Climate Change 2021: The Physical Science Basis — Chapter 9: Ocean, Cryosphere and Sea Level Change

Used in 2 entries

For each citing entry, the verbatim excerpt and Likelier's calculation notes (how the source's number was converted to the lifetime-probability framing) are shown below. Click through to read the full claim ledger.

1. [1] AMOC collapse Risk

   Statistic

   AMOC will 'very likely' weaken over the 21st century; abrupt collapse before 2100 is 'very unlikely' (0-10%) with medium confidence; projected AMOC weakening of 24-39% under low-to-high emissions scenarios

   “"There is medium confidence that the decline will not involve an abrupt collapse before 2100. The AMOC will very likely weaken over the 21st century (high confidence), although a collapse is very unlikely (medium confidence)."”

   Calculation notes

   IPCC calibrated language: "very unlikely" = 0-10% probability. Using 5% as the midpoint of this range for the headline. The "medium confidence" modifier (rather than high confidence) indicates limited model agreement on the abrupt-collapse question specifically. The IPCC does not give a numerical probability; 5% is the author's interpretation of the calibrated language midpoint. The 59-year adult lifetime window (age 18 to 77) maps roughly onto the "before 2100" horizon.
   

   Source date: 2021-08-09 · Accessed: 2026-05-09

2. [2] WAIS tipping point Risk

   Statistic

   Antarctic ice mass loss 'likely' to continue all century; MISI physically plausible and observed; 'low likelihood, high impact' storyline for >1m additional SLR from ice sheet instability; deep uncertainty acknowledged

   “"There is medium confidence that significant Antarctic ice mass loss will not involve an abrupt collapse before 2100... Marine Ice Sheet Instability (MISI) is physically plausible, has been observed in Thwaites Glacier, and would result in substantial irreversible sea level rise if triggered... Low-likelihood, high-impact storylines include scenarios leading to more than one metre of additional sea level rise from ice sheet instability before 2100."”

   Calculation notes

   IPCC AR6 does not assign a numerical probability to WAIS collapse before 2100, instead using "medium confidence that [abrupt collapse] will NOT happen" — implying some non-trivial probability that it will. The Bamber et al. 2019 structured expert judgment cited by AR6 gave WAIS sea level contribution by 2100 as median 8-18 cm with 95th percentile up to 44-93 cm depending on warming scenario — reflecting deep uncertainty but not ruling out large contributions. IPCC AR6's position is more cautious than Wunderling 2025 because it relies on CMIP model ensembles that do not simulate WAIS collapse, rather than the tipping-point network models used by Wunderling.
   

   Source date: 2021-08-09 · Accessed: 2026-05-09

Also cited in these entries

Natural

AMOC collapse

What are the odds of the AMOC experiencing an abrupt collapse before the end of your lifetime?

1 in 20 lifetime(5.0%)global

NaturalDirect

WAIS tipping point

What are the odds of the West Antarctic Ice Sheet's tipping point being crossed in your lifetime?

1 in 1.1 lifetime(90%)global