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

==== 12.4.7.2 Wet and Dry ====

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'''Mean precipitation:''' Observational datasets have generally revealed no significant long-term trends in rainfall in the Caribbean over the 20th century when analysed at seasonal and inter-decadal timescales, except for some areas where there is evidence for decreasing trends for the period 1901–2010 but not for the period 1951–2010 (Cross-Chapter Box Atlas.2, Table 1, and Atlas.10.2; [[#Knutson--2018|Knutson and Zeng, 2018]] ). Although there are spatial variations, annual rainfall trends in the western Indian Ocean are mostly decreasing, with generally non-significant trends in the western tropical Pacific since the 1950s ( ''low confidence'' ). Significant drying trends are noted in the southern Pacific subtropics and south-western French Polynesia during the 1951–2015 period ( [[#McGree--2019|McGree et al., 2019]] ), and in some areas of Hawaii during the 1920–2012 period ( ''medium confidence'' ) (Cross-Chapter Box Atlas.2, Table 1, and Atlas.10.2).

Atlas.10.4 projects precipitation reduction over the Caribbean ( ''high confidence'' ) ( [[#Almazroui--2021|Almazroui et al., 2021]] ) and parts of the Atlantic and Indian oceans, particularly in June to August, by end of 21st century under SSP5-8.5. Precipitation is generally projected to increase under SSP5-8.5 and for higher GWLs in the small islands in parts of the western and equatorial Pacific, but there is ''low confidence'' in broad changes given drier conditions projected for the southern subtropical and eastern Pacific Ocean ( ''limited agreement'' given spatial and seasonal variability) (Atlas.10.4 and Figure Atlas.28).

'''River flood:''' There is ''limited evidence'' on observed changes in river flooding in the small islands. Long-term records in Hawaii indicate no clear trends in peak flow, except for the significant decrease in peak streamflow in Hawaii Island over the period 1967–2016 ( [[#Bassiouni--2013|Bassiouni and Oki, 2013]] ; [[#Clilverd--2019|Clilverd et al., 2019]] ). Similarly, there is no significant trend in the frequency and height (after adjusting for average sea level rise) of river flood in Fiji over the period 1892–2013 ( [[#McAneney--2017|McAneney et al., 2017]] ). There is ''low confidence'' on the direction of future change of river flooding in the small islands due to the limited literature. In Oahu, Hawaii, extreme peak flow events with high return periods are projected to increase by end of the 21st century under RCP8.5, but there is also high uncertainty in these projections ( [[#Leta--2018|Leta et al., 2018]] ).

'''Heavy precipitation and pluvial flood:''' Heavy precipitation days in CAR have increased in magnitude, and have been more frequent in the northern part during the latter part of the 20th century ( ''low confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] and Table 11.14). The direction of change in extreme precipitation varies across the Pacific and depends on the season ( ''low confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] and Cross-Chapter Box Atlas.2, Table 1). Although pluvial flooding events have been observed in some islands, there is ''limited evidence'' for an assessment on past changes in pluvial flooding, unlike in other regions. There is ''low confidence'' in the projected change in magnitude of very heavy precipitation days in CAR across different GWLs (Table 11.14). On the other hand, there is ''high confidence'' in the increase in frequency and intensity of extreme rainfall events (i.e., 1-in-20-year rainfall events) in the western tropical Pacific in the 21st century, even for RCP2.6 scenario, based on model agreement and mechanistic understanding but ''low confidence'' in the magnitude of change in extreme rainfall due to model bias (BOM and CSIRO, 2014).

'''Landslide:''' Heavy rainfall, such as from tropical cyclones, can trigger landslides over steep terrain in the small islands ( [[#Bessette-Kirton--2019|Bessette-Kirton et al., 2019]] ). There is ''limited evidence'' to determine long-term trends in rainfall-induced landslides in the small islands ( [[#Kirschbaum--2015|Kirschbaum et al., 2015]] ; [[#Sepúlveda--2015|Sepúlveda and Petley, 2015]] ; [[#Froude--2018|Froude and Petley, 2018]] ; [[#Bessette-Kirton--2019|Bessette-Kirton et al., 2019]] ). There is ''low confidence'' in future changes in landslides in the small islands. The direction of change may depend on future changes in precipitation, tropical cyclones, climate modes (e.g., El Niño–Southern Oscillation, ENSO), as well as human disturbance, but more data and understanding of the complexity of these relationships are needed, especially in these vulnerable areas ( [[#Sepúlveda--2015|Sepúlveda and Petley, 2015]] ; [[#Gariano--2016|Gariano and Guzzetti, 2016]] ; [[#Froude--2018|Froude and Petley, 2018]] ).

'''Aridity:''' Current estimates identify many small islands as being under water stress and thus particularly sensitive to variations in rainfall and groundwater, population growth and demand, and land-use change, among others (Cross-Chapter Box Atlas.2; [[#Holding--2016|Holding et al., 2016]] ). From 1950 to 2016, a heterogeneous but prevalent drying trend is found in CAR ( ''low confidence'' ), where drought variability is modulated by the tropical Pacific and North Atlantic oceans (Table 11.15 and Cross-Chapter Box Atlas.2, Table 1; [[#Herrera--2017|Herrera and Ault, 2017]] ). In the future, increased aridity and decreased freshwater availability are projected in many small islands due to higher evapotranspiration in a warmer climate that partially offsets increases or exacerbates reductions in precipitation ( [[#Karnauskas--2016|Karnauskas et al., 2016]] , 2018b; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). Increased aridity is projected for the majority of the small islands, such as in CAR, southern Pacific and western Indian Ocean, by 2041–2059 relative to 1981–1999 under RCP8.5 or at 1.5°C and 2°C GWLs, which will further intensify by 2081–2099 ( ''medium confidence'' ) ( [[#Karnauskas--2016|Karnauskas et al., 2016]] , 2018b). Groundwater recharge is projected to increase in Maui, Hawaii except on the leeward side of the island, which underscores the importance of topography and elevation on freshwater availability in different island microclimates ( [[#Brewington--2019|Brewington et al., 2019]] ; [[#Mair--2019|Mair et al., 2019]] ).

'''Hydrological drought:''' There is ''low confidence'' of widespread changes to hydrological drought in CAR or Pacific small islands in recent decades, although an increasing number of studies document local changes. Records in Hawaii indicate downward trends in low streamflow and base flow from 1913 to 2008 ( [[#Bassiouni--2013|Bassiouni and Oki, 2013]] ). Decadal variability of Hawaiian streamflow coincides with rainfall fluctuations associated with the Pacific Decadal Variability although significant average declines in surface and baseflow runoff of about 8% and 11% per decade, respectively, have been noted during the 1987–2016 period ( [[#Clilverd--2019|Clilverd et al., 2019]] ).

There is ''low confidence'' in hydrological drought change projections, given low signal-to-noise ratios and the challenge in representing island scales in global analyses. [[#Prudhomme--2014|Prudhomme et al. (2014)]] recognized CAR as one of the regions with the highest increase in regional deficit index (RDI; a measure of the fraction of area in hydrological drought conditions) by the end of the 21st century under RCP8.5. Daily streamflow and extreme low flows in two watersheds in Oahu, Hawaii are projected to decline by mid- and end of the 21st century under RCP4.5 and RCP8.5, which would result in more frequent hydrological droughts in this area ( [[#Leta--2018|Leta et al., 2018]] ).

'''Agricultural and ecological drought:''' Recent trends toward more frequent and severe droughts have been noted in the small islands but only with ''low confidence'' in broad trend patterns, given high spatial variability including heightened drought on the leeward side of islands (e.g., [[#Frazier--2017|Frazier and Giambelluca, 2017]] ; [[#Herrera--2017|Herrera and Ault, 2017]] ; [[#McGree--2019|McGree et al., 2019]] ; see Table 11.15, Cross-Chapter Box Atlas.2, Table 1). Agricultural and ecological droughts are projected to increase in frequency, duration, magnitude, and extent in small islands, such as in CAR ( ''medium confidence'' ) and parts of the Pacific ( ''low confidence'' ), particularly where future declines in precipitation are compounded by higher evapotranspiration, under increasing levels of warming ( [[#Naumann--2018|Naumann et al., 2018]] ; [[#Taylor--2018|Taylor et al., 2018]] ; [[#Vichot-Llano--2021|Vichot-Llano et al., 2021]] ). Relative to the period 1985–2014, decreases in annual surface and total column soil moisture become more robust in more areas in CAR by 2071–2100 under SSP3-7.0 and SSP5-8.5 scenarios (B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ), but reliably representing drought features in small island domains with global simulations is challenging (see also [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ).

'''Fire weather:''' There is ''limited evidence'' on trends in wildfire in CAR and the Pacific. Records of wildfire in Hawaii from 2005 to 2011 indicate a peak in area burned during the hot and dry summer months, but [[#Trauernicht--2015|Trauernicht et al. (2015)]] note the difficulty in establishing the link between past climate and wildfire trends due to human activities and vegetation changes. Availability of literature limits assessment on future fire weather in the small islands. Drying and warming trends tend to increase fire probability aside from the climate impact on fuel loading, for example, grassland fires in Hawaii ( [[#Trauernicht--2019|Trauernicht, 2019]] ), and wildfires in Puerto Rico ( [[#Van%20Beusekom--2018|Van Beusekom et al., 2018]] ).

'''Observed and projected rainfall trends vary spatially across the small islands. Higher evapotranspiration under a warming climate are projected to partially offset future increases or amplify future reductions in rainfall, resulting in drier conditions and increased water stress in the small islands''' ( medium confidence ''').'''

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