which severely impact biodiversity (IPBES 2018) and affect well-being at the city level. These impacts affect areas that provide the ecosystem services needed to maintain urban function in cities.


Another illustration of the complex indirect impacts of land resource use and urbanization is the degradation of the land itself. Land degradation is a process that reduces many of the land’s characteristics, such as productivity, porosity, vegetation, biomass and biodiversity, in addition to the ecosystem services provided by land resources (UNCCD 2017). Value lost from land degradation and land-use change was estimated to be 10–17 per cent of global gross domestic product (GDP), based on figures for 2010 (ELD Initiative 2015). This has numerous impacts on urban areas. Land degradation reduces agricultural productivity, which can drive up food prices and force farmers in degraded rural areas to seek better opportunities by migrating to cities. It can also lead to expansion of the agricultural frontier as more land is needed for farming to satisfy the resource needs of human populations.


Other direct impacts of global environmental change on land include coastal erosion, which threatens urban infrastructure and increases the vulnerability of coastal dwellers. Climate-induced flooding and land subsidence are expected to cause significant loss to some of the densest coastal cities (Hallegatte et al. 2013). Moreover, as mangroves, marshes and other ecosystems are transformed into urban areas, these cities become more susceptible to erosion, storm surge damage and flooding. As urbanization continues, these direct and indirect impacts will likely increase unless urban form and function, equity, transportation, and water and food footprints are transformed to make cities more environmentally sustainable (UNCCD 2017; see also section 4.2).


3.3 How are cities affecting the environment locally and globally?


In parallel to the effects of global environmental change on cities, the current process of urbanization and development of cities is also affecting local, regional and global environments. Sewage pollution in rivers also pollutes coastal areas. Similarly, the effects of urban air and land pollution are not confined to cities: air pollution can degrade ecosystems through acid rain, contributing to biodiversity loss and land cover change beyond the city boundaries. Demand for certain products can also affect and transform land use and ecosystems through telecouplings. This section considers the main impacts of urban areas on the environment and how land-use planning can help minimize the environmental side effects of urbanization.


3.3.1 Air


Greenhouse gas emissions from cities have local, regional and global impacts. While cities cover just 2 per cent of the world’s land, their concentration of human consumption and activity means they are responsible for around 70 per cent of total human-induced emissions (UN-Habitat 2016). This means greenhouse gas emissions from cities have a


50 GEO for Cities


significant impact on the planet as a whole through their impact on climate change.


Emissions of air pollutants from cities impact human health, climate, ecosystems, and food and water security from the local to regional levels (WHO 2016a). Based on the available data, 98 per cent of cities with populations over 100,000 in low- income and middle-income countries exceed WHO guidelines for PM2.5 or PM10, compared with 56 per cent of cities in high-income countries (ibid.). While the density of urban areas increases the efficiency of transport and energy, their high population density and increased consumption and economic activity also lead to higher concentrations of emissions from transport, heating and cooling, waste and construction. These “density trade-offs” also mean more people are potentially exposed to poor air quality and noise (Mueller et al. 2020). Air pollution from cities is also transported beyond city boundaries and can be generated downwind through chemical reactions in the atmosphere. In India, for example, ground-level ozone pollution, primarily from air pollution emissions in cities, is responsible for crop yield losses that could feed about 94 million people (Ghude et al. 2014).


High levels of greenhouse gas emissions and air pollutant emissions in cities mean local governments and city residents can play a key role in mitigating climate change and improving the air we breathe. While the COVID-19 pandemic initially reduced global greenhouse gas and air pollutant emissions (Forster et al. 2020), the reductions did not necessarily translate into improved air quality (Kroll et al. 2020; Le et al. 2020; Shi and Brasseur 2020). Moreover, the temporary reduction in greenhouse gas emissions is unlikely to have a measurable impact on reducing climate change over the long term (Forster et al. 2020; Le Quéré et al. 2020). However, the pandemic has provided an opportunity to take stock, allowing researchers to study how reduced emissions in cities will impact local and global air quality and the climate in both the short and long term. This will provide insights into ways to simultaneously improve air quality and mitigate climate change in cities after the pandemic (Forster et al. 2020; Rosenbloom and Markard 2020) and research on the impact of reduced emissions during the COVID-19 pandemic may shape air quality and climate change policies for years to come (Schiermeier 2020).


3.3.2 Biodiversity


Cities impact biodiversity both directly, within the urban environment due to urban expansion, and indirectly, through the transformation of ecosystems to produce food and the other resources and provisioning services needed in the city (McDonald, Marcotullio and Güneralp 2013; Seto 2014, McDonald et al. 2020). The impacts mainly come from the destruction of natural landscapes and the fragmentation of habitats (for example, palm oil or soy bean production in South-East Asia and South America, respectively), as well as from changes in living conditions, such as temperature increases, changes in the availability and quality of water, soils, nutrients and biomass, and pollution (Pickett et al. 2001; Grimm et al. 2008). Urban growth is estimated to have caused 190,000 km2


of habitat loss between 1992 and 2000,


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