SSP climate scenarios explained: from SSP1-1.9 to SSP5-8.5

What are SSP scenarios?

SSPs are storylines of how the world could develop, each with different challenges for cutting emissions and for adapting to climate change. The IPCC summarizes them as SSP1 (sustainability), SSP2 (middle of the road), SSP3 (regional rivalry), SSP4 (inequality) and SSP5 (rapid growth). Each storyline was then turned into energy, land-use and emission pathways reaching different levels of radiative forcing by 2100.

The IPCC's Sixth Assessment Report uses five illustrative combinations, all starting in 2015: SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. The IPCC does not assess how likely each one is, and it notes that the SSPs do not cover every possible future.

What do the five main scenarios mean?

Other combinations exist, such as SSP4-3.4 and SSP4-6.0, and some data sets include them; the Köppen-Geiger maps by Beck et al. (2023), for example, cover seven scenarios.

• SSP1-1.9, very low emissions: CO₂ emissions fall to net zero around 2050 and then below zero. It was added to explore limiting warming to about 1.5 °C.
• SSP1-2.6, low emissions: CO₂ reaches net zero after 2050. It mirrors the earlier RCP2.6, and under it warming of 2 °C is unlikely to be exceeded this century.
• SSP2-4.5, intermediate: CO₂ emissions stay around current levels until mid-century and then fall, without reaching net zero by 2100.
• SSP3-7.0, high: a world of regional rivalry in which CO₂ emissions roughly double from current levels by 2100.
• SSP5-8.5, very high: fossil-fuel-intensive growth, with CO₂ emissions roughly doubling from current levels by 2050.

How much warming does each scenario mean?

The IPCC's best estimates of global surface warming relative to 1850–1900, as 20-year averages for 2021–2040, 2041–2060 and 2081–2100:

• SSP1-1.9: 1.5 °C, 1.6 °C and 1.4 °C.
• SSP1-2.6: 1.5 °C, 1.7 °C and 1.8 °C.
• SSP2-4.5: 1.5 °C, 2.0 °C and 2.7 °C.
• SSP3-7.0: 1.5 °C, 2.1 °C and 3.6 °C.
• SSP5-8.5: 1.6 °C, 2.4 °C and 4.4 °C.

How do SSPs relate to the older RCPs?

The Representative Concentration Pathways (RCPs) drove the CMIP5 projections of the IPCC's Fifth Assessment Report and older future weather files. The SSP-based scenarios cover the same forcing levels and add a new low one at 1.9 W/m², so SSP1-2.6, SSP2-4.5 and SSP5-8.5 broadly follow RCP2.6, RCP4.5 and RCP8.5.

The results are not identical, because CMIP6 uses updated climate models and updated emission and land-use inputs. You will still meet RCPs in practice: CIBSE's 2025 UK weather data, for example, uses UKCP18 projections for RCP2.6, RCP4.5 and RCP8.5.

How to choose scenarios for building design

• Start from the building's life. For a building completed around 2030, look at mid-century and at the end of its expected life; the scenarios differ little before about 2040 and a lot by 2081–2100.
• Use more than one scenario. A practical pairing is an intermediate scenario (SSP2-4.5) as the central case and a high one (SSP3-7.0 or SSP5-8.5) as a stress test for overheating and cooling capacity.
• Treat SSP5-8.5 as a high-end risk case. The IPCC's Working Group III notes that high-end scenarios have become considerably less likely since its Fifth Assessment Report but cannot be ruled out.
• Remember that global averages hide local change. Between 2011 and 2020, land had warmed by 1.59 °C against 0.88 °C over the ocean, relative to 1850–1900, and a site's summers and winters can change differently from the global figure.
• Record the scenario, period and data source in the design report, so the assumptions can be revisited when projections are updated.

SSP scenarios in C4B

C4B uses the five main scenarios, SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5, in two places. The climate zone map has a year slider from 1901 to 2100, with a choice of the five scenarios for future years, based on Beck et al. (2023). In projects on paid plans, the future climate module morphs the site's weather file to any year from 2025 to 2100 under any of the five scenarios, using the World Bank Climate Change Knowledge Portal's CMIP6 multi-model median for temperature, humidity and solar radiation, and prepares four scenarios automatically for the building's default mid-life year, 27 years ahead.