5.3 What are the IPCC scenarios?

In their own words:

Between its Second (1995) and Third (2001) Assessment Reports, the Intergovernmental Panel on Climate Change elaborated long-term greenhouse gas emissions scenarios, in part to drive global ocean-atmosphere general circulation models, and ultimately to assess the urgency of action to prevent the risk of climatic change. Using these scenarios led the IPCC to report a range of global warming over the next century from 1.4–5.8°C, without being able to report any likelihood considerations. This range turned out to be controversial, as it dramatically revised the top-range value, which was previously 3.5°C. Yet some combinations of values that lead to high emissions, such as high per-capita income growth and high population growth, appear less likely than other combinations.

The scenarios are still used, as the storylines still seem to apply to a wide range of possible futures. They are denoted by letters and numbers. There are four scenario families, two (coded A) for economic development and two (coded B) for environmental development. The families are subdivided to show whether the scenarios are globally (suffix 1) or regionally oriented (suffix 2).

So they are, broadly:

  • A1 – economic; global (further subdivided into A1F (fossil fuels), A1T (transition to non-fossil fuels) and A1B (balance – between A1F and A1T))

  • A2 – economic; regional

  • B1 – environmental; global

  • B2 – environmental; regional

The scenarios simply allow possible futures to be explored. Three important caveats apply to all of them:

  • they are descriptive

  • none is more likely than the others

  • all assume no new climate policies in the future.

The last one may need to be revised before too long. Actual emissions since 2000 have been higher than the worst-case A1F1 scenario. The IPCC Fourth Assessment Report estimates the range of temperature increase associated with this scenario to be 2.5°C to 6.4°C by 2100.

  • Report from the IPCC providing the rationale for the scenarios used in the Third and Fourth Assessment Reports (dated 2000).

  • Global Carbon Project's annual carbon budget report including a link to a slide show detailing actual emission trends compared with IPCC scenarios.

  • RealClimate discusses ‘What the IPCC models really say’.

5.2 Why do we use models and scenarios?

5.4 What is the relationship between emissions now and climate change in the future?