1.2.2 Current state of knowledge on cryosphere tipping points

In this section we assess available scientific literature relating to tipping points in the cryosphere, as summarised in Figure 1.2.2 and Table 1.2.1. We focus on the following systems: the ice sheets on Greenland and Antarctica, sea ice (in the Arctic and Antarctic), mountain glaciers, and permafrost.

Figure: 1.2.2
Figure 1.2.2: Map of cryosphere systems considered in this chapter (shading). The markers indicate which of the systems are in this report considered a tipping system (+++ high confidence, ++ medium confidence and + low confidence) and which are not (- – – high confidence, – – medium confidence and – low confidence), ▽ indicates systems for which a clear assessment is not possible based on the current level of understanding.

Table: 1.2.1 Summary of evidence for tipping dynamics, key drivers and biophysical impacts in each system considered in this chapter

Key: +++ Yes (high confidence), ++ Yes (medium confidence), + Yes (low confidence), – – – No (high confidence), – – No (medium confidence), No (low confidence) 
Primary drivers are bolded, DC: Direct Climate driver; CA: Climate-Associated driver (including second-order and related effects of climate change); NC: Non-Climate driver, PF: positive (amplifying) feedback (FB), NF: negative (damping) feedback. Drivers can enhance (↗) the tipping process or counter it (↘)

System (and potential tipping point)Key drivers Key biophysical impactsSelected key feedbacksAbrupt / large rate change?Critical threshold(s) (warming > preindustrial)Irreversible? (decadal / centennial)Tipping system? 
Ice Sheets
Greenland Ice Sheet (collapse)DC: atmospheric warming (↗)
DC: precipitation increase (↘)
DC: ocean warming and circulation changes (↗/↘)
DC: black carbon deposition (↗)
CA: sea ice decline (↗)
CA: atmospheric circulation changes (↗|↘)
• Sea level rise (up to 7m) over centuries to millennia
• Disruption of global ocean circulation
• Substantial shifts in atmospheric circulation patterns
• PF: melt-elevation
• PF: melt-albedo
+++0.8-3°C++++++
West Antarctic Ice Sheet (collapse)DC: ocean warming and circulation changes (↗)
DC: atmospheric warming (↗)
DC: precipitation increase (↘)
• Sea level rise (up to 3m) over centuries to millennia
• Disruption of global ocean circulation
• Substantial shifts in atmospheric circulation patterns
• PF: marine ice sheet instability
• NF: glacial isostatic adjustment
?: melt-stratification
+++1-3°C++++++
Marine basins
East Antarctica (collapse)
DC: ocean warming and circulation changes (↗)
DC: atmospheric warming (↗)
DC: precipitation increase (↘)
• Sea level rise (up to 19m) over centuries to millennia
• Disruption of global ocean circulation
• Substantial shifts in atmospheric circulation patterns
• PF: marine ice sheet instability 
• NF: glacial isostatic adjustment
?: melt-stratification
+++2-6°C++++++
Non-marine East Antarctic
Ice Sheet (collapse) 
DC: atmospheric warming (↗)
DC: precipitation increase (↘)
• Sea level rise (up to 34m) over centuries to millennia
• Disruption of global ocean circulation
• Substantial shifts in atmospheric circulation patterns
• PF: melt-elevation+++6-10°C++ ++
Sea Ice
Arctic summer
sea ice
(loss)
DC: atmospheric warming (↗)
DC: atmospheric circulation shifts (↗/↘)
DC: ocean warming (↗)
DC: ocean circulation shifts (↗/↘)
DC: black carbon deposition (↗)
DC: storminess increase (↗)
CA: ocean stratification increase (↘)
• Regional warming (polar amplification)
• Ecosystem disruption 
• Impacts on ocean circulation
• Impacts on atmospheric circulations
• Increased evaporation 
• PF: Ice-albedo
• FBNF: Snow
• FBNF: Growth
• FBNF: Radiation FB
– – –N/A– – –– – –
Arctic winter
sea ice
(loss)
+++3-6 °C– –– – (abrupt loss due to Arctic geometry)
Barents sea ice
(loss)
– (linear relationship in most models)unclearunclear
Antarctic sea ice (loss)unclearunclear+ (reversible over millennia)unclear
Glaciers
Glaciers (retreat)DC: atmospheric warming (↗)
DC: deposition of dust, black carbon, etc. (albedo) (↗)
DC: reduced snow (input and albedo) (↗)
DC: local thermokarst (↗)
• Water supply decline
• Ecosystem disruption (e.g. wetlands, water chemistry)
• Increase in number and size of glacier lakes
• Increase in slope instabilities
• Transition from glacial to para-glacial landscapes
• Sea level rise 
• PF: melt-elevation
• FBPF: calving front retreat 
• PF-: ice-dynamic FBs
• NF: retreat to higher altitudes
+ + (regional)
– – (global)
 
Regionally variable– –++ (regional) 
– – (global)
Permafrost
Land permafrost (thaw)DC: atmospheric warming (↗)
CA: vegetation increase (increase albedo ↗, increase summer shading ↘, and vice versa for forest die-back)
CA: wildfire intensity increase (↗)
CA: precipitation increase (rain extremes, snow cover albedo ↗)
• Greenhouse gas emissions
• Landscape disruption
• Ecosystem disruption
• PF: carbon-climate FB
• PF: thermokarst development
• PF: summer soil drying
•PF-: vegetation interaction
– – (global)
++ (regional)
N/A +++ (wrt carbon loss)
– – – (wrt frozen soil)
++ (regional)
– – (global, on 10s-100s year timescale)
Subsea permafrost (thaw)DC: ocean warming (↗)
CA: sea ice loss (↗)
CA: water pressure reduction (↗)
• Greenhouse gas emissions• PF: Carbon-climate FB
• NF: sediment sink
• NF: water column sink
+N/A+ + (w.r.t. gas hydrate dissociation)+ + (w.r.t. frozen sediment)– –
(global, on 10s-100s year timescale)
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