Environmental dimension
Biodiversity
What we know
Biodiversity is declining globally at all three key levels: genes, species and ecosystems (Pereira et al. 2010; Pimm et al. 2014; IPBES 2019). Historically, urban growth has been shown to contribute to natural habitat loss, encroach upon protected areas and reduce the habitat ranges of species on the International Union for Conservation of Nature Red List. These trends are projected to continue in the future (The Nature Conservancy 2018; McDonald et al. 2020). While biodiversity is decreasing on a global scale, at some local and regional scales in cities, species richness may actually be increasing, with “novel ecosystems” (Hobbs et al. 2006) formed from the addition of non-native species (Sax and Gaines 2003; Müller et al. 2013), some of which become invasive.
Freshwater
More and more urban dwellers are exposed to risks related to heavy precipitation and floods. Risks of water supply shortages are also rising in urban areas (Flörke, Schneider and McDonald 2018), aggravating challenges to access sufficient and clean water. Access to drinking water and sanitation has improved in urban areas but residents in informal settlements often remain excluded. SDG 6 progress shows there are still huge gaps in provision, especially in low-income countries. Larger cities increasingly rely on transfers between basins. Water shortages increase tensions and conflicts between urban and rural water users, and between different users in urban and peri-urban areas. Saline intrusion into freshwater supplies is a growing threat to coastal cities. Point source and diffuse pollution of freshwater bodies also remain a concern. Lakes and ponds in urban areas are natural sinks for stormwater drainage, which means they are especially sensitive to urban pollution from solid waste, sanitation and chemicals.
Oceans and coasts
Coastal populations are increasing globally and urban centres cover 10 per cent of low-lying coastal land (within 10 metres of the sea level) (Colenbrander et al. 2019). Increased coastal development in areas with rising populations threatens coastal ecosystems such as coral reefs, mangroves, salt marshes and sea grasses (Inniss et al. 2017; Muñoz Sevilla et al. 2019; UNEP 2019). Rising sea levels make urban populations more vulnerable to flooding, saltwater intrusion and coastal erosion (IPCC 2018; Masselink et al. 2020). Recent assessment of the impact of rising sea levels (under the high-emissions RCP 8.5 global warming scenario) indicates that by 2050, millions of coastal city dwellers could be displaced by floods (Kulp and Strauss 2019).
Ocean warming and acidification are predicted to adversely impact coastal fisheries and aquaculture (IPCC 2018). Cities are point sources of pollution (including plastic, other debris and chemicals) for the marine and coastal environment. Marine litter, much of which originates from poorly managed municipal waste, is increasing in some areas and declining in others (UNEP 2019). Coastal tourism is increasing, both in terms of the number of visitors the area occupied (Jarratt and Davies 2019). This increases pressure on services and the environment. Many large cities are port cities and marine traffic is increasing (Sardain, Sardain and Leung 2019).
Remaining gaps in knowledge
Although biodiversity is decreasing globally, the rate, magnitude and direction of change in biodiversity can vary depending on the scale and whether we are talking about urban or rural environments (Sax and Gaines 2003; The Nature Conservancy 2018). There remain gaps in biodiversity monitoring at the urban scale. Quantification of indirect urban impacts, such as food consumption, energy use and waste production, on biodiversity are less well studied than direct impacts like loss of habitat (McDonald et al. 2020). Recent IPBES assessments do not explicitly analyse the urban–rural relationship when it comes to biodiversity (IPBES 2019). Urban biodiversity studies need broader geographic coverage: there is a regional bias focused more on the Global North and temperate areas and less on the Global South, tropical areas and biodiversity hotspots (Aronson et al. 2014; McDonald et al. 2020).
A lack of reliable integrated data hinders assessing the impacts of hydrological disasters in terms of economic and infrastructural damage. Flood impact assessments (pluvial, fluvial, coastal and rising sea levels) and data on coping strategies is challenging due to the lack of detailed topographic and gradient data at the city, neighbourhood and smaller scales. This creates a gap between global data sets and scenarios and targeted local (re)actions. Groundwater data are generally lacking, which hinders estimates of urban groundwater deficits. Irregular groundwater extraction in cities creates additional challenges for estimating groundwater budgets (Flörke, Schneider and McDonald 2018). Data are lacking on the quality and quantity of freshwater, as well as on the quality of urban stormwater run-off.
Impacts on human health and ecosystems from emerging contaminants such as microplastics are not yet clear.
Data are required to better predict the timing and extent of coastal change and its impact on urban areas (including human well-being and livelihoods and coastal and ecosystems). Monitoring rising sea levels is inadequate in many coastal centres (especially in the Global South), hampering effective coastal planning and sustainable development.
The economic cost from the loss of services provided by coastal ecosystems is poorly understood (including food production and coastal protection) (Todd et al. 2019). There is also a lack of data on the impact of climate change on the marine food web and food security (Blasiak et al. 2017).
The State of the Environment in Cities
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