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

=== 3.3.3 Regional Precipitation, Including Heavy Precipitation and Monsoons ===

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This section addresses regional changes in precipitation on land, with a focus on heavy precipitation and consideration of changes to the key features of monsoons.

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==== 3.3.3.1 Observed and attributed changes in regional precipitation ====

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Observed global changes in the water cycle, including precipitation, are more uncertain than observed changes in temperature (Hartmann et al., 2013; Stocker et al., 2013) <sup>[[#fn:r95|95]]</sup> . There is ''high confidence'' that mean precipitation over the mid-latitude land areas of the Northern Hemisphere has increased since 1951 (Hartmann et al., 2013) <sup>[[#fn:r96|96]]</sup> . For other latitudinal zones, area-averaged long-term positive or negative trends have ''low confidence'' because of poor data quality, incomplete data or disagreement amongst available estimates (Hartmann et al., 2013) <sup>[[#fn:r97|97]]</sup> . There is, in particular, ''low confidence'' regarding observed trends in precipitation in monsoon regions, according to the SREX report (Seneviratne et al., 2012) <sup>[[#fn:r98|98]]</sup> and AR5 (Hartmann et al., 2013) <sup>[[#fn:r99|99]]</sup> , as well as more recent publications (Singh et al., 2014; Taylor et al., 2017; Bichet and Diedhiou, 2018; <sup>[[#fn:r100|100]]</sup> see Supplementary Material 3.SM.2).

For heavy precipitation, AR5 (Hartmann et al., 2013) <sup>[[#fn:r101|101]]</sup> assessed that observed trends displayed more areas with increases than decreases in the frequency, intensity and/or amount of heavy precipitation ( ''likely'' ). In addition, for land regions where observational coverage is sufficient for evaluation, it was assessed that there is ''medium confidence'' that anthropogenic forcing has contributed to a global-scale intensification of heavy precipitation over the second half of the 20th century (Bindoff et al., 2013a) <sup>[[#fn:r102|102]]</sup> .

Regarding changes in precipitation associated with global warming of 0.5°C, the observed record suggests that increases in precipitation extremes can be identified for annual maximum 1-day precipitation (RX1day) and consecutive 5-day precipitation (RX5day) for GMST changes of this magnitude (Supplementary Material 3.SM.2, Figure 3.SM.7; Schleussner et al., 2017) <sup>[[#fn:r103|103]]</sup> . 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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==== 3.3.3.2 Projected changes in regional precipitation at 1.5°C versus 2°C of global warming ====

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Figure 3.3 in Section 3.3.1 summarizes the projected changes in mean precipitation at 1.5°C and 2°C of global warming. Both warming levels display robust differences in mean precipitation compared to the pre-industrial period. Regarding differences at 2°C vs 1.5°C global warming, some regions are projected to display changes in mean precipitation at 2°C compared with that at 1.5°C of global warming in the CMIP5 multimodel average, such as decreases in the Mediterranean area, including southern Europe, the Arabian Peninsula and Egypt, or increases in high latitudes. The results, however, are less robust across models than for mean temperature. For instance, Déqué et al. (2017) <sup>[[#fn:r104|104]]</sup> investigated the impact of 2°C of global warming on precipitation over tropical Africa and found that average precipitation does not show a significant response, owing to two phenomena: (i) the number of days with rain decreases whereas the precipitation intensity increases, and (ii) the rainy season occurs later during the year, with less precipitation in early summer and more precipitation in late summer. The results from Déqué et al. (2017) <sup>[[#fn:r105|105]]</sup> regarding insignificant differences between 1.5°C and 2°C scenarios for tropical Africa are consistent with the results presented in Figure 3.3. For Europe, recent studies (Vautard et al., 2014; Jacob et al., 2018; Kjellström et al., 2018) <sup>[[#fn:r106|106]]</sup> have shown that 2°C of global warming was associated with a robust increase in mean precipitation over central and northern Europe in winter but only over northern Europe in summer, and with decreases in mean precipitation in central/southern Europe in summer. Precipitation changes reaching 20% have been projected for the 2°C scenario (Vautard et al., 2014) <sup>[[#fn:r107|107]]</sup> and are overall more pronounced than with 1.5°C of global warming (Jacob et al., 2018; Kjellström et al., 2018) <sup>[[#fn:r108|108]]</sup> .

Regarding changes in heavy precipitation, Figure 3.9 displays projected changes in the 5-day maximum precipitation (Rx5day) as a function of global temperature increase, using a similar approach as in Figure 3.5. Further analyses are available in Supplementary Material 3.SM.2. These analyses show that projected changes in heavy precipitation are more uncertain than those for temperature extremes. However, the mean response of model simulations is generally robust and linear (see also Fischer et al., 2014; Seneviratne et al., 2016) <sup>[[#fn:r109|109]]</sup> . As observed for temperature extremes, this response is also mostly independent of the considered emissions scenario (e.g., RCP2.6 versus RCP8.5; see also Section 3.2). This feature appears to be specific to heavy precipitation, possibly due to a stronger coupling with temperature, as the scaling of projections of mean precipitation changes with global warming shows some scenario dependency (Pendergrass et al., 2015) <sup>[[#fn:r110|110]]</sup> .

Robust changes in heavy precipitation compared to pre-industrial conditions are found at both 1.5°C and 2°C global warming (Figure 3.4). This is also consistent with results for, for example, the European continent, although different indices for heavy precipitation changes have been analysed. Based on regional climate simulations, Vautard et al. (2014) <sup>[[#fn:r111|111]]</sup> found a robust increase in heavy precipitation everywhere in Europe and in all seasons, except southern Europe in summer at 2°C versus 1971–2000. Their findings are consistent with those of Jacob et al. (2014) <sup>[[#fn:r112|112]]</sup> , who used more recent downscaled climate scenarios (EURO-CORDEX) and a higher resolution (12 km), but the change is not so pronounced in Teichmann et al. (2018) <sup>[[#fn:r113|113]]</sup> . There is consistent agreement in the direction of change in heavy precipitation at 1.5°C of global warming over much of Europe, compared to 1971–2000 (Jacob et al., 2018) <sup>[[#fn:r114|114]]</sup> .

Differences in heavy precipitation are generally projected to be small between 1.5°C and 2°C GMST warming (Figure 3.4 and 3.9 and Supplementary Material 3.SM.2, Figure 3.SM.10). Some regions display substantial increases, for instance southern Asia, but generally in less than two-thirds of the CMIP5 models (Figure 3.4, Supplementary Material 3.SM.2, Figure 3.SM.10). Wartenburger et al. (2017) <sup>[[#fn:r115|115]]</sup> suggested that there are substantial differences in heavy precipitation in eastern Asia at 1.5°C versus 2°C. Overall, while there is variation among regions, the global tendency is for heavy precipitation to increase at 2°C compared with at 1.5°C (see e.g., Fischer and Knutti, 2015 <sup>[[#fn:r116|116]]</sup> and Kharin et al., 2018 <sup>[[#fn:r117|117]]</sup> , as illustrated in Figure 3.10 from this chapter; see also Betts et al., 2018) <sup>[[#fn:r118|118]]</sup> .

AR5 assessed that the global monsoon, aggregated over all monsoon systems, is ''likely'' to strengthen, with increases in its area and intensity, while the monsoon circulation weakens (Christensen et al., 2013) <sup>[[#fn:r119|119]]</sup> . A few publications provide more recent evaluations of projections of changes in monsoons for high-emission scenarios (e.g., Jiang and Tian, 2013; Jones and Carvalho, 2013; Sylla et al., 2015, 2016 <sup>[[#fn:r120|120]]</sup> ; Supplementary Material 3.SM.2 ). However, scenarios at 1.5°C or 2°C global warming would involve a substantially smaller radiative forcing than those assessed in AR5 and these more recent studies, and there appears to be no specific assessment of changes in monsoon precipitation at 1.5°C versus 2°C of global warming in the literature. Consequently, the current assessment is that there is ''low confidence'' regarding changes in monsoons at these lower global warming levels, as well as regarding differences in monsoon responses at 1.5°C versus 2°C.

Similar to Figure 3.8, Figure 3.11 features an objective identification of ‘hotspots’ / key risks outlined in heavy precipitation indices subdivided by region, based on the approach by Wartenburger et al. (2017) <sup>[[#fn:r121|121]]</sup> . The considered regions follow the classification used in Figure 3.2 and also include global land areas. Hotspots displaying statistically significant changes in heavy precipitation at 1.5°C versus 2°C global warming are located in high-latitude (Alaska/western Canada, eastern Canada/Greenland/Iceland, northern Europe, northern Asia) and high-elevation (e.g., Tibetan Plateau) regions, as well as in eastern Asia (including China and Japan) and in eastern North America. Results are less consistent for other regions. Note that analyses for meteorological drought (lack of precipitation) are provided in Section 3.3.4.

In summary, observations and projections for mean and heavy precipitation are less robust than for temperature means and extremes ( ''high confidence'' ). Observations show that there are more areas with increases than decreases in the frequency, intensity and/or amount of heavy precipitation ''(high confidence'' ). Several large regions display statistically significant differences in heavy precipitation at 1.5°C versus 2°C GMST warming, with stronger increases at 2°C global warming, and there is a global tendency towards increases in heavy precipitation on land at 2°C compared with 1.5°C warming ( ''high confidence'' ). Overall, regions that display statistically significant changes in heavy precipitation between 1.5°C and 2°C of global warming are located in high latitudes (Alaska/western Canada, eastern Canada/Greenland/Iceland, northern Europe, northern Asia) and high elevation (e.g., Tibetan Plateau), as well as in eastern Asia (including China and Japan) and in eastern North America ( ''medium confidence'' ). There is ''low confidence'' in projected changes in heavy precipitation in other regions.

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'''Figure 3.9'''

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'''Projected changes in annual 5-day maximum precipitation (Rx5day) as a function of global warming for IPCC Special Report on the Risk of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) regions (see Figure 3.2), based on an empirical scaling relationship applied to Coupled Model Intercomparison Project Phase 5 (CMIP5) data together with projected changes from the HAPPI multimodel experiment (bar plots on regional analyses and central plot).'''
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The underlying methodology and data basis are the same as for Figure 3.5 (see Supplementary Material 3.SM.2 for more details).

Original Creation for this Report using CMIP5 multi-model ensemble output, HAPPI Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI) model intercomparison project

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'''Figure 3.10'''

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'''Probability ratio (PR) of exceeding (heavy precipitation) thresholds.'''
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[[File:01d5a0bd8735a97d388ee703a9c79a2f Figure_3.10-1024x576.jpg]]

(a) PR of exceeding the 99th (blue) and 99.9th (red) percentile of pre-industrial daily precipitation at a given warming level, averaged across land (from Fischer and Knutti, 2015) <sup>[[#fn:r122|122]]</sup> . (b) PR for precipitation extremes (RX1day) for different event probabilities (with RV indicating return values) in the current climate (1°C of global warming). Shading shows the interquartile (25–75%) range (from Kharin et al., 2018) <sup>[[#fn:r123|123]]</sup> .

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'''Figure 3.11'''

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'''Significance of differences in regional mean precipitation and range of precipitation indices between the 1.5°C and 2°C global mean temperature targets (rows).'''
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[[File:288b54c4bcd6a693ddf27e40e1992eb4 Figure_3.11-1024x452.png]]

Definition of indices: PRCPTOT: mean precipitation; CWD: consecutive wet days; R10mm: number of days with precipitation &gt;10 mm; R1mm: number of days with precipitation &gt;1 mm; R20mm: number of days with precipitation &gt;20 mm; R95ptot: proportion of rain falling as 95th percentile or higher; R99ptot: proportion of rain falling as 99th percentile or higher; RX1day: intensity of maximum yearly 1-day precipitation; RX5day: intensity of maximum yearly 5-day precipitation; SDII: Simple Daily Intensity Index. Columns indicate analysed regions and global land (see Figure 3.2 for definitions). Significant differences are shown in light blue (wetting tendency) or brown (drying tendency) shading, with increases indicated with ‘+’ and decreases indicated with ‘–’, while non-significant differences are shown in grey shading. The underlying methodology and the data basis are the same as in Figure 3.8 (see Supplementary Material 3.SM.2 for more details).

Original Creation for this Report using CMIP5 multi-model ensemble output data.

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