ecosystems and reduction in water quality (section 4.2). Ports and harbours are sources of pollution and are often sites of historical contamination that may pose an ongoing ecological threat. Examples of impacts that extend beyond the local marine environment include those related to the movement of litter, nutrients and other contaminants (both from discharge and run-off and releases to the atmosphere), as well as invasive alien species. As major sources of atmospheric CO2
, cities in
general drive global ocean warming and acidification (IPCC 2001; Licker et al. 2019).
Changes to coastal hydrodynamics from shoreline and catchment modifications can have major impacts on sediment deposition and transport. Research shows that coastal fortifications, such as sea walls, that are put in place to protect coastal infrastructure from storms and rising sea levels increase coastal erosion in other areas (Gracia et al. 2018). Large-scale engineering works, such as land reclamation, can radically alter the coastal environment. For example, Singapore has reportedly lost more than 65 per cent of local coral reef coverage due to land reclamation (Hilton and Manning 1995; Chou 2016). Despite action in many countries, mangroves, salt marshes, coral reefs and dune systems are still being lost to urban development. Urban population growth also exerts direct pressure on local fisheries around the world (through habitat loss and conversion), as well as indirect pressure, as seafood consumption continues to rise (Bange et al. 2017).
Many urban centres are struggling to manage the rise in solid waste. Up to 80 per cent of litter entering the oceans is thought to be from mismanaged urban waste (Li, Tse and Fok 2016). Estimates suggest that the biggest contributors are middle-income countries whose waste management systems have not kept pace with their economic development (Jambeck et al. 2015). Litter can accumulate on shorelines, degrade into microparticles that can enter the food chain, sink to the sea floor or remain in circulation, and act as a vector for invasive alien species. Ingestion and entanglement pose a threat to marine organisms and birdlife (UNEP and GRID-Arendal 2016) and can contribute to greenhouse gas emissions (Royer et al. 2018).
Cities are also a major source of chemicals in the ocean. Chemicals can be leached from discarded materials, such as plastics, or come directly from land-based and marine sources, such as riverine outflow, coastal run-off, storm water, sewage discharge, airborne particulates, shipping and fishing. High concentrations of persistent organic pollutants, heavy metals like mercury, microplastics and pharmaceuticals are routinely found in fish and shellfish near coastal urban centres (Milenkovic et al. 2019; Walkinshaw et al. 2020). The discharge or leakage of untreated sewage into the coastal zone is common in many coastal cities, especially in developing countries (UNEP 2016a). Despite continued improvement in wastewater treatment throughout the world, population growth is predicted to outpace any progress made, resulting in increasing nutrient discharge into surface waters, including estuaries and coasts (van Puijenbroek, Beusen and Bouwman 2019).
3.3.5 Land and soil
As a scarce resource, studies show that the physical footprint of urban areas tends to use much less land than other human settlements (UNCCD 2017). Cities house over half of the world’s population on less than 2 per cent of its habitable land (Ritchie and Roser 2013; OECD and European Commission 2020). This per capita “efficiency” of people per unit of land increases in line with settlement class (from village to city) and proximity to the city centre (suburban areas are half as efficient as urban centres) (European Commission Joint Research Centre [EC,JRC] 2019). However, “efficiency” needs to be considered more broadly.
Consumption-based ecological footprint studies indicate that an average urban resident’s indirect or “telecoupled” land-use (Leisz et al. 2016), accounting for urban needs like food, could be around 20 times their direct land-use (Zeng and Ramaswami 2020). These urban consumption patterns directly influence environmental outcomes and need to be transformed (section 4.2). While increased construction densities may promote per capita land-use “efficiency”, the inadequate provision of basic services may increase the risk of communicable diseases and reduce quality of life due to overcrowding. This has become particularly evident in some cities during the COVID-19 pandemic (Rocklöv and Sjödin 2020).
Studies indicate that the use of urban land in terms of size, form and the quality of the urban fabric has implications for the local and regional climate (Morote and Hernández 2016; Hanif 2018; Artmann, Inostroza and Fan 2019). While small, dispersed and spread-out cities may alleviate local urban heat islands (Zhou, Rybski and Kropp 2017), they are also associated with higher energy consumption, pollution and carbon footprints, cancelling out any local gains. In contrast, planned cities that are green and dense can mitigate the risk of heat islands at the same time as providing healthy living conditions (Li et al. 2020).
Industrial land uses within cities can pollute soil with chemicals containing elements like lead, arsenic and cadmium (Sharley et al. 2017; Kubier, Wilkin and Pichler 2019). The expansion of urban areas brings increased industrialization in the urban fringe, which can result in extreme soil pollution (Han et al. 2021). Nature-based infrastructure solutions are gaining prominence as part of the effort to address these impacts and promote ecosystem and human health, (Tzoulas et al. 2007; Morris et al. 2018). As the built environment and materials contribute significantly to increases in greenhouse gas emissions (Meng et al. 2017; Kayaçetin and Tanyer 2020), today’s urban development and infrastructure investment choices will affect carbon lock-in in the future (Seto et al. 2016; see also chapter 4).
One of the major telecouplings of urban areas is food production. About one-third of food grown throughout the world is wasted, either at source, on the way to markets or by consumers (Lipinski et al. 2013). With more than 55 per cent
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