use change, are increasing at 0.6 per cent a year. As of 2018, the resulting atmospheric CO2
concentration is 407.4 parts
per million and the global temperature has risen by of 1.0˚ C (IPCC 2018; Friedlingstein et al. 2019; World Meteorological Organization [WMO] 2019a). The second major human- influenced greenhouse gas is methane (CH4). A recent study estimated 575 million tons of CH4 were emitted per year from 2008 to 2017, with 60 per cent of emissions caused by direct human activity such as agriculture, waste management and activities related to fossil fuels (Saunois et al. 2020). The warming impacts cities by increasing their mean and extreme temperatures, changing precipitation patterns (resulting in droughts and floods) and increasing the frequency and intensity of cyclonic storms, as well as contributing to rising sea levels (IPCC 2018). These climate pressures impact human health through exposure to extreme temperatures, increased air pollution due to air circulation stagnation events, reduced quality food and water, changes in infectious agents, and population displacement (Balbus et al. 2016).
Air quality is currently the largest environmental health risk in cities (UNEP 2019). Globally, poor air quality contributes to 6–7 million premature deaths every year due to outdoor (ambient) and indoor (household) air pollution (Health Effects Institute 2020; World Health Organization [WHO] 2020). Air pollutants know no boundaries: the air quality of cities can be significantly impacted by human and natural activities outside their jurisdiction. The fact that cities cannot control emissions from outside their boundaries presents an urban governance challenge for air quality mitigation strategies. There are only a few examples of transboundary initiatives to improve air quality, including the California Air Resources Board in the United States and the 1979 Convention on Long-range Transboundary Air Pollution, a pioneering instrument that established a regional framework covering Europe, North America, and Russia and former East Bloc countries to reduce transboundary air pollution (United Nations Economic Commission for Europe [UNECE] 2019). More effort is needed to establish air pollution control strategies within cities and across jurisdictions to reduce the human health impacts of these pollutants. It is important to emphasize that air quality and climate change are linked, from their sources, which are primarily from the combustion of fossil fuels, through to their impacts (for example, urban heat islands affect the concentration of air pollutants in cities). As such, they must be addressed in a coordinated manner (Melamed, Schmale and von Schneidemesser 2016).
Dust storms are an example of global environmental changes impacting the climate and air quality in both urban and non-urban spaces (UNEP, WMO and UNCCD 2016) Events such as the Saharan dust storm in June 2020, are expected to become more intense due to increased warming in the North Atlantic and weaker Azores Highs (Clifford et al. 2019). The June 2020 dust storm was the largest such storm in decades. Atmospheric circulation patterns transported dust from the Sahara Desert across the Atlantic Ocean, increasing air pollution across the Caribbean, the south-east of the United States, Mexico and Central America (Çapraz and Deniz 2020; Farahani and Arhami
2020; Freedman, Cappucci and Samenow 2020; Soleimani et al. 2020). The impacts on human health, weather and climate are currently being studied (United States National Aeronautics and Space Administration 2020).
Other examples include the extreme wildfire seasons in Australia and the United States in 2020, fuelled by record temperatures and severe droughts. Smoke from the wildfires temporarily resulted in the worst air quality in the world in Canberra (Vardoulakis et al. 2020) and San Francisco (Cabanatuan 2020). Located at the wildland–urban interface, the fires destroyed many homes, which affected the composition of the smoke. There is still a significant amount to learn about the emissions and chemical processes that occur during wildland–urban interface fires and their impact on human health (National Academies 2020).
3.2.2 Biodiversity
Biodiversity is directly linked to the quality of life of urban citizens, providing a multitude of benefits for humans from ecosystem services and nature’s contributions to people.3 Such services include provisioning (food), regulating (filtering for clean air and water), supporting (underlying and enabling the production of all other ecosystem services) and culture (recreational and aesthetic enjoyment). Biodiversity comprises the biotic natural resources that underpin humanity and urban settlements and allow them to thrive. The world is currently experiencing rapid biodiversity loss (Díaz et al. 2019; Intergovernmental Science–Policy Platform on Biodiversity and Ecosystem Services [IPBES] 2019; UNEP 2019). The current rate of global species loss is estimated to be at least tens to hundreds of times greater than the average historical rate of extinction over the past 10 million years, and continues to accelerate, threatening a sixth planetary extinction event within a few centuries (Barnosky et al. 2011; Pimm et al. 2014; Ceballos et al. 2015; Ceballos, Ehrlich and Dirzo 2017; IPBES 2019, section 2.2.5.2.4).
Biodiversity loss impacts cities through the loss of ecosystem functions and the services they provide. Yet biodiversity loss is not inherent to cities: they can also be rich in biodiversity (Elmqvist et al. 2013; Aronson et al. 2014; Ives et al. 2016). Moreover, maintaining biodiversity can provide with significant cost savings. It is key for maintaining functional ecosystems, adapting to other environmental challenges like climate change and providing benefits for humans (Millennium Ecosystem Assessment 2003; Millennium Ecosystem Assessment 2004; Haines-Young and Potschin 2010; Díaz et al. 2018; Díaz et al. 2019). For example, the loss of insect species that pollinate crops and other vegetation outside the city (a regulating ecosystem service) lowers agricultural productivity, leading to food insecurity (Food and Agriculture Organization of the United
3 Although they are similar, it is possible to distinguish between ecosystem services and Nature’s Contributions to People. The former focus on the benefits society receives from ecosystems (see, for example Millennium Ecosystem Assessment 2003; The Economics of Ecosystems and Biodiversity [TEEB] 2011; International Union for the Conservation of Nature 2013; Convention on Biological Diversity 2015; MacKinnon et al. 2019). In contrast, the latter considers both their positive and negative contributions (see, for example, Pascual et al. 2017; Díaz et al. 2018; Intergovernmental Science–Policy Platform on Biodiversity and Ecosystem Services 2019). While this distinction is important, for clarity, we have opted to use the term ecosystem services, since it is more familiar. However, as Kadykalo et al. (2019) note, Nature’s Contributions to People is a broader concept.
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