Why Climate Scientists Dropped the SSP5‑8.5 Scenario and What the New CMIP7 Climate Models Mean for 2100 Warming

Key changes include:

1. A redesigned scenario set focused on plausible futures
The CMIP7 framework introduces a new set of scenarios ranging from very low emissions to high emissions, intended to cover futures that have a realistic chance of occurring based on current knowledge and policy trends.

2. A shift toward emissions‑driven modeling
Most CMIP7 simulations are designed to run in emissions‑driven mode, meaning models directly simulate how emissions translate into atmospheric concentrations. This approach helps better represent uncertainties in the carbon cycle and Earth system responses.

3. Greater focus on peak‑and‑decline pathways
Several scenarios explicitly include trajectories where greenhouse‑gas concentrations peak and decline later in the century, allowing researchers to study temperature overshoot and potential reversal through deep emissions cuts and carbon removal.

Together, these changes reflect a shift from exploring extreme theoretical futures to studying policy‑relevant pathways and the consequences of realistic mitigation strategies.

Why the SSP5‑8.5 “worst‑case” scenario was dropped

For more than a decade, one scenario dominated climate research headlines: SSP5‑8.5, a pathway in which fossil‑fuel use expands rapidly and radiative forcing reaches 8.5 W/m² by 2100.

In the CMIP7 design process, researchers concluded that this level of emissions “has become implausible” as a central benchmark scenario.

Several developments contributed to that assessment:

• Real‑world emissions trends and policies suggest slower growth than the extreme assumptions embedded in SSP5‑8.5.
• Technological change—particularly the rapid expansion of clean energy—makes a massive global return to coal‑heavy energy systems less consistent with observed trajectories.
• Scenario designers wanted models to prioritize futures with non‑negligible likelihood, rather than extremely unlikely outcomes.

Importantly, removing SSP5‑8.5 from the core scenario set does not eliminate the possibility of severe warming. The new high‑emissions pathway still explores adverse futures but is expected to produce lower forcing than SSP5‑8.5.

Why 1.5°C without overshoot is increasingly unlikely

The Paris Agreement aims to limit warming to 1.5°C above pre‑industrial levels, but many recent analyses indicate that holding temperatures below that level throughout the century is no longer feasible under realistic pathways.

The new CMIP7 scenarios reflect this reality by explicitly studying overshoot pathways, where global temperatures exceed 1.5°C before being reduced later through aggressive emissions cuts and carbon dioxide removal.

Research assessing mitigation pathways already suggested a large gap between current policy trajectories and the emissions reductions needed for a strict 1.5°C limit. IPCC assessments find that implemented policies as of 2020 could lead to median warming of roughly 2.2°C–3.5°C by 2100.

As a result, many scientists now focus on questions such as:

• how high temperatures might overshoot 1.5°C
• how quickly emissions must fall afterward
• whether large‑scale carbon removal can help lower temperatures later in the century

What current policies imply for warming by 2100

Estimates based on today’s policies generally fall well above the Paris targets.

Climate Action Tracker’s global analysis projects that current policies could lead to about 2.6–2.8°C of warming by 2100, depending on methodological assumptions.

These estimates broadly align with the wider IPCC policy range of 2.2°C–3.5°C, illustrating the persistent gap between current policies and climate goals.

Even under improved pledges and targets, many projections still remain above the 1.5°C limit.

The role of clean energy progress

Despite the challenging outlook, the trajectory is not fixed. Rapid deployment of clean technologies could still significantly reduce warming.

Climate Action Tracker estimates that three major actions could lower projected warming this century by about 0.9°C:

• tripling global renewable‑energy capacity
• doubling the rate of energy‑efficiency improvement
• sharply reducing methane emissions

Together, these steps could shift projections from roughly 2.6°C toward about 1.7°C, potentially bringing global warming below 2°C.

The bigger picture

The CMIP7 scenario update represents a methodological shift rather than a relaxation of climate concerns.

Researchers are moving away from highly extreme baseline scenarios and toward policy‑relevant, plausible futures. At the same time, the core message remains consistent: current policies are not yet sufficient to meet the Paris targets.

In practical terms, the emerging consensus looks roughly like this:

• extreme fossil‑fuel expansion scenarios like SSP5‑8.5 are now considered unlikely
• current policies still point to warming around the high‑2°C range this century
• limiting warming close to 1.5°C likely requires temporary overshoot and large‑scale carbon removal
• rapid expansion of clean energy and methane reductions could still significantly reduce the final temperature outcome

The next few years of policy and technology deployment will determine where within that range the world ultimately lands.