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

==== 3.3.4.2 Projected changes in drought and dryness at 1.5°C versus 2°C ====

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There is ''medium confidence'' in projections of changes in drought and dryness. This is partly consistent with AR5, which assessed these projections as being ‘ ''likely'' ( ''medium confidence'' )’ (Collins et al., 2013; Stocker et al., 2013) <sup>[[#fn:r143|143]]</sup> . However, given this ''medium confidence'' , the current assessment does not include a likelihood statement, thereby maintaining consistency with the IPCC uncertainty guidance document (Mastrandrea et al., 2010) <sup>[[#fn:r144|144]]</sup> and the assessment of the IPCC SREX report (Seneviratne et al., 2012) <sup>[[#fn:r145|145]]</sup> . The technical summary of AR5 (Stocker et al., 2013) <sup>[[#fn:r146|146]]</sup> assessed that soil moisture drying in the Mediterranean, southwestern USA and southern African regions was consistent with projected changes in the Hadley circulation and increased surface temperatures, and it concluded that there was ''high confidence'' in ''likely'' surface drying in these regions by the end of this century under the RCP8.5 scenario. However, more recent assessments have highlighted uncertainties in dryness projections due to a range of factors, including variations between the drought and dryness indices considered, and the effects of enhanced CO <sub>2</sub> concentrations on plant water-use efficiency (Orlowsky and Seneviratne, 2013; Roderick et al., 2015) <sup>[[#fn:r147|147]]</sup> . Overall, projections of changes in drought and dryness for high-emissions scenarios (e.g., RCP8.5, corresponding to about 4°C of global warming) are uncertain in many regions, although a few regions display consistent drying in most assessments (e.g., Seneviratne et al., 2012; Orlowsky and Seneviratne, 2013) <sup>[[#fn:r148|148]]</sup> . Uncertainty is expected to be even larger for conditions with a smaller signal-to-noise ratio, such as for global warming levels of 1.5°C and 2°C.

Some published literature is now available on the evaluation of differences in drought and dryness occurrence at 1.5°C and 2°C of global warming for (i) precipitation minus evapotranspiration (P–E, a general measure of water availability; Wartenburger et al., 2017; Greve et al., 2018) <sup>[[#fn:r149|149]]</sup> , (ii) soil moisture anomalies (Lehner et al., 2017; Wartenburger et al., 2017), <sup>[[#fn:r150|150]]</sup> , (iii) consecutive dry days (CDD) (Schleussner et al., 2016b; Wartenburger et al., 2017) <sup>[[#fn:r151|151]]</sup> , (iv) the 12-month standardized precipitation index (Wartenburger et al. 2017) <sup>[[#fn:r152|152]]</sup> , (v) the Palmer drought severity index (Lehner et al., 2017) <sup>[[#fn:r153|153]]</sup> , and (vi) annual mean runoff (Schleussner et al., 2016b <sup>[[#fn:r154|154]]</sup> , see also next section). These analyses have produced consistent findings overall, despite the known sensitivity of drought assessments to chosen drought indices (see above paragraph). These analyses suggest that increases in drought, dryness or precipitation deficits are projected at 1.5°C or 2°C global warming in some regions compared to the pre-industrial or present-day conditions, as well as between these two global warming levels, although there is substantial variability in signals depending on the considered indices or climate models (Lehner et al., 2017; Schleussner et al., 2017; Greve et al., 2018) <sup>[[#fn:r155|155]]</sup> ( ''medium confidence'' ). Generally, the clearest signals are found for the Mediterranean region ( ''medium confidence'' ).

Greve et al. (2018 <sup>[[#fn:r156|156]]</sup> , Figure 3.12) derives the sensitivity of regional changes in precipitation minus evapotranspiration to global temperature changes. The simulations analysed span the full range of available emission scenarios, and the sensitivities are derived using a modified pattern scaling approach. The applied approach assumes linear dependencies on global temperature changes while thoroughly addressing associated uncertainties via resampling methods. Northern high-latitude regions display robust responses tending towards increased wetness, while subtropical regions display a tendency towards drying but with a large range of responses. While the internal variability and the scenario choice play an important role in the overall spread of the simulations, the uncertainty stemming from the climate model choice usually dominates, accounting for about half of the total uncertainty in most regions (Wartenburger et al., 2017; Greve et al., 2018) <sup>[[#fn:r157|157]]</sup> . The sign of projections, that is, whether there might be increases or decreases in water availability under higher global warming levels, is particularly uncertain in tropical and mid-latitude regions. An assessment of the implications of limiting the global mean temperature increase to values below (i) 1.5°C or (ii) 2°C shows that constraining global warming to the 1.5°C target might slightly influence the mean response but could substantially reduce the risk of experiencing extreme changes in regional water availability (Greve et al., 2018) <sup>[[#fn:r158|158]]</sup> .

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<span id="figure-3.12"></span>
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'''Figure 3.12'''

<span id="summary-of-the-likelihood-of-increasesdecreases-in-precipitation-minus-evapotranspiration-pe-in-coupled-model-intercomparison-project-phase-5-cmip5-simulations-considering-all-scenarios-and-a-representative-subset-of-14-climate-models-one-from-each-modelling-centre."></span>
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'''Summary of the likelihood of increases/decreases in precipitation minus evapotranspiration (P–E) in Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations considering all scenarios and a representative subset of 14 climate models (one from each modelling centre).'''
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[[File:6dfb5aab2d005c0a10a2edc3c9e6ce6b Figure_3.12-1024x791.jpg]]

Panel plots show the uncertainty distribution of the sensitivity of P–E to global temperature change, averaged for most IPCC Special Report on Managing the Risk of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) regions (see Figure 3.2) outlined in the map (from Greve et al., 2018) <sup>[[#fn:r159|159]]</sup> .

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The findings from the analysis for the mean response by Greve et al. (2018) <sup>[[#fn:r160|160]]</sup> are qualitatively consistent with results from Wartenburger et al. (2017) <sup>[[#fn:r161|161]]</sup> , who used an ESR (Section 3.2) rather than a pattern scaling approach for a range of drought and dryness indices. They are also consistent with a study by Lehner et al. (2017) <sup>[[#fn:r162|162]]</sup> , who assessed changes in droughts based on soil moisture changes and the Palmer-Drought Severity Index. Notably, these two publications do not provide a specific assessment of changes in the tails of the drought and dryness distribution. The conclusions of Lehner et al. (2017) <sup>[[#fn:r163|163]]</sup> are that (i) ‘risks of consecutive drought years show little change in the US Southwest and Central Plains, but robust increases in Europe and the Mediterranean’, and that (ii) ‘limiting warming to 1.5°C may have benefits for future drought risk, but such benefits are regional, and in some cases highly uncertain’.

Figure 3.13 features projected changes in CDD as a function of global temperature increase, using a similar approach as for Figures 3.5 (based on Wartenburger et al., 2017) <sup>[[#fn:r164|164]]</sup> . The figure also include results from the HAPPI experiment (Mitchell et al., 2017) <sup>[[#fn:r165|165]]</sup> . Again, the CMIP5-based ESR estimates and the results of the HAPPI experiment agree well. Note that the responses vary widely among the considered regions.

Similar to Figures 3.8 and 3.11, Figure 3.14 features an objective identification of ‘hotspots’ / key risks in dryness indices subdivided by region, based on the approach by Wartenburger et al. (2017) <sup>[[#fn:r166|166]]</sup> . This analysis reveals the following hotspots of drying (i.e. increases in CDD and/or decreases in P–E, soil moisture anomalies (SMA) and 12-month Standardized Precipitation Index (SPI12), with at least one of the indices displaying statistically significant drying): the Mediterranean region (MED; including southern Europe, northern Africa, and the Near East), northeastern Brazil (NEB) and southern Africa.

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<span id="figure-3.13"></span>
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'''Figure 3.13'''

<span id="projected-changes-in-consecutive-dry-days-cdd-as-a-function-of-global-warming-for-ipcc-special-report-on-managing-the-risk-of-extreme-events-and-disasters-to-advance-climate-change-adaptation-srex-regions-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-respectively."></span>
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'''Projected changes in consecutive dry days (CDD) as a function of global warming for IPCC Special Report on Managing the Risk of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) regions, 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, respectively).'''
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[[File:1860c37e6a4c700da6ed5b38bdc25b84 Figure_3.13-1024x717.jpg]]

The underlying methodology and the 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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<span id="figure-3.14"></span>
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'''Figure 3.14'''

<span id="significance-of-differences-in-regional-drought-and-dryness-indices-between-the-1.5c-and-2c-global-mean-temperature-targets-rows."></span>
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'''Significance of differences in regional drought and dryness indices between the 1.5°C and 2°C global mean temperature targets (rows).'''
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[[File:29300bb8cc468e0fb508399d406dde79 FGD_Fig3p14-1024x154.jpg]]

Definition of indices: CDD: consecutive dry days; P–E: precipitation minus evapotranspiration; SMA: soil moisture anomalies; SPI12: 12-month Standarized Precipitation Index. Columns indicate analysed regions and global land (see Figure 3.2 for definitions). Significant differences are shown in light blue/brown shading (increases indicated with +, decreases indicated with –; light blue shading indicates decreases in dryness (decreases in CDD, or increases in P–E, SMA or SPI12) and light brown shading indicates increases in dryness (increases in CDD, or decreases in P–E, SMA or SPI12). Non—significant differences are shown in grey shading. The underlying methodology and the data basis are the same as for Figure 3.7 (see Supplementary Material 3.SM.2 for more details).

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Consistent with this analysis, the available literature particularly supports robust increases in dryness and decreases in water availability in southern Europe and the Mediterranean with a shift from 1.5°C to 2°C of global warming ( ''medium confidence'' ) (Figure 3.13; Schleussner et al., 2016b; Lehner et al., 2017; Wartenburger et al., 2017; Greve et al., 2018; Samaniego et al., 2018 <sup>[[#fn:r167|167]]</sup> ). This region is already displaying substantial drying in the observational record (Seneviratne et al., 2012; Sheffield et al., 2012; Greve et al., 2014; Gudmundsson and Seneviratne, 2016; Gudmundsson et al., 2017) <sup>[[#fn:r168|168]]</sup> , which provides additional evidence supporting this tendency and suggests that it will be a hotspot of dryness change at global warming levels beyond 1.5°C (see also Box 3.2). The other identified hotspots, southern Africa and northeastern Brazil, also consistently display drying trends under higher levels of forcing in other publications (e.g., Orlowsky and Seneviratne, 2013) <sup>[[#fn:r169|169]]</sup> , although no published studies could be found reporting observed drying trends in these regions. There are substantial increases in the risk of increased dryness ( ''medium confidence'' ) in both the Mediterranean region and Southern Africa at 2°C versus 1.5°C of global warming because these regions display significant changes in two dryness indicators (CDD and SMA) between these two global warming levels (Figure 3.14); the strongest effects are expected for extreme droughts ( ''medium confidence'' ) (Figure 3.12). There is ''low confidence'' elsewhere, owing to a lack of consistency in analyses with different models or different dryness indicators. However, in many regions there is ''medium confidence'' that most extreme risks of changes in dryness are avoided if global warming is constrained at 1.5°C instead of 2°C (Figure 3.12).

In summary, in terms of drought and dryness, limiting global warming to 1.5°C is expected to substantially reduce the probability of extreme changes in water availability in some regions compared to changes under 2°C of global warming ( ''medium confidence'' ). For shift from 1.5°C to 2°C of GMST warming, the available studies and analyses suggest strong increases in the probability of dryness and reduced water availability in the Mediterranean region (including southern Europe, northern Africa and the Near East) and in southern Africa ( ''medium confidence'' ). Based on observations and modelling experiments, a drying trend is already detectable in the Mediterranean region, that is, at global warming of less than 1°C ( ''medium confidence'' ).

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