Although the scale of impending urbanization is well-acknowledged, we have a limited understanding of how urban forms will change and what their impact will be on building energy use. Using both topdown and bottom-up approaches and scenarios, we examine building energy use for heating and cooling. Globally, the energy use for heating and cooling by the middle of the century will be between 45 and 59 exajoules per year (corresponding to an increase of 7-40% since 2010). Most of this variability is due to the uncertainty in future urban densities of rapidly growing cities in Asia and particularly China. Dense urban development leads to less urban energy use overall. Waiting to retrofit the existing built environment until markets are ready in about 5 years to widely deploy the most advanced renovation technologies leads to more savings in building energy use. Potential for savings in energy use is greatest in China when coupled with efficiency gains. Advanced efficiency makes the least difference compared with the business-as-usual scenario in South Asia and Sub-Saharan Africa but significantly contributes to energy savings in North America and Europe. Systemic efforts that focus on both urban form, of which urban density is an indicator, and energyefficient technologies, but that also account for potential co-benefits and trade-offs with human well-being can contribute to both local and global sustainability. Particularly in growing cities in the developing world, such efforts can improve the well-being of billions of urban residents and contribute to mitigating climate change by reducing energy use in urban areas.urbanization | cities | urban form | climate change | mitigation U rban areas account for 67-76% of global final energy consumption and 71-76% of fossil fuel-related CO 2 emissions (1). With the global urban population expected to increase by an additional 2.5 billion people between 2010 and 2050 (2) and concomitant expansion of urban areas (3), the urban shares in total energy use and greenhouse gas (GHG) emissions are also expected to increase. It is not, however, just the rate or scale of urbanization that matters for urban energy use. An important, and often underexamined, factor is the future spatial patterns of urban development.The most recent Intergovernmental Panel on Climate Change (IPCC) assessment report identifies urban form, the 2D and 3D relationships between the physical elements, spaces, and activities that constitute urban settlements, as a key determinant of urban energy use (4). Urban form significantly affects both direct (operational) and indirect (embodied) energy (5). Beyond energy use, urban form also affects two other dimensions of sustainability: human well-being and economic productivity. Urban form that enables nonvehicular transport, characterized by smaller city blocks, higher street connectivity, mixed land use, and higher population and built-up densities, has been shown to be beneficial for health by promoting more physical activity, such as walking and bicycling (6, 7)...

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Clean air policies in China have substantially reduced PM2.5 air pollution in recent years, primarily by curbing end-of-pipe emissions. However, further reaching the WHO guideline may instead depend upon the air quality co-benefits of ambitious climate action. Here, we assess pathways of Chinese PM2.5 air quality from 2015 to 2060 under a combination of scenarios which link Global and China's climate mitigation pathways (i.e. global 2°C- and 1.5°C-pathways, NDC pledges, and carbon neutrality goals) to local clean air policies. We find that China can achieve both its near-term climate goals (peak emissions) and PM2.5 air quality annual standard (35 μg/m3) by 2030 by fulfilling its NDC pledges and continuing air pollution control policies. However, the benefits of end-of-pipe control reductions are mostly exhausted by 2030, and reducing PM2.5 exposure of the majority of the Chinese population to below 10 μg/m3 by 2060 will likely require more ambitious climate mitigation efforts such as China's carbon neutrality goals and global 1.5°C-pathways. Our results thus highlight that China's carbon neutrality goals will play a critical role in reducing air pollution exposure to the WHO guideline and protecting public health.