “emissions-driven” mode for CO2, calculating atmospheric concentrations themselves based on emissions.

This is a significant improvement. It means that the substantial uncertainty in carbon-cycle feedbacks will now show up directly in the range of projected warming, rather than being overlooked. (The change applies to CO2 only; methane, nitrous oxide and halocarbons remain prescribed as concentrations.)

Second, emissions match observations up to 2023. IAM modellers were asked to stay close to observed trends up to 2025 to avoid emissions diverging from reality before models were run. Scenario differences only open up after 2026, avoiding an earlier problem of scenarios diverging from reality years before the models were even run.

Finally, the period over which models are being run has been extended from 2100 to 2150. This is important as the world is already more than a quarter of the way through the 21st century.

The extended model runs out to 2150 will provide a more thorough exploration of the warming that people born in the coming decades may experience within their lifetimes.

In addition, all scenarios have extensions to 2500 where temperatures are eventually stabilised. These allow scientists to explore changes to long-term Earth-system processes, such as ice sheets and sea level, as well as whether warming is reversible.

A narrower range of future CO2 emissions

Overall, the new scenarios provide a notably more narrow range of future CO2 emissions than the SSP scenarios used in CMIP6.

The figure below shows net global CO2 emissions (combining fossil-fuel and land-use emissions) for the seven new scenarios, alongside the five SSP scenarios used for climate model runs in CMIP6 (e.g. SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5).

At the bottom of the range, the new scenarios closely track their predecessors: the very-low scenario reaches net-zero CO2 around mid-century much like SSP1-1.9, while the low scenario lands close to SSP1-2.6.

The chart below shows total emissions for the same scenarios for the period 2024-2100.

The lowest emissions scenarios now have somewhat higher total emissions, reflecting the failure of the world to rapidly reduce emissions after 2020 that occurred in the lower SSP emissions scenarios, such as SSP1-1.9 and SSP1-2.6. The very-low scenario results in 310bn tonnes of CO2 (GtCO2) cumulative emissions between 2024 and 2100, compared to around 110GtCO2 in SSP1-1.9.

At the top end, the change is particularly dramatic. The high scenario in CMIP7 reaches 55GtCO2 per year in 2100. The previous high scenario, SSP5-8.5, by contrast, reached around 126GtCO2 per year in 2100.

In cumulative terms – which is what matters most for global warming – high reaches around 3,820GtCO2 over 2024-2100, half the roughly 7,600GtCO2 of SSP5-8.5 and about three-quarters of the 5,140GtCO2 of SSP3-7.0.

To put it another way: the top of the new scenario range sits between SSP2-4.5 and SSP3-7.0 in cumulative emissions terms, which is territory that CMIP6 treated as its middle ground.

To make the scale of this shift clear, Carbon Brief analysed the CO2 emissions trajectories in each of the prior generations of high-end emissions scenarios, using the same simple climate model – FaIR – to calculate future warming.

Below, four different generations of emissions scenarios are examined. The SRES scenarios were originally published in 2000 and used in the IPCC’s third (2001) and fourth (2007) assessment reports (and the corresponding CMIP3 model runs). The RCPs were developed in the early 2010s and used in the IPCC fifth assessment report (AR5; 2013) and CMIP5, while the SSPs were developed in the late 2010s and used in the IPCC AR6 report and CMIP6.

Over the past two decades, the highest emissions scenarios all resulted in comparable amounts of warming in 2100: SRES A1FI (the highest SRES scenario) reached 4.6C in 2100 (5-95% range; 3.5-6.1C), RCP8.5 reached 4.9C (3.7-6.5C) and SSP5-8.5 reached 4.6C (3.5-6.2C).

(RCP8.5 edges out its successor despite lower CO2 emissions because it assumed considerably more methane and nitrous oxide.)