Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.5.2 Emergence of Climatic Impact-drivers Across Time and Scenarios ===

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The emergence of a climate change signal occurs when that signal exceeds some critical threshold (usually taken to be a measure of natural variability; see for example, [[#Hawkins--2012|Hawkins and Sutton, 2012]] ) or when the probability distribution of an indicator becomes significantly different to that over a reference period (e.g., [[#Chadwick--2019|Chadwick et al., 2019]] ; see also [[IPCC:Wg1:Chapter:Chapter-10|Chapter 10]] and [[IPCC:Wg1:Chapter:Chapter-1#1.4.2|Section 1.4.2]] ), in which case external anthropogenic forcings can be detected as causal factors. The ‘time of emergence’ (ToE) or ‘temperature of emergence’ is the time or global warming level thresholds associated with this exceedance. Emergence is particularly relevant to impacts, risk assessment and adaptation because human and natural systems are largely adapted to natural variability but may be vulnerable if exposed to changes that go beyond this variability range; this is not to say that changes within natural variability have no impact, as occurrence of damaging extremes proves. Emergence also informs the timing of adaptation measures. The emergence of a change is always relative to a reference period (e.g., the pre-industrial period or a recent past), depending on the framing question. In the former case, the goal is to estimate the amplitude of an anthropogenically driven change while in the latter, it is to estimate the amplitude of change relative to a baseline that is familiar to stakeholders. Both questions are important for risk assessment, but the former may be more directly interpretable in a mitigation context. The variability also refers to a time scale, generally interannual to inter-decadal. See ( [[IPCC:Wg1:Chapter:Chapter-1#1.4.2|Section 1.4.2]] and [[IPCC:Wg1:Chapter:Chapter-10|Chapter 10]] for more details about how emergence is defined and used in the literature.

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'''Table 12.12 |''' '''Emergence of CIDs in different time periods, as assessed in this section.''' The colour corresponds to the confidence of the region with the highest confidence: white cells indicate where evidence is lacking or the signal is not present, leading to overall ''low confidence'' of an emerging signal.

[[File:fd050b1890340bd9cf8ad2435a50cca8 IPCC_AR6_WGI_Chapter12_Table_12_12_1.jpg]] [[File:3a65d20c43a262742bf56749e9ece726 IPCC_AR6_WGI_Chapter12_Table_12_12_2.jpg]]

Changes in climatic impact-drivers may remain within the range of natural variability or have a time of emergence that varies by region and scenario. This section assesses the evidence for the effects of anthropogenic climate change on the emergence of changes in CID index, past, present and future, as evidenced by the literature assessed in other chapters, as well as additional literature assessed here, at both global and regional scales. In many cases, however, sufficient literature for a robust region-by-region assessment of ToE is lacking. The assessment herein is made by CID. Regional emergence assessment is reported in Tables 12.3–12.11 but is undertaken in this section.

Estimations of ToE must be done with caution given the many sources of inherent uncertainties, such as observations representing only a single realization of climate history, internal variability (whose frequency – e.g., annual or decadal – needs to be precisely defined), model biases, and potential low-frequency changes in variability (Chapter 10; [[#Lehner--2017|Lehner et al., 2017]] ). In addition, a homogeneous interpretation of multiple studies is hampered by heterogeneous methodologies used to calculate emergence. In this section, we assess emergence and its confidence level based on such multiple methods as provided by the literature, and unless specified otherwise, emergence here refers to a signal to noise ration S/N &gt; 1 relative to a pre-industrial baseline and interannual variability (the ‘noise’). Furthermore, observed trends and attribution are taken into account in combination with climate simulations (historical or projections) for assessing whether a trend has already emerged in the historical period.

'''Mean air temperature:''' Warming of mean annual temperatures has already emerged in all land regions, as obtained from past observations and confirmed by historical simulations ( ''high confidence'' ) (Figure 1.13; [[#King--2015|King et al., 2015]] ; [[#Hawkins--2020|Hawkins et al., 2020]] ), with S/N ratios larger than two. In the current climate, the highest S/N ratios exceed five over Central Africa, Amazonia, East and South East Asia. Seasonal warming emergence depends on the season. Because the temperature variability in the mid-latitudes is higher in winter than in summer, the emergence of seasonal warming occurs for summer but not for winter in most of this part of the world. In Europe, summer warming has emerged in all regions ( ''medium confidence'' , ''medium agreement'' ), and in North America, it has emerged only over Eastern and Western regions while in winter there is ''low confidence'' of an emergence in warming in all regions for both Europe and North America ( [[#Lehner--2017|Lehner et al., 2017]] ; [[#Hawkins--2020|Hawkins et al., 2020]] ). When considering the climate of the end of the 20th century (i.e., recent past) as a baseline, the emergence of mean temperature is projected at very different times depending on the scenario. For instance, emergence is reached by 2050 under RCP8.5 in most areas of Europe, Australia or East Asia, but it does not occur within the 21st century under RCP2.6 ( ''medium confidence'' ) ( [[#Sui--2014|Sui et al., 2014]] ; [[#Im--2021|Im et al., 2021]] ). This means that under RCP2.6, mean temperatures stay within the recent climate variability range observed in the mid-latitudes. However, even under RCP2.6, mean temperatures in tropical regions that have not already emerged are projected to emerge before 2050 ( ''medium confidence'' ).

'''Extreme heat and cold:''' An increase in heat extremes has emerged or will emerge in the coming three decades in most land regions ( ''high confidence'' ) (Chapter 11; [[#King--2015|King et al., 2015]] ; [[#Seneviratne--2020|Seneviratne and Hauser, 2020]] ), relative to the pre-industrial period, as found by testing significance of differences in distributions of yearly temperature maxima in simulated 20-year periods. In tropical regions, wherever observed changes can be established with statistical significance, and in most mid-latitude regions, there is ''high confidence'' that hot and cold extremes have emerged in the historical period, but only ''medium confidence'' elsewhere. In other regions emergence is projected at the latest in the first half of the 21st century under RCP8.5 ( ''high confidence'' ) ( [[#King--2015|King et al., 2015]] ; [[#Seneviratne--2020|Seneviratne and Hauser, 2020]] ). Relative to the end of 20th-century conditions, changes in humid heat stress as characterized by wet bulb temperature, indicates a ToE as early as in the first two decades of the 21st century in RCP8.5 at least in many tropical regions (most of Africa in the band 20°S–20°N, South Asia and South East Asia) ( ''medium confidence'' ) ( [[#Im--2021|Im et al., 2021]] ). By 2050 and under RCP8.5, wet bulb temperature is projected to emerge in many other areas such as Southern Africa, North Africa, Europe, and most of Central, Southern and Eastern Asia and Northern and Eastern Australia, while under RCP2.6, emergence is either reached later in the century (Europe, Central Asia, Northern Australia), or never reached in the century ( [[#Im--2021|Im et al., 2021]] ). Decrease of cold spells has already emerged above the interannual variability in Australasia, Africa and most of Northern South America, and they are projected to emerge before 2050 in the northern mid-latitudes and in Southern South America ( [[#King--2015|King et al., 2015]] ) under RCP8.5 ( ''medium confidence, limited evidence'' and ''high agreement'' ).

'''Mean precipitation:''' Mean precipitation changes only emerged over a few regions in the historical period (increase in Northern and Eastern Europe and decrease in West Africa and Amazonia) from observations with an S/N ratio larger than one ( ''low confidence'' ) ( [[#Hawkins--2020|Hawkins et al., 2020]] ). The emergence of increasing precipitation before the middle of the 21st century is found across scenarios in Siberian regions, Russian Far East, Northern Europe, Arctic regions and the northernmost parts of North America ( ''high confidence'' ) and later in other northern mid-latitude areas, depending on the scenario, albeit with different methods and emergence definitions used in climate projections (Chapter 8; [[#Giorgi--2009|Giorgi and Bi, 2009]] ; [[#Maraun--2013|Maraun, 2013]] ; [[#King--2015|King et al., 2015]] ; [[#Akhter--2018|Akhter et al., 2018]] ; [[#Kumar--2018|Kumar and Ganguly, 2018]] ; [[#Nguyen--2018|Nguyen et al., 2018]] ; [[#Barrow--2019|Barrow and Sauchyn, 2019]] ; [[#Rojas--2019|Rojas et al., 2019]] ; [[#Kusunoki--2020|Kusunoki et al., 2020]] ; [[#Pohl--2020|Pohl et al., 2020]] ; W. [[#Li--2021|]] [[#Li--2021|Li et al., 2021]] ). Decreases in mean precipitation are projected to emerge in parts of Africa by the middle of the century, and later in the Mediterranean and Southern Australia, but the emergence depends on the scenario, and specific seasons for crop growth ( [[#Nguyen--2018|Nguyen et al., 2018]] ; [[#Rojas--2019|Rojas et al., 2019]] ). Mean precipitation does not emerge in any of these regions at any time in the 21st century under RCP2.6, but emerges in all under RCP8.5. ToE under RCP4.5 is projected to be around 25 years later relative to RCP8.5 in many of the early emergence regions, highlighting the importance of mitigation to gain more time for adaptation.

'''Heavy precipitation and floods:''' There is ''low confidence'' in the emergence of heavy precipitation and pluvial and river flood frequency in observations, despite trends that have been found in a few regions (Chapters 8 and Chapter 11, and across ( [[#12.4|Section 12.4]] ). In climate projections, the emergence of increase in heavy precipitation strongly depends on the scale of aggregation ( [[#Kirchmeier-Young--2019|Kirchmeier-Young et al., 2019]] ), with, in general, no emergence before a 1.5°C or 2°C warming level, and before the middle of the century ( ''medium confidence'' ), but results depend on the method used for the calculation of the ToE ( [[#Maraun--2013|Maraun, 2013]] ; [[#King--2015|King et al., 2015]] ; [[#Kusunoki--2020|Kusunoki et al., 2020]] ). Emergent increases in heavy precipitation are found in several regions when aggregated at a regional scale in Northern Europe, Northern Asia and East Asia, at latest by the end of the century in SRES A1B or RCP8.5 scenarios or when considering the decadal variability as a reference ( ''medium confidence'' ) ( [[#Maraun--2013|Maraun, 2013]] ; W. [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] , 2021; [[#Kusunoki--2020|Kusunoki et al., 2020]] ). There have been few emergence studies for streamflow and flooding, although one study showed emergence of different hydrological regimes at different times during the 21st century across the USA ( [[#Leng--2016|Leng et al., 2016]] ). Variability in extreme streamflows from year to year can be high relative to a trend ( [[#Zhuan--2018|Zhuan et al., 2018]] ). Given the heterogeneity of methods and results, there is only ''low confidence'' in the emergence of heavy precipitation and flood signals in any region when considering the S/N ratio.

'''Droughts, aridity and fire weather:''' There is ''low confidence'' in the emergence of drought frequency in observations, for any type of drought, in all regions. Even though significant drought trends are observed in several regions with at least ''medium confidence'' (Sections 11.6 and 12.4), agricultural and ecological drought indices have interannual variability that dominates trends, as can be seen from their time series ( ''medium confidence'' ) (H. [[#Guo--2018|]] [[#Guo--2018|Guo et al., 2018]] ; [[#Spinoni--2019|Spinoni et al., 2019]] ; [[#Haile--2020|Haile et al., 2020]] ; M. [[#Wu--2020|]] [[#Wu--2020|Wu et al., 2020]] ). Studies of the emergence of drought with systematic comparisons between trends and variability of indices are lacking, precluding a comprehensive assessment of future drought emergence. Historical climate simulations indicate that fire weather indices have already emerged in several regions (the Amazon basin, Mediterranean, Central America, West and Southern Africa) ( ''low confidence'' , ''limited evidence'' ) ( [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ), and emergence is projected with ''low confidence'' by the middle of the century in several other regions (Southern Australia, Siberia, most of North America and Europe) when considering several indices together.

'''Wind:''' Observed mean surface wind speed trends are present in many areas ( [[#12.4|Section 12.4]] ), but the emergence of these trends from the interannual natural variability and their attribution to human-induced climate change remains of ''low confidence'' due to various factors such as changes in the type and exposure of recording instruments, and their relation to climate change is not established. For future conditions, there is ''limited evidence'' of the emergence of trends in mean wind speeds due to the lack of studies quantifying wind speed changes and their interannual variability. The same limitation also holds for wind extremes (severe storms, tropical cyclones, sand and dust storms).

'''Snow and ice:''' The decrease in the Northern Hemisphere snow cover extent in spring has already emerged from natural variability ( [[IPCC:Wg1:Chapter:Chapter-3#3.4.2|Section 3.4.2]] ). The snow cover duration period is projected to emerge over large parts of Eastern and Western North America and Europe by the mid-century both in spring and autumn, and emergence is expected in the second half of the 21st century in the Arctic regions in the high RCP8.5 scenario ( ''medium confidence'' ) (Chapter 9, SROCC). For snow depth or snow water equivalent, there is ''low confidence'' ( ''limited evidence'' ) of the emergence of a decrease before 2050 because climate change also increases the variability of the snow depth signal, for example in Europe ( [[IPCC:Wg1:Chapter:Chapter-3#3.4.2|Section 3.4.2]] ; [[#Willibald--2020|Willibald et al., 2020]] ). Terrestrial permafrost is warming worldwide due to climate change (Sections [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|2.3.2.5]] and [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|9.5.2]] ). Due to weak interannual variability of permafrost temperatures, terrestrial permafrost warming has emerged above natural variability in almost all observed time series of the Northern Hemisphere ( ''medium confidence'' , ''limited evidence'' , ''high agreement'' ) ( [[#Biskaborn--2019|Biskaborn et al., 2019]] ), but the active layer thickness exhibits considerable interannual variability inhibiting evidence for emergence (Chapter 9).

'''Sea ice:''' Sea ice area decrease in the Arctic in all seasons has already emerged from the interannual variability ( ''high confidence'' ) (Chapter 9). By contrast, the Antarctic sea ice area shows no significant trend, and therefore no emergence.

For other snow and ice CIDs (heavy snowfall and ice storm, hail, snow avalanche), there is ''limited evidence'' of emerging signals.

'''Relative sea level, coastal flood and coastal erosion:''' Near-coast RSLR will emerge before 2050 for RCP4.5 along the coasts of all AR6 regions (with coasts) except East Asia, the Russian Far East, Madagascar, the southern part of Eastern North America and the Antarctic regions ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.1.4|Section 9.6.1.4]] ; [[#Bilbao--2015|Bilbao et al., 2015]] ). Under RCP8.5, emergence of near-coast RSLR is projected by mid-century along the coasts of all AR6 regions (with coasts), except WAN where emergence is projected to occur before 2100 ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.1.4|Section 9.6.1.4]] ; [[#Lyu--2014|Lyu et al., 2014]] ) ( ''medium confidence'' ). Emergence studies for ETWL and coastal erosion are lacking and hence it is not currently possible to robustly assess emergence in these CIDs.

'''Mean ocean temperature and marine heatwave:''' The emergence of the sea surface temperature increase signal has been observed in global oceans over the last century, and the largest S/N values are found in the tropical Atlantic and tropical Indian oceans ( [[#Hawkins--2020|Hawkins et al., 2020]] ). There is ''high confidence'' in the widespread occurrence of marine heatwaves in all basins and marginal seas over the last decades (Chapter 9), but the emergence of this signal above the natural variability has not yet been addressed in detail.

'''Ocean acidity, ocean salinity and dissolved oxygen:''' The global ocean pH decline has ''very likely'' emerged from natural variability for more than 95% of the global open ocean (SROCC, Chapter 2). The regional signals are more variable, but in all ocean basins, the signal of ocean acidification in the surface ocean is projected to emerge in the early 21st century (Chapter 5). The mean ToE for acidity in the coastal subtropical to temperate north-east Pacific and north-west Atlantic is above two decades ( ''high agreement'' , ''medium evidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-5#5.3.5.2|Section 5.3.5.2]] ). Salinity change signals have already emerged with 20–45% of the zonally averaged basin in the Atlantic, 20–55% in the Pacific and 25–50% in the Indian oceans and will be reaching 35–55% in the Atlantic in 2050 to 55–65% in 2080; 45–65% to 60–75% in the Pacific; and 45–65% to 60–80% in the Indian oceans (Chapter 9; [[#Silvy--2020|Silvy et al., 2020]] ). Deoxygenization has already emerged in many open oceans. The signal is most evident in the Pacific and Southern oceans but not evident in the North Atlantic Ocean ( [[#Andrews--2013|Andrews et al., 2013]] ; [[#Levin--2018|Levin, 2018]] ). However, there is ''medium confidence'' in the emergence of the anthropogenic signal in many other oceanic regions by 2050 ( [[#Henson--2017|Henson et al., 2017]] ; [[#Levin--2018|Levin, 2018]] ).

'''There is''' high confidence '''that several CID changes have already emerged above historical period natural variability in many regions (e.g., mean temperature in most regions, heat extremes in tropical areas, sea ice, salinity). Heat and cold CIDs (excluding frost) that have not already emerged will emerge by 2050 whatever the scenario''' '''in almost all land regions''' ( medium confidence '''). The emergence of increasing precipitation before the middle of the century is also projected in Siberian regions, Russian Far East, Northern Europe and the northernmost parts of North America and Arctic regions across scenarios with the various methods and emergence definitions used''' ( high confidence '''). Studies are missing to properly assess S/N emergence for droughts and for wind CIDs. Arctic sea ice extent declines have mostly emerged above noise level''' ( medium '''to''' high confidence '''), and the emergence of declining snow cover is expected by the end of the century under RCP8.5. There is''' medium confidence '''that, under RCP8.5, the anthropogenic forced signal in near-coast relative sea level change will emerge by mid-century in all regions with coasts, except in the West Antarctic region where emergence is projected to occur before 2100. In all ocean basins, the signal of ocean acidification in the surface ocean is projected to emerge before 2050''' ( high confidence ''').'''

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'''Cross-Chapter Box 12.1 | Projections by Warming Levels of Hazards Relevant to the Assessment of Representative Key Risks and Reasons for Concern'''

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'''Contributors:''' Claudia Tebaldi (United States of America), Guofinna Aoalgeirsdottir (Iceland), Sybren Drijfhout (United Kingdom), John Dunne (United States of America), Tamsin Edwards (United Kingdom), Erich Fischer (Switzerland), John Fyfe (Canada), Richard Jones (United Kingdom), Robert Kopp (United States of America), Charles Koven (United States of America), Gerhard Krinner (France), Friederike Otto (United Kingdom/Germany), Alex C. Ruane (United States of America), Sonia I. Seneviratne (Switzerland), Jana Sillmann (Norway/Germany), Sophie Szopa (France), Prodromos Zanis (Greece)

A consistent risk framework ( [[#Reisinger--2020|Reisinger et al., 2020]] ) has been adopted across the three Working Groups (WGs) in IPCC AR6 while recognizing the diversity of risk concepts across disciplines. WGI is assessing changes in climatic impact-drivers (CIDs), which are physical climate system conditions (e.g., means, events and extremes) that affect an element of society or ecosystems. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements and regions (Sections 12.1–12.3). In the assessment of Representative Key Risk (RKR) categories and Reasons for Concern (RFCs) in WGII Chapter 16, the focus lies on the adverse consequences of climate change, for which many types of CIDs (i.e., ‘hazards’ in the context of identified risks) play a key role. This box synthesizes the assessment of such hazards according to global warming levels (GWLs) from various chapters of WGI to inform understanding of their potential changes and associated risks with temperature levels in general, and in particular to facilitate WGII integrated assessments of RKRs and RFCs. Cross-Chapter Box 11.1, connects the organization of regional information according to GWLs to the other common dimension along which future projections are organized: that is, scenarios. [[IPCC:Wg1:Chapter:Chapter-1#1.6|Section 1.6]] describes all dimensions of integration adopted in this Report, adding cumulative carbon emissions to GWLs and scenarios.

Eight RKRs are identified within WGII Chapter 16:

* RKR-A: risk to the low-lying coastal socio-ecological systems;
* RKR-B: risk to terrestrial and ocean ecosystems;
* RKR-C: risks associated with critical physical infrastructure, networks and services;
* RKR-D: risk to living standards;
* RKR-E: risk to human health;
* RKR-F: risk to food security;
* RKR-G: risk to water security; and
* RKR-H: risks to peace and to human mobility.

RFCs further synthesize the landscape of risks from climatic changes into five categories (from the IPCC Third Assessment Report onward; [[#Smith--2001|Smith et al., 2001]] ):

* RFC1: risks to unique and threatened systems;
* RFC2: risks associated with extreme weather events;
* RFC3: risks associated with the distribution of impacts;
* RFC4: risks associated with global aggregate impacts; and
* RFC5: risks associated with large-scale singular events.

Importantly, the assessment of risk in WGII considers hazards as only one component of an integrated assessment that involves their complex interaction with exposure and vulnerability of the systems at risk ( [[#Reisinger--2020|Reisinger et al., 2020]] ).

Hazards relevant to RKRs and RFCs are identified among aspects of the climate system that have an episodic, short-term nature, like extreme events (particularly relevant to RFC2 but contributing to many other risk categories). Increasing GWLs translate into changing characteristics of frequency, duration, intensity, seasonality and spatial extent for many of these hazards that are also apparent in scenario-based results (Chapters 11 and 12, and Sections 12.4 and 12.5.1). Other relevant hazards coincide with long-term trends embodying a gradual change that may result in unfavourable environmental conditions. Also, increasing GWLs increase the likelihood of compound temporal or spatial occurrence of similar or different hazards ( [[IPCC:Wg1:Chapter:Chapter-11#11.8|Section 11.8]] ). Furthermore, RFC5’s focus on singular events includes concern surrounding potential tipping points and irreversible behaviour in the physical climate system.

Cross-Chapter Box 12.1, Table 1 organizes information by hazard and presents current state and future change assessments with increasing GWLs (defined by increasing global surface air temperature, GSAT; see Cross-Chapter Box 2.3). We draw on individual chapters across the WGI report for the assessment of how these hazards vary with GWL. Hazards for which a relation to GWLs has not been assessed are not reported in the table.

Cross-Chapter Box 12.1

'''Cross-Chapter Box 12.1, Table 1'''

'''|'''

'''Summary of CIDs/hazards that are identified as driving RKRs and RFCs.''' The behaviour of each (in most cases considered at the global scale, but for some types in terms of spatially resolved patterns) as a function of GWLs is described and when possible quantified, together with the level of confidence of the assessment, to be found in more detail in the chapter/sections indicated in the corresponding column. For the relation with GSAT levels, two columns detail current state, which can be associated to about 1°C of global warming, and future behaviour. Tipping Points and Irreversibility are comprehensively assessed with CMIP6 models up to GWL = 3°C, with fewer studies and lower confidence at higher GWLs up to 5°C.
{| class="wikitable"
|-
! '''Hazard Category'''

'''Sub-category'''

! '''RKR/RFC Relevance'''

! '''Behaviour at About 1°C (Present)'''

! '''Behaviour as a Function of GWL (Future)'''

! '''WGI Chapter References'''

|-
| colspan="5"| '''Extreme Events'''

|-
| Hot and Cold Extremes

| All RKRs; RFC2, RFC3

| Frequency and intensity of hot extremes increased and cold extremes decreased at the global scale and in most regions since 1950 (GSAT change about 0.6°C) ( ''virtually certain'' ). Number of warm days and nights increased; intensity and duration of heatwaves increased; number of cold days and nights decreased ( ''virtually certain'' ). Regional-to-continental scale trends generally consistent with global-scale trends ( ''high confidence'' ). Limited data in a few regions (especially Africa) hampers trend assessment.

| Strong linear relation between magnitude and intensity of heat and cold extremes and GSAT, detectable from warming as low as 1.5°C; changes in the extreme metrics twice as large (in mid-latitude regions) or more (in high-latitude regions) than GSAT warming ( ''very likely'' ) ; metrics related to frequency of exceedance may show stronger than linear relationships (exponential) ( ''very likely'' ) . Compared to today, changes in extremes at +2°C at least two times larger than at +1.5°C, and four times larger at +3°C.

| Sections 11.3, 11.9, 12.4; Figures 11.3, 11.4, 11.6, 11.11, 11.12; Table 11.2.

|-
| Extreme Precipitation Events

| All RKRs; RFC2, RFC3

| Frequency and intensity of heavy precipitation events increased at the global scale over a majority of land regions with good observational coverage ( ''high confidence'' ) and at the continental scale in North America, Europe and Asia. Larger percentage increases in heavy precipitation observed in the northern high latitudes in all seasons, and in the mid-latitudes in the cold season ( ''high confidence'' ). Regional increases in the frequency and/or intensity of heavy rainfall also observed in most parts of Asia, north-west Australia, northern Europe, South-Eastern South America, and most of the USA ( ''high confidence'' ), and West and Southern Africa, Central Europe, the eastern Mediterranean region, Mexico, and North-Western South America ( ''medium confidence'' ). GHGs ''likely'' the main cause.

| Precipitation events – including those associated with tropical cyclones (TCs) – increase with GSAT. For GWLs &gt;2°C very rare (e.g., 1-in-10 or more years) heavy precipitation events more frequent and more intense over all continents ( ''virtually certain'' ) and nearly all AR6 regions ( ''likely'' ) . Likelihood lower at lower GWLs and for less-rare events. At the global scale, intensification of heavy precipitation generally follows Clausius–Clapeyron (about 6–7% per °C of GSAT warming; ''high confidence'' ). Increase in frequency of heavy precipitation events accelerates with warming, higher for rarer events ( ''high confidence'' ), with approximately a doubling and tripling frequency of 10-year and 50-year events, respectively, at 4°C of global warming.

| Sections 11.4, 11.7, 12.4; Figures 11.4, 11.7, 11.15, 11.16; Table 11.2.

|-
| Drought

| All RKRs; RFC2, RFC3

| Increased atmospheric evaporative demand in dry seasons over a majority of land areas due to human-induced climate change ( ''medium confidence'' ). Especially observed in dry summer climates in Europe, North America and Africa ( ''high confidence'' ).

| Upward trend with GSAT ( ''high confidence'' ).

| Sections 11.6, 12.4; Figure 11.18; Table 11.2.

|-
| Inland Floods

| All RKRs; RFC2, RFC3

|
| Upward trend with GSAT for flooded area extent, starting from 2°C compared with 1.5°C and higher levels. Increase in the frequency and magnitude of pluvial floods ( ''high confidence'' ). Increasing flood potential in urban areas where heavy precipitation projected to increase, especially at high GWLs ( ''high confidence'' ).

| Sections 11.5, 12.4; Table 11.2.

|-
| Tropical Cyclones (TCs)

| All RKRs; RFC2, RFC3

| Human contribution to extreme rainfall amount from specific TC events ( ''high confidence'' ). Global proportion of major TC intensities ''likely'' increased over the past four decades.

| Increase in precipitation from TC with GSAT; average peak TC wind speeds, proportion of intense TCs, and peak wind speeds of most intense TCs increase globally with GSAT ( ''high confidence'' ). Decrease or lack of change in global frequency of TCs (all categories) with GSAT ( ''medium confidence'' ).

| Sections 11.7.1, 12.4; Table 11.2.

|-
| Marine Heatwaves (MHWs)

| RKR A,B,F; RFC1,2,3

| ''High confidence'' that MHWs have increased in frequency over the 20th century, with an approximate doubling from 1982 to 2016, and ''medium confidence'' that they have become more intense and longer since the 1980s.

| MHWs ''very likely'' become 2–9 times more frequent in 2081–2100 compared to 1985–2014 under SSP1-2.6 corresponding to a GWL of 2.0 [1.3 to 2.8] °C (95% CI), or 3–15 times more frequent under SSP5-8.5 corresponding to a GWL of 4.8 [3.6 to 6.5] °C. Spatial heterogeneity with larger changes in the tropical oceans and Arctic Ocean ( ''medium confidence'' ).

| [[#12.4|Section 12.4]] ; Box 9.2.

|-
| Concurrent Events in Time and Space

| All RKRs; RFC2, RFC3

| Higher frequency already detected: more frequent concurrent heatwaves and droughts. Increased compound flooding risk (storm surge, extreme rainfall and/or river flow) in some locations; the probability of concurrent events ''likely'' increased.

| Higher frequency with increasing GSAT. Increasing trend in more frequent concurrent heatwaves and droughts with GSAT ( ''high confidence'' ). More frequent concurrent (in time) extreme events at different locations with increasing GSAT, for GWLs &gt; 2°C ( ''high confidence'' ). Compound flooding risk (storm surge, extreme rainfall and/or river flow) increasing with GSAT ( ''high confidence'' ).

| [[IPCC:Wg1:Chapter:Chapter-11#11.8|Section 11.8]] ; Table 11.2; Boxes 11.2, 11.4.

|-
| colspan="5"| '''Trends'''

|-
| Fire Weather Trends

| RKR-B, C; RFC1,2,3

| Weather conditions that promote wildfire (compound hot, dry and windy events) more probable in some regions over the last century ( ''medium confidence'' ).

| Weather conditions promoting wildfire (compound hot, dry and windy events) ''likely'' more frequent with GSAT.

| [[#12.4|Section 12.4]] ; Table 11.2.

|-
| Air Pollution Weather

| RKR-E; RFC3

| Not discernible.

| Behaviour to first order controlled by emissions and policies, not by meteorology. Ozone decreases with GSAT in low-polluted regions (−0.2 to –2 ppbv per °C). Ozone increases with GSAT in regions close to sources of precursors (0.2 to 2 ppbv per °C).

| Sections 6.5, 12.4.

|-
| Patterns of Mean Warming

| RKR-B, D, F, RFC1,3,4

| Spatial patterns of temperature changes associated with the 0.5°C difference in GMST warming between 1991–2010 and 1960–1970 consistent with projected changes under 1.5°C and 2°C of global warming.

| Temperatures scale approximately linearly with GSAT, largely independently of scenario ( ''high confidence'' ). High latitudes of Northern Hemisphere warm faster ( ''virtually certain'' ). Antarctic polar amplification smaller than Arctic ( ''high confidence'' ). Arctic annual mean temperatures warm between 2 and 2.4 times faster for GWLs between 1.5°C and 4°C. In the Southern Hemisphere relatively high rates of warming in subtropical continental areas of South America, Southern Africa and Australia ( ''high confidence'' ).

| Sections 4.6.1.1, 12.4; Atlas; Figures 4.31, Atlas.13 and all ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] Sections’ figures for mean temperature changes.

|-
| Arctic Warming Trends

| RKR-A,C,G,H; RFC1, RFC3

| Emerged already from internal variability.

| ''Very likely'' more pronounced (2–2.4 times faster) than the global average over the 21st century ( ''high confidence'' ).

| Sections 4.6.1, 7.4.4.1, 12.4.9 Atlas.11; Figures 4.19, 4.31, Atlas.29; Table 4.2.

|-
| Patterns of Precipitation Change

| RKR-B, D, F, RFC1, RFC3

| Regional patterns of recent trends, over at least the past three decades, consistent with documented increase in precipitation over tropical wet regions and decrease over dry areas.

| Changes in large-scale atmospheric circulation and precipitation with each 0.5°C of warming ( ''high confidence'' ). Stable pattern of change over time and scenarios. Some departures from linearity possible at regional scale ( ''medium confidence'' ). Precipitation increase on land higher at 3°C and 4°C compared to 1.5°C and 2°C. Precipitation increases in large parts of the monsoon regions, tropics and high latitudes, decreases in the Mediterranean and large parts of the subtropics ( ''high confidence'' ).

| Sections 2.3.1.3.4, 4.5.1, 4.6.1, 12.4;

Figures 2.15, 4.32, Atlas.13.

|-
| Sea Surface Temperature (SST) Warming

| RKR-A, B, D; RFC1, RFC4

| Increased 0.81 (0.65–0.94) per °C of GSAT (1850–1900 average compared with 2009–2018 average).

| Models and observations show globally averaged SSTs warming at a lower rate of about 80% that of GSAT. It is ''virtually certain'' that SST will continue to increase at a rate depending on future emissions scenario ranging from 0.4°C–1.5°C in 2081–2100 relative to 1995–2014 under SSP1-2.6, corresponding to a GWL of 2.0 [1.3 to 2.8] °C, to 2°C–4°C under SSP5-8.5, corresponding to a GWL of 4.8 [3.6 to 6.5] °C.

| Sections 2.3.1.1.3, 9.2.1, 12.4.

|-
| Ocean Acidification/pH

| RKR-A,B; RFC1, RFC4

| ''Virtually certain'' decline of surface pH globally over the last 40 years at a rate of 0.017–0.027 pH units per decade; decline also in the subsurface over the past 2–3 decades ( ''medium confidence'' ). Surface pH now the lowest of at least the last 26,000 years ( ''very high confidence'' ).

| Increase of net ocean carbon flux throughout the century irrespective of the emissions scenario considered ( ''high confidence'' ). Decrease of ocean surface pH through the 21st century, except for SSP1-1.9 and SSP1-2.6 where values increase slightly starting from 2070–2100 ( ''high confidence'' ).

| Sections 2.3.3.5, 4.3.2.4, 5.3.4, 5.4.2, 12.4; Figure 4.8.

|-
| SPEI Index Global

| RKR-B,F,G,H; RFC3

| 9.4% chance of at least three months of drought in a year at current levels (about 1°C).

| Increase at the global scale in the chance of at least three months of drought in a year to about 20 [15 to 30] % at 1.5°C, 35 [20 to 45] % at 2°C to 60 [45 to 75] % at 4°C.

| Sections 12.4, 12.5.1.

|-
| El Niño–Southern Oscillation (ENSO) Variability

| RKR-B,D,F,G; RFC2,3,5

| ''Medium confidence'' that both ENSO amplitude and frequency of high-magnitude events since 1950 higher than over the pre-industrial period (before 1850) but ''low confidence'' of this being outside the range of internal variability. No clear evidence shifts in ENSO or associated features or its teleconnections.

| No change in the amplitude of ENSO variability ( ''medium confidence'' ); enhanced ENSO-related variability of precipitation under SSP2-4.5 and higher ( ''high confidence'' ). ''Likely'' shift eastward of the pattern of teleconnection over North Pacific and North America.

| Sections 2.4.2, 4.3.3.2, 4.5.3.2; Figure 4.10.

|-
| Sea Ice Loss

| RKR-A, B, H; RFC1,3,5

| Arctic sea ice area decreased for all months since 1970s; strongest decrease in summer ( ''very high confidence'' ). Arctic sea ice younger, thinner and faster moving ( ''very high confidence'' ). Current pan-Arctic sea ice levels unprecedented since 1850 ( ''high confidence'' ). ''Low confidence'' in all aspects of Antarctic sea ice prior to the satellite era. Antarctic sea ice area experienced little net change since 1979 ( ''high confidence'' ).

| The Arctic Ocean will ''likely'' become sea ice-free in September before 2050 in all considered SSP scenarios; such disappearance consistently occurring in most years at 2°C–3°C ( ''medium confidence'' ) and including several months in most years at 3°C–5°C ( ''high confidence'' ).

| Sections 2.3.2, 4.3.2.1, 9.3.1, 9.3.2, 12.4.9; Figures 4.2c, 4.5.

|-
| Permafrost Thaw

| RKR-A,C; RFC3,5

| Increases in permafrost temperatures in the upper 30 m over the past three to four decades throughout the permafrost regions ( ''high confidence'' ).

| Global permafrost volume in the top 3 m decreasing by about 25 ± 5% per °C for GWLs &lt;4°C. Relative to 1995–2014: at 1.5°C and 2°C decreasing by less than 40% ( ''medium confidence'' ), at 2°C and 3°C by less than 75% ( ''medium confidence'' ), at 3°C and 5°C by more than 60% loss ( ''medium confidence'' ).

| Sections 2.3.2.5, 9.5.2, 12.4.9.

|-
| Sea Level Change

| RKR-A,C,D,E,F,G,H; RFC1,3,4

| Gobal mean sea level (GMSL) is rising at an accelerated rate since the 19th century ( ''high confidence'' ) '','' almost doubled during past two decades (about 0.1 mm yr <sup>–2</sup> ). GMSL increase over the 20th century faster than over any preceding century in at least the last three millennia ( ''high confidence'' ).

| Up to 2050, limited scenario/GWL dependency ( ''likely'' sea level rise about 0.15–0.30 m). By 2100, ''likely'' GMSL rise with respect to 1995–2014 of 0.51 (0.40–0.69) m, 0.62 (0.50–0.81) m and 0.70 (0.58–0.91) m for, respectively, GWLs of 2.0°C, 3.0°C, and 4.0°C ( ''medium confidence'' ). Deep uncertainty in projections for GWLs &gt;3°C because of ice-sheet behaviour. For example, incorporation of ''low confidence'' ice-sheet processes under SSP5-8.5 (approximately 5°C) leads to a rise of 0.6-1.6 m rather than 0.7–1.1 m.

| Sections 9.6.1.2, 9.6.3.3, 9.6.3.4, 12.4.

|-
| Sea-Level Change Commitment (2,000 years after peak GWL)

| RFC5

|
| GMSL commitment (over the 2000-year-long period following peak warming) of 2–6 m for 2°C peak warming, 4–10 m for 3°C peak warming, 12–16 m for 4°C peak warming, and 19–22 m for 5°C peak warming ( ''medium agreement, limited evidence'' ).

| [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.5|Section 9.6.3.5]] .

|-
| Northern Hemisphere (NH) Spring Snow Cover

| RKR-G, RFC1,3

| Substantial reductions in spring snow cover extent in the NH since 1978 ( ''very high confidence'' ) ''.'' Since 1981, general decline in NH spring snow water equivalent ( ''high confidence'' ).

| Linear change of NH snow cover in spring of about 8% (area) per °C ofglobal warming (for GWLs &lt;4°C). Relative to 1995–2014: at 1.5°C–2°C NH spring snow cover extent ''likely'' decreases by less than 20% ( ''medium confidence'' ); at 2°C–3°C ''likely'' decreases by less than 30%; at 3°C–5°C, ''likely'' decreases by more than 25%.

| Sections 2.3.2.2,

9.5.3, 12.4.

|-
| Mass Loss of Glaciers

| RKR-B,G; RFC1, RFC3

| ''Very high confidence'' global glaciers continuing retreat since about 1850. Current global glacier mass loss highly unusual over at least the last 2000 years ( ''medium confidence'' ). Increased rate of glacier mass loss over the last 3 to 4 decades ( ''high confidence'' ). Glaciers not in balance with respect to current climate conditions and will continue to lose mass for at least several decades.

| For 1.5°C–2°C about 50–60% ( ''low confidence'' ) of glacier mass outside the two ice sheets and excluding peripheral glaciers in Antarctica remaining, predominantly in the polar regions. At 2°C–3°C about 40–50% ( ''low confidence'' ) of current glacier mass outside Antarctica remaining. At sustained 3°C–5°C 25–40% ( ''low confidence'' ) of current glacier mass outside Antarctica remaining. Likely nearly all glacier mass lost in low latitudes, Central Europe, Caucasus, western Canada and USA, North Asia, Scandinavia and New Zealand.

| Sections 2.3.2.3, 9.5.1, 12.4.

|-
| colspan="5"| '''Tipping Points/Irreversibility'''

|-
| Amazon Forest Dieback

| RFC1, RFC5

| Highly dependent on human disturbance.

| Amazon drying and deforestation expected to cause a rapid change in the regional water cycle, possibly linked to the crossing of a climate threshold. ''Low confidence'' change will occur by 2100.

| Sections 5.4.9, 8.6.2.1, 12.4.10; Table 4.10.

|-
| Boreal Forest Dieback

| RFC1, RFC5

| Highly dependent on human disturbance.

| Possible if climate threshold is exceeded, but counteracted by poleward expansion.

| [[IPCC:Wg1:Chapter:Chapter-5#5.4.9|Section 5.4.9]] .

|-
| Ice Sheets

| RFC5

| Greenland Ice Sheet mass-loss rate increased substantially since the turn of the 21st century ( ''high confidence'' ). The Antarctic Ice Sheet has lost mass between 1992 and 2017 ( ''very high confidence'' ), with an increasing mass-loss rate over this period ( ''medium confidence'' ).

| At sustained warming levels between 1.5°C and 2°C, the ice sheets will continue to lose mass ( ''high confidence'' ); on time scales of multiple centuries, the Greenland and West Antarctic ice sheets will partially be lost ( ''medium confidence'' ); there is ''limited evidence'' that the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia; at sustained warming levels between 2°C and 3°C, there is ''limited evidence'' that the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia, and ''high confidence'' in increasing risk of complete loss and increasing rate of mass loss for higher warming; At sustained warming levels between 3°C and 5°C, near-complete loss of the Greenland Ice Sheet and complete loss of the West Antarctic Ice Sheet will occur irreversibly over multiple millennia ( ''medium confidence'' ); substantial parts or all of Wilkes Subglacial Basin in East Antarctica will be lost over multiple millennia ( ''low confidence'' ).

| Sections 2.3.2.4,

9.4.1, 9.4.2; Table 4.10.

|-
| Glaciers

| RFC5

| See Trends section of this table.

| Continuing substantial global mass loss.

| Sections 9.5.1, 12.4.9.

|-
| Global Ocean Temperature

| RFC5

| See Trends section of this table.

| Centennial-scale irreversibility of ocean warming.

| Sections 4.7.2, 9.6.3; Table 4.10.

|-
| Sea Level Rise (SLR)

| RFC5

| See Trends section of this table.

| Centennial-scale irreversibility of sea level rise. Tipping point linked to ice-sheet behaviour. Deep uncertainty on SLR above 3°C warming.

| Sections 4.7.2, 9.6.3; Table 4.10.

|-
| Atlantic Meridional Overturning Circulation (AMOC)

| RFC5

| ''Low agreement'' on 20th century trend between models and most reconstructions. Observed decline since the mid-2000s cannot be distinguished from internal variability ( ''high confidence'' ).

| There is ''medium confidence'' an abrupt collapse will not occur before 2100; for 1.5–2°C, 2–3°C, 3–5°C warming in 2100, AMOC decline is 29, 32 and 39%, respectively, of its pre-industrial strength.

| Sections 2.3.3.4.1,

9.2.3.1; Table 4.10.

|-
| Permafrost Carbon

| RFC5

| See Trends section of this table.

| Will contribute as a feedback with warming, of approximately 18 ± 12 PgC per °C. Possibly non-linear but ''low confidence'' in the value of any threshold for such behaviour. Likely irreversible at centennial time scales.

| [[IPCC:Wg1:Chapter:Chapter-5#5.4.9|Section 5.4.9]] ; Table 4.10;

Box 5.1.

|-
| Arctic Sea Ice

| RFC5

| Abrupt change already observed.

| Reversible within years to decades; no tipping point or threshold beyond which loss of ice becomes irreversible ( ''high confidence'' ).

| Sections 4.3.2, 9.3.1; Table 4.10.

|-
| Snow Cover of Northern Hemisphere

| RFC5

| See Trends section of the table

| Not anticipated to present tipping point/irreversible behaviour.

| [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] .

|-
| Global Monsoon

| RFC5

| Has ''likely'' increased over the last 40 years ( ''medium confidence'' ) and can be explained by a phase change in Atlantic Multi-decadal Variability.

| Not anticipated to present tipping point/irreversible behaviour, unless AMOC collapse occurs.

| Sections 4.4.1.4, 4.5.1.5, 8.6.1; Table 4.10.

|-
| ENSO

| RFC5

| See Trends section of the table.

| Not anticipated to present tipping point/irreversible behaviour.

| [[IPCC:Wg1:Chapter:Chapter-4#4.5.3.2|Section 4.5.3.2]] .

|-
| Methane Clathrates

| RFC5

| Methane release from shelf clathrates is &lt;10 TgCH <sub>4</sub> yr <sup>–1</sup> .

| Not anticipated to present tipping point/irreversible behaviour.

| [[IPCC:Wg1:Chapter:Chapter-5#5.4.9|Section 5.4.9]] ; Table 4.10.

|}

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