IPCC’s 2023 climate scenarios

World Outlooks

Link to World Outlook’s article on the AR6 Synthesis Report.

Link to IPCC’s AR6 Synthesis Report microsite.

Link to IPCC’s AR6 Synthesis Report press release of 20 March 2023.

Summary

There are five scenarios, based primarily on different levels of ‘radiative forcing’, i.e. the difference between the earth’s incoming and outgoing energy, measured in watts per square metre.

  • Very low: 1.9
  • Low: 2.6
  • Intermediate: 4.5
  • High: 7.0
  • Very high: 8.5

Each is associated with a set of socio-economic trends (a Shared Socio-economic Pathway or ‘SSP’):

  • Very low: SSP1
  • Low: SSP1
  • Intermediate: SSP2
  • High: SSP3
  • Very high: SSP5

SSP1 is used twice and SSP4 not at all. The five SSPs are:

  • SSP1: Sustainability – Taking the Green Road (low challenges to mitigation and adaptation)
  • SSP2: Middle of the Road (medium challenges to mitigation and adaptation)
  • SSP3: Regional Rivalry – A Rocky Road (high challenges to mitigation and adaptation)
  • SSP4: Inequality – A Road Divided (low challenges to mitigation, high challenges to adaptation)
  • SSP5: Fossil-fueled Development – Taking the Highway (high challenges to mitigation, low challenges to adaptation)

They were derived from the following matrix:

Source

Descriptions

IPCC’s descriptions of the main scenarios used in its 2023 Synthesis Report are very poor, e.g. no indication of important assumptions in the Shared Socio-economic Pathways. Its best attempt is in the ‘Category description’ column of the following (p.10 of the full report):

WGI, WGII and WGIII: three IPCC working groups. GHG: greenhouse gas. Percentages are confidence levels. RCP: Representative Concentration Pathway, a separate basis for climate change scenarios but not directly comparable to SSPs.

Hence:

  • SSP1-1.9: limit warming to 1.5oC (>50% confidence)
  • SSP1-2.6: limit warming to 2oC (>67% confidence)
  • SSP2-4.5: limit warming to 3oC (>50% confidence)
  • SSP3-7.0: limit warming to 4oC (>50% confidence)
  • SSP3-8.5: exceed warming of 4oC (>50% confidence)

Overview

Emissions and temperatures from the AR6 Synthesis Report (p.65):

Gt: gigatonnes. C1-C8: see ‘Descriptions’ section above.

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