🌡️ Research committee that provides input to the IPCC drops the worst climate scenarios

• The research committee ScenarioMIP, which develops the climate scenarios that form the basis for the IPCC's reports, has officially classified the previous worst-case scenarios RCP8.5, SSP5-8.5 and SSP3-7.0 as implausible.
• The new highest scenario, HIGH, results in 0.9°C less warming by 2100 than the previous SSP5-8.5 in apples-to-apples comparisons.
• The change is due in part to falling costs for renewable energy, the emergence of climate policy, and actual emission trends in recent years.

New generation of climate scenarios

The Coupled Model Intercomparison Project (CMIP) is run by the World Climate Research Programme together with the World Meteorological Organization, the International Science Council, and UNESCO's Intergovernmental Oceanographic Commission. Under CMIP, now in its seventh iteration, sits the subcommittee ScenarioMIP, which is responsible for developing the scenarios used by climate models. ScenarioMIP has now published seven new scenarios that will form the basis for the IPCC's Seventh Assessment Report (AR7) and much of the research it builds on.

In a paper by Van Vuuren et al., the authors write that the previous high emission levels in CMIP6, quantified by SSP5-8.5, have become implausible. They cite trends in the costs of renewable energy, the emergence of climate policy, and recent years' emission trends as reasons.

Large differences from previous scenarios

The new HIGH scenario reaches 71 gigatons of carbon dioxide per year from fossil fuels and industry by 2100. This compares with SSP5-8.5, which stood at 128 gigatons in 2100. None of the new CMIP7 scenarios come close to SSP5-8.5. HIGH also sits about 9 percent below SSP3-7.0 measured in cumulative emissions through 2100.

In terms of temperature, the new HIGH scenario is 0.9°C cooler than SSP5-8.5 in apples-to-apples comparisons, and 1.4°C cooler compared with the IPCC's AR6 values for SSP5-8.5. Compared with SSP3-7.0, HIGH is 0.2°C cooler.

Updated emissions data

Part of the change comes from the fact that historical emissions are now calibrated against 2023 data, compared with 2014 data used in AR6. The more moderate emission trajectories in recent years have led to lower projected end-of-century warming even for the old SSP scenarios when run through the updated climate model FaIR.

The IEA's scenarios are lower

The two main reference scenarios from the International Energy Agency's World Energy Outlook 2025, which extends to 2050, both sit below both the new MEDIUM scenario and the previous SSP2-4.5. The IEA's STEPS scenario, based on current and announced policies, shows emissions falling to under 30 gigatons of carbon dioxide per year by 2050 and leading to about 2.5°C of warming by 2100.

Scenarios that shape policy

The scenarios now classified as implausible have been widely used in policy work. National climate impact assessments in the United States, the United Kingdom, Germany, Canada, Australia, Japan, and the Netherlands have used RCP8.5 or SSP5-8.5 as a reference. The Network for Greening the Financial System, used by more than 140 central banks, has applied scenarios calibrated against RCP8.5 in stress tests at institutions including the European Central Bank, the Bank of England, and the US Federal Reserve. The World Bank's Climate Change Knowledge Portal, which provides the basis for climate reports in more than 100 countries, has used SSP5-8.5 and SSP3-7.0 as defaults.

Nine years from research to confirmation

It has been known since 2017 that the upper-end climate scenarios have fundamental flaws. Nine years later, this has now been officially recognized by the committee responsible for the scenarios used in the IPCC's reports.

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