Through increasing urbanization, a large share of world's population is already now living in urban areas such as cities. Urban infrastructures will be of utmost importance to support the everyday life of the urban population. Their adequacy related to delivering everyday services needed by the citizens may sometimes be stressed to their limits, which may lead to issues with power grid reliability and energy supply, environmental damage, or jeopardizing human health. A common concern to all urban regions presently is the greenhouse gas emissions (GHG) from various energy uses. Often the energy needed is still produced with fossil fuels, but the Paris Climate Agreement from December 2015 calls for rapid cuts in emissions. For example, the European Union has committed to reach a net-zero of all GHG by the middle of this century.

The above-described challenges are even more serious in the densely populated regions of the emerging economies such as China and India with huge populations and fast pace in urbanization. How these large countries will deal with their urban infrastructures aiming at better sustainability will have worldwide impacts, in particular for climate change mitigation. Due to the urgency of sustainability and climate protection, this Wiley Interdisciplinary Reviews' (WIREs) collection of papers is focusing on the urban infrastructure sustainability in China, which is perhaps the most important country in the world from the climate and environmental perspective taken its size. The collection aims at providing state-of-the-art knowledge from the latest stand of science in relevant topics and helping to figure out possible better pathways into the future.

WIREs is a series of interdisciplinary review journals that also serve as encyclopedic resources across a range of topics. We have chosen eight contributions into this collection covering three of the WIREs (Energy and Environment, Data Mining and Knowledge Discovery, and Water). The papers have a strong focus on the environment and climate, but view it from different perspectives, to provide a more versatile view on the urban infrastructure sustainability issues in China.

In the next, we provide short summaries of the key observations from the eight articles in the collection to highlight their relevance:

Zhu et al. (2020) reviewed the concept of smart city and resilient city and their differences and connections. Smart cities often relate to intensive use of information and communication technologies, but also to sustainable development. Whereas the resilient city relates mainly to its ability to absorb, adapt, and transform external pressures and ensure urban safety in the event of any crisis, hazards, or disasters. Both aspects are important for city development, but it seems that based on the research literature the smart cities develop faster than the resilient city concept. In spite of their differences, the authors conclude that both are necessary for the urban development and should be paid equal attention before decision-making.

Smart energy systems assisted by data and information technology are regarded as promising solutions for energy system integration and have been put into regional practices as part of a low-carbon and sustainable energy transition. Zhao et al. (2021) reviewed concepts and regional practices of smart energy systems. They found that current practices were limited to small areas and that understanding the concept of smart energy system is still confusing, in particular in China. The authors therefore proposed development of multiregional smart energy systems, which provides a coordinated effort of subsmart energy systems, for example, of industrial parks, transportation networks, towns, and farms. The authors also observed that in China smart energy often includes also nuclear and fossil energy and emphasizes energy hubs.

China has the highest GHG in the world with energy production dominated by coal leading to high emissions. Therefore, energy is definitely one of the key infrastructures to be addressed to cut emissions. China has indeed announced in 2020 to strive toward carbon neutrality by 2060, which will necessitate huge investments in energy infrastructures and clean energy. Turning more to solar photovoltaics (PV) and wind power in the power production will, however, require much more system flexibility. Gradually, renewable electricity sources will overtake the role fossil fuel sources. Zhang, Dai, et al. (2021) show in their review the importance of source-network-demand-storage coordinated planning as well as integration of multiple urban infrastructures in China to manage the energy transition optimally and to provide the flexibilities needed when using massive amounts of variable renewables. The authors foresee stronger coupling between power systems and other urban energy-related systems and infrastructures, such as heat systems, gas systems, information and communication infrastructure, and transportation infrastructures.

Zhang et al. (2020) discussed in their review the transformation of the coal power industry. Coal is the key energy question for China. They show how the specific coal consumption in coal power plants in China has actually constantly decreased due to efficiency improvements, for example, the CO₂ emissions have dropped by more than 20% from 2005. The authors perceive a diminishing role of coal, but it will still play a crucial role as part of the energy infrastructure for a long time to come, also in the urban context, for example, in Combined Heat and Power plants in cities, but also due to power balancing needs. They conclude that it will be necessary to establish a decommissioning schedule for coal power, but also to prepare for a longer transition period than, for example, in Europe.

The present transport system is one of the sectors in the urban context that create major emissions and other local environmental damage. Li et al. (2020) discussed in their review the implementation of a widespread electric vehicle scheme in Shenzen, China. The city has now the most electrical vehicles in the world, for example, more than 16,000 electric buses. Shenzen's success can be attributed to visionary municipal policies which included a comprehensive set of policies to promote electric vehicles (EVs) via three main channels: (a) vehicle purchase and use, (b) charging facility installation and services, and (c) traffic and parking management. These policies were refined in three piloting phases during 2009–2018. These policies initially included financial subsidies which have now gradually been removed.

Densely populated urban areas often suffer from water shortage. This is also the case of China due to the rapidly expanding urban populations, industrial use, and irrigation-intensive agriculture. Climate change puts even more pressure on the water resources. Kattel et al. (2021) discussed the water situation in the Beijing–Hebei–Tianjin region and conceptualized an ideal eco-city making use of novel approaches in water resources systems integrating perspectives of experts from a broad range of disciplines. Importantly, such concepts integrate social, ecological, and hydrological systems based on evidence-based scientific advice.

Some urban regions may suffer from droughts and water shortage, but at the same time the intensity and frequency of extreme precipitation have increased in many regions in the past century due to climate change. Intensity–duration–frequency (IDF) curves summarizing the relationships between the intensity and frequency of extreme precipitation for different durations are presently used for urban infrastructure design and stormwater management, but may not adequately reflect on the impacts of climate change. Yan et al. (2021) reviewed possible approaches to update the IDFs which could better mitigate the risks under future climate.

Municipal water and wastewater are complex sources of GHG, for which reason quantifying their roles in tackling global environmental challenges is important. Zhang, Smith, et al. (2021) provided a systemic review of the state-of-the-art of the characterization of GHG emissions in China's urban water infrastructure. They highlighted the hotspots of GHG emission sources in drinking water and wastewater treatment systems. In the former case, the electricity use is the largest source for emissions, whereas in wastewater the non-CO₂ GHG emissions are substantial vary much.

We hope that this collection of papers on urban infrastructure in China will draw more attention globally to the importance of sustainable infrastructures of urban areas, in particular those of highly populated regions and countries where urbanization is strong. Infrastructures will have a huge impact on climate change mitigation, environment, and prosperity of the urban inhabitants. We hope that this collection will stimulate more research on these themes.

Finally, we thank the authors for their valuable efforts in preparing the papers of the collection. Dr Meghana Hemphill, the Publisher of WIREs at Wiley, is gratefully acknowledged for raising the question of urban infrastructures and initiating this collection.