Uncertain futures: IPCC projections, feedbacks and tipping points

IPCC projections and why they are a range

IPCC AR6 projects best-estimate warming for 2081 to 2100 (against 1850 to 1900) of 1.4 C under a very low emissions scenario (SSP1-1.9), 2.7 C under an intermediate scenario (SSP2-4.5) and 4.4 C under a very high scenario (SSP5-8.5).

Global mean sea level rose 0.20 m between 1901 and 2018, reaching 3.7 mm per year in 2006 to 2018; further rise is virtually certain.

The range comes from:

• human uncertainty: future population, economic growth, energy choices and mitigation (the scenarios)
• scientific uncertainty: how sensitive the climate is and how feedbacks behave (the error bars around each scenario).

Skill (7): reading projections critically

• Identify the scenario lines and the shaded likely or very likely ranges (error bars).
• Note that ranges widen further into the future.
• Explain why: emissions pathways diverge, and feedback uncertainty grows.
• Remember that limiting warming to 1.5 C needs GHG emissions to fall by a median 43% by 2030 relative to 2019 (IPCC Table SPM.1).

Feedbacks and tipping points

• Ice-albedo feedback: melting sea ice and snow expose darker ocean and land, which absorb more heat (positive feedback).
• Ocean carbon sinks: oceans absorb much human CO2, but warmer water holds less, so the sink may weaken.
• Forest die-back: drought and heat could turn forests such as the Amazon from carbon stores into carbon sources.

A tipping point is a threshold beyond which a change becomes self-sustaining and possibly irreversible, such as large ice sheet loss.

Worked example

A student says 'scientists cannot agree, so the projections are useless.' Respond using the IPCC ranges.

1. The ranges mainly reflect different possible human choices (scenarios), not disagreement.
2. Every scenario shows further warming; even the lowest gives about 1.4 C.
3. Uncertainty is a reason for caution, because the high end includes severe risks.

Answer: Projections are ranges because future emissions are a choice; all scenarios warm, so they are useful for planning.

Check yourself

1. What is the difference between IPCC best-estimate warming for 2081 to 2100 under SSP5-8.5 (4.4 C) and SSP1-1.9 (1.4 C)?
   Show answer

   3.0 C. 4.4 - 1.4 = 3.0 C.

2. Sea level rose 0.20 m between 1901 and 2018. Calculate the mean rate in mm per year, to 1 decimal place.
   Show answer

   1.7 mm per year. 200 mm / 117 years = 1.7 mm per year, about half the 2006 to 2018 rate of 3.7 mm per year.

3. To limit warming to 1.5 C, GHG emissions must fall by a median 43% by 2030 from 2019 levels. If a country emitted 500 Mt in 2019, what is its 2030 level on this pathway?
   Show answer

   285 Mt. 500 x 0.57 = 285 Mt.

4. Why do the IPCC projections for 2100 cover such a wide range?
   • Scientists measure temperature inaccurately today
   • Volcanoes are certain to erupt
   • Future emissions depend on population, economic development and mitigation choices
   • Sea level will fall
   Show answer

   Future emissions depend on population, economic development and mitigation choices. Scenarios represent different possible human futures.

5. A climate tipping point is
   • a threshold beyond which change becomes self-sustaining and hard to reverse
   • the hottest day of the year
   • the point at which emissions peak
   • a seasonal change in CO2
   Show answer

   a threshold beyond which change becomes self-sustaining and hard to reverse. Crossing it triggers further change.

6. Why might the ocean carbon sink weaken as the climate warms?
   • Oceans stop evaporating
   • Sea ice adds CO2 to the water
   • Warmer water holds less dissolved CO2
   • Salt absorbs CO2
   Show answer

   Warmer water holds less dissolved CO2. Gas solubility falls as temperature rises.

7. Explain how forest die-back could act as a positive feedback on global warming.
   Show answer

   Heat and drought kill trees, which release their stored carbon as CO2 when they burn or decay, adding to greenhouse gases and causing more warming.. Dying forests release carbon, increasing warming further.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification (Issue 3, September 2025)
• IPCC AR6 Synthesis Report: Summary for Policymakers (2023)

Last updated 8 November 2026. Spotted a mistake? Tell us.