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

=== 11.3.2 Observed Trends ===

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The SREX (IPCC, 2012) reported a ''very likely'' decrease in the number of cold days and nights and increase in the number of warm days and nights at the global scale. Confidence in trends was assessed as regionally variable ( ''low to medium confidence'' ) due to either a lack of observations or varying signals in sub-regions.

Since SREX (IPCC, 2012) and AR5 (IPCC, 2014), many regional-scale studies have examined trends in temperature extremes using different metrics that are based on daily temperatures, such as the Commission for Climatology/World Climate Research Program/Commission for Oceanography and Marine Meteorology joint Expert Team on Climate Change Detection and Indices (ETCCDI) indices ( [[#Dunn--2020|Dunn et al., 2020]] ). The additional observational records, along with a stronger warming signal, show very clearly that changes observed at the time of AR5 (IPCC, 2014) continued, providing strengthened evidence of an increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes. While the magnitude of the observed trends in temperature-related extremes varies depending on the region, spatial and temporal scales, and metric assessed, evidence of a warming effect is overwhelming, robust, and consistent. In particular, an increase in the intensity and frequency of hot extremes is almost always associated with an increase in the hottest temperatures and in the number of heatwave days. It is also the case for changes (decreases) in cold extremes. For this reason, and to simplify the presentation, the phrase ‘increase in the intensity and frequency of hot extremes’ is used to represent, collectively, an increase in the magnitude of extreme day and/or night temperatures, in the number of warm days and/or nights, and in the number of heatwave days. Changes in cold extremes are assessed similarly.

On the global scale, evidence of an increase in the number of warm days and nights and a decrease in the number of cold days and nights, and an increase in the coldest and hottest extreme temperatures is very robust and consistent among all variables. Figure 11.2 displays time series of globally averaged TXx and TNn on land. Warming of land mean TXx is similar to the mean temperature warming on land, which is about 45% higher than global warming ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] ). Warming of land mean TNn is even higher, with about 3°C of warming since 1960 (Figure 11.2). Figure 11.9 shows maps of linear trends over 1960–2018 in TXx, TNn, and frequency of warm days (TX90p). The maps for TXx and TNn show trends consistent with overall warming in most regions, with a particularly high warming of TXx in Europe and north-western South America, and a particularly high warming of TNn in the Arctic. Consistent with the observed warming in global surface temperature ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2|Section 2.3.1.2]] ) and the observed trends in TXx and TNn, the frequency of TX90p has increased, while that of cold nights (TN10p) has decreased since the 1950s: Nearly all land regions showed statistically significant decreases in TN10p ( [[#Alexander--2016|Alexander, 2016]] ; [[#Dunn--2020|Dunn et al., 2020]] ), though trends in TX90p are variable with some decreases in Southern South America, mainly during austral summer ( [[#Rusticucci--2017|Rusticucci et al., 2017]] ). A decrease in the number of cold spell days is also observed over nearly all land surface areas ( [[#Easterling--2016|Easterling et al., 2016]] ) and in the northern mid-latitudes in particular ( [[#van%20Oldenborgh--2019|van Oldenborgh et al., 2019]] ). These observed changes are also consistent when a new global land surface daily air temperature dataset is analysed (P. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ). Warming trends in temperature extremes globally, and in most land areas, over the path century are also found to be consistent in a range of observation-based datasets ( [[#Fischer--2014|Fischer and Knutti, 2014]] ; [[#Donat--2016a|Donat et al., 2016a]] ; [[#Dunn--2020|Dunn et al., 2020]] ), with the extremes related to daily minimum temperatures changing faster than those related to daily maximum temperatures ( [[#Dunn--2020|Dunn et al., 2020]] ; see Figure 11.2). Seasonal variations in trends in temperature-related extremes have been demonstrated. A warming in warm-season temperature extremes is detected, even during the ‘slower surface global warming’ period from the late 1990s to early 2010s (Cross-Chapter Box 3.1; [[#Kamae--2014|Kamae et al., 2014]] ; [[#Seneviratne--2014|Seneviratne et al., 2014]] ; [[#Imada--2017|Imada et al., 2017]] ). Many studies of past changes in temperature extremes for particular regions or countries show trends consistent with this global picture, as summarized below and in Tables 11.4, 11.7, 11.10, 11.13, 11.16 and 11.19.

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[[File:9f3091f4b853f00de2588be1834e43f8 IPCC_AR6_WGI_Figure_11_9.png]]

'''Figure 11.9 |''' '''Linear trends over 1960–2018 for three temperature extreme indices: (a)''' the annual maximum daily maximum temperature (TXx), '''(b)''' the annual minimum daily minimum temperature (TNn), and '''(c)''' the annual number of days when daily maximum temperature exceeds its 90th percentile from a base period of 1961–1990 (TX90p); based on the HadEX3 dataset (Dunn et al. , 20 20). Linear trends are calculated only for grid points with at least 66% of the annual values over the period and which extend to at least 2009. Areas without sufficient data are shown in grey. No overlay indicates regions where the trends are significant at the p = 0.1 level. Crosses indicate regions where trends are not significant. Further details on data sources and processing are available in the chapter data table (Table 11.SM.9).

In Africa (Table 11.4), while it is difficult to assess changes in temperature extremes in parts of the continent because of a lack of data, evidence of an increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes is clear and robust in regions where data are available. These include an increase in the frequency of warm days and nights and a decrease in the frequency of cold days and nights with ''high confidence'' ( [[#Donat--2013a|Donat et al., 2013a]] , 2014b; [[#Kruger--2013|Kruger and Sekele, 2013]] ; [[#Chaney--2014|Chaney et al., 2014]] ; [[#Filahi--2016|Filahi et al., 2016]] ; [[#Moron--2016|Moron et al., 2016]] ; [[#Ringard--2016|Ringard et al., 2016]] ; [[#Barry--2018|Barry et al., 2018]] ; [[#Gebrechorkos--2018|Gebrechorkos et al., 2018]] ) and an increase in heatwaves ( [[#Russo--2016|Russo et al., 2016]] ; [[#Ceccherini--2017|Ceccherini et al., 2017]] ). The increase in TNn is more notable than in TXx (Figure 11.9). Cold spells occasionally strike subtropical areas, but are ''likely'' to have decreased in frequency ( [[#Barry--2018|Barry et al., 2018]] ). The frequency of cold events has ''likely'' decreased in South Africa ( [[#Song--2014|Song et al., 2014]] ; [[#Kruger--2017|Kruger and Nxumalo, 2017]] ), North Africa ( [[#Filahi--2016|Filahi et al., 2016]] ; [[#Driouech--2021|Driouech et al., 2021]] ), and the Sahara ( [[#Donat--2016a|Donat et al., 2016a]] ). Over the whole continent, there is ''medium confidence'' in an increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes; it is ''likely'' that similar changes have also occurred in areas with poor data coverage, as warming is widespread and as projected future changes are similar over all regions ( [[#11.3.5|Section 11.3.5]] ).

In Asia (Table 11.7), there is very ''robust evidence'' for a ''very likely'' increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes in recent decades. This is clear in global studies (e.g., [[#Alexander--2016|Alexander, 2016]] ; [[#Dunn--2020|Dunn et al., 2020]] ), as well as in numerous regional studies (Table 11.7). The area fraction with extreme warmth in Asia increased during 1951–2016 ( [[#Imada--2018|Imada et al., 2018]] ). The frequency of warm extremes increased and the frequency of cold extremes decreased in East Asia ( [[#Zhou--2016|]] [[#Zhou--2016|B. Zhou et al., 2016]] ; [[#Chen--2017|Chen and Zhai, 2017]] ; [[#Yin--2017|Yin et al., 2017]] ; W. [[#Lee--2018|]] [[#Lee--2018|]] [[#Lee--2018|Lee et al., 2018]] ; [[#Qian--2019|Qian et al., 2019]] ) and west Asia (Acar Deniz and Gönençgil, 2015; [[#Erlat--2016|Erlat and Türkeş, 2016]] ; [[#Rahimi--2018|Rahimi and Hejabi, 2018]] ; [[#Rahimi--2018|Rahimi et al., 2018]] ) with ''high confidence'' . The duration of heat extremes has also lengthened in some regions, for example, in southern China ( [[#Luo--2016|Luo and Lau, 2016]] ), but there is ''medium confidence'' of heat extremes increasing in frequency in South Asia ( [[#AlSarmi--2014|AlSarmi and Washington, 2014]] ; [[#Sheikh--2015|Sheikh et al., 2015]] ; [[#Mazdiyasni--2017|Mazdiyasni et al., 2017]] ; [[#Zahid--2017|Zahid et al., 2017]] ; [[#Nasim--2018|Nasim et al., 2018]] ; [[#Khan--2019|Khan et al., 2019]] ; [[#Sen%20Roy--2019|Sen Roy, 2019]] ). Warming trends in daily temperature extremes indices have also been observed in central Asia ( [[#Hu--2016|Hu et al., 2016]] ; [[#Feng--2018|Feng et al., 2018]] ), the Hindu Kush Himalaya ( [[#Sun--2017|Sun et al., 2017]] ), and South East Asia ( [[#Supari--2017|Supari et al., 2017]] ; [[#Cheong--2018|Cheong et al., 2018]] ). The intensity and frequency of cold spells in all Asian regions have been decreasing since the beginning of the 20th century ( ''high confidence'' ) ( [[#Sheikh--2015|Sheikh et al., 2015]] ; [[#Donat--2016a|Donat et al., 2016a]] ; [[#Dong--2018|Dong et al., 2018]] ; [[#van%20Oldenborgh--2019|van Oldenborgh et al., 2019]] ).

In Australasia (Table 11.10), there is very ''robust evidence'' for ''very likely'' increases in the number of warm days and warm nights and decreases in the number of cold days and cold nights since 1950 ( [[#Lewis--2015|Lewis and King, 2015]] ; [[#Jakob--2016|Jakob and Walland, 2016]] ; [[#Alexander--2017|Alexander and Arblaster, 2017]] ). The increase in extreme minimum temperatures occurs in all seasons over most of Australia and typically exceeds the increase in extreme maximum temperatures (X.L. [[#Wang--2013|Wang et al., 2013]] b; [[#Jakob--2016|Jakob and Walland, 2016]] ). However, some parts of Southern Australia have shown stable or increased numbers of frost days since the 1980s ( [[#Dittus--2014|Dittus et al., 2014]] ) (see also [[#11.3.4|Section 11.3.4]] ). Similar positive trends in extreme minimum and maximum temperatures have been observed in New Zealand, in particular in the autumn and winter seasons, although they generally show higher spatial variability ( [[#Caloiero--2017|Caloiero, 2017]] ). In the tropical Western Pacific region, spatially coherent warming trends in maximum and minimum temperature extremes have been reported for the period 1951–2011 ( [[#Whan--2014|Whan et al., 2014]] ; [[#McGree--2019|McGree et al., 2019]] ).

In Central and South America (Table 11.13), there is ''high confidence'' that observed hot extremes (TN90p, TX90p) have increased, and cold extremes (TN10p, TX10p) have decreased over recent decades, though trends vary among different extremes types, datasets, and regions ( [[#Skansi--2013|Skansi et al., 2013]] ; [[#Dittus--2016|Dittus et al., 2016]] ; [[#Rusticucci--2017|Rusticucci et al., 2017]] ; [[#Meseguer-Ruiz--2018|Meseguer-Ruiz et al., 2018]] ; [[#Salvador--2018|Salvador and de Brito, 2018]] ; [[#Dereczynski--2020|Dereczynski et al., 2020]] ; [[#Dunn--2020|Dunn et al., 2020]] ; [[#Olmo--2020|Olmo et al., 2020]] ). An increase in the intensity and frequency of heatwave events was also observed between 1961 and 2014 in an area covering most of South America ( [[#Ceccherini--2016|Ceccherini et al., 2016]] ; [[#Geirinhas--2018|Geirinhas et al., 2018]] ). However, there is ''medium confidence'' that warm extremes (TXx and TX90p) have decreased in the last decades over the central region of South-Eastern South America (SES) during austral summer ( [[#Tencer--2012|Tencer and Rusticucci, 2012]] ; [[#Skansi--2013|Skansi et al., 2013]] ; [[#Rusticucci--2017|Rusticucci et al., 2017]] ; [[#Wu--2017|Wu and Polvani, 2017]] ). There is ''medium confidence'' that TNn extremes are warming faster than TXx extremes, with the largest warming rates observed over North-East Brazil (NEB) and Northern South America (NSA) for cold nights ( [[#Skansi--2013|Skansi et al., 2013]] ).

In Europe (Table 11.16), there is very ''robust evidence'' for a ''very likely'' increase in maximum temperatures and the frequency of heatwaves. The increase in the magnitude and frequency of high maximum temperatures has been observed consistently across regions, including in central Europe ( [[#Twardosz--2013|Twardosz and Kossowska-Cezak, 2013]] ; [[#Christidis--2015|Christidis et al., 2015]] ; [[#Lorenz--2019|Lorenz et al., 2019]] ) and southern Europe ( [[#Croitoru--2013|Croitoru and Piticar, 2013]] ; [[#El%20Kenawy--2013|El Kenawy et al., 2013]] ; [[#Christidis--2015|Christidis et al., 2015]] ; [[#Nastos--2015|Nastos and Kapsomenakis, 2015]] ; [[#Fioravanti--2016|Fioravanti et al., 2016]] ; [[#Ruml--2017|Ruml et al., 2017]] ). In Northern Europe, a strong increase in extreme winter warming events has been observed ( [[#Matthes--2015|Matthes et al., 2015]] ; [[#Vikhamar-Schuler--2016|Vikhamar-Schuler et al., 2016]] ). Temperature observations for winter cold spells show a long-term decreasing frequency in Europe (Brunner et al., 2018; [[#van%20Oldenborgh--2019|van Oldenborgh et al., 2019]] ), and typical cold spells, such as that observed during the 2009–2010 winter, had an occurrence probability two times smaller currently than if climate change had not occurred ( [[#Christiansen--2018|Christiansen et al., 2018]] ).

In North America (Table 11.19), there is very ''robust evidence'' for a ''very likely'' increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes for the whole continent, though there are substantial spatial and seasonal variations in the trends. Minimum temperatures display warming consistently across the continent, while there are more contrasting trends in the annual maximum daily temperatures in parts of the USA (Figure 11.9; [[#Lee--2014|Lee et al., 2014]] ; [[#van%20Oldenborgh--2019|van Oldenborgh et al., 2019]] ; [[#Dunn--2020|Dunn et al., 2020]] ). In Canada, there is a clear increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes ( [[#Vincent--2018|Vincent et al., 2018]] ). In Mexico, a clear warming trend in TNn was found, particularly in the northern arid region ( [[#Montero-Martínez--2018|Montero-Martínez et al., 2018]] ). The number of warm days has increased and the number of cold days has decreased ( [[#García-Cueto--2019|García-Cueto et al., 2019]] ). Cold spells have undergone a reduction in magnitude and intensity in all regions of North America ( [[#Bennett--2015|Bennett and Walsh, 2015]] ; [[#Donat--2016a|Donat et al., 2016a]] ; [[#Grotjahn--2016|Grotjahn et al., 2016]] ; [[#Vose--2017|Vose et al., 2017]] ; [[#García-Cueto--2019|García-Cueto et al., 2019]] ; [[#van%20Oldenborgh--2019|van Oldenborgh et al., 2019]] ).

Extreme heat events have increased around the Arctic since 1979, particularly over Arctic North America and Greenland ( [[#Matthes--2015|Matthes et al., 2015]] ; [[#Dobricic--2020|Dobricic et al., 2020]] ), which is consistent with summer melt ( [[IPCC:Wg1:Chapter:Chapter-9#9.4.1|Section 9.4.1]] ). Observations north of 60˚N show increases in winter warm days and nights over 1979–2015, while cold days and nights declined ( [[#Sui--2017|Sui et al., 2017]] ). Extreme heat days are particularly strong in winter, with observations showing the warmest mid-winter temperatures at the North Pole rising at twice the rate of mean temperature ( [[#Moore--2016|Moore, 2016]] ), as well as increases in Arctic winter warm days ( [[#Vikhamar-Schuler--2016|Vikhamar-Schuler et al., 2016]] ; [[#Graham--2017|Graham et al., 2017]] ). Arctic annual minimum temperatures have increased at about three times the rate of global surface temperature since the 1960s (Figures 11.2 and 11.9), consistent with the observed mean cold season (October–May) warming of 3.1°C in the region ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] 11.2).

Trends in some measures of heatwaves are also observed at the global scale. Globally averaged heatwave intensity, heatwave duration, and the number of heatwave days have significantly increased from 1950–2011 ( [[#Perkins--2015|Perkins, 2015]] ). There are some regional differences in trends in characteristics of heatwaves, with significant increases reported in Europe ( [[#Russo--2015|Russo et al., 2015]] ; [[#Forzieri--2016|Forzieri et al., 2016]] ; [[#Sánchez-Benítez--2020|Sánchez-Benítez et al., 2020]] ) and Australia (CSIRO and BOM, 2016; [[#Alexander--2017|Alexander and Arblaster, 2017]] ). In Africa, there is ''medium confidence'' that heatwaves, regardless of the definition, have been becoming more frequent, longer-lasting, and hotter over more than three decades ( [[#Fontaine--2013|Fontaine et al., 2013]] ; [[#Mouhamed--2013|Mouhamed et al., 2013]] ; [[#Ceccherini--2016|Ceccherini et al., 2016]] , 2017; [[#Forzieri--2016|Forzieri et al., 2016]] ; [[#Moron--2016|Moron et al., 2016]] ; [[#Russo--2016|Russo et al., 2016]] ). The majority of heatwave characteristics examined in China between 1961 and 2014 show increases in heatwave days, consistent with warming ( [[#You--2017|You et al., 2017]] ; [[#Xie--2020|Xie et al., 2020]] ). Increases in the frequency and duration of heatwaves are also observed in Mongolia ( [[#Erdenebat--2016|Erdenebat and Sato, 2016]] ) and India ( [[#Ratnam--2016|Ratnam et al., 2016]] ; [[#Rohini--2016|Rohini et al., 2016]] ). In the UK, the lengths of short heatwaves have increased since the 1970s, while the lengths of long heatwaves (more than 10 days) have decreased over some stations in the south-east of England (M. [[#Sanderson--2017|]] [[#Sanderson--2017|Sanderson et al., 2017]] ). In Central and South America, there are increases in the frequency of heatwaves ( [[#Barros--2015|Barros et al., 2015]] ; [[#Bitencourt--2016|Bitencourt et al., 2016]] ; [[#Ceccherini--2016|Ceccherini et al., 2016]] ; [[#Piticar--2018|Piticar, 2018]] ), although decreases in Excess Heat Factor (EHF), which is a metric for heatwave intensity, are observed in South America in data derived from HadGHCND ( [[#Cavanaugh--2015|Cavanaugh and Shen, 2015]] ).

In summary, it is ''virtually certain'' that there has been an increase in the number of warm days and nights and a decrease in the number of cold days and nights on the global scale since 1950. Both the coldest extremes and hottest extremes display increasing temperatures. It is ''very likely'' that these changes have also occurred at the regional scale in Europe, Australasia, Asia, and North America. It is ''virtually certain'' that there has been increases in the intensity and duration of heatwaves and in the number of heatwave days at the global scale. These trends ''likely'' occur in Europe, Asia, and Australia. There is ''medium confidence'' in similar changes in temperature extremes in Africa and ''high confidence'' in South America; the lower confidence is due to reduced data availability and fewer studies. Annual minimum temperatures on land have increased about three times more than global surface temperature since the 1960s, with particularly strong warming in the Arctic ( ''hi'' ''gh confidence'' ).

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