Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.3.2.1 Observed and attributed changes in regional temperature means and extremes ====

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While the quality of temperature measurements obtained through ground observational networks tends to be high compared to that of measurements for other climate variables (Seneviratne et al., 2012) <sup>[[#fn:r56|56]]</sup> , it should be noted that some regions are undersampled. Cowtan and Way (2014) <sup>[[#fn:r57|57]]</sup> highlighted issues regarding undersampling, which is most problematic at the poles and over Africa, and which may lead to biases in estimated changes in GMST (see also Supplementary Material 3.SM.2 and Chapter 1). This undersampling also affects the confidence of assessments regarding regional observed and projected changes in both mean and extreme temperature. Despite this partly limited coverage, the attribution chapter of AR5 (Bindoff et al., 2013a) <sup>[[#fn:r58|58]]</sup> and recent papers (e.g., Sun et al., 2016; Wan et al., 2018) <sup>[[#fn:r59|59]]</sup> assessed that, over every continental region and in many sub-continental regions, anthropogenic influence has made a substantial contribution to surface temperature increases since the mid-20th century.

Based on the AR5 and SREX, as well as recent literature (see Supplementary Material 3.SM), there is ''high confidence'' ( ''very likely'' ) that there has been an overall decrease in the number of cold days and nights and an overall increase in the number of warm days and nights at the global scale on land. There is also ''high confidence'' ( ''likely'' ) that consistent changes are detectable on the continental scale in North America, Europe and Australia. There is ''high confidence'' that these observed changes in temperature extremes can be attributed to anthropogenic forcing (Bindoff et al., 2013a) <sup>[[#fn:r60|60]]</sup> . As highlighted in Section 3.2, the observational record can be used to assess past changes associated with a global warming of 0.5°C. Schleussner et al. (2017) <sup>[[#fn:r61|61]]</sup> used this approach to assess observed changes in extreme indices for the 1991–2010 versus the 1960–1979 period, which corresponds to just about a 0.5°C GMST difference in the observed record (based on the Goddard Institute for Space Studies Surface Temperature Analysis (GISTEMP) dataset, Hansen et al., 2010) <sup>[[#fn:r62|62]]</sup> . They found that substantial changes due to 0.5°C of warming are apparent for indices related to hot and cold extremes, as well as for the Warm Spell Duration Indicator (WSDI). In particular, they identified that one-quarter of the land has experienced an intensification of hot extremes (maximum temperature on the hottest day of the year, TXx) by more than 1°C and a reduction in the intensity of cold extremes by at least 2.5°C (minimum temperature on the coldest night of the year, TNn). In addition, the same study showed that half of the global land mass has experienced changes in WSDI of more than six days, as well as an emergence of extremes outside the range of natural variability (Schleussner et al., 2017) <sup>[[#fn:r63|63]]</sup> . Analyses from Schleussner et al. (2017) <sup>[[#fn:r64|64]]</sup> for temperature extremes are provided in the Supplementary Material 3.SM, Figure 3.SM.6. It should be noted that assessments of attributed changes in the IPCC SREX and AR5 reports were generally provided since 1950, for time frames also approximately corresponding to a 0.5°C global warming (3.SM).

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