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Megacities (metropolitan areas with populations over 10 million) and large urban centers present a major challenge for the global environment. Transportation, industrial activities, and energy demand have increased in megacities due to population growth and unsustainable urban development, leading to increasing levels of air pollution that subject the residents to the health risks associated with harmful pollutants, and impose heavy economic and social costs. Although much progress has been made in reducing air pollution in developed and some developing world megacities, there are many remaining challenges in achieving cleaner and breathable air for their residents. As centers of economic growth, scientific advancement, and technology innovation, however, these urban settings also offer unique opportunities to capitalize on the multiple benefits that can be achieved by optimizing energy use, reducing atmospheric pollution, minimizing greenhouse gas emissions, and bringing many social benefits. Realizing such benefits will, however, require strong and wide-ranging institutional cooperation, public awareness, and multi-stakeholder involvement. This is especially critical as the phenomenon of urbanization continues in virtually all countries of the world, and more megacities will be added to the world, with the majority of them located in developing countries. The air quality and emission mitigation strategies of eight megacities—Mexico City, Beijing, Shanghai, Shenzhen, Chengdu, Delhi, Kolkata, and Mumbai—are presented as examples of the environmental challenges experienced by large urban centers. While these megacities share common problems of air pollution due to the rapid growth in population and urbanization, each city has its own unique circumstances—geographical location, meteorology, sources of emissions, human and financial resources, and institutional capacity—to address them. Nevertheless, the need for an integrated multidisciplinary approach to air quality management is the same. Mexico City’s air pollution problem was considered among the worst in the world in the 1980s due to rapid population growth, uncontrolled urban development, and energy consumption. After three decades of implementing successive comprehensive air quality management programs that combined regulatory actions with technological change and were based on scientific, technical, social, and political considerations, Mexico City has made significant progress in improving its air quality; however, ozone and particulate matter are still at levels above the respective Mexican air quality standards. Beijing, Shanghai, Shenzhen, and Chengdu are microcosms of megacities in the People’s Republic of China, with rapid socioeconomic development, expanding urbanization, and swift industrialization since the era of reform and opening up began in the late 1970s, leading to severe air pollution. In 2013, the Chinese government issued the Action Plan for Air Pollution Prevention and Control. Through scientific research and regional coordinated air pollution control actions implemented by the Chinese government authority, the concentration of atmospheric pollutants in several major cities has decreased substantially. About 20% of total megacities’ populations in the world reside in Indian megacities; the population is projected to increase, with Delhi becoming the largest megacity by 2030. The increased demands of energy and transportation, as well as other sources such as biomass burning, have led to severe air pollution. The air quality trends for some pollutants have reduced as a result of emissions control measures implemented by the Indian government; however, the level of particulate matter is still higher than the national standards and is one of the leading causes of premature deaths. The examples of the eight cities illustrate that although most air pollution problems are caused by local or regional sources of emissions, air pollutants are transported from state to state and across international borders; therefore, international coordination and collaboration should be strongly encouraged. Based on the available technical-scientific information, the regulations, standards, and policies for the reduction of polluting emissions can be formulated and implemented, which combined with adequate surveillance, enforcement, and compliance, would lead to progressive air quality improvement that benefits the population and the environment. The experience and the lessons learned from the eight megacities can be valuable for other large urban centers confronting similar air pollution challenges.

Urban green infrastructure (GI) has been promoted as an approach to respond to major urban environmental and social challenges such as reducing the ecological footprint, improving human health and well-being, and adapting to climate change. Various definitions of GI have been proposed since its emergence more than two decades ago. This article aims to provide an overview of the concept of GI as a strategic planning approach that is based on certain principles. A variety of green space types exist in urban areas, including remnants of natural areas, farmland on the fringe, designed green spaces, and derelict land where successional vegetation has established itself. These green spaces, and especially components such as trees, can cover significant proportions of urban areas. However, their uneven distribution raises issues of social and environmental justice. Moreover, the diverse range of public, institutional, and private landowners of urban green spaces poses particular challenges to GI planning. Urban GI planning must consider processes of urban change, especially pressures on green spaces from urban sprawl and infill development, while derelict land may offer opportunities for creating new, biodiverse green spaces within densely built areas. Based on ample evidence from the research literature, it is suggested that urban GI planning can make a major contribution to conserving and enhancing biodiversity, improving environmental quality and reducing the ecological footprint, adapting cities to climate change, and promoting social cohesion. In addition, GI planning may support the shift toward a green economy. The benefits derived from urban green spaces via the provision of ecosystem services are key to meeting these challenges. The text argues that urban GI planning should build on seven principles to unlock its full potential. Four of these are treated in more detail: green-gray integration, multifunctionality, connectivity, and socially inclusive planning. Considering these principles in concert is what makes GI planning a distinct planning approach. Results from a major European research project indicate that the principles of urban GI planning have been applied to different degrees. In particular, green-gray integration and approaches to socially inclusive planning offer scope for further improvement In conclusion, urban GI is considered to hold much potential for the transition toward more sustainable and resilient pathways of urban development. While the approach has developed in the context of the Western world, its application to the rapidly developing cities of the Global South should be a priority.