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' decline over the 21st century for all SSP scenarios; there is 'medium confidence' that the decline will not involve an abrupt collapse before 2100

   “"The Atlantic Meridional Overturning Circulation (AMOC) will very likely decline over the 21st century for all SSP scenarios. There is medium confidence that the decline will not involve an abrupt collapse before 2100."”

   Calculation notes

   AR6 WG1 Ch9 does not itself assign a numerical probability, nor does it use the "very unlikely" likelihood term for AMOC collapse — it states only "medium confidence that the decline will not involve an abrupt collapse before 2100." The "very unlikely (medium confidence)" characterization of an AMOC collapse is the IPCC SROCC (2019) SPM B.2.7 assessment (see the SROCC source below), which AR6 Ch9 is consistent with. Under the IPCC calibrated-language scale "very unlikely" = 0-10% probability; 5% is taken as the midpoint of that SROCC-defined range for the headline. 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 Sheet 'likely' to keep losing mass all century; MISI/MICI excluded from likely-range projections as 'deep uncertainty'; 'low likelihood, high impact' storyline for >1m additional SLR from these instability processes by 2100

   “"Both the Greenland Ice Sheet (virtually certain) and the Antarctic Ice Sheet (likely) will continue to lose mass throughout this century under all considered SSP scenarios... Under high-emissions scenarios, poorly understood processes related to marine ice sheet instability and marine ice cliff instability, characterized by deep uncertainty, have the potential to strongly increase Antarctic mass loss on century to multi-century time scales... These likely range projections do not include those ice-sheet-related processes that are characterized by deep uncertainty... In a low-likelihood, high-impact storyline, under high emissions such processes could in combination contribute more than one additional metre of sea level rise by 2100."”

   Calculation notes

   IPCC AR6 does not assign a numerical probability to WAIS collapse before 2100. Instead it explicitly excludes the abrupt, widespread onset of marine ice sheet instability (MISI) and marine ice cliff instability (MICI) from its likely-range projections because those processes are "characterized by deep uncertainty arising from limited process understanding." It does assess that the Antarctic Ice Sheet is likely to keep losing mass throughout this century, and treats a >1 m additional sea-level-rise contribution from these instability processes by 2100 as a low-likelihood, high-impact storyline under high emissions. 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