Significance: Our quantification of energy and material flows for the world’s 27 megacities is a major undertaking, not previously achieved. The sheer magnitude of these flows (e.g., 9% of global electricity, 10% of gasoline; 13% of solid waste) shows the importance of megacities in addressing global environmental challenges. In aggregate the resource flows through megacities are consistent with scaling laws for cities. Statistical relations are established for electricity use, heating/industrial fuels, ground transportation, water consumption, waste generation, and steel production in terms of heating-degree days, urban form, economic activity, and population growth. Analysis at the microscale shows that electricity use is strongly correlated with building floor area, explaining the macroscale correlation between per capita electricity use and urbanized area per capita.

Credibly and consistently reporting greenhouse gas (GHG) emissions from cities and urban areas enables policy-makers and practitioners to contribute to addressing the challenge of climate change by meeting mitigation targets, and is critical to overall good municipal management. Good reporting allows for transparency, verification, and replication over time. This study provides an understanding of the GHG emissions inventory protocols and methodologies as they apply to cities. Though the inventories generally use common terminology, the differences in inventorying approaches are many, and the implications of the inventorying results at the city level are important to climate change policy and decision-makers. A compilation of GHG emissions inventory protocols is developed along with an analysis of their characteristics and inherent differences. Seven protocols are investigated: four are applied to Shanghai's community emissions; four to New York City's corporate emissions (i.e. those from municipal activities); and two to the reporting of Paris' emissions, including upstream components. The results show a significant degree of variability among the protocols.

As drivers of climate action, cities are taking measures to reduce greenhouse gas (GHG) emissions, which if left unabated pose a challenge to meeting long-term climate targets. The economics of climate action needs to be at the forefront of climate dialogue to prioritize investments among competing mitigation measures. A marginal abatement cost (MAC) curve is an effective visualization of climate action that initiates a technical and economic discussion of the cost-effectiveness and abatement potential of such actions among local leaders, policy makers, and climate experts. More commonly demonstrated for countries, MAC curves need to be developed for cities because of their heterogeneity, which vary in their urban activities, energy supply, infrastructure stock, and commuting patterns. The methodology for constructing bottom-up MAC curves for cities is presented for technologies that offer fuel switching and/or energy efficiencies, while considering technology lifetimes, city-specific electricity and fuel prices, and emission intensities. Resulting MAC curves are unique to every city, and chart the pathway towards low-carbon growth by prioritizing measures based on cost-effectiveness. A case study of Toronto’s climate targets demonstrates the prioritization of select technologies. Leveraging MAC curves to support climate programs enables cities to strategically invest in financing climate action and designing incentives.