Editing IPCC:AR6/WGI/Chapter-4 (section)

=== 4.7.2 Potential for Abrupt and Irreversible Climate Change ===

<div id="h2-26-siblings" class="h2-siblings"></div>

Similar to AR5 and SROCC, AR6 defines an abrupt climate change as a large-scale abrupt change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades and causes substantial impacts in human and/or natural systems (Glossary). Further, AR6 considers such a perturbed state of a dynamical system as irreversible on a given time scale, if the recovery time scale from this state due to natural processes takes substantially longer than the time scale of interest (Glossary). The AR6 adopts the related definition of a tipping point as a critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly, and a tipping element as a component of the Earth system that is susceptible to a tipping point (Glossary). Tipping points may involve global or regional climate changes from one stable state to another stable state or to changes that occur faster than the rate of change of forcing ( [[#Alley--2003|Alley et al., 2003]] ) and include shifts from one equilibrium state to another and other responses of the climate system to external forcing ( [[IPCC:Wg1:Chapter:Chapter-1#1.2.4.2|Section 1.2.4.2]] ). While reversibility has been defined alternatively in the literature with respect to the response specifically to idealized CO <sub>2</sub> forcing and generally GSAT change, AR6 considers both definitions synonymous, because it has been widely demonstrated that the GSAT change is reversible in models with respect to CO <sub>2</sub> with a several-year lag ( [[#Boucher--2012|Boucher et al., 2012]] ).

Abrupt and irreversible changes in the climate system are assessed across multiple chapters in AR6. This section provides a cross-chapter synthesis of these assessments as an update to Table 12.4 in AR5 and Table 6.1 in SROCC. Understanding of abrupt climate change and irreversibility has advanced considerably since AR5 with many of the projected changes in proposed Tipping Elements having grown more confident (Table 4.10). Many aspects of the physical climate changes induced by GHG warming previously demonstrated to be reversible in a single model have been confirmed in multiple models ( [[#Boucher--2012|Boucher et al., 2012]] ; [[#Tokarska--2015|Tokarska and Zickfeld, 2015]] ) with others such as sea level rise or terrestrial ecosystems confirmed to continue to respond on long time scales ( [[#Clark--2016|Clark et al., 2016]] ; [[#Zickfeld--2017|Zickfeld et al., 2017]] ; [[#Pugh--2018|Pugh et al., 2018]] ).

<div id="_idContainer102" class="Basic-Text-Frame"></div>

'''Table 4.10''' '''|''' '''Cross-chapter assessment updating AR5 and SROCC of components in the Earth system that have been proposed as susceptible to tipping points/abrupt change, irreversibility, projected 21st century change, and overall change in assessment from previous IPCC reports.''' Also provided are confidence levels and, in parentheses, the main section(s) of this Report in which proposed tipping elements are assessed.

{| class="wikitable"
|-
| Earth System Component/Tipping Element

| Potential Abrupt Climate Change?

| Irreversibility if Forcing Reversed (Time Scales Indicated)

| Projected 21st Century Change Under Continued Warming

| Change in Assessment

|-
| Global Monsoon (4.5.1.5; 8.6)

| Yes, under AMOC collapse, ''medium confidence''

| Reversible within years to decades, ''medium confidence''

| ''Medium confidence'' in global monsoon increase; ''medium confidence'' in Asian-African strengthening and North American weakening

| More lines of evidence than AR5

|-
| Tropical Forest (5.4.8; 8.6.2)

| Yes, ''low confidence''

| Irreversible for multi-decades, ''medium confidence''

| ''Medium confidence'' of increasing vegetation carbon storage depending on human disturbance

| More confident rates than AR5

|-
| Boreal Forest (5.4.8)

| Yes, ''low confidence''

| Irreversible for multi-decades, ''medium confidence''

| ''Medium confidence'' in offsetting lower latitude dieback and poleward extension depending on human disturbance

| More confident rates than AR5

|-
| Permafrost Carbon (5.4.8)

| Yes, ''high confidence''

| Irreversible for centuries, ''high confidence''

| ''Virtually certain'' decline in frozen carbon; ''low confidence'' in net carbon change

| More confident rates than SROCC

|-
| Arctic Summer Sea Ice (4.3.2; 4.6.2.1; 9.3.1)

| No, ''high confidence''

| Reversible within years to decades, ''high confidence''

| ''Likely complete loss''

| More specificity than SROCC

|-
| Arctic Winter Sea Ice (4.3.2; 9.3.1)

| Yes, ''high confidence''

| Reversible within years to decades, ''high confidence''

| ''High confidence'' in moderate winter declines

| More specificity than SROCC

|-
| Antarctic Sea Ice (9.3.2)

| ''Yes, low confidence''

| Unknown, ''low confidence''

| ''Low confidence'' in moderate winter and summer declines

| Improved CMIP6 simulation

|-
| Greenland Ice Sheet (9.4.1)

| No, ''high confidence''

| Irreversible for millennia, ''high confidence''

| ''Virtually certain'' mass loss under all scenarios

| More lines of evidence than SROCC

|-
| West Antarctic Ice Sheet and Shelves (9.4.2; Box 9.4)

| ''Yes, high confidence''

| Irreversible for decades to millennia, ''high confidence''

| ''Likely'' mass loss under all scenarios; ''deep uncertainty'' in projections for above 3°C

| Added deep uncertainty at GWL &gt;3°C

|-
| Global Ocean Heat Content (4.5.2.1; 4.6.2.1; 9.2.2; CCBox 7.1)

| No, ''high confidence''

| Irreversible for centuries, ''very high confidence''

| ''Very high confidence'' oceans will continue to warm

| Better consistency with ECS/TCR

|-
| Global Sea-Level Rise (4.6.2.1; 4.6.3.2; 9.6.3.5; Box 9.4)

| Yes, ''high confidence''

| Irreversible for centuries, ''very high confidence''

| ''Very high confidence'' in continued rise; ''deep uncertainty'' in projections above 3°C

| Added deep uncertainty at GWL &gt;3°C

|-
| AMOC (4.6.3.2; 8.6.1; 9.2.3.1)

| Yes, ''medium confidence''

| Reversible within centuries, ''high confidence''

| ''Very likely decline; medium confidence of no collapse''

| More lines of evidence than SROCC

|-
| Southern MOC (9.2.3.2)

| Yes, ''medium confidence''

| Reversible within decades to centuries, ''low confidence''

| ''Medium confidence'' in decrease in strength

| More lines of evidence than SROCC

|-
| Ocean Acidification (4.3.2.5; 5.4.2; 5.4.4)

| Yes, ''high confidence''

| Reversible at surface; irreversible for centuries to millennia at depth, ''very high confidence''

| ''Virtually certain'' to continue with increasing CO <sub>2</sub> ; likely polar aragonite undersaturation

| More lines of evidence than SROCC

|-
| Ocean Deoxygenation (5.3.3.2)

| Yes, ''high confidence''

| Reversible at surface; irreversible for centuries to millennia at depth, ''medium confidence''

| ''Medium confidence'' in deoxygenation rates and increased hypoxia

| Improved CMIP6 simulation

|}

The Carbon Dioxide Removal Model Intercomparison Project (CDR-MIP; [[#Keller--2018|Keller et al., 2018]] ) comprises a set of 1% ramp-up, ramp-down simulations aimed at establishing a multi-model assessment of reversibility of Earth system components. Preliminary results from CDRMIP are presented in [[#4.6.3|Section 4.6.3]] . Results from the SSP5-3.4-Overshoot scenario and other quantities of climate change at the same CO <sub>2</sub> level before and after overshoot are assessed in [[#4.6.2|Section 4.6.2]] . Forcing reversal is followed by reversal of ocean surface and land temperature along with land and ocean precipitation, snow cover, and Arctic sea ice with a lag of a few years to decades (Table 4.10). Other tipping elements have much longer time scales of reversibility from decades to millennia. [[#Drijfhout--2015|Drijfhout et al. (2015)]] provided an assessment of 13 regional mechanisms of abrupt change, finding abrupt changes in sea ice, oceanic flows, land ice, and terrestrial ecosystem response, although with little consistency among the models. The potential for abrupt changes in ice sheets, the AMOC, tropical forests, and ecosystem responses to ocean acidification were also recently reviewed by ( [[#Good--2018|Good et al., 2018]] ). They found that some degree of irreversible loss of the West Antarctic Ice Sheet (WAIS) may have already begun, that tropical forests are adversely affected by drought, and rapid development of aragonite undersaturation at high latitudes affecting calcifying organisms.

New since AR5 is the fundamental recognition in SRCCL and in this Report (Chapter 5) that projected changes in forests strongly depend on the human disturbance and that tropical forest dieback in the absence of disturbance is largely driven by the increased potential for drought, while that in boreal forests includes both thermal and hydrological factors ( [[#Drijfhout--2015|Drijfhout et al., 2015]] ). For some proposed tipping elements, the role of seasonal change has become better understood. For example, the lack of a tipping point in the reduction of summer Arctic sea ice area (Stroeve and [[#Notz--2015|Notz, 2015]] ) has been further substantiated. The role of abrupt change at the edges ( [[#Bathiany--2020|Bathiany et al., 2020]] ) has also been clarified, as has been the importance of distinguishing summer from winter mechanisms and associated abruptness, because ice area reduces gradually in summer, but not necessarily in winter ( [[#Bathiany--2016|Bathiany et al., 2016]] ). For other tipping elements including AMOC ( [[IPCC:Wg1:Chapter:Chapter-19#9.2.3.1|Section 9.2.3.1]] ), mixed layer depth ( [[IPCC:Wg1:Chapter:Chapter-19#9.2.1.3|Section 9.2.1.3]] ), and sea level rise ( [[IPCC:Wg1:Chapter:Chapter-19#9.6.3.5|]] ), an increase in the diversity of model structure and sensitivity to multiple factors has led to a better understanding of the complexity of the problem, with some increase in assessed uncertainty and an assessed deep uncertainty (Glossary) related to projected sea level rise with global warming levels above 3°C ( [[IPCC:Wg1:Chapter:Chapter-19#9.6.3.5|Section 9.6.3.5]] ). In still other cases such as Antarctic sea ice ( [[IPCC:Wg1:Chapter:Chapter-19#9.3.2|Section 9.3.2]] ) and Southern Ocean Meridional Overturning Circulation (MOC; [[IPCC:Wg1:Chapter:Chapter-19#9.2.3.1|Section 9.2.3.1]] ), uncertainty remains high. Finally, it has also been postulated that models may be prone to being too stable ( [[#Valdes--2011|Valdes, 2011]] ) based on the limitations of models as well as other lines of evidence such paleo-evidence of abrupt events ( [[#Dakos--2008|Dakos et al., 2008]] ; [[#Klus--2018|Klus et al., 2018]] ; [[#Sime--2019|Sime et al., 2019]] ).

<div id="4.8" class="h1-container"></div>

<span id="low-likelihood-high-warming-storylines"></span>