5.1 Introduction

Emissions generated by human activity have changed the composition of the Earth’s atmosphere, with consequences for the health of people and the planet. The impacts of human activity on the atmosphere are often framed in terms of four separate challenges: air pollution; climate change; stratospheric ozone depletion; and persistent, bioaccumulative, toxic substances (PBT) (Abelkop, Graham and Royer 2017). The causes of these four challenges, their effects on atmospheric composition and meteorological processes, and their impacts on humans and ecosystems are closely intertwined (see Figure 5.1). Solutions to these challenges are also interrelated, as changes in lifestyle, technology and policy alter emissions of multiple pollutants simultaneously with a variety of interrelated implications. This chapter describes these four challenges together following the Drivers, Pressures, State, Impact, Response (DPSIR) framework (see Section 1.6).

Since the fifth Global Environment Outlook (GEO-5) was published in 2012, a number of developments have focused international attention on changing atmospheric composition. Estimates of the global burden of disease contributed by air pollution have doubled (comparing assessments published in 2004, 2012 and 2017) primarily due to new exposure estimates informed by satellite-borne instruments (Lim et al. 2012; Cohen et al. 2017). The United Nations Environment Assembly of the United Nations Environment Programme (UNEA) (2014; 2017) and World Health Assembly of the World Health Organization (WHO) (2015) have responded with resolutions to encourage national-level actions to address air pollution. Concentrations of major GHGs are still growing strongly (World Meteorological Organization [WMO] 2017a) and indicators of climate change have continued to accumulate. Targets in the Kyoto Protocol of the United Nations Framework Convention on Climate Change (UNFCCC) expired but were replaced by new ones under the Doha Amendment and new commitments under the Paris Agreement (UNFCCC 2016). Complementing the work of the UNFCCC, new efforts have targeted reductions of short-lived climate pollutants (SLCPs) from specific sectors with benefits for climate change mitigation and human health (Climate and Clean Air Coalition [CCAC] 2015). As stratospheric ozone (O₃) has continued its recovery, the Kigali Amendment to the Montreal Protocol (United Nations 2016a) has harnessed this successful international agreement to help mitigate the climate impacts of hydrofluorocarbons (HFCs), originally introduced as substitutes for ozone-depleting substances (ODS). Emissions of mercury (Hg) have declined in some regions and increased in others. Emissions of some banned persistent organic pollutants (POPs) have declined due to the implementation of international agreements. However, atmospheric burdens of other POPs and PBTs remain at levels of concern, and new chemical risks have been identified (United Nations Environment Programme [UNEP] 2017a).

Efforts to achieve each of the Sustainable Development Goals (SDGs) are linked directly or indirectly to mitigating air emissions and changes to atmospheric composition, as shown in Figure 5.2.

Figure 5.1: Primary linkages between pressures, state and impacts of atmospheric change

This figure is intended as a road map for the reader, showing the relationships between the main topics and pollutants discussed in this chapter. Chemical symbols and abbreviations are defined in Table 5.1.

Figure 5.2: Linkages between changes in atmospheric composition and achievement of the Sustainable Development Goals

Direct linkages are shown with bold arrows, indirect linkages with light arrows.

In the GEO-6 regional assessments, air pollution, climate change and energy development, as well as the intersection of these three issues, were identified as top priorities in every region. Growing cities, energy, and transportation demand were consistently identified as issues of concern. Indoor air pollution and access to clean household energy were priorities in Africa and Asia. Other regional priorities highlight differences in the institutional capacities of governments in different regions: improving observational networks (Africa, Latin America and the Caribbean, West Asia), strengthening governance (Asia, Latin America and the Caribbean), and understanding costs and benefits of mitigation measures (Asia). The following sections build upon the GEO-6 regional assessments to explore the state of these challenges from a global perspective.