Global scenarios of urban density and its impacts on SPECIAL FEATURE building energy use through 2050 Burak Güneralpa,b,1, Yuyu Zhouc, Diana Ürge-Vorsatzd, Mukesh Guptad, Sha Yue, Pralit L. Patele, Michail Fragkiasf, Xiaoma Lic,g,h, and Karen C. Setog aDepartment of Geography, Texas A&M University, College Station, TX 77843; bCenter for Geospatial Science, Applications and Technology (GEOSAT), Texas A&M University, College Station, TX 77843; cDepartment of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011; dCentre for Climate Change and Sustainable Energy Policy, Central European University, 1051 Budapest, Hungary; eJoint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD 20740; fDepartment of Economics, Boise State University, Boise, ID 83725; gYale School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511; and hState Key Laboratory of Urban and Regional Ecology, Research Center for Eco- Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People’s Republic of China Edited by Jay S. Golden, Duke University, Durham, NC, and accepted by Editorial Board Member B. L. Turner, November 29, 2016 (received for review July 6, 2016) Although the scale of impending urbanization is well-acknowledged, and vibrant street life (10). Overall, more compact urban forms are we have a limited understanding of how urban forms will change and important levers for targeting transportation energy use reductions what their impact will be on building energy use. Using both top- (11). However, there are trade-offs because higher urban densities down and bottom-up approaches and scenarios, we examine building are also associated with disproportionately larger embodied energy energy use for heating and cooling. Globally, the energy use for in buildings and other infrastructure (5), higher exposure to air heating and cooling by the middle of the century will be between 45 pollutants (12), and traffic congestion (13). and 59 exajoules per year (corresponding to an increase of 7–40% However, we have little understanding of how future urban since 2010). Most of this variability is due to the uncertainty in future growth in different parts of the world will manifest in terms of urban densities of rapidly growing cities in Asia and particularly spatial development and what its implications will be for human China. Dense urban development leads to less urban energy use well-being and the environment. Our primary goals in this study are overall. Waiting to retrofit the existing built environment until mar- to develop possible scenarios of future urbanization, using urban SCIENCE population density as the metric, and to estimate the energy im- SUSTAINABILITY kets are ready in about 5 years to widely deploy the most advanced plications of these different urban futures. We aim to answer the renovation technologies leads to more savings in building energy following questions: What are likely future trajectories in urban use. Potential for savings in energy use is greatest in China when densities, and what is the potential for cities worldwide to alter their coupled with efficiency gains. Advanced efficiency makes the least densities significantly? Globally, what are the relative energy savings difference compared with the business-as-usual scenario in South from increasing building energy efficiencies versus increasing urban Asia and Sub-Saharan Africa but significantly contributes to energy densities? Where might concerted efforts to alter urban densities savings in North America and Europe. Systemic efforts that focus on yield the greatest benefits in terms of energy savings? both urban form, of which urban density is an indicator, and energy- Our analysis is a global-scale study that provides scenarios of efficient technologies, but that also account for potential co-benefits the spatial dimension of urbanization and associated energy use and trade-offs with human well-being can contribute to both local in the built environment. Of the three major forms of urban and global sustainability. Particularly in growing cities in the devel- energy—embodied, operational, and transport (5), the scope of oping world, such efforts can improve the well-being of billions of our paper is limited to operational building energy use. We use urban residents and contribute to mitigating climate change by re- two global energy/climate models: One is a top-down regionally ducing energy use in urban areas. Significance urbanization | cities | urban form | climate change | mitigation Urban density significantly impacts urban energy use and the rban areas account for 67–76% of global final energy con- quality of life of urban residents. Here, we provide a global- – Usumption and 71 76% of fossil fuel-related CO2 emissions scale analysis of future urban densities and associated energy (1). With the global urban population expected to increase by an use in the built environment under different urbanization additional 2.5 billion people between 2010 and 2050 (2) and scenarios. The relative importance of urban density and energy- concomitant expansion of urban areas (3), the urban shares in efficient technologies varies geographically. In developing re- total energy use and greenhouse gas (GHG) emissions are also gions, urban density tends to be the more critical factor in expected to increase. It is not, however, just the rate or scale of building energy use. Large-scale retrofitting of building stock urbanization that matters for urban energy use. An important, later rather than sooner results in more energy savings by the and often underexamined, factor is the future spatial patterns of middle of the century. Reducing building energy use, improv- urban development. ing the local environment, and mitigating climate change can The most recent Intergovernmental Panel on Climate Change be achieved through systemic efforts that take potential co- (IPCC) assessment report identifies urban form, the 2D and 3D benefits and trade-offs of both higher urban density and build- relationships between the physical elements, spaces, and activities ing energy efficiency into account. that constitute urban settlements, as a key determinant of urban energy use (4). Urban form significantly affects both direct (op- Author contributions: B.G., Y.Z., D.Ü.-V., and K.C.S. designed research; M.G., S.Y., P.L.P., erational) and indirect (embodied) energy (5). Beyond energy use, M.F., and X.L. performed research; Y.Z., D.Ü.-V., S.Y., and P.L.P. contributed new reagents/ urban form also affects two other dimensions of sustainability: analytic tools; B.G., Y.Z., D.Ü.-V., M.G., M.F., and X.L. analyzed data; and B.G. and K.C.S. human well-being and economic productivity. Urban form that wrote the paper. enables nonvehicular transport, characterized by smaller city blocks, The authors declare no conflict of interest. higher street connectivity, mixed land use, and higher population and This article is a PNAS Direct Submission. J.S.G. is a Guest Editor invited by the Editorial built-up densities, has been shown to be beneficial for health by Board. promoting more physical activity, such as walking and bicycling (6, 7). 1To whom correspondence should be addressed. Email: [email protected]. Higher levels of population density, one key feature of urban form, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. are associated with economic co-benefits (8), higher productivity (9), 1073/pnas.1606035114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1606035114 PNAS Early Edition | 1of6 Downloaded by guest on September 27, 2021 retrofit technology become the “standard” after a transitionary pe- riod of market adjustment, i.e., in about 5 years. Results Urban Population Densities Are Likely to Continue to Decline Through 2050. For the past three decades, urban population densities have been declining across all countries, income levels, and geographies (1). Our analysis shows across the three future urban density scenarios that urban population densities are likely to continue to decline for nearly all geographic regions through the first half of this century (Fig. 1A). The range across the three scenarios tends to be very large in all developing regions, exhibiting evidence of their ongoing urbanization dynamics, with their cities exhibiting a wide variety of expansion patterns from more compact to more dispersed. This range is particularly large in Centrally Planned Asia and China (henceforth, China, in short, to refer to the whole region), whose urban population density was about 90 persons per hectare in 2010. Its urban population density in 2050 is forecasted to range from 10 to 250 persons per hectare, primarily due to the rapid increase of urban population density in the most compact urban form (S75) scenario. Of all of the developing regions, only South Asia is forecasted to exhibit more dispersed urban forms in Fig. 1. Urban population density (A) and respective floor area (B) forecasts by all three scenarios. Both North America and the two European region through 2050 under three urban density scenarios used in the top- regions exhibit trajectories toward more dispersed urban forms, down analysis. The 11 International Energy Agency regions used in the analysis but with little difference between the three scenarios. The pop- are Centrally Planned Asia and China (CPA), Central and Eastern Europe (EEU), ulation densities in these regions remain below 50 persons per newly independent states of the former Soviet Union (FSU), Latin America and B the Caribbean (LAC), the Middle East and North Africa (MNA), North America hectare through 2050 in all three scenarios (Fig. 1 ). (NAM), Other Pacific Asia (PAS), Pacific countries that are members of the Or- Despite the relatively wide ranges across the urban density sce- ganization for Economic Cooperation and Development (POECD), South Asia narios, the ranges of the corresponding floor area projections are (SAS), Sub-Saharan Africa (SSA), and Western Europe (WEU). Results are also remarkably narrow (Fig. 1B). There are, however, significant dif- provided in tabular format in Dataset S1.