Editing IPCC:AR6/WGI/Chapter-11 (section)

== Executive Summary ==

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'''This chapter assesses changes in weather and climate extremes on regional and global scales, including observed changes and their attribution, as well as projected changes.''' The extremes considered include temperature extremes, heavy precipitation and pluvial floods, river floods, droughts, storms (including tropical cyclones), as well as compound events (multivariate and concurrent extremes). The assessment focuses on land regions excluding Antarctica. Changes in marine extremes are addressed in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] and Cross-Chapter Box 9.1. Assessments of past changes and their drivers are from 1950 onward, unless indicated otherwise. Projections for changes in extremes are presented for different levels of global warming, supplemented with information for the conversion to emissions scenario-based projections (Cross-Chapter Box 11.1 and Table 4.2). Since the IPCC Fifth Assessment Report (AR5), there have been important new developments and knowledge advances on changes in weather and climate extremes, in particular regarding human influence on individual extreme events, on changes in droughts, tropical cyclones, and compound events, and on projections at different global warming levels (1.5°C–4°C). These, together with new evidence at regional scales, provide a stronger basis and more regional information for the AR6 assessment on weather and climate extremes.

'''It is an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since pre-industrial time, in particular for temperature extremes.''' Evidence of observed changes in extremes and their attribution to human influence (including greenhouse gas and aerosol emissions and land-use changes) has strengthened since AR5, in particular for extreme precipitation, droughts, tropical cyclones and compound extremes (including dry/hot events and fire weather). Some recent hot extreme events would have been ''extremely unlikely'' to occur without human influence on the climate system. {11.2, 11.3, 11.4, 11.6, 11.7, 11.8}

'''Regional changes in the intensity and frequency of climate extremes generally scale with global warming. New evidence strengthens the conclusion from the IPCC Special Report on Global Warming of 1.5°C (SR1.5) 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''' ( ''high confidence'' '''). In particular, this is the case for temperature extremes''' ( ''very'' ''likely'' '''), the intensification of heavy precipitation''' ( ''high confidence'' ''') including that associated with tropical cyclones''' ( ''medium confidence'' '''), and the worsening of droughts in some regions''' ( ''high confidence'' ''').''' The occurrence of extreme events unprecedented in the observed record will rise with increasing global warming, even at 1.5°C of global warming. Projected percentage changes in frequency are higher for the rarer extreme events ( ''high confidence'' ). {11.1, 11.2, 11.3, 11.4, 11.6, 11.9, Cross-Chapter Box 11.1}

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=== Methods and Data for Extremes ===

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'''Since AR5, the confidence about past and future changes in weather and climate extremes has increased due to better physical understanding of processes, an increasing proportion of the scientific literature combining different lines of evidence, and improved accessibility to different types of climate models''' ( ''high confidence'' '''). There have been improvements in some observation-based datasets, including reanalysis data''' ( ''high confidence'' '''). Climate models can reproduce the sign (direction) of changes in temperature extremes observed globally and in most regions, although the magnitude of the trends may differ''' ( ''high confidence'' ''').''' Models are able to capture the large-scale spatial distribution of precipitation extremes over land ( ''high confidence'' ). The intensity and frequency of extreme precipitation simulated by Coupled Model Intercomparison Project Phase 6 (CMIP6) models are similar to those simulated by CMIP5 models ( ''high confidence'' ). Higher horizontal model resolution improves the spatial representation of some extreme events (e.g., heavy precipitation events), in particular in regions with highly varying topography ( ''high confidence'' ). {11.2, 11.3, 11.4}

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=== Temperature Extremes ===

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'''The frequency and intensity of hot extremes (including heatwaves) have increased, and those of cold extremes have decreased on the global scale since 1950''' ( ''virtually certain'' '''). This also applies at regional scale, with more than 80% of AR6 regions''' [[#footnote-011|1]] '''showing similar changes assessed to be at least''' ''likely'' '''.''' In a few regions, ''limited evidence'' (data or literature) prevents the reliable estimation of trends. {11.3, 11.9}

'''Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale''' ( ''virtually certain'' ''') and on most continents''' ( ''very likely'' ''').''' The effect of enhanced greenhouse gas concentrations on extreme temperatures is moderated or amplified at the regional scale by regional processes such as soil moisture or snow/ice-albedo feedbacks, by regional forcing from land-use and land-cover changes, or aerosol concentrations, and decadal and multi-decadal natural variability. Changes in anthropogenic aerosol concentrations have ''likely'' affected trends in hot extremes in some regions. Irrigation and crop expansion have attenuated increases in summer hot extremes in some regions, such as the Midwestern USA ( ''medium confidence'' ). Urbanization has ''likely'' exacerbated changes in temperature extremes in cities, in particular for nighttime extremes. {11.1, 11.2, 11.3}

'''The frequency and intensity of hot extremes will continue to increase and those of cold extremes will continue to decrease, at global and continental scales and in nearly all inhabited regions''' <sup>1</sup> '''with increasing global warming levels.''' This will be the case even if global warming is stabilized at 1.5°C. Relative to present-day conditions, changes in the intensity of extremes would be at least double at 2°C, and quadruple at 3°C of global warming, compared to changes at 1.5°C of global warming. The number of hot days and hot nights and the length, frequency, and/or intensity of warm spells or heatwaves will increase over most land areas ( ''virtually certain'' ). In most regions, future changes in the intensity of temperature extremes will ''very likely'' be proportional to changes in global warming, and up to two to three times larger ( ''high confidence'' ). The highest increase of temperature of hottest days is projected in some mid-latitude and semi-arid regions and in the South American Monsoon region, at about 1.5 times to twice the rate of global warming ( ''high confidence'' ). The highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming ( ''high confidence'' ). The frequency of hot temperature extreme events will ''very likely'' increase nonlinearly with increasing global warming, with larger percentage increases for rarer events. {11.2, 11.3, 11.9; Table 11.1; Figure 11.3}

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=== Heavy Precipitation and Pluvial Floods ===

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'''The frequency and intensity of heavy precipitation events have''' ''likely'' '''increased at the global scale over a majority of land regions with good observational coverage.''' '''Heavy precipitation has''' ''likely'' '''increased on the continental scale over three continents: North America, Europe, and Asia.''' Regional increases in the frequency and/or intensity of heavy precipitation have been observed with at least ''medium confidence'' for nearly half of AR6 regions, including WSAF, ESAF, WSB, SAS, ESB, RFE, WCA, ECA, TIB, EAS, SEA, NAU, NEU, EEU, GIC, WCE, SES, CNA, and ENA. {11.4, 11.9}

'''Human influence, in particular greenhouse gas emissions, is''' ''likely'' '''the main driver of the observed global-scale intensification of heavy precipitation over land regions.''' It is ''likely'' that human-induced climate change has contributed to the observed intensification of heavy precipitation at the continental scale in North America, Europe and Asia. Evidence of a human influence on heavy precipitation has emerged in some regions ( ''high confidence'' ). {11.4, 11.9, Table 11.1}

'''Heavy precipitation will generally become more frequent and more intense with additional global warming. At a global warming level of 4°C relative to the pre-industrial level, very rare (e.g., one in 10 or more years) heavy precipitation events would become more frequent and more intense than in the recent past, on the global scale''' ( ''virtually certain'' ''') and in all continents and AR6 regions. The increase in frequency and intensity is''' ''extremely likely'' '''for most continents and''' ''very likely'' '''for most AR6 regions.''' At the global scale, the intensification of heavy precipitation will follow the rate of increase in the maximum amount of moisture that the atmosphere can hold as it warms ( ''high confidence'' ), of about 7% per 1°C of global warming. The increase in the frequency of heavy precipitation events will be non-linear with more warming and will be higher for rarer events ( ''high confidence'' ), with a ''likely'' doubling and tripling in the frequency of 10-year and 50-year events, respectively, compared to the recent past at 4°C of global warming. Increases in the intensity of extreme precipitation at regional scales will vary, depending on the amount of regional warming, changes in atmospheric circulation and storm dynamics ( ''high confidence'' ). {11.4, Box 11.1}

'''The projected increase in the intensity of extreme precipitation translates to an increase in the frequency and magnitude of pluvial floods – surface water and flash floods –''' ( ''high confidence'' '''), as pluvial flooding results from precipitation intensity exceeding the capacity of natural and artificial drainage''' '''systems.''' {11.4}

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=== River Floods ===

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'''Significant trends in peak streamflow have been observed in some regions over the past decades''' ( ''high confidence'' ). The seasonality of river floods has changed in cold regions where snow-melt is involved, with an earlier occurrence of peak streamflow ( ''high conf'' ''idence'' ). {11.5}

'''Global hydrological models project a larger fraction of land areas to be affected by an increase in river floods than by a decrease in river floods''' ( ''medium confidence'' ''').''' Regional changes in river floods are more uncertain than changes in pluvial floods because complex hydrological processes and forcings, including land cover change and human water management, are involved. {11.5}

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=== Droughts ===

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'''Different drought types exist, and they are associated with different impacts and respond differently to increasing greenhouse gas concentrations.''' Precipitation deficits and changes in evapotranspiration govern net water availability. A lack of sufficient soil moisture, sometimes amplified by increased atmospheric evaporative demand, results in agricultural and ecological drought. Lack of runoff and surface water result in hydrological drought. {11.6}

'''Human-induced climate change has contributed to increases in a''' '''gricultural and''' '''ecological droughts in some regions due to evapotranspiration increases''' ( ''medium confidence'' ''').''' Increases in evapotranspiration have been driven by increases in atmospheric evaporative demand induced by increased temperature, decreased relative humidity and increased net radiation ( ''high confidence'' ). Trends in precipitation are not a main driver in affecting global-scale trends in drought ( ''medium confidence'' ), but have induced increases in meteorological droughts in a few AR6 regions (NES: ''high confidence'' ; WAF, CAF, ESAF, SAM, SWS, SSA, SAS: ''medium confidence'' ). Increasing trends in agricultural and ecological droughts have been observed on all continents (WAF, CAF, WSAF, ESAF, WCA, ECA, EAS, SAU, MED, WCE, WNA, NES: ''medium confidence'' ), but decreases only in one AR6 region (NAU: ''medium confidence'' ). Increasing trends in hydrological droughts have been observed in a few AR6 regions (MED: ''high confidence'' ; WAF, EAS, SAU: ''medium confidence'' ). Regional-scale attribution shows that human-induced climate change has contributed to increased agricultural and ecological droughts (MED, WNA), and increased hydrological drought (MED) in some regions ( ''medium confidence'' ). {11.6, 11.9}

'''More regions are affected by increases in agricultural and ecological droughts with increasing global warming''' ( ''high confidence'' ''').''' Several regions will be affected by more severe agricultural and ecological droughts even if global warming is stabilised at 2°C, including MED, WSAF, SAM and SSA ( ''high confidence'' ), and ESAF, MDG, EAU, SAU, SCA, CAR, NSA, NES, SWS, WCE, NCA, WNA and CNA ( ''medium confidence'' ). Some regions are also projected to be affected by more severe agricultural and ecological droughts at 1.5°C (MED, WSAF, ESAF, SAU, NSA, SAM, SSA, CNA, ''medium confidence'' ). At 4°C of global warming, about 50% of all inhabited AR6 regions would be affected by increases in agricultural and ecological droughts (WCE, MED, CAU, EAU, SAU, WCA, EAS, SCA, CAR, NSA, NES, SAM, SWS, SSA, NCA, CNA, ENA, WNA, WSAF, ESAF, MDG: ''medium confidence'' or higher), and only two regions (NEAF, SAS) would experience decreases in agricultural and ecological drought ( ''medium confidence'' ). There is ''high confidence'' that the projected increases in agricultural and ecological droughts are strongly affected by evapotranspiration increases associated with enhanced atmospheric evaporative demand. Several regions are projected to be more strongly affected by hydrological droughts with increasing global warming (at 4°C of global warming: NEU, WCE, EEU, MED, SAU, WCA, SCA, NSA, SAM, SWS, SSA, WNA, WSAF, ESAF, MDG: ''medium confidence'' or higher). There is ''low confidence'' that effects of enhanced atmospheric carbon dioxide (CO <sub>2</sub> ) concentrations on plant water-use efficiency alleviate extreme agricultural and ecological droughts in conditions characterized by limited soil moisture and enhanced atmospheric evaporative demand. There is also ''low confidence'' that these effects will substantially reduce global plant transpiration and the severity of hydrological droughts. There is ''high confidence'' that the land carbon sink will become less efficient due to soil moisture limitations and associated drought conditions in some regions in higher-emissions scenarios, in particular under global warming levels above 4°C. {11.6, 11.9, Cross-Chapter Box 5.1}

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=== Extreme Storms, Including Tropical Cyclones ===

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'''The average and maximum rain rates associated with tropical cyclones (TCs), extratropical cyclones and atmospheric rivers across the globe, and severe convective storms in some regions,''' '''increase in a warming world''' ( ''high confidence'' ''')''' ''.'' Available event attribution studies of observed strong TCs provide ''medium confidence'' for a human contribution to extreme TC rainfall. Peak TC rain rates increase with local warming at least at the rate of mean water vapour increase over oceans (about 7% per 1°C of warming) and in some cases exceeding this rate due to increased low-level moisture convergence caused by increases in TC wind intensity ( ''medium confidence'' ). {11.7, 11.4, Box 11.1}

'''It is''' ''likely'' '''that the global proportion of Category 3–5 tropical cyclone instances''' [[#footnote-010|2]] '''has increased over the past four decades.''' The average location where TCs reach their peak wind intensity has ''very likely'' migrated poleward in the western North Pacific Ocean since the 1940s, and TC translation speed has ''likely'' slowed over the conterminous USA since 1900. Evidence of similar trends in other regions is not robust. The global frequency of TC rapid intensification events has ''likely'' increased over the past four decades. None of these changes can be explained by natural variability alone ( ''medium'' ''confidence'' ).

'''The proportion of intense TCs, average peak TC wind speeds, and peak wind speeds of the most intense TCs will increase on the global scale with increasing global warming''' ( ''high confidence'' ''').''' The total global frequency of TC formation will decrease or remain unchanged with increasing global warming ( ''medium confide'' ''nce'' ). {11.7.1}

'''There is''' ''low confidence'' '''in past changes of maximum wind speeds and other measures of dynamical intensity of extratropical cyclones. Future wind speed changes are expected to be small, although poleward shifts in the storm tracks could lead to substantial changes in extreme wind speeds in some regions''' ( ''medium confidence'' ''').''' There is ''low confidence'' in past trends in characteristics of severe convective storms, such as hail and severe winds, beyond an increase in precipitation rates. The frequency of spring severe convective storms is projected to increase in the USA, leading to a lengthening of the severe convective storm season ( ''medium confidence'' ); evidence in other regions is limited. {11.7.2, 11.7.3} .

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=== Compound Events, Including Dry/Hot Events, Fire Weather, Compound Flooding, and Concurrent Extremes ===

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'''The probability of compound events has''' ''likely'' '''increased in the past due to human-induced climate change and will''' ''likely'' '''continue to increase with further global warming.''' Concurrent heatwaves and droughts have become more frequent, and this trend will continue with higher global warming ( ''high confidence'' ). Fire weather conditions (compound hot, dry and windy events) have become more probable in some regions ( ''medium confidence'' ) and there is ''high confidence'' that they will become more frequent in some regions at higher levels of global warming. The probability of compound flooding (storm surge, extreme rainfall and/or river flow) has increased in some locations ( ''medium confidence'' ), and will continue to increase due to sea level rise and increases in heavy precipitation, including changes in precipitation intensity associated with tropical cyclones ( ''high confidence'' ). The land area affected by concurrent extremes has increased ( ''high confidence'' ). Concurrent extreme events at different locations, but possibly affecting similar sectors (e.g., critical crop-producing areas for global food supply) in different regions, will become more frequent with increasing global warming, in particular above 2°C of global warming ( ''high confidence'' ). {11.8, Box 11.2, Box 11.4} .

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=== Low-likelihood, High-impact Events Associated With Climate Extremes ===

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'''The future occurrence of low-likelihood, high-impact events linked to climate extremes is generally associated with''' ''low confidence'' ''', but cannot be excluded, especially at global warming levels above 4°C.''' Compound events, including concurrent extremes, are a factor increasing the probability of low-likelihood, high-impact events ( ''high confidence'' ). With increasing global warming, some compound events with low likelihood in past and current climates will become more frequent, and there is a higher chance of occurrence of historically unprecedented events and surprises ( ''high confidence'' ). However, even extreme events that do not have a particularly low probability in the present climate (at more than 1°C of global warming) can be perceived as surprises because of the pace of global warming ( ''high confidence'' ). {Box 11.2}

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