Urban activities have profound and lasting effects on the global carbon balance. Here we develop a consistent metabolic approach that combines two complementary carbon accounts, the physical carbon balance and the fossil fuel-derived gaseous carbon footprint, to track carbon coming into, being added to urban stocks, and eventually leaving the city. We find that over 88% of the physical carbon in 16 global cities is imported from outside their urban boundaries, and this outsourcing of carbon is notably amplified by virtual emissions from upstream activities that contribute 33-68% to their total carbon inflows. While 13-33% of the carbon appropriated by cities is immediately combusted and released as CO2, between 8 and 24% is stored in durable household goods or becomes part of other urban stocks. Inventorying carbon consumed and stored for urban metabolism should be given more credit for the role it can play in stabilizing future global climate.

Center for Energy and Environmental Sciences University of Groningen Groningen 9747 AG The Netherlands

Department of Cultural History and Theory and Department of Social Sciences Humboldt University Berlin Unter den Linden 6 D 10117 Berlin Germany

Department of Environmental Studies Masaryk University Brno Czech Republic

Department of Geographical Sciences University of Maryland College Park MD 20742 USA

Department of Mathematics Quaid 1 Azam University Islamabad 44000 Pakistan

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology Guangzhou 510275 China

Institutes of Science and Development Chinese Academy of Sciences Beijing China

International Institute for Applied Systems Analysis Schlossplatz 1 A 2361 Laxenburg Austria

Ministry of Education Key Laboratory for Earth System Modeling Department of Earth System Science Tsinghua University Beijing 100084 China

NAAM Research Group Faculty of Science King Abdulaziz University Jeddah 21589 Saudi Arabia

Potsdam Institute for Climate Impact Research Potsdam 14473 Germany

Resnick Sustainability Institute California Institute of Technology Pasadena CA 91125 USA

School of Environmental Science and Engineering Sun Yat sen University Guangzhou 510275 China

State Key Joint Laboratory of Environment Simulation and Pollution Control School of Environment Beijing Normal University Beijing 100875 China

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