Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.4.4 Projected Changes in Floods ===

<div id="h2-22-siblings" class="h2-siblings"></div>

SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) concluded with ''medium confidence'' that global warming of 2°C would lead to an expansion of the area affected by flood hazards, compared to conditions at 1.5°C global warming. Both AR5 ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ) and SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) concluded that spring snowmelt floods would be earlier ( ''high confidence'' ), and hazards from floods involving meltwater will gradually diminish, particularly at low elevation ( ''medium confidence'' ). SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) and AR6 WGI [[IPCC:Wg2:Chapter:Chapter-9|Chapter 9]] stated that given ''limited evidence'' and the complexity of the process, the changes of glacier-related floods under climate change are not clear. AR6 WGI Chapters 8 and 11 summarised that there is ''medium confidence'' for a general increase in flooding due to warming, but there are significant regional and seasonal variations.

There is ''high confidence'' that the frequency and magnitude of river floods are projected to change at a global scale. For example, the frequency of river floods is projected to increase in many regions, including Asia, central Africa, western Europe, Central and South America and eastern North America, and decrease in northern North America, southern South America, the Mediterranean and eastern Europe in 2050 and beyond ( [[#Koirala--2014|Koirala et al., 2014]] ; [[#Arnell--2016|Arnell et al., 2016]] ) (Figure 4.17). There is ''low agreement'' in projections in changes to snowmelt flood magnitude. A negative trend in snowmelt flood magnitude, together with an increase in rain-fed winter floods, is projected with ''medium confidence'' , for example, in mid-latitude and low-altitude basins of Scandinavia ( [[#Arheimer--2015|Arheimer and Lindström, 2015]] ; [[#Vormoor--2016|Vormoor et al., 2016]] ) and throughout Europe as a whole ( [[#Kundzewicz--2017|Kundzewicz et al., 2017]] ), and northeastern North America ( [[#Arnell--2014|Arnell and Lloyd-Hughes, 2014]] ). With ''medium confidence'' , a positive trend is projected in high-latitude basins, for example, for large Arctic rivers such as Lena and Mackenzie ( [[#Eisner--2017|Eisner et al., 2017]] ; [[#Gelfan--2017|Gelfan et al., 2017]] ; [[#Pechlivanidis--2017|Pechlivanidis et al., 2017]] ) and high-altitude upstreams, such as the Ganges, Brahmaputra, Salween, Mekong and the upper Indus Basin ( [[#Lutz--2014|Lutz et al., 2014]] ) and alpine catchments ( [[#Hall--2014|Hall et al., 2014]] ). Moderate decreasing trends or insignificant changes are projected for snowmelt floods in the Fraser River Basin of British Columbia ( [[#Shrestha--2017|Shrestha et al., 2017]] ).

<div id="_idContainer065" class="Figure"></div>

[[File:22f37989981eaf6f07d34c57a2537fe2 IPCC_AR6_WGII_Figure_4_017.png]]

'''Figure 4.17 |''' '''Multi-model median return period (years) in the 2080s for the 20th-century 100-year river flood, based on a global river and inundation model, CaMa-Flood, driven by runoff output of nine CMIP6 Models in the SSP1-2''' '''.''' '''6 (a), SSP2-4.5 (b) and SSP5-8.5 (c) scenario respectively.''' All changes are estimated in 2071–2100 relative to 1970-–2000. A dot indicates regions with high model consistency (more than seven models out of nine show the same direction of change).

'''(d)''' Global or regional potential exposure (% to the total population affected by flooding) under different global warming levels with a constant population scenario and climate of CMIP5-HELIX (circle, [[#Alfieri--2017|Alfieri et al., 2017]] ) and CMIP6 (triangle, [[#Hirabayashi--2021b|Hirabayashi et al., 2021b]] ), and with the population scenario of SSP5 and climate of CMIP6 (bar chart, [[#Hirabayashi--2021b|Hirabayashi et al., 2021b]] ). Inundation is calculated when the magnitude of flood exceeds current flood protection ( [[#Scussolini--2016|Scussolini et al., 2016]] ). Note that number of GCMs used to calculate global warming level (GWL) 4.0 is less than that for other GWLs, as the global mean temperature change of some GCMs did not exceed 4°C.

There is ''high confidence'' that climate change and projected socioeconomic development would increase exposure in inundation areas (Figure 4.17), resulting in a large increase in direct flood damages as several times more in all warming levels (Table 4.6). [[#Alfieri--2017|Alfieri et al. (2017)]] estimated a 120 and 400% increase in population affected by river flooding for 2°C and 4°C warming, respectively, and a 170% increase in damage for 2°C warming without socioeconomic impact development ( [[#4.7.5|Section 4.7.5]] ). [[#Dottori--2018|Dottori et al. (2018)]] estimated the same but with a 134% increase in fatalities with population increase under the SSP3 scenario. The highest numbers of people affected by river flooding are projected for countries in southern, eastern and southeastern Asia, with tens of millions of people per year per country projected to be affected (Figure 4.17; [[#Alfieri--2017|Alfieri et al., 2017]] ; [[#Hirabayashi--2021b|Hirabayashi et al., 2021b]] ). [[#Kinoshita--2018|Kinoshita et al. (2018)]] showed that climate change contributes a 2.8–28.8% increase in global fatality for the period 2071–2100 compared to 1991–2005, but socioeconomic change (~131.3% increase) and associated vulnerability change (~72.1% reduction) have a greater impact of the projected flood-related fatality rate than climate change alone. [[#Winsemius--2016|Winsemius et al. (2016)]] discussed that projected flood damage could be reduced to 1/20th in absolute value with adequate adaptation strategies. Direct flood damages are projected to increase by 4–5 times at 4°C compared to 1.5°C, highly depending on scenarios and assumptions (Table 4.6; Box 4.7).

'''Table 4.6 |''' Projected economic impact by river flooding in billion USD in different emission scenarios or for different global warming levels (GWLs). The percentage of the total GDP of the region is given in brackets.

{| class="wikitable"
|-
! Description

! The economic impact in billion USD (% of GDP)

! Reference

|-
| No adaptation with current flood protection, no economic development (fixed at the level of 2010), USD at 2010 purchasing power parity (PPP), mean of 7 GCMs with the RCP8.5 scenario

| * Current (1976–2005): 75 (0.11%)
* GWL 1.5°C: 145 (0.22%)
* (Asia 92, Australasia 8, Europe 29, Africa 7, North America 3, Central and South America 5)
* GWL 2°C: 172 (0.26%)
* (Asia 114, Australasia 7, Europe 32, Africa 9, North America 4, Central and South America 7)
* GWL 3°C: 249 (0.37%)
* (Asia 176, Australasia 9, Europe 38, Africa 11, North America 4, Central and South America 11)
* GWL 4°C: 343 (0.51%)
* (Asia 241, Australasia 19, Europe 55, Africa 9, North America 6, Central and South America 14)

| [[#Alfieri--2017|Alfieri et al. (2017)]] , with regional aggregation and currency conversion

|-
| No adaptation with current flood protection, USD at 2010 PPP, mean of five CMIP5 GCMs and 10 hydrological models

| * Current (1976–2005): 142 (0.21%)
* GWL 1.5°C, SSP3: 370 (0.55%), SSP5: 485 (0.72%)
* GWL 2°C, SSP3: 597 (0.89%), SSP5: 888 (1.32%)
* GWL 3°C, SSP3: 1024 (1.52%), SSP5: 1616 (2.40%)

| [[#Dottori--2018|Dottori et al. (2018)]] with currency conversion

|-
| No adaptation and no flood protection, mean value in 2030 (2010–2030) and 2080 (2010–2080), USD at 2010 PPP, mean of five CMIP5 GCMs

| * Current (1960–1999): 1,032 (1.6%)
* RCP2.6, SSP1: 2030: 2366 (1.44%), 2080: 7429 (1.43%)
* RCP6.0, SSP3: 2030: 1987 (1.44%), 2080: 3353(1.14%)
* RCP8.5, SSP5: 2030: 2304 (1.37%), 2080: 3684(1.77%)

| [[#Winsemius--2016|Winsemius et al. (2016)]]

|-
| Partial adaptation (protected against 100-year floods in high-income countries, against 5-year floods for all others), mean value in 2030 (2010–2030) and 2080 (2010–2080), USD at 2010 PPP, mean of five CMIP5 GCMs

| * Current (1960–1999): 163 (0.25%)
* RCP2.6, SSP1: 2030: 558 (0.34%), 2080: 851 (0.48%)
* RCP6.0, SSP3: 2030: 418 (0.29%), 2080: 413(0.32%)
* RCP8.5, SSP5: 2030: 418 (0.33%), 2080: 441 (0.57%)

| [[#Winsemius--2016|Winsemius et al. (2016)]]

|-
| A model calibrated to fit reported damages, future vulnerability scenarios considering autonomous adaptation, USD at 2005 PPP, mean of 11 CMIP5 GCMs,

| * Current (1991–2005): 14 (0.044%)
* RCP2.6, SSP1: 2081–2100, 121 (0.037%)
* RCP6.0, SSP2: 2081–2100, 133 (0.042%)
* RCP8.5, SSP3: 2081–2100, 130 (0.063%)

| [[#Kinoshita--2018|Kinoshita et al. (2018)]]

|-
| No adaptation and current flood protection, USD at 2005 PPP, mean of five CMIP5 GCMs

| * Current (1961–2005): 102 (0.39%)
* RCP2.6, SSP1: 2020–2100, 2333 (0.99%)
* RCP4.5, SSP2: 2020–2100, 2221 (0.99%)
* RCP6.0, SSP3: 2020–2100, 1328 (0.80%)
* RCP8.5, SSP5: 2020–2100, 4007 (1.21%)

| [[#Tanoue--2021|Tanoue et al. (2021)]]

|-
| Optimised adaptation, USD at 2005 PPP, mean of five CMIP5 GCMs

| * Current (1961–2005): 102 (0.39%)
* RCP2.6, SSP1: 2020–2100, 1621 (0.69%)
* RCP4.5, SSP2: 2020–2100, 1567 (0.70%)
* RCP6.0, SSP3: 2020–2100, 872 (0.52%)
* RCP8.5, SSP5: 2020–2100, 2558 (0.77%)

| [[#Tanoue--2021|Tanoue et al. (2021)]]

|}

In all climate scenarios projected, earlier snowmelt leads to earlier spring floods ( ''high confidence'' ), for example, in northern and eastern Europe ( [[#Gobiet--2014|Gobiet et al., 2014]] ; [[#Hall--2014|Hall et al., 2014]] ; [[#Etter--2017|Etter et al., 2017]] ; [[#Lobanova--2018|Lobanova et al., 2018]] ), northern North America ( [[#Vano--2015|Vano et al., 2015]] ; [[#Musselman--2018|Musselman et al., 2018]] ; [[#Islam--2019b|Islam et al., 2019b]] ), large Arctic rivers ( [[#Gelfan--2017|Gelfan et al., 2017]] ; [[#Pechlivanidis--2017|Pechlivanidis et al., 2017]] ) and high-altitude Asian basins ( [[#Lutz--2014|Lutz et al., 2014]] ; [[#Winsemius--2016|Winsemius et al., 2016]] ). There is ''high confidence'' that snowmelt floods will occur 25–30 d earlier in the year by the end of the 21st century with RCP8.5, but there is only ''low agreement'' in the projected magnitude of snowmelt flood ( [[#Arheimer--2015|Arheimer and Lindström, 2015]] ; [[#Vormoor--2016|Vormoor et al., 2016]] ; [[#Islam--2019b|Islam et al., 2019b]] ).

Challenges to projecting flood risk are large because of the complexity of the projecting snowmelt, high-intensity rainfall and soil wetness in large river basins. Even though increases in the number and area of glacier lakes may cause increases in glacier-related floods ( [[#4.2.2|Section 4.2.2]] ), knowledge of the frequency or magnitude of glacier-related projected floods is limited. Some local studies indicate that the severity of ice-jam flooding is projected to decrease ( [[#Rokaya--2019|Rokaya et al., 2019]] ; [[#Das--2020|Das et al., 2020]] ), but a model study in Canada projected increases in damage of ice-jam floods ( [[#Turcotte--2020|Turcotte et al., 2020]] ). While most flood risk projections do not consider the impact of urban expansion, Güneralp et al. (2015) estimate that urban areas exposed to flooding will increase by a factor of 2.7 due to urban growth by 2030 ( [[#4.5.4|Section 4.5.4]] ). Given the significant differences in assumption in flood protection, exposure or vulnerability scenario among studies, uncertainties in the global estimation of flood losses and damages are large are large (Table 4.6, 4.7.5).

Floods and their societal impacts, especially the enhancement of hazards and increase in vulnerability, depend on complex political, economic and cultural processes ( [[#Carey--2017|Carey et al., 2017]] ; [[#Caretta--2021|Caretta et al., 2021]] ). Thus, assessments that analyse long-term flood impacts need to account for the interplay of water and society relations. Unfortunately, such studies remain scarce ( [[#Pande--2017|Pande and Sivapalan, 2017]] ; [[#Ferdous--2018|Ferdous et al., 2018]] ; [[#Caretta--2021|Caretta et al., 2021]] ). In particular, projected socioeconomic, cultural and political impacts on the vulnerable group are understudied, as is their resourcefulness through LK, adaptive capacity and community-led adaptation (Sections 4.6.9; 4.8.4; Cross-Chapter Box INDIG in Chapter 18).

In summary, there is ''high confidence'' that the magnitude and frequency of floods are projected to increase in many regions, including Asia, central Africa, western Europe, Central and South America and eastern North America, and decrease in northern North America, southern South America, the Mediterranean and eastern Europe. Projected increases in flooding pose increasing risks, with a 1.2–1.8 and 4–5 times increase in global GDP loss at 2°C and 4°C compared to 1.5°C warming, respectively ( ''medium confidence'' ). Without adaptation, projected increases in flooding are 1.4 to 2.5 and 2.5 to 3.9 times in global GDP loss at 2°C and 3°C compared to 1.5°C warming, respectively ( ''medium confidence'' ). However, regional differences in risks are large because of the strong influence of socioeconomic conditions and significant uncertainty in flood hazard projection. In small river basins and urban areas, there is ''medium confidence'' that projected increases in heavy rainfall would contribute to increases in rain-generated local flooding. However, the snowmelt floods are projected to decrease ( ''medium confidence'' ) and occur 25–30 d earlier in the year by the end of the 21st century with RCP8.5 ( ''high confidence'' ).

<div id="4.4.5" class="h2-container"></div>

<span id="projected-changes-in-droughts"></span>