Editing IPCC:AR6/SROCC/Chapter-6 (section)

=== 6.2.1 Introduction ===

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Some potentially abrupt or irreversible events are assessed in other chapters, hence Table 6.1 presents a cross-chapter summary of those. Subsection numbers indicate where detailed information may be found.

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'''Table 6.1'''

Cross-Chapter assessment of abrupt and irreversible phenomena related to the ocean and cryosphere. The column on the far right of the table indicates the likelihood of an abrupt/irreversible change based on the assessed literature which, in general, assesses Representative Concentration Pathway (RCP) scenarios. Assessments of likelihood and confidence are made according to IPCC guidance on uncertainties.

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{| class="wikitable"
|-
| Change in system component
| Potentially abrupt
| Irreversibility if forcing reversed (time scales indicated)
| Impacts on natural and human systems; global ''vs.'' regional ''vs.'' local
| Projected likelihood and/or confidence level in 21st century under scenarios considered
|-
| colspan="5"| Ocean
|-
| Atlantic Meridional Overturning Circulation (AMOC) collapse (Section 6.7)
| Yes
| Unknown
| Widespread; increased winter storms in Europe, reduced Sahelian rainfall and agricultural capacity, variations in tropical storms, increased sea levels on Atlantic coasts
| ''Very unlikely'' ''',''' but physically plausible
|-
| Subpolar gyre (SPG) cooling (Section 6.7)
| Yes
| Irreversible within decades
| Similar to AMOC impacts but considerably smaller.
| ''Medium confidence''
|-
| Marine heatwave (MHW) increase (Section 6.4)
| Yes
| Reversible within decades to centuries
| Coral bleaching, loss of biodiversity and ecosystem services, harmful algal blooms, species redistribution
| ''Very likely (very high confidence)'' for physical changes. ''High confidence'' for impacts
|-
| Arctic sea ice retreat (Section 3.3)
| Yes
| Reversible within decades to centuries
| Coastal erosion in Arctic (may take longer to reverse), impact on mid-latitude storms ( ''low confidence'' ); rise in Arctic surface temperatures ( ''high confidence'' )
| ''High confidence''
|-
| Ocean deoxygenation and hypoxic events (Section 5.2)
| Yes
| Reversible at surface, but irreversible for centuries to millennia at depth
| Major changes in ocean productivity, biodiversity and biogeochemical cycles
| ''Medium confidence ''
|-
| Ocean acidification (Section 5.2)
| Yes
| Reversible at surface, but irreversible for centuries to millennia at depth
| Changes in growth, development, calcification, survival and abundance of species, for example, from algae to fish
| ''Virtually certain (very high confidence)''
|-
| colspan="5"| Cryosphere
|-
| Methane release from permafrost (Section 3.4)
| Yes
| Reversible due to short lifetime of methane in the atmosphere
| Further increased global temperatures through climate feedback
| ''Medium confidence''
|-
| CO 2 release from permafrost (Section 3.4)
| Yes
| Irreversible for millennia due to long lifetime of CO 2 in the atmosphere
| Further increased global temperatures through climate feedback
| ''Low confidence''
|-
| Partial West Antarctic Ice Sheet (WAIS) collapse (Cross Chapter Box 2 in Chapter 1, Section 4.2)
| Yes (late 21st century, under RCP8.5 only)
| Irreversible for decades to millennia
| Significant contribution to sea level rise (SLR) and local decrease in ocean salinity
| ''Low confidence''
|-
| Greenland Ice sheet (GIS) decay (Cross Chapter Box 8, Section 4.2)
| No
| Irreversible for millennia
| Significant contribution to SLR, shipping (icebergs)
| ''High confidence'' for decay contributing 10s of cm of SLR
|-
| Ice-shelf collapses (Cross Chapter Box 8, Sections 3.3, 4.2}
| Yes
| Possibly irreversible for centuries
| May lead to SLR from contributing glaciers. Some shelves more prone than others.
| ''Low confidence''
|-
| Glacier avalanches, surges, and collapses (Section 2.3)
| Yes
| Variable
| Local hazard; may accelerate SLR; local iceberg production; local ecosystems
| ''Medium confidence'' for occurrence; ''Low confidence'' for increase in frequency/magnitude
|-
| Strong shrinkage or disappearance of individual glaciers (Sections 2.2, 3.3)
| Yes
| Reversible within decades to centuries
| Regional impact on water resources, tourism, ecosystems and global sea level
| ''Medium confidence''
|-
| Landslides related to glaciers and permafrost, glacier lake outbursts (Section 2.3)
| Yes
| Irreversible for rock slopes; reversible within decades to centuries for glaciers, debris and lakes
| Local direct impact on humans, land use, infrastructure (hazard), and ecosystems
| ''Medium confidence for increase in frequency''
|-
| Change in biodiversity in high mountain areas (impact – Section 2.4)
| Yes
| In many cases irreversible (e.g., extinction of species)
| Local impacts on ecosystems and ecosystem services
| ''Medium confidence''
|}
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