Editing IPCC:AR6/WGII/Chapter-16 (section)

==== 16.6.3.2 Extreme Weather Events (RFC2) ====

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This RFC addresses the risks to human health, livelihoods, assets and ecosystems from extreme weather events such as heatwaves, heavy rain, drought and associated wildfires, and coastal flooding ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ). Previous assessments of this RFC have focused mainly on changes to the hazard component of the risk, using the projected increase in hazard as an indicator of higher risk. However, in AR6 an expanding (although still smaller) body of evidence now allows also incorporation of the exposure and/or vulnerability components of risk and, to a limited extent, their trends.

AR5 identified a transition from undetectable to moderate risk below ‘recent’ temperatures (i.e., during 1986–2005, which then corresponded to a global warming of 0.6°C above pre-industrial levels). SR15 [[IPCC:Wg2:Chapter:Chapter-3#3.5.2|Section 3.5.2.2]] ( [[#Hoegh-Guldberg--2018b|Hoegh-Guldberg et al., 2018b]] ) concluded that differences of 0.5°C in global warming led to detectable changes in extreme weather and climate events on the global scale and for large regions. IPCC WGI AR6 [[IPCC:Wg2:Chapter:Chapter-11|Chapter 11]] confirms this assessment and concludes that ‘new evidence strengthens the conclusion from SR15 that even relatively small incremental increases in global warming (+0.5°C) cause statistically significant changes in extremes on the global scale and for large regions’. Substantial literature is available for comparisons at +1.5°C versus +2°C of global warming, but the conclusions are assessed to also apply at lower global warming levels and smaller increments of global warming given the identified linearity of regional responses of several extremes in relation to global warming ( [[#Seneviratne--2016|Seneviratne et al., 2016]] ; [[#Wartenburger--2017|Wartenburger et al., 2017]] ; [[#Tebaldi--2018|Tebaldi and Knutti, 2018]] ) and the identification of emergence of global signals in climate extremes for global warming levels as small as 0.1°C ( [[#Seneviratne--2020|Seneviratne and Hauser, 2020]] , WGI AR6, Chapter 11, Figure 11.8; WGI Cross-Chapter Box 12.1). Further analyses are consistent with this assessment, based on model simulations ( [[#Fischer--2015|Fischer and Knutti, 2015]] ; [[#Schleussner--2017|Schleussner et al., 2017]] ; [[#Kirchmeier-Young--2019a|Kirchmeier-Young et al., 2019a]] ; [[#Seneviratne--2020|Seneviratne and Hauser, 2020]] ) and observational evidence ( [[#Zwiers--2011|Zwiers et al., 2011]] ; [[#Dunn--2020|Dunn et al., 2020]] ). A global warming of +0.5°C above pre-industrial conditions corresponds approximately to climate conditions in the 1980s (Chapter 2, Figure 2.11), a time frame at which detectable changes in some extremes were established at the global scale based on observations ( [[#Dunn--2020|Dunn et al., 2020]] ). Heat-related mortality has also been assessed to have increased considerably because of climate change ( [[#Ebi--2021|Ebi et al., 2021]] ; [[#Vicedo-Cabrera--2021|Vicedo-Cabrera et al., 2021]] ). The onset, and also median location of the transitions of risk (Figure 16.15) from undetectable to moderate, is therefore considered to be 0.5°C. Further strong new evidence shows that changes in extremes emerged during the 1990s and 2000s ( [[#Dunn--2020|Dunn et al., 2020]] ) by which time +0.7°C of global warming had taken place (IPCC SR15, Chapter 1; WGI AR6, Chapter 2). In AR5 Section 19.6.3.3 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ), a transition to moderate risk was assessed to have taken place at the then ‘recent’ global warming level of 0.6°C, with ''high confidence'' . Owing to the increase in evidence, there is now ''very high confidence'' that the median value of the transition from undetectable to moderate risk is at 0.5°C and led by heat extremes, with the lower estimate set at 0.5°C as well, and upper estimate at 0.7°C.

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'''Figure 16.15 |''' '''The dependence of risk associated with the Reasons for Concern (RFCs) on the level of climate change, updated by expert elicitation and reflecting new literature and scientific evidence since AR5 and SR15.'''

'''(a)''' Global surface temperature (GST), relative to pre-industrial, 1850–1900 (WGI AR6 Figure SPM.8d). ( [[#IPCC--2021|IPCC, 2021]] a).

'''(b)''' Embers are shown for each RFC, assuming low to no adaptation (i.e., adaptation is fragmented, localised, incremental adjustments to existing practices). The dashed horizontal line denotes the present global warming of 1.09°C (IPCC WGI Figure SPM.8a ) which is used to separate the observed, past impacts below the line from the future projected risks above it. '''RFC1 Unique and threatened systems''' : ecological and human systems that have restricted geographic ranges constrained by climate-related conditions and have high endemism or other distinctive properties. Examples include coral reefs, the Arctic and its Indigenous People, mountain glaciers and biodiversity hotspots. '''RFC2 Extreme weather events''' : risks/impacts to human health, livelihoods, assets and ecosystems from extreme weather events such as heatwaves, heavy rain, drought and associated wildfires, and coastal flooding. '''RFC3 Distribution of impacts''' : risks/impacts that disproportionately affect particular groups owing to uneven distribution of physical climate change hazards, exposure or vulnerability. '''RFC4 Global aggregate impacts''' : impacts to socio-ecological systems that can be aggregated globally into a single metric, such as monetary damages, lives affected, species lost or ecosystem degradation at a global scale. '''RFC5 Large-scale singular events''' : relatively large, abrupt and sometimes irreversible changes in systems caused by global warming, such as ice sheet disintegration or thermohaline circulation slowing. Comparison of the increase of risk across RFCs indicates the relative sensitivity of RFCs to increases in GSAT. The levels of risk illustrated reflect the judgements of IPCC author experts from WGI and WGII.

Further evidence of more recent observed changes in extreme weather and climate events, and their potential for associated adverse consequences across many aspects of society and ecosystems, has continued to accrue (WGI AR6 Chapter 11; WGI AR6 Chapter 12). Since a necessary condition for ‘moderate’ levels of risk is the detection and attribution of observed impacts, the following text provides an overview of some salient examples of this evidence. In particular, WGI AR6 [[IPCC:Wg2:Chapter:Chapter-11|Chapter 11]] ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) concludes that some recent hot extreme events that happened in the past decade (2010s) would have been ''extremely unlikely'' to occur without human influence on the climate system. Global warming in that decade reached approximately 1.09°C on average (IPCC WGI AR6 Chapter 2).

Assessment of a high level of risk requires a higher level of magnitude, severity and spatial extent of the risks. Events prior to that already had substantial impacts, such as the 2003 European heatwave (IPCC SREX Chapter 9). Examples of impactful events in the early 2010s (at ca. 0.95°C of global warming; WGI AR6 Chapter 2, [[#Gulev--2021|Gulev et al., 2021]] ) include the 2010 Russian heatwave ( [[#Barriopedro--2011|Barriopedro et al., 2011]] ) and the 2010 Amazon drought ( [[#Lewis--2011|Lewis et al., 2011]] ). Later impactful events include, among others, the 2013 heatwave in eastern China ( [[#Sun--2014|Sun et al., 2014]] ), the 2017 tropical cyclone Harvey ( [[#Risser--2017|Risser and Wehner, 2017]] ; [[#Van%20Oldenborgh--2017|Van Oldenborgh et al., 2017]] ) and the 2018 concurrent North Hemisphere heatwaves in Europe, North America and Asia ( [[#Vogel--2019|Vogel et al., 2019]] ). Very recent events with severe and unprecedented impacts attributed to anthropogenic climate change indicate that thresholds to high risks may already have been crossed at recent levels of global warming (ca. 1.1–1.2°C), including the Siberian fires and the 2019 Australian bushfires that were linked to extreme heat and drought conditions ( [[#Van%20Oldenborgh--2017|Van Oldenborgh et al., 2017]] ) and extreme precipitation linked to increased storm activity in the USA ( [[#Van%20Oldenborgh--2017|Van Oldenborgh et al., 2017]] ). Severe and unprecedented impacts occurred with current low levels of adaptation ( [[#16.2.3.4|Section 16.2.3.4]] ). The global-scale risk of wildfire considerably degrading ecosystems and increasing illnesses and death of people has been assessed to transition from undetectable to moderate over the range 0.6–0.9°C with ''high confidence'' (Chapter 2, Table SM2.5, Figure 2.11).

In addition, long-term trends in various types of extremes are now detectable (WGI AR6 Chapter 11, [[#Seneviratne--2021|Seneviratne et al., 2021]] ). This includes increases in hot extremes over most land regions ( ''virtually certain'' ), increases in heavy precipitation at the global scale and over most regions with sufficient observations ( ''high confidence'' ), and increases in agricultural and ecological droughts in some regions ( ''medium confidence'' ) (WGI AR6 Chapter 11). There has also been overall a ''likely'' increase in the probability of compound events, such as an increase in concurrent heatwaves and droughts ( ''high confidence'' ) (WGI AR6 Chapter 11). There is ''medium confidence'' that weather conditions that promote wildfires (fire weather) have become more probable in southern Europe, northern Eurasia, the USA and Australia over the last century (WGI AR6 Chapter 11; SRCCL Chapter 2, [[#Jolly--2015|Jolly et al., 2015]] ; [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ). Furthermore, food security and livelihoods are being affected by short-term food shortages caused by climate extremes ( [[IPCC:Wg2:Chapter:Chapter-5#5.12.1|Section 5.12.1]] ; Chapter 16, Food Security RKR) which have affected the productivity of all agricultural and fishery sectors ( ''high confidence'' ). The frequency of sudden food production losses has increased since at least mid-20th century on land and sea ( ''medium evidence'' , ''high agreement'' ). Droughts, floods and marine heatwaves contribute to reduced food availability and increased food prices, threatening food security, nutrition and livelihoods of millions ( ''high confidence'' ). Changes in sea surface temperatures drive simultaneous variation in climate extremes, increasing the risk of multi-breadbasket failures ( [[#Cai--2014|Cai et al., 2014]] ; [[#Perry--2017|Perry et al., 2017]] ). Droughts induced by the 2015–2016 El Niño, partially attributable to human influences ( ''medium confidence'' ), caused acute food insecurity in various regions, including eastern and southern Africa and the dry corridor of Central America ( ''high confidence'' ). Human-induced climate change warming also worsened the 2007 drought in southern Africa, causing food shortages, price spikes and acute food insecurity in Lesotho ( [[#Verschuur--2021|Verschuur et al., 2021]] ). In the fisheries and aquaculture sector, marine heatwaves are estimated to have doubled in frequency between 1982 and 2016, as well as increasing in intensity and length, with consequences for fish mortality (Chapter 5; [[#Smale--2019|Smale et al., 2019]] ; [[#Laufkötter--2020|Laufkötter et al., 2020]] ). In the northeast Pacific, a recent 5-year warm period impacted the migration, distribution and abundance of key fish resources ( ''high confidence'' ). At 1°C warming, the number of people affected by six categories of extreme events was found to have already increased by a factor of 2.3 relative to pre-industrial ( [[#Lange--2020|Lange et al., 2020]] ).

The general picture is one of annual or more frequent occurrences of severe extremes with widespread impacts (as also reflected in [[#16.2|Section 16.2]] ), and of multiple extremes, meeting the criteria for the ‘severe and widespread’ nature of risks that is required for classification at a ‘high’ level of risk. This is consistent with AR5 Chapter 19 ( [[#Oppenheimer--2014|Oppenheimer et al., 2014]] ), and gives ''high confidence'' that the lower threshold for entering high risks associated with extreme weather events is +1°C, and that the best estimate is that this transition already occurred now that global warming has reached its present-day level of ca. 1.2°C ( [[#WMO--2020|WMO, 2020]] ), slightly above the 1.09°C average conditions in the 2010s, that is, 2011–2020 (IPCC WGI AR6 Chapter 2, [[#Gulev--2021|Gulev et al., 2021]] ).

A range of literature projects further substantial increases in several extreme event types with a global warming of +1.5°C, notably hot extremes in most regions, heavy precipitation in several regions, and drought in some regions (IPCC SR15; WGI AR6 , Chapter 11). In particular, heavy precipitation and associated flooding are projected to intensify and be more frequent in most regions in Africa and Asia ( ''high confidence'' ), North America ( ''medium'' to ''high confidence'' depending on the region) and Europe ( ''medium confidence'' ). Also, more frequent and/or severe agricultural and ecological droughts are projected in a few regions in all continents except Asia, compared with 1850–1900 ( ''medium confidence'' ); increases in meteorological droughts are also projected in a few regions ( ''medium confidence'' ). Increases at 1.5°C of global warming are projected in marine heatwaves ( [[#Laufkötter--2020|Laufkötter et al., 2020]] ) and the occurrence of fire weather ( [[#IPCC--2019a|IPCC, 2019a]] ). Heat-related mortality is assessed to increase from moderate to high levels of risk under about 1.5°C warming under SSP3, a socioeconomic scenario with large challenges to adaptation ( [[#Ebi--2021|Ebi et al., 2021]] ) especially in urban centres (Chapter 6). An additional 350 million people living in urban areas are estimated to be exposed to water scarcity from severe droughts at 1.5°C warming (Sections 6.1, 6.2.2; CCP2 Coastal Cities). In summary, there is ''high confidence'' that the best estimate for the transition from moderate to high risk is 1.2°C of global warming, with 1°C as lower estimate and 1.5°C as upper estimate. The latter would be set to 1.3°C for an assessment at ''medium confidence'' .

As in RFC1, one of the criteria for identification of very high risks is limits to adaptation. Though the literature explicitly considering societal adaptation to extreme weather events is limited, there is evidence that investments in hydro-meteorological information, early-warning systems and anticipatory forecast-based finance are a cost-effective way to prevent some of the most adverse effects of extreme events ( [[#Coughlan%20de%20Perez--2016|Coughlan de Perez et al., 2016]] ; [[#Fakhruddin--2019|Fakhruddin and Schick, 2019]] ; [[#Merz--2020|Merz et al., 2020]] ). Despite a lack of systematic methods for assessing general adaptation effectiveness, there is some evidence of risk reduction for particular places and hazards, especially flood and heat vulnerability ( [[#16.3.2.4|Section 16.3.2.4]] ), including investment in flood protection, building design and monitoring and forecasting, air conditioning, reduced social vulnerability, and improved population health. One study finds declining global mortality and economic loss due to extreme weather events over the past four decades ( [[#Formetta--2019|Formetta and Feyen, 2019]] ) especially in low-income countries. Using SSP2 as a proxy for expanded adaptation, [[#Ebi--2021|Ebi et al. (2021)]] assess that the transition to high risk for heat-related mortality increases to 1.8°C (compared with 1.5°C with less adaptation under SSP3). There is evidence of adaptation avoiding heat-related mortality at low levels of global warming, using early-warning and response systems and sustainable alterations of the thermal environment at the individual, building, urban and landscape levels ( [[#Jay--2021|Jay et al., 2021]] ). Despite the evidence that adaptation can reduce risks of heat stress, the impact of projected climate change on temperature-related mortality is expected to be a net increase under a wide range of climate change scenarios, even with adaptation (Chapter 7, ''high confidence'' ) ''.'' Much of the adaptation literature focuses on coping with long-term gradual climate change and largely does not take into account the increased difficulty of adapting to climate extremes and general higher variability in climate that is projected to occur in the future. However, expanding and more coordinated adaptation, including wider implementation and multi-level coordination, has the potential to reduce the risks to crops from heatwaves at intermediate (but not high) levels of warming.(IPCC AR5 Ch7, [[#Ahmed--2018|Ahmed et al., 2018]] ; [[#Ahmed--2019|Ahmed et al., 2019]] , [[#16.3.2.2|Section 16.3.2.2]] ; [[#EEA--2019|EEA, 2019]] ; [[#Raza--2019|Raza et al., 2019]] ; [[#Tripathi--2020|Tripathi and Sindhi, 2020]] ).

The transition from high to very high risk for the RFC2 was not assessed in the AR5 or in SR15. Some new evidence suggests, however, that very high risks associated with weather and climate extremes would be reached at higher levels of global warming. In particular, changes in several hazards would be more widespread and pronounced at 2°C compared with 1.5°C global warming, including increases in multiple and concurrent extremes (IPCC WGI AR6 SPM; IPCC WGI AR6 Chapter 11, IPCC WGI AR6 Chapter 12). On average over land, high temperature events that would have occurred once in 50 years in the absence of anthropogenic climate change are projected to become 13.9 times more likely with 2°C warming, and 39.2 times more likely with 4°C warming (IPCC AR6 WGI SPM Figure SPM.6, [[#IPCC--2021|IPCC, 2021]] ), indicating a nonlinear increase with warming. [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] assessed that risk of wildfire transitions from moderate to high over the range 1.5°C to 2.5°C warming ( ''medium confidence'' , Table SM2.5 , Figure 2.11). The intensity of heavy precipitation events increases overall by about 7% for each additional degree of global warming (IPCC AR6 WGI SPM), while their frequency increases nonlinearly. Events that would have occurred once every 10 years in a climate without human influence are projected to become 1.7 times more likely with 2°C warming, and 2.7 times more likely with 4°C warming (IPCC AR6 WGI SPM Figure SPM.6). Several AR6 regions are projected to be affected by increases in agricultural and ecological droughts at 2°C of global warming, including western North America, central North America, northern Central America, southern Central America, the Caribbean, northern South America, northeastern South America, South American Monsoon, southwestern South America, southern South America, West and Central Europe, the Mediterranean, western Southern Africa, eastern Southern Africa, Madagascar, eastern Australia and southern Australia (IPCC WGI AR6, Chapter 11, [[#Seneviratne--2021|Seneviratne et al., 2021]] ). This is a substantially larger number compared with projections at 1.5°C (IPCC WGI AR6, Chapter 11, [[#Seneviratne--2021|Seneviratne et al., 2021]] ). In these drying regions, events that would have occurred once every 10 years in a climate without human influence are projected to happen 2.4 times more frequently at 2°C of global warming (IPCC WGI AR6 SPM Figure SPM.6). Urban land exposed to floods and droughts is ''very likely'' to have more than doubled between 2000 and 2030, and the risk of flooding accelerates after 2050 (Chapter 4). At 2°C of global warming, there are also significant projected increases in fluvial flood frequency and resultant risks associated with higher populations exposed to these flood risks ( [[#Alfieri--2017|Alfieri et al., 2017]] ; [[#Dottori--2018|Dottori et al., 2018]] ).

Heat-related mortality is assessed to increase from high to very high by 3°C under SSP3, a socioeconomic scenario with large challenges to adaptation ( [[#Ebi--2021|Ebi et al., 2021]] ). SRCCL assessed that very high risks would be reached in association with wildfire above 3°C of global warming ( [[#IPCC--2019a|IPCC, 2019a]] ). [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] has assessed that risk of fire weather itself transitions from high to very high over the range 3°C to 4.5°C warming ( ''medium confidence'' , Table SM2.5, Figure 2.11). Matthews et al. (2017) show that, at 1.5°C of global warming, about 40% of all megacities would be affected at least 1 d yr −1 with a heat index above 40.6°C (i.e., with 40.6°C ‘feels-like’ temperatures, accounting for moisture effects). This number would reach about 65% of megacities at 2.7°C and close to 80% at 4°C. In addition, there is evidence for a higher risk of concurrent heat extremes at different locations with increasing global warming ( [[#Vogel--2019|Vogel et al., 2019]] ), meaning that several cities could be affected by deadly heatwaves simultaneously. Laufkötter et al. (2020) found that marine heatwave events would become annual to decadal events under 3°C of global warming, with consequences for aquaculture (Chapter 5). [[#Gaupp--2019|Gaupp et al. (2019)]] conclude that risks of simultaneous crop failure across worldwide breadbasket regions, due to changes in maximum temperatures in the crop-growth-relevant season or cumulative precipitation in relevant time frames, increase disproportionately between 1.5°C and 2°C of global warming. Populations exposed to extreme weather and climate events may consume inadequate or insufficient food, leading to malnutrition and increasing the risk of disease (Chapter 5, ''high confidence'' ). Hence, there is the potential for very high risks associated with changes in climate extremes for food security in the low adaptation case, already above 2°C of global warming. Finally, studies suggest that regional thresholds for climate extremes could be reached at 2°C of global warming, for instance in the Mediterranean ( [[#Guiot--2016|Guiot and Cramer, 2016]] ). [[#Samaniego--2018|Samaniego et al. (2018)]] conclude that soil moisture droughts in that region would become two to three times longer than at the end of the 20th century at 2°C, and three to four times longer (125 d long yr –1 ) at 3°C of global warming. There is clear evidence of very high risk at 3°C global warming for wildfires, marine heatwaves and heatwaves in megacities (the latter being set at 2.7°C).

Based on the available evidence, we assess that there is ''medium confidence'' that the transition to very high risk would happen at a median value 2°C of global warming, considering the increased risk for breadbasket failure and irreversible impacts associated with changes in extremes at this warming level (e.g., damages to ecosystems, health impacts, severe coastal storms), but that due to the disproportionate increases in risk between 1.5°C and 2°C this transition begins already at 1.8°C. The higher range for this transition is set with ''medium confidence'' at 2.5°C in this low/no adaptation scenario, owing to the further projected nonlinear increases in risks associated with high temperature events above 2°C (WGI AR6 Figure SPM.6, [[#IPCC--2021|IPCC, 2021]] ; Cross-Chapter Box 12.1, [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), and also the limits to adaptation associated with dealing with a rapid escalation of extreme weather events globally during this century; extreme events are particularly difficult to adapt to and thus more often exceed hard limits to adaptation, particularly in natural ecosystem settings ( [[#16.4|Section 16.4]] ).

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