Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.2.2.4 Other Dynamic Interactions ====

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A range of other dynamic climate interactions are relevant for cities, settlements and infrastructure: cold spells, landslides, wind, fire and air pollution.

'''Cold spells.''' Although frequencies and intensities of cold spells/cold waves are ''virtually certain'' to have decreased globally, and are projected to consistently decrease for most warming levels ( ''high confidence'' ; WGI Table 11.2), cold weather events can periodically occur and impact urban areas and their connected infrastructures. For cities in eastern Canada, the intra-annual distribution of freezing rain events may become more frequent from December to February, and less frequent in other months by 2100 (Cheng, Li and Auld, 2011). Freezing rain is also a risk to urban populations and infrastructure. In general, higher population mortality rates ''likely'' occur during the winter season, while more temperature-attributable deaths are caused by cold than by heat in cities located in temperate climates (Gasparrini et al., 2015; Chen et al., 2017; Ryti, Guo and Jaakkola, 2016). Winter mortality is ''unlikely'' to significantly decrease due to warming trends, partly because a range of other medical factors (e.g., influenza seasons and elevations in cardiac risk factors) also drive this winter-excess mortality (Kinney et al., 2015). However, the evidence is unclear whether mortality related to cold waves will decrease in coming decades in European (Smid et al., 2019) or US cities (Wang et al., 2016). While projected global cold extremes are expected to decrease in frequency and intensity, the higher regional variability of future climates means that cold waves may remain locally important threats, including in milder regions where there are larger temperature differences between ‘normal’ winter days and extreme cold events, and where there is less capacity to adapt (Ma, Chen and Kan, 2014; Ho et al., 2019). This will be accentuated in many cities, particularly in Europe, by anticipated demographic changes that result in a more elderly population susceptible to cold wave health risks (Smid et al., 2019).

The effects of cold waves on the energy sector include breakdowns in power plants and reduced oil and gas production ( [[#Jendritzky--1999|Jendritzky, 1999]] ), as well as failures in overhead power lines and towers leading to outages in Moscow and Bucharest ( [[#Panteli--2015|Panteli and Mancarella, 2015]] ; Andrei et al., 2019). Six major power outages associated with cold shocks and ice storms have been recorded since 2010, the majority recorded from large cities in the USA (Añel et al., 2017). Cold waves can also significantly increase energy demand. A cold wave that affected the Iberian Peninsula in January 2017 caused electricity prices to peak at a mean price of 112.8 €/MWh, the highest ever recorded in Spain ( [[#AEMET--2017|AEMET, 2017]] ).

'''Landslides.''' While geomorphological events (e.g., land subsidence from permafrost thaw at high latitudes or from groundwater extraction) and factors associated with the built environment (e.g., settlement location adjacent to steep slopes and zonation laws for building construction) are major factors determining urban landslide risk, these can also be influenced by a range of climatic variables, namely precipitation (frequency, intensity and duration), snow melt and temperature change. Some 48 million people are exposed to landslide risk in Europe alone, with the majority in smaller urban centres (Mateos et al., 2020). [[#Travassos--2020|Travassos et al. (2020)]] also documented all landslide deaths in the São Paulo Macro Metropolis Region from 2016 to 2019 that occurred from extreme rainfall events in vulnerable areas prone to landslides. An increase in the number of people exposed to urban landslide risks is projected for landslide-prone settlements lying within regions projected to experience a corresponding increase in extreme rainfall ( [[#Gariano--2016|Gariano and Guzzetti, 2016]] ). In addition, human factors such as expansion of towns onto unstable land and land use changes within settlements (e.g., road building, deforestation) are increasing human exposure to landslides and the likelihood of landslides occurring (Kirschbaum, Stanley and Zhou, 2015). Rainfall triggered landslides kill at least 5000 people per year, and at least 11.7% of these landslides occurred on road networks ( [[#Froude--2018|Froude and Petley, 2018]] ). Although the spatial footprint of an individual landslide might be small (i.e., &lt; 1 km 2 ), the ‘vulnerability shadow’ cast over an area in terms of regional transport network disruptions can be a significant proportion of a region, and cascade to other infrastructures (Winter et al., 2016).

Landslides tend to occur on moderate to steep slopes, and are thus particularly prevalent in mountainous regions which are also characterised by low infrastructure redundancy (i.e., few alternative routes) and increased impacts from climate change (Schlögl et al., 2019). More robust forecasts of landslides driven by climate risk requires (a) more complete long-term records of previous landslides and (b) baseline studies of the Global South which are currently missing from the literature (Gariano et al., 2017).

'''Wind''' . Urban morphology alters wind conditions at multiple spatial scales; generally, increased surface roughness in settlements have resulted in declining trends in both measured wind speed and frequency of extremely windy days (Mishra et al., 2015; Peng et al., 2018; [[#Ahmed--2014|Ahmed and Bharat, 2014]] ; WGI Box 10.3).

Urban wind risks can also be affected by location adjacent to mountains, lakes or coasts with localised wind systems (WGI 10.3.3.4.2; WGI 10.3.3.4.3). In large cities with significant urban heat island, an urban-driven thermal circulation can enhance pollution dispersion under calm conditions (Fan, Li and Yin, 2018) or advect heat to areas downwind of the city (Bassett et al., 2016). Microscale wind conditions within urban canyons also strongly affect ventilation of air pollution dispersion and thermal comfort at pedestrian level, especially in cities located in warm climates (Rajagopalan, Lim and Jamei, 2014; Middel et al., 2014; [[#Lin--2016|Lin and Ho, 2016]] ).

In cities, wind risks from climate change hazards can arise from increased exposure from the expanding built environment. Very high wind speeds associated with severe weather systems, for example, tropical cyclones or derechos can cause significant structural damage to buildings and key infrastructure with insufficient wind load, as well as causing human injury through flying debris (Burgess et al., 2014). In particular, there is evidence from North American cities that tornado damage are ''likely'' fundamentally driven by growing built-environment exposure ( ''medium confidence'' ) (Ashley et al., 2014; [[#Rosencrants--2015|Rosencrants and Ashley, 2015]] ; [[#Ashley--2016|Ashley and Strader, 2016]] ).

Extreme winds in urban areas can have particularly damaging effects on poorly constructed buildings, including low-income houses in African cities ( [[#Okunola--2019|Okunola, 2019]] ), and on urban trees that may be uprooted by strong wind gusts from downbursts ( [[#Ordóñez--2015|Ordóñez and Duinker, 2015]] ; [[#Pita--2016|Pita and de Schwarzkopf, 2016]] ; Brandt et al., 2016), as well as on disrupting transportation along urban road and railway networks (Koks et al., 2019; Pregnolato et al., 2016).

'''Fire.''' Hotter and drier climates in several regions, for example Australia, the Western USA, the Mediterranean and Russia ( [[#IPCC--2018|IPCC, 2018]] ), ''likely'' enable weather conditions driving fire events impacting cities within these regions ( [[IPCC:Wg2:Chapter:Chapter-2#2.4.4.2|Section 2.4.4.2]] , 2.5.5.2). These include wildfires along the margins where cities are adjacent to wildlands, that is, the wildland-urban interface (WUI) ( [[#Bento-Gonçalves--2020|Bento-Gonçalves and Vieira, 2020]] ; Radeloff et al., 2018), or fires in cities with a high degree of informal settlements having greater vulnerability to fire hazards (Kahanji, Walls and Cicione, 2019; [[#Walls--2017|Walls and Zweig, 2017]] ; Sections 8.3.3.2). This vulnerability is considerable; over 95% of urban fire related deaths and injuries occur within informal settlements in low- and middle-income countries (Rush et al., 2020).

For wildfires at the WUI, anthropogenic climate change, natural weather variability, expansion of human settlement and a legacy of fire suppression are key factors in determining fire risk ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ; Knorr, Arneth and Jiang, 2016; van Oldenborgh et al., 2020). Recent wildfires in Australia and California both occurred under hot and dry weather conditions exacerbated by climate change, and resulted in substantial property damage along the WUI, ecosystem destruction and lives lost (Brown et al., 2020; Lewis et al., 2020; Yu et al., 2020). Future climate risk of fires at the WUI are ''likely'' ( ''medium confidence'' ), and are compounded by projected urban development along the WUI within several regions, such as in the Western USA (Syphard et al., 2019), Australia (Dowdy et al., 2019) and the Bolivian Chiquitania (Devisscher et al., 2016).

'''Air Pollution.''' Despite recent observed improvements in air quality arising from COVID-19 restrictions (Krecl et al., 2020; Naik et al., 2021 Cross-Chapter Box 6.1), significant risks to human health in cities leading to premature mortality ''very likely'' arise from exposure to decreased outdoor air quality from a combination of biogenic (e.g., wildfires at the WUI that advect into the urban atmosphere [Reddington et al., 2014; Naik et al., 2021 [[IPCC:Wg2:Chapter:Chapter-12|Chapter 12]] Box 12.1]) and anthropogenic sources that are influenced by climate change (e.g., fine particulate matter such as PM 2.5 , tropospheric ozone, oxides of nitrogen and volatile organic compounds [Burnett et al., 2018; Knight et al., 2016; Turner et al., 2016; West et al., 2016; Chang et al., 2019b; Li et al., 2019a; Alexader, Luisa and Molina, 2016; Naik et al., 2021 Sections 6.5.1, 6.7.1.1, 6.7.1.2]). Risks of premature mortality from indoor air pollution in cities, arising from biomass burning for heating in winter or cooking, indoor pesticide use or exposure to volatile organic compounds from poor thermal insulation in buildings, are also ''likely'' to occur ( [[#Leung--2015|Leung, 2015]] ; Peduzzi et al., 2020; Cross-Chapter Box HEALTH in Chapter 7).

The mortality risk for several pollutants, for example PM 2.5, is considerable ( ''high confidence'' ). Current estimates indicate that 95% of global population live in areas where ambient PM 2.5 exceeds the WHO guideline of annual average exposure of 10 µg m −3 (Shaddick et al., 2018a; Shaddick et al., 2018b; Chang et al., 2019b). Among the 250 most populous urban areas, estimated PM 2.5 concentrations are generally highest in cities in Africa, South Asia, the Middle East and East Asia; PM 2.5 in many cities in North Africa and the Middle East is ''likely'' due mainly to wind-blown dust, whereas that in South Asia and East Asia are mainly anthropogenic in origin (Anenberg et al., 2019). However, data on PM 2.5 concentrations are unavailable in many cities in low- and middle-income countries owing to a lack of measurements (Martin et al., 2019).

For some air pollutants, for example concentrations of PM 2.5 in several US, Western European and Chinese cities have recently decreased as a result of clean air regulations that have controlled emissions from sources such as motor vehicles, fossil fuel power plants and major industries (Zheng et al., 2018a; Fleming et al., 2018). These decreases have brought substantial improvements in public health in settlements within these regions (Ciarelli et al., 2019; Zhang et al., 2018). In South Asia, Southeast Asia and Africa, however, concentrations of other air pollutants, for example tropospheric ozone, oxides of nitrogen and volatile organic compounds are ''likely'' to continue to grow and peak by mid-century before they subside due to global urbanisation assumptions embedded in the SSPs (Naik et al., 2021 Sections 6.2.1; 6.7.1.2). Broadly, future air pollutant emissions are projected to decline globally by 2050 as societies become wealthier and more willing to invest in air pollution controls, but the trajectories vary among pollutants, world regions and scenarios (Silva et al., 2016b; Rao et al., 2017; Silva et al., 2016c). Whereas cities in East Asia and South Asia currently have large exposure to anthropogenic air pollution, African cities may emerge by 2050 as the most polluted because of growing populations and demand for energy, increased urbanisation and relatively weak regulations to control emissions (Liousse et al., 2014).

Studies modelling climate change impacts on air quality find that the spatiotemporal patterns of concentration changes vary strongly at urban scales, and that often those patterns differ among the different years modelled due to internal variability (Saari et al., 2019) and different models used (Weaver et al., 2009). Changes in PM 2.5 due to climate change are less clear than for ozone and may be relatively smaller (Westervelt et al., 2019) as climate change can affect PM 2.5 species differently (Fiore, Naik and Leibensperger, 2015). For Beijing, climate change is expected to cause a 50% increase in the frequency of meteorological conditions conducive to high PM 2.5 concentrations (Cai et al., 2017). The impacts of future climate change on air quality and consequent risks on human health have been studied at urban (Knowlton et al., 2004; Physick, Cope and Lee, 2014) and national scales (Fann et al., 2015; Orru et al., 2013; Doherty, Heal and O’Connor, 2017); globally, these studies have found a ''likely'' net increased risk of climate change on air pollution-related health ( ''low confidence'' ). They have focused mainly on the USA and Europe, with few studies elsewhere (Orru, Ebi and Forsberg, 2017), although the relationship between climate and air quality in megacities is particularly complex (Baklanov, Luisa and Molina, 2016). [[#Silva--2017|Silva et al. (2017)]] found that global premature mortality attributable to climate change (and not from urbanisation) from ozone and PM 2.5 will increase by about 260,000 deaths per year in 2100 under RCP8.5, but substantial variance in results exists between individual models.

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