A global city, also called a power city, world city, alpha city or world center, is a city which is a primary node in the global economic network. The concept comes from geography and urban studies, and the idea that globalization is created, facilitated, and enacted in strategic geographic locales according to a hierarchy of importance to the operation of the global system of finance and trade.
The most complex node is the "global city", with links binding it to other cities having a direct and tangible effect on global socio-economic affairs. The term "megacity" entered common use in the late 19th or early 20th centuries; one of the earliest documented uses of the term was by the University of Texas in 1904. The term "global city", rather than "megacity", was popularized by sociologist Saskia Sassen in her 1991 work, The Global City: New York, London, Tokyo. "World city", meaning a city heavily involved in global trade, appeared in the May 1886 description of Liverpool, by The Illustrated London News. Patrick Geddes later used the term "world city" in 1915. More recently, the term has focused on a city's financial power and high technology infrastructure, with other factors becoming less relevant.
Global city status is considered beneficial and desirable. Competing groups have developed multiple alternative methods to classify and rank world cities and to distinguish them from non-world cities. Although there is a consensus upon leading world cities, the chosen criteria affect which other cities are included. Selection criteria may be based on a yardstick value (e.g., if the producer-service sector is the largest sector then city X is a world city) or on an imminent determination (if the producer-service sector of city X is greater than the combined producer-service sectors of N other cities then city X is a world city.)
Cities can fall from ranking, as in the case of cities that have become less cosmopolitan and less internationally renowned in the current era.
Although criteria are variable and fluid, typical characteristics of world cities are:
Global city rankings are numerous, with one study suggesting as many as 300. Ranked cities tend to be concentrated in North America and Europe.
In 2015, the second Global Economic Power Index, a meta list compiled by Richard Florida, was published by The Atlantic (distinct from a namesake list published by the Martin Prosperity Institute), with city composite rank based on five other lists.
The Institute for Urban Strategies at The Mori Memorial Foundation in Tokyo issued a comprehensive study of global cities in 2018. They are ranked based on six categories: economy, research and development, cultural interaction, livability, environment, and accessibility, with 70 individual indicators among them. The top ten world cities are also ranked by subjective categories including manager, researcher, artist, visitor and resident.
Jon Beaverstock, Richard G. Smith and Peter J. Taylor established the Globalization and World Cities Research Network (GaWC). A roster of world cities in the GaWC Research Bulletin 5 is ranked by their connectivity through four "advanced producer services": accountancy, advertising, banking/finance, and law. The GaWC inventory identifies three levels of global cities and several sub-ranks, although the authors caution that "concern for city rankings operates against the spirit of the GaWC project" (emphasis in original).
The 2004 rankings added several new indicators while continuing to rank city economics more heavily than political or cultural factors. The 2008 roster, similar to the 1998 version, is sorted into categories of Alpha world cities (with four sub-categories), Beta world cities (three sub-categories), Gamma world cities (three sub-categories) and additional cities with High sufficiency or Sufficiency presence. The cities in the top two classifications in the 2018 edition are:
In 2008, the American journal Foreign Policy, in conjunction with the consulting firm A.T. Kearney and the Chicago Council on Global Affairs, published a ranking of global cities, based on consultation with Saskia Sassen, Witold Rybczynski, and others. Foreign Policy noted that "the world’s biggest, most interconnected cities help set global agendas, weather transnational dangers, and serve as the hubs of global integration. They are the engines of growth for their countries and the gateways to the resources of their regions." The ranking is based on 27 metrics across five dimensions: business activity, human capital, information exchange, cultural experience, and political engagement and was updated in 2010, 2012, 2014, 2015, 2016, 2017, 2018 and 2019. Since 2015 it has been published together with a separate index called the Global Cities Outlook: a projection of a city’s potential based on rate of change in 13 indicators across four dimensions: personal well-being, economics, innovation, and governance.
"The Wealth Report" (a global perspective on prime property and wealth) is made by the London-based estate agent Knight Frank LLP together with the Citi Private Bank. The report includes a "Global Cities Survey", evaluating which cities are considered the most important to the world’s HNWIs (high-net-worth individuals, having over $25 million of investable assets). For the Global Cities Survey, Citi Private Bank’s wealth advisors, and Knight Frank’s luxury property specialists were asked to name the cities that they felt were the most important to HNWIs, in regard to: "economic activity", "political power", "knowledge and influence" and "quality of life".
In 2012, the Economist Intelligence Unit (The Economist Group) ranked the competitiveness of global cities according to their demonstrated ability to attract capital, businesses, talent and visitors.
A study by Brookings Institution conducted in 2016 introduced its own typology defining global cities into seven categories: Global Giants, Asian Anchors, Emerging Gateways, Factory China, Knowledge Capitals, American Middleweights, and International Middleweights 
The Global Giants classification includes wealthy, extremely large metro areas that the largest cities from developed nations. They are hubs for financial markets, major corporations, and serve as key nodes in global capital and talent flows.
A study by ING Media, a London-based built environment communications firm, has ranked 250 global cities by total online mentions across social media and online news for 2019. It found that one in every five digital mentions were for Tokyo, New York City, London and Paris, identifying these as the world’s super brands. The "Top 10 in the 2019 edition are:
Our new ranking puts the Big Apple firmly on top.
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