Editing IPCC:AR6/WGI/TS (section)

== Box TS.3 | Low-likelihood, High-warming Storylines ==

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'''Future global warming exceeding the assessed ''very likely'' range cannot be ruled out and is potentially associated with the highest risks for society and ecosystems. Such low-likelihood, high-warming storylines tend to exhibit substantially greater changes in the intensity of regional drying and wetting than the multi-model mean. Even at levels of warming within the ''very likely'' range, global and regional low-likelihood outcomes might occur, such as large precipitation changes, additional sea level rise associated with collapsing ice sheets (see Box TS.4), or abrupt ocean circulation changes. While there is ''medium confidence'' that the Atlantic Meridional Overturning Circulation (AMOC) will not experience an abrupt collapse before 2100, if it were to occur, it would ''very likely'' cause abrupt shifts in regional weather patterns and water cycle. The probability of these low-likelihood outcomes increases with higher global warming levels. If the real-world climate sensitivity lies at the high end of the assessed range, then global and regional changes substantially outside the ''very likely'' range projections occur for a given emissions scenario. With increasing global warming, some very rare extremes and some compound events (multivariate or concurrent extremes) with low likelihood in past and current climate will become more frequent, and there is a higher chance that events unprecedented in the observational record occur (''high confidence''). Finally, low-likelihood, high-impact outcomes may also arise from a series of very large volcanic eruptions that could substantially alter the 21st century climate trajectory compared to SSP-based Earth system model (ESM) projections. Links to chapters Cross-Chapter Box 4.1, 4.3, 4.4, 4.8, 7.3, 7.4, 7.5, 8.6, 9.2, 9.6, Box 9.4, Box 11.2, Cross-Chapter Box 12.1'''

Previous IPCC reports largely focused their assessment on the projected ''very likely'' range of future surface warming and associated climate change. However, a comprehensive risk assessment also requires considering the potentially larger changes in the physical climate system that are ''unlikely'' or ''very unlikely'' but possible and potentially associated with the highest risks for society and ecosystems (Figure TS.6). Since AR5, the development of physical climate storylines of high warming has emerged as a useful approach for exploring the future risk space that lies outside of the IPCC ''very likely'' range projections. Links to chapters 4.8

Uncertainty in the true values of equilibrium climate sensitivity (ECS) and transient climate response (TCR) dominate uncertainty in projections of future warming under moderate to strong emissions scenarios (Section TS.3.2). A real-world ECS higher than the assessed ''very likely'' range (2°C–5°C) would require a strong historical aerosol cooling and/or a trend towards stronger warming from positive feedbacks linked to changes in SST patterns (pattern effects), combined with a strong positive cloud feedback and substantial biases in paleoclimate reconstructions – each of which is assessed as either ''unlikely'' or ''very unlikely'' , but not ruled out. Since CMIP6 contains several ESMs that exceed the upper bound of the assessed ''very likely'' range in future surface warming, these models can be used to develop low-likelihood, high warming storylines to explore risks and vulnerabilities, even in the absence of a quantitative assessment of likelihood. Links to chapters 4.3.4, 4.8, 7.3.2, 7.4.4, 7.5.2, 7.5.5, 7.5.7

CMIP6 models with surface warming outside, or close to, the upper bound of the ''very likely'' range exhibit patterns of large widespread temperature and precipitation changes that differ substantially from the multi-model mean in all scenarios. For SSP5-8.5, the high-warming models exhibit widespread warming of more than 6°C over most extratropical land regions and parts of the Amazon. In the Arctic, annual mean temperatures increase by more than 10°C relative to present-day, corresponding to about 30% more than the best estimate of warming. Even for SSP1-2.6, high-warming models show on average 2°C–3°C warming relative to present-day conditions over much of Eurasia and North America (about 40% more than the best estimate of warming) and more than 4°C warming relative to the present over the Arctic in 2081–2100 (Box TS.3, Figure 1). Such a high global warming storyline would imply that the remaining carbon budget consistent with a 2°C warming is smaller than the assessed ''very likely'' range. Put another way, even if a carbon budget that ''likely'' limits warming to 2°C is met, a low-likelihood, high-warming storyline would result in warming of 2.5°C or more. Links to chapters 4.8

CMIP6 models with global warming close to the upper bound of the assessed ''very likely'' warming range tend to exhibit greater changes in the intensity of regional drying and wetting than the multi-model mean. Furthermore, these model projections show a larger area of drying and tend to show a larger fraction of strong precipitation increases than the multi-model mean. However, regional precipitation changes arise from both thermodynamic and dynamic processes so that the most pronounced global warming levels are not necessarily associated with the strongest precipitation response. Abrupt human-caused changes to the water cycle cannot be ruled out. Positive land surface feedbacks, involving vegetation and dust, can contribute to abrupt changes in aridity, but there is only ''low confidence'' that such changes will occur during the 21st century. Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century (''low confidence''). (See also Box TS.9). Links to chapters 4.8, 8.6.2

While there is ''medium confidence'' that the projected decline in the AMOC (Section TS.2.4) will not involve an abrupt collapse before 2100, such a collapse might be triggered by an unexpected meltwater influx from the Greenland Ice Sheet. If an AMOC collapse were to occur, it would ''very likely'' cause abrupt shifts in the regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and could result in weakening of the African and Asian monsoons, strengthening of Southern Hemisphere monsoons, and drying in Europe. (See also Boxes TS.9 and TS.13). Links to chapters 4.7.2, 8.6.1, 9.2.3

Very rare extremes and compound or concurrent events, such as the 2018 concurrent heatwaves across the Northern Hemisphere, are often associated with large impacts. The changing climate state is already altering the likelihood of extreme events, such as decadal droughts and extreme sea levels, and will continue to do so under future warming. Compound events and concurrent extremes contribute to increasing probability of low-likelihood, high-impact outcomes and will become more frequent with increasing global warming (''high confidence''). Higher warming levels increase the likelihood of events unprecedented in the observational record. Links to chapters 9.6.4, Box 11.2

Finally, low likelihood storylines need not necessarily relate solely to the human-induced changes in climate. A low-likelihood, high-impact outcome, consistent with historical precedent in the past 2500 years, would be to see several large volcanic eruptions that could greatly alter the 21st century climate trajectory compared to SSP-based Earth system model projections. Links to chapters Cross-Chapter Box 4.1

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'''Box TS.3, Figure 1 |'''

'''High-warming storylines.''' ''The intent of this figure is to illustrate high warming storylines compared to the CMIP6 multi-model-mean.'' '''(a)''' Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model mean linearly scaled to the assessed best global surface temperature estimate for SSP1-2.6 in 2081–2100 relative to 1995–2014, '''(b)''' mean across five high-warming models with global surface temperature changes nearest to the upper bound of the assessed very likely range, and '''(c)''' mean across five very high-warming models with global surface temperature changes higher than the assessed ''very likely'' . '''(d–f)''' Same as (a–c) but for SSP5-8.5. Note the different colour bars in (a–c) and (d–f). Links to chapters 4.7, Figure 4.41
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=== TS.2.4 The Ocean ===

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'''Observations, models and paleo-evidence indicate that recently observed changes in the ocean are unprecedented for centuries to millennia (''high confidence''). Over the past four to six decades, it is ''virtually certain'' that the global ocean has warmed, with human influence ''extremely likely'' the main driver since the 1970s, making climate change irreversible over centuries to millennia (''medium confidence''). It is ''virtually certain'' that upper ocean salinity contrasts have increased since the 1950s and ''extremely likely'' that human influence has contributed. It is ''virtually certain'' that upper ocean stratification has increased since 1970 and that sea water pH has declined globally over the last 40 years, with human influence being the main driver of the observed surface open ocean acidification (''virtually certain''). A long-term increase in surface open ocean pH occurred over the past 50 million years (''high confidence''), and surface ocean pH as low as recent times is uncommon in the last 2 million years (''medium confidence''). There is ''high confidence'' that marine heatwaves have become more frequent in the 20th century, and most of those since 2006 have been attributed to anthropogenic warming (''very likely'') . There is ''high confidence'' that oxygen levels have dropped in many regions since the mid 20th century and that the geographic range of many marine organisms has changed over the last two decades.'''

'''The amount of ocean warming observed since 1971 will ''likely'' at least double by 2100 under a low warming scenario (SSP1-2.6) and will increase by 4–8 times under a high warming scenario (SSP5-8.5). Stratification (''virtually certain''), acidification (''virtually certain''), deoxygenation (''high confidence'') and marine heatwave frequency (''high confidence'') will continue to increase in the 21st century. While there is ''low confidence'' in 20th century AMOC change, it is ''very likely'' that AMOC will decline over the 21st century (Figure TS.11). Links to chapters 2.3, 3.5, 3.6, 4.3.2, 5.3, 7.2, 9.2, Box 9.2, 12.4'''
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'''Figure TS.11 |''' '''Past and future ocean and ice-sheet changes.''' ''The intent of this figure is to show that observed and projected time series of many ocean and cryosphere indicators are consistent.'' Observed and simulated historical changes and projected future changes under varying greenhouse gas emissions scenarios. Simulated and projected ocean changes are shown as Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble mean, and 5–95% range (shading) is provided for scenarios SSP1-2.6 and SSP3-7.0 (except in panel a where the range is provided for scenario SSP1-2.6 and SSP5-8.5). Mean and 5–95% range in 2100 are shown as vertical bars on the right-hand side of each panel. (a) Change in multiplication factor in surface ocean marine heatwave days relative to 1995–2014 (defined as days exceeding the 99th percentile in sea surface temperature (SST) from 1995–2014 distribution). Assessed observational change span 1982–2019 from AVHRR satellite SST. (b) Atlantic Meridional Overturning Circulation (AMOC) transport relative to 1995–2014 (defined as maximum transport at 26°N). Assessed observational change spans 2004–2018 from the RAPID array smoothed with a 12-month running mean (shading around the mean shows the 12-month running standard deviation around the mean). (c) Global mean percent change in ocean oxygen (100–600 m depth), relative to 1995–2014. Assessed observational trends and ''very likely'' range are from the SROCC assessment, and span 1970–2010 centred on 2005. (d) Global mean surface pH. Assessed observational change spans 1985–2019, from the CMEMS SOCAT-based reconstruction (shading around the global mean shows the 90% confidence interval). (e), (f) : Ice sheet mass changes. Projected ice-sheet changes are shown as median, 5–95% range (light shading), and 17–83% range (dark shading) of cumulative mass loss and sea level equivalent from ISMIP6 emulation under SSP1-2.6 and SSP5-8.5 (shading and bold line), with individual emulated projections as thin lines. Median (dot), 17–83% range (thick vertical bar), and 5–95% range (thin vertical bar) in 2100 are shown as vertical bars on the right-hand side of each panel, from ISMIP6, ISMIP6 emulation, and LARMIP-2. Observation-based estimates: For Greenland (e), for 1972–2018 (Mouginot), for 1992–2016 (Bamber), for 1992–2020 (IMBIE) and total estimated mass loss range for 1840–1972 (Box). For Antarctica (f), estimates based on satellite data combined with simulated surface mass balance and glacial isostatic adjustment for 1992–2020 (IMBIE), 1992–2016 (Bamber), and 1979–2017 (Rignot). Left inset maps: mean Greenland elevation changes 2010–2017 derived from CryoSat-2 radar altimetry (e) and mean Antarctica elevation changes 1978–2017 derived from restored analogue radar records (f). Right inset maps: ISMIP6 model mean (2093–2100) projected changes under the MIROC5 climate model for the RCP8.5 scenario. Links to chapters 2.3.3; 2.3.4; 3.5.4; 4.3.2; 5.3.2; 5.3.3; 5.6.3; 9.2.3; 9.4.1; 9.4.2; Box 9.2; Box 9.2, Figure 1; Figures 9.10, 9.17 and 9.18

It is ''virtually certain'' that the global ocean has warmed since at least 1971, representing about 90% of the increase in the global energy inventory (Section TS.3.1). The ocean is currently warming faster than at any other time since at least the last deglacial transition (''medium confidence''), with warming extending to depths well below 2000 m (''very high confidence''). It is ''extremely likely'' that human influence was the main driver of this recent ocean warming. Ocean warming will continue over the 21st century (''virtually certain''), and will ''likely'' continue until at least to 2300 even for low CO <sub>2</sub> emissions scenarios. Ocean warming is irreversible over centuries to millennia (''medium confidence''), but the magnitude of warming is scenario-dependent from about the mid-21st century (''medium confidence''). The warming will not be globally uniform, with heat primarily stored in Southern Ocean water-masses and weaker warming in the subpolar North Atlantic (''high confidence''). Limitations in the understanding of feedback mechanisms limit our confidence in future ocean warming close to Antarctica and how this will affect sea ice and ice shelves. Links to chapters 2.3.3, 3.5.1, 4.7.2, 7.2.2, 9.2.2, 9.2.3, 9.2.4, 9.3.2, 9.6.1, Cross-Chapter Box 9.1

Global mean SST has increased since the beginning of the 20th century by 0.88 [0.68 to 1.01] °C, and it is ''virtually certain'' it will continue to increase throughout the 21st century, with increasing hazards to marine ecosystems (''medium confidence''). Marine heatwaves have become more frequent over the 20th century (''high confidence''), approximately doubling in frequency (''high confidence'') and becoming more intense and longer since the 1980s (''medium confidence''). Most of the marine heatwaves over 2006–2015 have been attributed to anthropogenic warming (''very likely'') . Marine heatwaves will continue to increase in frequency, with a ''likely'' global increase of 2–9 times in 2081–2100 compared to 1995–2014 under SSP1-2.6, and 3–15 times under SSP5-8.5 (Figure TS.11a), with the largest changes in the tropical and Arctic ocean. Links to chapters 2.3.1, Cross-Chapter Box 2.3, 9.2.1, Box 9.2, 12.4.8

Observed upper-ocean stratification (0–200 m) has increased globally since at least 1970 ''('' ''virtually certain'' '')'' . Based on recent refined analyses of the available observations, there is ''high confidence'' that it increased by 4.9 ± 1.5% from 1970–2018, which is about twice as much as assessed in SROCC, and will continue to increase throughout the 21st century at a rate depending on the emissions scenario (''virtually certain''). Links to chapters 2.3.3, 9.2.1

It is ''virtually certain'' that since 1950 near-surface high-salinity regions have become more saline, while low-salinity regions have become fresher, with ''medium confidence'' that this is linked to an intensification of the hydrological cycle (Box TS.6). It is ''extremely likely'' that human influence has contributed to this salinity change and that the large-scale pattern will grow in amplitude over the 21st century (''medium confidence''). Links to chapters 2.3.3, 3.5.2, 9.2.2, 12.4.8

The AMOC was relatively stable during the past 8000 years (''medium confidence''). There is ''low confidence'' in the quantification of AMOC changes in the 20th century because of ''low agreement'' in quantitative reconstructed and simulated trends, missing key processes in both models and measurements used for formulating proxies, and new model evaluations. Direct observational records since the mid-2000s are too short to determine the relative contributions of internal variability, natural forcing and anthropogenic forcing to AMOC change (''high confidence''). An AMOC decline over the 21st century is ''very likely'' for all SSP scenarios (Figure TS.11b); a possible abrupt decline is assessed further in Box TS.3. Links to chapters 2.3.3, 3.5.4, 4.3.2, 8.6.1, 9.2.3, Cross-Chapter Box 12.3

There is ''high confidence'' that many ocean currents will change in the 21st century in response to changes in wind stress. There is ''low confidence'' in 21st century change of Southern Ocean circulation, despite ''high confidence'' that it is sensitive to changes in wind patterns and increased ice-shelf melt. Western boundary currents and subtropical gyres have shifted poleward since 1993 (''medium confidence''). Subtropical gyres, the East Australian Current Extension, the Agulhas Current, and the Brazil Current are projected to intensify in the 21st century in response to changes in wind stress, while the Gulf Stream and the Indonesian Throughflow are projected to weaken (''medium confidence''). All of the four main eastern boundary upwelling systems are projected to weaken at low latitudes and intensify at high latitudes in the 21st century (''high confidence''). Links to chapters 2.3.3, 9.2.3

It is ''virtually certain'' that surface pH has declined globally over the last 40 years and that the main driver is uptake of anthropogenic CO <sub>2</sub> . Ocean acidification and associated reductions in the saturation state of calcium carbonate – a constituent of skeletons or shells of a variety of marine organisms – is expected to increase in the 21st century under all emissions scenarios (''high confidence''). A long-term increase in surface open ocean pH occurred over the past 50 million years (''high confidence''), and surface ocean pH as low as recent times is uncommon in the last 2 million years (''medium confidence''). There is ''very high confidence'' that present-day surface pH values are unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at least that time. Over the past 2–3 decades, a pH decline in the ocean interior has been observed in all ocean basins (''high confidence'') (Figure TS.11d). Links to chapters 2.3.3, 2.3.4, 3.6.2, 4.3.2, 5.3.2, 5.3.3, 5.6.3, 12.4.8

Open-ocean deoxygenation and expansion of oxygen minimum zones have been observed in many areas of the global ocean since the mid 20th century (''high confidence''), in part due to human influence (''medium confidence''). Deoxygenation is projected to continue to increase with ocean warming (''high confidence'') (Figure TS.11c). Higher climate sensitivity and reduced ocean ventilation in CMIP6 compared to CMIP5 results in substantially greater projections of subsurface (100–600 m) oxygen decline than reported in SROCC for the period 2080–2099. Links to chapters 2.3.3, 2.3.4, Cross-Chapter Box 2.4, 3.6.2, 5.3.3, 12.4.8

Over at least the last two decades, the geographic range of many marine organisms has shifted towards the poles and towards greater depths (''high confidence''), indicative of shifts towards cooler waters. The range of a smaller subset of organisms has shifted equatorward and to shallower depths (''high confidence''). Phenological metrics associated with the life cycles of many organisms have also changed over the last two decades or longer (''high confidence''). Since the changes in the geographical range of organisms and their phenological metrics have been observed to differ with species and location, there is the possibility of disruption to major marine ecosystems. Links to chapters 2.3.4

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=== TS.2.5 The Cryosphere ===

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'''Over recent decades, widespread loss of snow and ice has been observed, and several elements of the cryosphere are now in states unseen in centuries (''high confidence''). Human influence was ''very likely'' the main driver of observed reductions in Arctic sea ice since the late 1970s (with late-summer sea ice loss ''likely'' unprecedented for at least 1000 years) and the widespread retreat of glaciers (unprecedented in at least the last 2,000 years, ''medium confidence''). Furthermore, human influence ''very likely'' contributed to the observed Northern Hemisphere spring snow cover decrease since 1950.''' '''By contrast, Antarctic sea ice area experienced no significant net change since 1979, and there is only ''low confidence'' in its projected changes. The Arctic Ocean is projected to become practically sea ice-free in late summer under high CO <sub>2</sub> emissions scenarios by the end of the 21st century (''high confidence''). It is ''virtually certain'' that further warming will lead to further reductions of Northern Hemisphere snow cover, and there is ''high confidence'' that this is also the case for near-surface permafrost volume.''' '''Glaciers will continue to lose mass at least for several decades even if global temperature is stabilized (''very high confidence''), and mass loss over the 21st century is ''virtually certain'' for the Greenland Ice Sheet and ''likely'' for the Antarctic Ice Sheet. Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet within the 21st century and beyond, in particular due to the potential instability of the West Antarctic Ice Sheet. Links to chapters 2.3, 3.4, 4.3, 8.3, 9.3–9.6, Box 9.4, 12.4'''

Current Arctic sea ice coverage levels (both annual and late summer) are at their lowest since at least 1850 (''high confidence''), and for late summer for the past 1000 years (''medium confidence''). Since the late 1970s, Arctic sea ice area and thickness have decreased in both summer and winter, with sea ice becoming younger, thinner and more dynamic (''very high confidence''). It is ''very likely'' that anthropogenic forcing, mainly due to greenhouse gas increases, was the main driver of this loss, although new evidence suggests that anthropogenic aerosol forcing has offset part of the greenhouse gas-induced losses since the 1950s (''medium confidence''). The annual Arctic sea ice area minimum will ''likely'' fall below 1 million km <sup>2</sup> at least once before 2050 under all assessed SSP scenarios. This practically sea ice-free state will become the norm for late summer by the end of the 21st century in high CO <sub>2</sub> emissions scenarios (''high confidence''). Arctic summer sea ice varies approximately linearly with global surface temperature, implying that there is no tipping point and observed/projected losses are potentially reversible (''high'' ''confidence''). Links to chapters 2.3.2, 3.4.1, 4.3.2, 9.3.1, 12.4.9

For Antarctic sea ice, there is no significant trend in satellite-observed sea ice area from 1979 to 2020 in both winter and summer, due to regionally opposing trends and large internal variability. Due to mismatches between model simulations and observations, combined with a lack of understanding of reasons for substantial inter-model spread, there is ''low confidence'' in model projections of future Antarctic sea ice changes, particularly at the regional level. Links to chapters 2.3.2, 3.4.1, 9.3.2

In permafrost regions, increases in ground temperatures in the upper 30 m over the past three to four decades have been widespread (''high confidence''). For each additional 1°C of warming (up to 4°C above the 1850–1900 level), the global volume of perennially frozen ground to 3 m below the surface is projected to decrease by about 25% relative to the present volume (''medium confidence''). However, these decreases may be underestimated due to an incomplete representation of relevant physical processes in ESMs (''low confidence''). Seasonal snow cover is treated in Section TS.2.6. Links to chapters 2.3.2, 9.5.2, 12.4.9

There is ''very high confidence'' that, with few exceptions, glaciers have retreated since the second half of the 19th century; this behaviour is unprecedented in at least the last 2000 years (''medium confidence''). Mountain glaciers ''very likely'' contributed 67.2 [41.8 to 92.6] mm to the observed GMSL change between 1901 and 2018. This retreat has occurred at increased rates since the 1990s, with human influence ''very likely'' being the main driver. Under RCP2.6 and RCP8.5, respectively, glaciers are projected to lose 18% ± 13% and 36% ± 20% of their current mass over the 21st century (''medium confidence''). Links to chapters 2.3.2, 3.4.3, 9.5.1, 9.6.1

The Greenland Ice Sheet was smaller than at present during the Last Interglacial period (roughly 125,000 years ago) and the mid-Holocene (roughly 6,000 years ago) (''high confidence''). After reaching a recent maximum ice mass at some point between 1450 and 1850, the ice sheet retreated overall, with some decades ''likely'' close to equilibrium (i.e., mass loss approximately equalling mass gained). It is ''virtually certain'' that the Greenland Ice Sheet has lost mass since the 1990s, with human influence a contributing factor (''medium confidence''). There is ''high confidence'' that annual mass changes have been consistently negative since the early 2000s. Over the period 1992–2020, Greenland ''likely'' lost 4890 ± 460 Gt of ice, contributing 13.5 ± 1.3 mm to GMSL rise. There is ''high confidence'' that Greenland ice mass losses are increasingly dominated by surface melting and runoff, with large interannual variability arising from changes in surface mass balance. Projections of future Greenland ice-mass loss (Box TS.4, Table 1; Figure TS.11e) are dominated by increased surface melt under all emissions scenarios (''high confidence''). Potential irreversible long-term loss of the Greenland Ice Sheet, and of parts of the Antarctic Ice Sheet, is assessed in Box TS.9. Links to chapters 2.3.2, 3.4.3, 9.4.1, 9.4.2, 9.6.3, Atlas.11.2

It is ''likely'' that the Antarctic Ice Sheet has lost 2670 ± 530 Gt, contributing 7.4 ± 1.5 mm to GMSL rise over 1992–2020. The total Antarctic ice mass losses were dominated by the West Antarctic Ice Sheet, with combined West Antarctic and Peninsula annual loss rates increasing since about 2000 (''very high confidence''). Furthermore, it is ''very likely'' that parts of the East Antarctic Ice Sheet have lost mass since 1979. Since the 1970s, snowfall has ''likely'' increased over the western Antarctic Peninsula and eastern West Antarctica, with large spatial and interannual variability over the rest of Antarctica. Mass losses from West Antarctic outlet glaciers, mainly induced by ice shelf basal melt (''high confidence''), outpace mass gain from increased snow accumulation on the continent (''very high confidence''). However, there is only ''limited evidence'' , with ''medium agreement'' , of anthropogenic forcing of the observed Antarctic mass loss since 1992 (with ''low confidence'' in process attribution). Increasing mass loss from ice shelves and inland discharge will ''likely'' continue to outpace increasing snowfall over the 21st century (Figure TS.11f). Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet along high-end mass-loss storylines within the 21st century and beyond, primarily related to the abrupt and widespread onset of marine ice sheet instability and marine ice cliff instability. (See also Boxes TS.3 and TS.4). Links to chapters 2.3.2, 3.4.3, 9.4.2, 9.6.3, Box 9.4, Atlas.11.1

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