Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.6.2 Wet and Dry ====

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'''Mean precipitation:''' Atlas.9.2 found that trends in annual precipitation over 1960–2015 are generally non-significant, though there are consistent positive trends over parts of ENA and CNA, together with significant decreases in precipitation in parts of the south-western USA and north-western Mexico.

Atlas.9.4 assessed ''very high confidence'' in increases in mean precipitation over most of northern and Eastern North America, with ''medium confidence'' of decrease over Northern Central America and ''low confidence'' elsewhere (see Figure Atlas.26, and Cross-Chapter Box Atlas.1, Figure 1). Changes are most dramatic in the spring and winter, when wet conditions are projected to extend from the northern portions of the continent as far south as the central Great Plains, while Mexico becomes drier; in contrast, summer changes are uncertain across most of the continent other than wetter conditions in northern Canada ( [[#Easterling--2017|Easterling et al., 2017]] ; [[#Bukovsky--2020|Bukovsky and Mearns, 2020]] ; [[#Almazroui--2021|Almazroui et al., 2021]] ; [[#Teichmann--2021|Teichmann et al., 2021]] ).

'''River flood:''' There is ''limited evidence'' and ''low agreement'' on observed climate change influences for river floods in North America ( [[IPCC:Wg1:Chapter:Chapter-11#11.5|Section 11.5]] ). Trends in streamflow indices are mixed and difficult to separate from river engineering influences, with large changes but little spatial coherence across the USA, making it difficult to identify trends with confidence ( [[#Peterson--2013|Peterson et al., 2013]] ; [[#Mallakpour--2015|Mallakpour and Villarini, 2015]] ; [[#Archfield--2016|Archfield et al., 2016]] ; [[#Wehner--2017|Wehner et al., 2017]] ; [[#Villarini--2018|Villarini and Slater, 2018]] ; [[#Hodgkins--2019|Hodgkins et al., 2019]] ; [[#Neri--2019|Neri et al., 2019]] ). There is ''high confidence'' in historical shifts in the timing of peak streamflow towards higher winter and earlier spring flows in snowmelt-driven basins in Canada ( [[#Burn--2016|Burn and Whitfield, 2016]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ) and the USA ( [[#Dudley--2017|Dudley et al., 2017]] ; [[#Wehner--2017|Wehner et al., 2017]] , [[#Neri--2020|Neri et al., 2020]] ). Some rivers show ice-jam floods occurring a week earlier, but changes are mixed, given localized positive and negative changes across the continent ( [[#Rokaya--2018|Rokaya et al., 2018]] ). There is ''medium confidence'' that climate change will increase river floods over the USA and Canada but ''low confidence'' for changes in Mexico. [[#Wobus--2017a|Wobus et al. (2017a)]] used a regional hydrologic model for 57,000 streams to project more than a doubling in the frequency of current 1-in-100-year flow events in many portions of the USA for RCP8.5 2050 with additional contributions from earlier snowmelt. CMIP6 projections for SSP5-8.5 2065–2099 show strongest peak USA runoff increases in the east ( [[#Villarini--2020|Villarini and Zhang, 2020]] ); however, several studies applying global hydrological models disagree with regional streamflow projections, indicating a decrease in the magnitude or frequency of floods over a large portion of North America (e.g., [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; see Figure 12.10a,c).

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[[File:2789e0f641f8b8383f22e976ab34b8be IPCC_AR6_WGI_Figure_12_10.png]]

'''Figure 12.10''' '''|''' '''Projected changes in selected climatic impact-driver indices for North America. (a)''' Mean change in 1-in-100-year river discharge per unit catchment area (Q100, m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ), and '''(b)''' median change in the number of days with snow water equivalent (SWE) over 100 mm (from November to March), from CORDEX-North America models for 2041–2060 relative to 1995–2014 and RCP8.5. Diagonal lines indicate where less than 80% of models agree on the sign (direction) of change. '''(c)''' Bar plots for Q100 (m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ) averaged over land areas for the AR6 WGI Reference Regions (defined in Chapter 1). The left-hand column within each panel (associated with the left-hand y-axis) shows the ‘recent past’ (1995–2014) Q100 absolute values in grey shades. The other columns (associated with the right-hand y-axis) show the Q100 changes relative to the recent past values for two time periods (‘mid’ 2041–2060 and ‘long’ 2081–2100) and for three global warming levels (defined relative to the pre-industrial period 1850–1900): 1.5°C (purple), 2°C (yellow) and 4°C (brown). The bars show the median (dots) and the 10–90th percentile range of model ensemble values across each model ensemble. CMIP6 is shown by the darkest colours, CMIP5 by medium, and CORDEX by light. SSP5-8.5/RCP8.5 is shown in red and SSP1-2.6/RCP2.6 in blue. '''(d)''' As for (c) but showing absolute values for number of days with SWE &gt; 100 mm, masked to grid cells with at least 14 such days in the recent past. See Technical ( [[IPCC:Wg1:Chapter:Annex-vi|Annex VI]] for details of indices. A Caribbean (CAR) Q100 bar plot is included here but assessed in the Small Islands section ( [[#12.4.7|Section 12.4.7]] ). Further details on data sources and processing are available in the chapter data table (Table 12.SM.1).

'''Heavy precipitation and pluvial flood:''' Section 11.4 assessed ''high confidence'' in observed increases in extreme precipitation events (including hourly totals) in Central and Eastern North America with ''low confidence'' in broad trends elsewhere in the continent despite observational increases in some portions of each region ( [[#Vincent--2018|L.A. Vincent et al., 2018]] ; [[#García-Cueto--2019|García-Cueto et al., 2019]] ; X. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ).

( [[IPCC:Wg1:Chapter:Chapter-11#11.4|Section 11.4]] found that high precipitation is projected to increase across North America ( ''high confidence'' ) except for portions of Western North America where projections are mixed ( ''medium confidence'' of increase). [[#Maxwell--2018|Maxwell et al. (2018)]] identified regional ‘heavy precipitation day’ thresholds for five cities across the USA and projected that a tripling (or more) of these events is possible by RCP8.5 mid-century. Projections indicate changes to intensity-duration-frequency (IDF) curves typically used for construction design and automobile hazards, as well as increases in the 10-year recurrence level of 24-hour rainfall intensities that challenge storm water drainage systems ( [[#Hambly--2013|Hambly et al., 2013]] ; [[#Cheng--2015|Cheng and AghaKouchak, 2015]] ; J.E. [[#Neumann--2015|]] [[#Neumann--2015|Neumann et al., 2015]] ; [[#Prein--2017b|Prein et al., 2017b]] ; [[#Hettiarachchi--2018|Hettiarachchi et al., 2018]] ; [[#Ragno--2018|Ragno et al., 2018]] ). Precise levels of regional IDF characteristics may still depend substantially on the method and resolution of downscaling applied ( [[#DeGaetano--2017|DeGaetano and Castellano, 2017]] ; L.M. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ).

'''Landslide:''' There is growing yet ''limited evidence'' for unique climate-driven changes in landslide and rockfall hazards in North America, even as theory suggests decreases in slope and rockface stability due to more intense rainfall, rain-on-snow events, mean warming, permafrost thaw, glacier retreat, and coastal erosion ( [[#Cloutier--2017|Cloutier et al., 2017]] ; [[#Coe--2018|Coe et al., 2018]] ; [[#Handwerger--2019|Handwerger et al., 2019]] ; [[#Hock--2019|Hock et al., 2019]] ; [[#Patton--2019|Patton et al., 2019]] ) although dry trends can decelerate mass movements ( [[#Bennett--2016|Bennett et al., 2016]] ). Landslide frequency has increased in British Columbia (Canada; [[#Geertsema--2006|Geertsema et al., 2006]] ) and is expected to increase in North-Western North America given the combination of these factors ( ''medium confidence'' ) ( [[#Gariano--2016|Gariano and Guzzetti, 2016]] ). [[#Cloutier--2017|Cloutier et al. (2017)]] projected an increase in landslides in western Canada due to wetter overall conditions and reduced return period for extreme rainfall. [[#Robinson--2017|Robinson et al. (2017)]] used scenarios based upon projection of 50-year recurrence of 7-day precipitation periods to highlight the potential for increased landslide hazards near Seattle (USA). Broad projections for the USA are more uncertain given increases in evapotranspiration that will counteract precipitation changes over much of the country ( [[#Coe--2016|Coe, 2016]] ) ''.''

'''Aridity:''' [[IPCC:Wg1:Chapter:Chapter-8|Chapter 8]] showed that aridity in North America generally moves opposite to mean precipitation change with an added evaporative demand from warmer temperatures ( ''high confidence'' in aridity increase for Northern Central America; ''medium confidence'' for an increase in Central North America; ''high confidence'' for a decrease in North-Eastern North America; ''medium confidence'' decreases in Eastern and North-Western North America; see also [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). Projected soil moisture declines (Figure 12.4j–l) are most widespread across North America during the summer, with the largest declines in Mexico and the southern Great Plains but also extending into Canada ( [[IPCC:Wg1:Chapter:Chapter-8#8.4.1.6|Section 8.4.1.6]] ; [[#Swain--2015|Swain and Hayhoe, 2015]] ; [[#Easterling--2017|Easterling et al., 2017]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ; J. [[#Lu--2019|]] [[#Lu--2019|Lu et al., 2019]] ). [[#Yoon--2018|Yoon et al. (2018)]] found net reduction in southern Great Plains groundwater storage in RCP8.5 mid-century projections despite increases in mean precipitation and both wet and dry extremes. Soil moisture drying could reach unprecedented levels by the CMIP6 RCP8.5 end-of-century, even when evaluating deeper soil columns relevant for crop rooting depth (B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ). Projected changes in the aridity index portray a shift the geographic range of temperate drylands northward and eastward in Central and Western North America ( [[#Schlaepfer--2017|Schlaepfer et al., 2017]] ; [[#Seager--2018|Seager et al., 2018]] ) which also diminishes aquifer recharge rates in the southern Great Plains and in some western regions where snowpack is reduced ( [[#Meixner--2016|Meixner et al., 2016]] ).

'''Hydrological drought:''' Section 11.9 asssessed ''low confidence'' of significant observational trends and projected future changes in the characteristics of episodic hydrological drought in North America given ''limited evidence'' and ''low agreement'' in modeled changes. [[#Zhao--2020|]] [[#Zhao--2020|C. Zhao et al. (2020)]] found that increases in hydrological drought frequency (particularly the 100 yr drought) were far more prevalent than for meteorological drought across 5797 watersheds in the USA and Canada, indicating a strong influence of evaporative demand. Reductions in the overall supply of meltwater from a declining snowpack also increase the potential for intermittent hydrological droughts in the western USA ( [[#Mote--2018|Mote et al., 2018]] ; [[#Livneh--2020|Livneh and Badger, 2020]] ).

'''Agricultural and ecological drought:''' Section 11.9 assessed ''medium confidence'' for an increase in agricultural and ecological drought in Western North America but otherwise found ''limited evidence'' for broadly observed changes in North American agricultural and ecological drought, even as increasing evaporative demand intensified vegetation stress and soil moisture deficits in recent events (Sections 11.6, 11.9). [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] asssessed ''medium confidence'' for more intense agricultural and ecological drought conditions over North Central America, Western North America and Central North America in a 2°C global warming level (about mid-century), with ''medium confidence'' extending to Eastern North America and ''high confidence'' for Northern Central America and Central North America under a 4°C global warming level associated with higher emissions scenarios past 2050. Figure 12.4g–i shows that the frequency of meteorological droughts (which often initiate hydrological, agricultural and ecological droughts) is largely projected to increase in North American areas where total precipitation decreases (and vice versa; see [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] and [[#Coppola--2021b|Coppola et al., 2021b]] ), and higher evaporative demand will extend the regions where more intense ecological and agricultural droughts develop when meteorological droughts occur ( [[#Wehner--2017|Wehner et al., 2017]] ; B.I. [[#Cook--2019|Cook et al., 2019]] , 2020). Studies utilizing a variety of drought indices and soil moisture projections consistently project increased drought extending from Mexico into the southern Canadian Plains during the summer ( [[#Swain--2015|Swain and Hayhoe, 2015]] ; [[#Ahmadalipour--2017|Ahmadalipour et al., 2017]] ; [[#Feng--2017|Feng et al., 2017]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ; [[#Tam--2019|Tam et al., 2019]] ; B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ).

'''Fire weather:''' Climatic conditions conducive to wildfire have increased in Mexico, Western and North-Western North America, primarily due to warming ( ''high confidence'' ). [[#Abatzoglou--2016|Abatzoglou and Williams (2016)]] found climate change led to higher values for eight fuel aridity indices over the western USA in recent decades, with 2000–2015 changes exposing 75% more forested area to high fuel aridity and adding nine more high fire-potential days each year, similar to 1979–2013 western USA and Mexico fire season expansion reported in [[#Jolly--2015|Jolly et al. (2015)]] . Increases in lightning-initiated fires have been distinguished from trends in man-made fire in western Canada and the USA ( [[#Balch--2017|Balch et al., 2017]] ; [[#Hanes--2019|Hanes et al., 2019]] ). [[#Jain--2017|Jain et al. (2017)]] identified a 1979–2015 expansion in fire weather season in eastern Canada and the south-western USA (with a smaller reduction in the northern Mountain West) along with regional shifts in the 99th percentile Canadian Fire Weather Index (FWI) and potential fire-spread days. [[#Girardin--2009|Girardin and Wotton (2009)]] noted that 1951–2002 trends in the Monthly Drought Code fire index in eastern Canada could hardly be distinguished from decadal variability.

Climate change drives future increases in North American fire weather, particularly in the south-west ( ''high confidence'' ), although further studies on shifts in exposure and vulnerability are needed to understand overall fire risks (see WGII Chapter 14). A significant increase of FWI is apparent before 2050 under RCP8.5 in much of North America, including the frequency of 95th-percentile FWI days, peak seasonal FWI average, fire weather season length, and maximum fire weather index ( [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ), and fire season across North America expands dramatically beyond 2°C global warming levels (Q. [[#Sun--2019|]] [[#Sun--2019|]] [[#Sun--2019|Sun et al., 2019]] ; [[#Jain--2020|Jain et al., 2020]] ). X. [[#Wang--2017b|Wang et al. (2017b)]] simulated fire-spread days across Canada and found increases across most of the areas studied by 2071–2100, with median changes of –20 to +140% (RCP2.6), –20 to +250% (RCP4.5) and 40 to 360% (RCP8.5) compared to 1976–2015. [[#Prestemon--2016|Prestemon et al. (2016)]] found more conducive conditions for lightning-ignited fires in the south-eastern USA by mid-century, while warming conditions in Alaska increasingly push July temperatures above 13.4°C, a threshold for fire danger across Alaska’s tundra and boreal forest ( [[#Partain--2016|Partain et al., 2016]] ; [[#Young--2017|Young et al., 2017]] ). Longer and more intense fire seasons would also raise particulate matter and black carbon concentrations in the western USA, reducing visibility at many National Parks ( [[#Yue--2013|Yue et al., 2013]] ; [[#Val%20Martin--2015|Val Martin et al., 2015]] ).

'''Changes in North American wet and dry climatic impact-drivers are largely organized by the ‘north-east (more wet) to south-west (more dry)’ pattern of mean precipitation change, although heavy precipitation increases are widespread and increasing evaporative demand expands aridity, agricultural and ecological drought, and fire weather (particularly in summer)''' ( high confidence ''').'''

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