Nations [FAO] 2015; Mbow et al. 2019). Similarly, urban trees substantially reduce pollution (a regulating ecosystem service). This brings major economic benefits, with one estimate based on 10 megacities calculating average annual health savings of $482 million (Endreny et al. 2017).


Species diversity also supports ecological and functional redundancy and resilience in the face of environmental shifts (Walker 1992; Rosenfeld 2002; Elmqvist et al. 2003; Luck, Daily and Ehrlich 2003; Mori, Furukawa and Sasaki 2013; Oliver et al. 2015). For example, the presence of multiple urban coastal wetland plant species can help maintain the ecosystem services of water quality provision and protection from erosion, tidal currents and flooding from storms, even if populations of one particular species are eliminated (Boyer and Polasky 2004).


Urban biodiversity can also help to reduce the impact of infectious diseases, such as when organisms prey upon disease vectors (for example bats, spiders and dragonflies prey upon mosquitoes). It can also include organisms that absorb or “dilute” some of the burden of disease on humans (Epstein 1995; Ostfeld and Keesing 2000; Chivian and Bernstein 2004; Campbell et al. 2011; Taylor and Hochuli 2015). A good example of this phenomenon would be the reduction in the transmission of Lyme disease by ticks biting other animals rather than humans. Shifts in biodiversity in cities can have negative effects, or result in ecosystem “disservices” (Lyytimäki and Sipilä 2009) caused by environmental transformations in urban environments (Villa et al. 2014). For example, increased proximity between humans and animals in urban settings can also contribute to the spread of zoonotic diseases (Lyytimäki et al. 2008; Dobbs, Escobedo and Zipperer 2011; Escobedo, Kroeger and Wagner 2011), of which the current COVID-19 pandemic is a case in point (Leite Júnior et al. 2020; Platto et al. 2020).


3.2.3 Freshwater


Cities and their environmental dimensions of sustainable development depend on access to sufficient and safe freshwater resources. This explains why cities have historically developed near freshwater bodies. Moreover, prolonged droughts, devastating floods and water mismanagement have led to the fall of multiple civilizations (van den Brandeler and Gupta 2020). Disasters caused by extreme hydrological events have increased significantly in recent decades (partly as a result of climate change) and cities are particularly vulnerable to weather and climate extremes such as droughts, floods and the resulting water quality problems (Pahl-Wostl 2015; see also chapter 4). For instance, 79 large cities have suffered extensively from droughts since 2000 (Xiang et al. 2019), including megacities such as São Paulo in Brazil. Similarly, in 2018, Cape Town narrowly escaped “day zero” on which it would effectively have run out of water, albeit at the cost of severe restrictions on water use for residents (Rodina 2019).4


Many cities face challenges to adapt to more frequent water shortages as a 4 Section 5.3 examines how Cape Town has built resilience to tackle this situation in future.


result of rapid and unplanned urban growth and inadequate water management. This trend is exacerbated by the changes in precipitation caused by climate change (IPCC 2018). Rising sea levels can also lead to saltwater intrusion that threatens urban groundwater supplies in coastal cities and their surroundings (Safi et al. 2018). Finally, deforestation and other land-use changes in the watersheds of cities are further stressing urban water supplies and increasing the intensity of flooding (McDonald and Shemie 2014).


These pressures have led to tensions between urban and rural water users, especially given that agriculture accounts for an average of 70 per cent of global freshwater withdrawals (FAO 2017). There are also pressures within cities between different peri-urban water users and social groups. Water scarcity and droughts affect the availability of ecosystem services in cities, with negative effects on the health of residents, which can have knock-on effects on social stability (Xiang et al. 2019). Chronic water stress and extended droughts contribute to increased migration of people from rural to urban areas (often into informal settlements) and can drive international migration that can fuel or aggravate refugee crises and conflicts (King 2015; Berchin et al. 2017).


Moreover, the proportion of urban land subject to frequent flooding is likely to increase from 30 per cent in 2000 to 40 per cent in 2030 (Güneralp, Güneralp and Liu 2015). However, many developing countries lack accurate data on flood risks in cities, a situation that hampers efforts to build resistance (Frick-Trzebitzky, Baghel and Bruns 2017; Osuteye, Johnson and Brown 2017). Finally, while land-use and urban development factors (especially the expansion of impervious areas) are major drivers of increased flood risk, climate change puts additional stress on urban storm and wastewater infrastructure (Güneralp, Güneralp and Liu 2015; Avashia and Garg 2020).


3.2.4 Oceans and coasts


Human settlements have historically grown up around natural harbours. In 2018, a total of 21 of the world’s 33 megacities were located in low-lying coastal areas (United Nations 2018). More than 700 million people are estimated to live in urban or quasi-urban areas that are 10 metres or less above sea level (Colenbrander et al. 2019). These coastal communities are increasingly vulnerable to the effects of human-induced climate change. Increases in mean sea level and extreme weather events are predicted to continue throughout the century and beyond (IPCC 2018). Models of the vulnerability of coastal populations indicate that, even the rise in sea levels predicted under a low-carbon emissions scenario will threaten almost 200 million people who currently live in areas that will be under water at high tide by the end of the century (Kulp and Strauss 2019). Damaging saltwater intrusion caused by over-extraction of water resources in coastal cities and exacerbated by rising sea levels can infiltrate groundwater (often a source of drinking water), impede drainage and contribute to the contraction and disappearance of shoreline protecting coastal habitats.


48 GEO for Cities


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