Skyscrapers in the Twenty-First Century City: A Global Snapshot

: The ﬁrst two decades of the twenty-ﬁrst century represent a major milestone in skyscraper developments. By analyzing extensive data, the research presented here contrasts building activities of skyscrapers before and after the turn of the 21st century. It examines tall buildings in the world’s major continents (Asia, Europe, North America, Oceana, Middle East, South America, Central America, and Africa) and their respective cities including Shanghai, Beijing, Shenzhen, Bangkok, London, Moscow, New York, Chicago, Miami, San Francisco, Melbourne, Sydney, Dubai, Doha, Riyadh, Tel Aviv, S ã o Paulo, Panama City, Mexico City, and Nairobi. By using nearly 40 tables and 80 maps, the paper highlights the rapid activities of building signiﬁcant skyscrapers at greater heights, elucidates the changes in functions and services, and delineates shifts in spatial patterns and visual impact.


Introduction
This paper reviews recent skyscraper developments globally. In the past two decades, the world has witnessed an unprecedented construction boom of tall buildings. Despite financial difficulties and construction challenges, cities have been building an increasingly greater number of tall buildings with greater heights. This paper intends to map out recent construction developments in global cities and attempts to answer basic questions, which are shown below.

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What are the most active regions in constructing tall buildings? • What are the tallest buildings? • What are the driving forces of building skyscrapers? • Are there changes in the functionality of this building typology? • How do new skyscraper developments alter the spatial patterns of their respective cities?
For answering these questions, the paper conducts an extensive literature review and uses the Skyscraper Center's database of the Council on Tall Buildings and Urban Habitat (CTBUH) [1]. Therefore, all data reported in this paper including the building's height, use, number of floors, square footages, and date of project completion are based on this database. Reported building heights refer to the architectural tip of the building and the reported numbers of floors refer to those that are above ground. All tables and maps in this paper are produced by the same database. Each city is represented by four maps grouped with the following order. Two maps at the top compare the larger geographic coverage and two maps at the bottom compare the smaller geographic coverage of the same city between 2000 and 2020. These comparisons highlight changes and recent activities regarding building skyscrapers in these cities.
Detailed definitions of tall buildings are offered in previous research [2,3]. For the sake of brevity, this paper embraces the following definitions.

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The project has a specific site, an owner, and a developer who are seriously interested in executing the project.

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The professional design and planning team has passed the conceptual design stage of the project and are progressing toward completing construction drawings.

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The project has obtained construction permission or in the process to do so.

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The announcement of the "proposed" building comes from a credible source.

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Vision. A building project earns this title when it meets one of the following three criteria: • A project idea that does not fulfill the "proposed" criteria mentioned above. • A "proposed" project that developers could not advance to the construction stage, or • A project idea whom architects conceived to be an inspirational proposition.

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Demolished. When a building is destroyed by an authority at the end of its life cycle or by a nature-made (e.g., earthquake, hurricane, etc.) or a human-made disaster (e.g., terrorist attack, war, etc.), it earns the "demolished" title.
The paper holds the following structure. First, it provides a global overview of the rapid pace of constructing tall buildings, highlighting major activities, and the tallest skyscrapers. Then it discusses the driving forces of building tall construction projects. Next, it examines case studies within each world continent. The examined case studies compare the spatial patterns and tallest skyscrapers in 2000 and 2020. Afterward, the paper discusses key findings and closes with a summary conclusion.
Among all Asian countries, China stands out as the epicenter of high-rise construction. Many of the towers that China built or are under construction have lent a global recognition. Over the past two decades, tall building construction has increased dramatically in China, which is a phenomenon that moved from first-tier to second-tier and third-tier cities. In this regard, Quanhong Li (2017) explains, "High-density vertical urban developments are shaping the future identities of these cities by way of their strategic locations, massive scale, significant functional mix, and large social, economic, and environmental impact" [9] (p. 32). Meanwhile, tall building projects have been proliferating in established skyscraper cities.
Starting during the 21st century, China has built 52% of all Asia's tall buildings (1995/3788). The rest of Asia (31 countries) shares the remaining 48%. The CTBUH enlightens that China leads the world by concentrating on completions of 200 m+ (656 ft+) buildings. While in 2015, China counted for 58% of the world's tall buildings of this height category (62/106), in 2016, it counted for 66% (84/128) [5]. Specifically, China achieved significant leaps in this height category in the past few years where the total number of 200 m+ (656 ft+) buildings more than doubled between 2000 and 2020.
Remarkably, despite expecting an economic slowdown for China, the CTBUH predicts that China will continue to rank first in building skyscrapers for years to come. "With the closest national contender, the US, in 2016 having only seven completions and China having 84, it is clear that the gap will take a number of years to close, if it ever does" [10].
European cities that have historically banned tall buildings to protect their valuable built heritage-e.g., London, Paris, Frankfurt, Amsterdam, Moscow, and Warsaw-have resumed constructing tall buildings. In her article "Development of High-Rise Buildings in Europe in the 20th and 21st centuries," Joanna Pietrzak writes [11] (p. 31), "It was not until the 1950s that Europe began to construct buildings taller than 100 meters. By 2013, skyscrapers had been constructed in over 100 European cities located in 30 different countries, and the trend towards the expansion of high-rise construction continues." European cities have completed a record number of skyscrapers since the turn of the 21 st century and this trend is likely to continue in the coming years. Europe had merely two 250 m+ (820 ft+) tall buildings before the 21 st century. However, by 2020, Europe will have 23 buildings of this height category.
Since the turn of the 21 st century, Russian cities have been experiencing a period of extensive high-rise construction. Russian cities such as Moscow, St. Petersburg, and Groznyj have been constructing some of Europe's tallest buildings. This construction boom happened partially because of the prosperous recovery of the national economy after the 1998 crisis. These cities are trying to convey to the world that the country is reclaiming its glory and joining the global economy. After the 20 th century, Russia has built fourteen 200 m+ (656 ft+) buildings, which equals 40% of all Europe's new buildings of this height category (14/35). There are now six buildings of this category under construction as well. Currently, Russia has seven of the 10 tallest skyscrapers in Europe and will have eight of the 10 tallest buildings in 2020. Overall, 64% of the tallest buildings in Russia were built in the 2000s [2].
Sustainable land use and transportation are different from the suburban auto-dependent sprawl development that has dominated North America's landscape since 1950. Compact, mixed use, walkable, transit-oriented places offer significant environmental, economic, and social benefits. Consequently, major cities in North America such as New York City, Chicago, Miami, Philadelphia, Seattle, San Francisco, Los Angeles, Toronto, Vancouver, and Calgary have also been constructing significant tall buildings around mass transit nodes. Australian cities are also progressively building upward due to multiple reasons such as increased land value. Sydney, Melbourne, Gold Coast, and Brisbane are taking the lead.
Significantly, before the turn of the century, the Middle East built only 106 tall buildings and in the first two decades of the 21 st century, it will construct over 647 tall buildings. Currently, the Middle East has 28 supertalls and two megatalls. By 2020, it will have 46 supertalls and three megatalls. Cities in the Middle East such as Dubai, Abu Dubai, Doha, Jeddah, Mecca, Riyadh, Kuwait, Tel Aviv, and Beirut have been among the most active in building significant skyscrapers. For example, Mecca and Dubai have built two of the world's three megatalls, Makkah Royal Clock Tower and Burj Khalifa, respectively-the third is Shanghai Tower in Shanghai, China. By 2020, Jeddah will add to the Middle East another megatall, which is the new world's tallest building called Jeddah Tower soaring 1000 m (3280 ft). Doha, Qatar had no skyscrapers before the turn of the century and, by 2020, it will have completed 39 skyscrapers. Likewise, Abu Dhabi, UAE, had merely one skyscraper before the turn of the century and now has 31 skyscrapers. Similarly, before the turn of the century, Manama, Bahrain had no skyscrapers. However, it has now 15 skyscrapers.
Since the beginning of the 21st Century, South America has built 350 tall buildings-more than it did before (286 buildings). The most active cities in building skyscrapers are Santiago (Chile), Bucaramanga, Cartagena (Colombia), Buenos Aires (Argentina), São Paulo, Balneario Camboriu, and Curitiba (Brazil). Notably, in 2014, South America built its two tallest towers including the 300 m (1000 ft) Torre Costanera in Santiago (Chile) and the 219 m (719 ft) Millennium Palace in Balneario Camboriu (Brazil). However, there are six buildings currently under construction of remarkable heights including: One Tower (280 m (919 ft)), Yachthouse Residence Club towers (270 m each (886 ft)), Infinity Coast Tower (237 m (778 ft)), Epic Tower (220 m (722 ft)), and Boreal Tower (220 m (772 ft)) [2,11]. Although South America will continue to build taller, it lags significantly on buildings' height when compared to other continents including Asia, Middle East, North America, and Europe.
Central America has also been active in constructing tall buildings. Since the turn of the century, it built 201 tall buildings, which is more than it did before (88 buildings). Regarding height, Central America built all of its 26 200 m+ buildings post the turn of the century except one. Among the cities constructing tall buildings are Panama City (Panama) and Mexico City, Monterrey, and Guadalajara  [2]. These cities surpass other cities in the continent regarding counts and heights. Currently, the continent has no supertall and will have no supertall by 2020 as well [12,13].
Comparing to other continents, Africa has the smallest share of tall buildings. It has constructed 182 tall buildings, 132 of which were completed before the turn of the century and 50 after. Regarding height, its buildings are shorter than that of other continents. It has no supertall and merely one building that exceeds 200 m (656 ft). By 2020, Africa will also have no supertall and will have merely two 200 m+ buildings. Cities engaged in constructing tall buildings include Johannesburg, Pretoria, and Sandton (South Africa), Dar es Salaam (Tanzania), Nairobi (Kenya), and Lagos (Nigeria) [14,15]. Overall, the pace of construction is slow and fewer buildings are of significant height in these cities.

Population Increase and Migration
Among the pressing issues that have spurred tall building development, and will likely continue, is the exponential increase in urban population worldwide in conjunction with wealth accumulation. Currently, more than half of the world is urban when 20 years ago it was only one-

Population Increase and Migration
Among the pressing issues that have spurred tall building development, and will likely continue, is the exponential increase in urban population worldwide in conjunction with wealth accumulation. Currently, more than half of the world is urban when 20 years ago it was only one-third [16,17]. By 2030, it is expected that about 60% of the world's population will be urban. In 2050, about 75% of the world population will live in urban areas when the world's population is expected to reach 9 billion.
At that time, all major cities of the world especially those in Asia, Africa, and Latin America will have enormous populations likely ranging from 30 million to 50 million or more [18]. Accommodating such a large population in cities will be a colossal challenge. The horizontal scale of cities is continually being strained with no alternative but to build upward to accommodate city dwellers. Rural-to-urban migration is one of the causes of the urban population increase. In China, it is projected that, by 2025, 350 million people will migrate from a rural to an urban environment. Marcos Fava Neves predicted, "This will require five million buildings . . . equivalent of 10 cities the size of New York" [19]. In other words, Chinese cities need to build an equivalent amount to the U.S. population in merely 13 years to accommodate the population increase [19]. In such cases, high-rise development is almost certain to be part of the solution.

Demographic Change
Demographic shifts demonstrate that many of the millennials prefer living in urban centers that offer cultural amenities, vibrant social life, and mass transit. They prefer a car-free lifestyle enabled by having a workplace near home. The US housing market has expected that demand for single-family homes will continue to grow tremendously. Nonetheless, this did not occur. Instead, many people, particularly millennials and boomers, have decided to move into high-rises of central cities (e.g., New York City, Chicago, San Francisco, Atlanta, and Austin), seeking urban living. While millennials, or the Creative Class as described by the urban theorist Richard Florida, are finding "hot" jobs in the city, downsizing retirees are moving to walkable neighborhoods to free themselves from the maintenance burden of suburban homes. Both crowds, nevertheless, are enjoying urban amenities, services, mass transit, walkable places, and civic life offered in these city centers. In San Francisco, developers are building residential towers near Twitter's headquarters. Similarly, Seattle's developers are building residential towers near Amazon's global headquarters. Overall, "after 50 years of an immense suburban sprawl, there is a noteworthy flight back to cities, an increasing acceptance of density and demand on urban cores, as well as reemphasis on public transit as a key to a sustainable future," state Rebecca Leonard and Joe Porter [20] (p. 12).

Global Competition and Globalization
The ongoing trend for constructing tall buildings around the world reflects the increasing impact of global competition on the development of the world's major cities. These cities compete on the global stage to have the title of tallest building in which to announce the confidence and global stature of their growing economies. An iconic tall building enhances the global image of the city. It is likely to put the city on the world map and signal or promote its significant economic progress and advancement. Political leaders have supported constructing tall buildings to present their countries as emerging global economic powers. In some parts of the world, globalization has immensely promoted the local economy and, consequently, the construction of tall buildings. The City of Shenzhen, China, for example, was a small fishing village in the 1970s. Due to global forces and rapid foreign investment, it was transformed into a modern city of skyscrapers. Foreign nationals have invested billions of dollars in building factories and forming joint ventures. Today, the city is home to the headquarters of numerous high-tech companies that house their offices in major tall and supertall buildings [2].

Urban Regeneration
As explained earlier, city centers in developed countries that suffered from the migration of their population to the suburbs in the 1970-1990s have witnessed a major return to their centers in recent years. Therefore, cities are witnessing an urban renaissance and a desire to return to high-rise living in the urban cores. Tall buildings are viewed as tools to encourage central living and working. Construction of new attractive high-rises can also beautify and revitalize dilapidated districts and neighborhoods within the urban core and surrounding areas. This improves the quality of life in these areas by minimizing or eliminating social ills such as crime that might have been prevalent there [2].

Agglomeration
The tallness of buildings is also a matter of agglomeration in business districts. Urban agglomeration hinges on the proximity of activities and tall buildings do just that. Clustering tall buildings fosters urban synergy for diverse activities and specialized services. The high concentration of activities creates "knowledge spillovers" between firms in the same sector and across sectors that lead to increased innovation. In a denser and varied environment, knowledge can spill into unintended fields and a significant share of knowledge transfer occurs informally. David Audretsch explains: "Since knowledge is generated and transmitted more efficiently via local proximity, economic activity based on new knowledge has a high propensity to cluster within a geographic region . . . . Greater geographic concentration of production leads to more, and not less, dispersion of innovative activity . . . " [21] (p. 21). Clearly, the presence of a large concentration of firms offering similar products spurs competition, innovation, and efficiency. Agglomeration improves the economy of scale and can increase productivity through access to denser markets. Access to competing suppliers helps firms procure more efficient, cheaper, and more appropriate inputs. Researchers have attempted to quantify the impact of agglomeration. Colin Buchanan's research reveals that "a doubling of employment density within a given area can lead to a 12.5% additional increase in output per worker in that area. For the service sector, the figure is far higher at 22%" [22] (p. 590).

Land Prices
Land prices have been a prime driver for constructing tall buildings. A phrase for skyscrapers came from Cass Gilbert in 1900, "A skyscraper is a machine that makes the land pay" [23] (p. 71). In large cities, properties are expensive and buildings logically grow upward. Cheaper land keeps buildings closer to the ground. Tall buildings are not an attractive option for small towns where land is cheap [24]. Land prices recently have been significant drivers of tall building development in cities seeking to re-populate their urban centers with residential-recreational complexes inserted in the predominantly commercial-retail Central Business Districts (CBD). These relatively new markets drive up city center land prices, which make building upward for investment return increasingly necessary. In some cities such as London, Singapore, and Hong Kong land prices are very high, at about $30,000 per square meter. Therefore, developers maximize the site by building ultra-tall buildings between 50 to 80 floors [25]. In the case of New York City, Rem Koolhaas, in his book Delirious New York, explained that Manhattan has no choice but extruding the city grid vertically [26]. Similarly, in Mecca, Saudi Arabia, land nearby the Sacred Mosque (Al-Masjid Al-Haram) is limited and exceedingly expensive and, therefore, has recently witnessed significant high-rise development including the ultra-tall Abraj Al-Bait Towers.

Land Preservation
Sustainability promotes compact urban living and vertical density is viewed as a tool to create a more sustainable city. Urban planners and institutions such as the Urban Land Institute in the U.S. are supporting this view: "By strategically increasing the number of dwelling units per acre, cities not only will go a long way toward meeting their sustainability objectives but also will be competitive, resilient, and great places to live" [27] (p. 67). Dense arrangements help preserve open space-a core goal of sustainability-that aims at preserving different types of open spaces including natural areas in and around cities and localities that provide a habitat for plants and animals, recreational spaces, farm and ranch lands, places of natural beauty, critical environmental areas (e.g., wetlands), and recreational community spaces. The availability of open space provides significant environmental quality and health benefits that include improving air pollution, attenuating noise, controlling the wind, providing erosion control, and moderating temperatures. Open space also protects surface and ground water resources by filtering trash, debris, and chemical pollutants before they enter a water system. Through the "Towers-in-the-Park" model, Le Corbusier advocated the high-density city mainly for increasing access to nature. For example, accommodating the same number of people in a tall building of 50 stories versus five stories requires about one-tenth of the land [28][29][30].

Climate Change and Energy Conservation
Global warming could change climate patterns, which makes droughts and excessive rainfall more frequent and severe. According to a NASA (National Aeronautics and Space Administration) study, the Arctic perennial sea ice has been decreasing at a rate of 9% per decade since the 1970s and is caused likely by climate change. These issues are significant for living conditions since they can profoundly affect our cities. For example, a 6 m (20 ft) rise in sea level would submerge all of South Florida [31]. Consequently, fighting global warming and reducing CO 2 emissions are becoming prime goals of countless cities. The Kyoto Protocol was created in 1997 to fight global warming and more than 180 states joined the protocol by 2009. In 2015, at the Paris climate conference (COP21), 195 countries adopted the "Paris Agreement," which is the first-ever universal, legally binding global climate treaty. Therefore, the increase in emissions will result in damaging the climate and, hence, the desperate need to stabilize carbon emission can hardly be overemphasized. In this context, tall buildings have the potential to consume less energy than low-rise complexes since they have several energy-effective attributes such as agglomeration, savings in auto fuel and travel time, and a reduction in losses in power lines [32]. The roof is a prime source of energy loss in a building in addition to the façade. As such, a 50-story building of 10 apartments per floor has one roof and 500 single-family homes with each having the same floor area of an apartment with 500 roofs. Energy loss from 500 roofs is greater than that from one roof [33]. Overall, vertical development not only reduces carbon emissions but also provides opportunities to create compact environments that feature efficient mobility and accessibility while offering a higher quality of life. "Compact, mixed use, walkable, transit-oriented places offer significant environmental, economic, and social benefits" [34].

Infrastructure and Transportation
The high cost of maintaining expansive infrastructure hurts taxpayers and contributes to the fiscal crisis that local governments face. The costs of providing and maintaining public infrastructure and services for a given community in a new sprawling development is higher than to service the same community in a "smart growth" or infill development. Largely, vertically configured buildings facilitate more efficient infrastructure. Simply put, a 500-unit single-family subdivision requires many more roads, sidewalks, sewers, hydro lines, power and gas lines, light standards, and fire hydrants than that of a tall building, which allows integrating these systems efficiently in a dense manner. A comprehensive review of dozens of studies published by the Urban Land Institute uncovered that since 1980, the number of miles Americans drive has grown three times faster than the population and almost twice as fast as vehicle registrations. The researchers conclude that one of the greatest ways to reduce carbon emission is to build compact places where people can accomplish more with less driving. Compact development reduces driving from 20% to 40% [35,36].

International Finance
Wealthy international people increasingly invest in the global economy and real estate. They find cities such as New York City and London to be safe places to park their money as opposed to keeping their wealth in their insecure and politically unstable home countries. International investors view tall and supertall buildings as excellent investments because they involve placing a huge amount of capital in one place. In these skyscrapers, international owners can choose to rent their residential units, live there temporary or permanently, or leave them vacant [2].

Air Rights
Buying "air right," also called transferable development rights (TDR), has become popular in dense cities in recent years due to high demand in areas that have fewer developable lots. Zoning codes assign the greatest height for each zone and specify air right rules. The air right concept enables a low-rise building's owner to sell the air above a property (e.g., church, school, museum, apartment building) to a neighboring building, which can boost its allowable height. Consequently, the air above an "underbuilt" structure or "vertical" real estate space becomes a valuable commodity. Overall, the air right concept allows a neighborhood to tap into its "unused potential." New York City offers an illustrative example. NYC introduced the air right concept in 1961 and today, it witnesses a renewed demand [2]. For example, One57 (306 m/1004 ft) in Midtown Manhattan added to its height allotment by buying air rights from its neighboring buildings including the nine-story 140 West 58th Street built in 1930. Completed in 2014, the 92-story ultra-tall building offers splendid views of the nearby Central Park. Similarly, 220 Central Park South (nearing completion) will reach new heights by buying air rights from neighboring buildings. Remarkably, as land prices soar, air right prices soar as well [37,38].

Human Aspirations and Ego
According to Roberto Assagioli, a pioneer of psycho-synthesis theories, the conception of height has to do with "self-realization," "self-actualization," and "human potential." Consequently, humans have admired tall structures since ancient times. A commonality of the "seven wonders" of the ancient worlds (e.g., the Temple of Artemis at Ephesus, the Lighthouse of Alexandria, and the Great Pyramids of Egypt) is that all are tall and visible. Human spirit and resilience were the driving forces behind the skyscraper phenomenon that started in the late 19th century. Tall buildings can project a sense of socio-economic power and promote the city as a leading and modern commercial center. Skyscrapers epitomize people's pride in their cities and display the achievements of warm architectural passion and cold engineering logic [15]. Tall structures provide an identity for a city such as Big Ben and the Shard in London, the Eiffel Tower in Paris, the Space Needle in Seattle, Willis Tower in Chicago, Burj Khalifa in Dubai, and so on. Observation decks celebrate human ascendance over the sky and the surrounding landscape by providing unique panoramic views of the world below. Humanity has a preoccupation with building large and tall constructs to defy gravity. "Tall has power." Imagining modern cities without skyscrapers is antithetical to the human spirit, pride, and identity. Therefore, human ego has a role in building skyscrapers.

Emerging Technologies
The evolution of the tall building has led to major advancements in engineering and technology. As today's technology becomes increasingly sophisticated, architects have an opportunity to build taller and implement their desire for the latest and utmost aesthetic expressions of tall buildings. Simultaneously, the desire for achieving higher quality tall buildings has encouraged research in areas such as building service systems, computer sciences, façade engineering, glazing, daylight and heat control, structural framing systems, ceiling systems, lighting, ventilation, exit strategies, water recycling systems, and more. Tall buildings have challenged technology itself and allowed to build towers more efficiently and sustainably and to create internal environments that are comfortable, productive, and energy-efficient. The prevalent green movement has propelled the design of high-performance tall buildings by employing intelligent technologies and smart materials.

Shanghai
As a major global city, Shanghai has been among the most active cities in constructing tall buildings. Overall, in the first two decades of the 21st Century, Shanghai will add 156 skyscrapers to the 69 skyscrapers it built previously. Furthermore, 63 of these skyscrapers are in the Lujiazui Financial District. About 40 skyscrapers are located immediately west of the district across the Huangpu River and the rest proliferates throughout the city. In addition, 37 of the added 47 200 m+ skyscrapers are located in the Lujiazui district and the rest are placed immediately west of the district across the river. Three of the five added supertalls (Shanghai Tower, Shanghai World Financial Center, and Jin Mao) are also located in the Lujiazui district, which creates a strong focal point in the city's skyline [39]. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 235 m to 365 m, equaling a 160% increase (Table 13). Certainly, Shanghai is one of the most active cities in building significant skyscrapers and it has been transformed from a polycentric city to a super polycentric city ( Figure 3). Financial District. About 40 skyscrapers are located immediately west of the district across the Huangpu River and the rest proliferates throughout the city. In addition, 37 of the added 47 200 m+ skyscrapers are located in the Lujiazui district and the rest are placed immediately west of the district across the river. Three of the five added supertalls (Shanghai Tower, Shanghai World Financial Center, and Jin Mao) are also located in the Lujiazui district, which creates a strong focal point in the city's skyline [39]. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 235 m to 365 m, equaling a 160% increase (Table 13). Certainly, Shanghai is one of the most active cities in building significant skyscrapers and it has been transformed from a polycentric city to a super polycentric city ( Figure 3).  Beijing, China's capital, has recently engaged in a race with its rival city, Shanghai, in building significant tall buildings. Scheduled for completion in 2018, China Zun Tower will be the flagship building of a 30-hectare master plan for the central business district. Twenty tall buildings ranging from 150 m to 350 m in height will cluster around Zun Tower [8,39]. China Zun Tower will rise to 528 m (1732 ft) and will become the tallest building in Beijing and the eighth tallest in China. Overall, in the first two decades of the 21st century, Beijing will add 55 skyscrapers to the 10 skyscrapers it  Beijing, China's capital, has recently engaged in a race with its rival city, Shanghai, in building significant tall buildings. Scheduled for completion in 2018, China Zun Tower will be the flagship building of a 30-hectare master plan for the central business district. Twenty tall buildings ranging from 150 m to 350 m in height will cluster around Zun Tower [8,39]. China Zun Tower will rise to 528 m (1732 ft) and will become the tallest building in Beijing and the eighth tallest in China. Overall, in the first two decades of the 21st century, Beijing will add 55 skyscrapers to the 10 skyscrapers it built previously. Almost all of these buildings are located in close proximity (within a square mile) in the east side of the city center. Eight of the 200 m+ buildings are located in this area including one supertall building (Figure 4). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 147 m to 291 m, equaling a 200% increase (Table 14). built previously. Almost all of these buildings are located in close proximity (within a square mile) in the east side of the city center. Eight of the 200 m+ buildings are located in this area including one supertall building (Figure 4). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 147 m to 291 m, equaling a 200% increase (Table 14).  An important emerging global skyscraper city in China is Shenzhen. Since becoming China's first Special Economic Zone, Shenzhen has witnessed an unprecedented urban growth and expanded from 300,000-people to more than 10 million-city in 35 years. Shenzhen has built significant skyscrapers. Located in the center of Shenzhen and completed recently, Ping An Finance Center (PAFC) rises to 599 m (1965 ft), which ranks as the second tallest building in China after Shanghai Tower. Overall, in the first two decades of the 21st Century, Shenzhen will add 172 skyscrapers to the 20 skyscrapers it built previously. This sudden and massive increase in skyscrapers has not only reinforced existing modest clusters but created totally new ones [13,39,40]. The locations of the new 79 200 m+ skyscrapers stretch across the two far ends of the city, east and west ( Figure 5). The locations of the newly constructed five supertalls reinforce "downtown" clusters. The average height  An important emerging global skyscraper city in China is Shenzhen. Since becoming China's first Special Economic Zone, Shenzhen has witnessed an unprecedented urban growth and expanded from 300,000-people to more than 10 million-city in 35 years. Shenzhen has built significant skyscrapers. Located in the center of Shenzhen and completed recently, Ping An Finance Center (PAFC) rises to 599 m (1965 ft), which ranks as the second tallest building in China after Shanghai Tower. Overall, in the first two decades of the 21st Century, Shenzhen will add 172 skyscrapers to the 20 skyscrapers it built previously. This sudden and massive increase in skyscrapers has not only reinforced existing modest clusters but created totally new ones [13,39,40]. The locations of the new 79 200 m+ skyscrapers stretch across the two far ends of the city, east and west ( Figure 5). The locations of the newly constructed five supertalls reinforce "downtown" clusters. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 231 m to 393 m, equaling a 170% increase (Table 15). of the 10 tallest skyscrapers in 2000 and 2020 will increase from 231 m to 393 m, equaling a 170% increase (Table 15).

Bangkok
Bangkok, the capital of Thailand, is also an emerging skyscraper city in Asia. Recently, Bangkok has completed its tallest building, which is the 314-m (1031-ft) MahaNakhon. The Magnolias Waterfront Residences Tower 1, however, will snatch the tallest building title from MahaNakhon soon by rising higher by merely one meter. Other important towers under construction include Capella Residences (300 m (983 ft)), Magnolias Waterfront Residences Tower 2 (280 m (919 ft)), and Canapaya Residences I (253 m (830 ft)). Lately, SOM has unveiled a vision for a 16-hectare "skyscraper village" in the city [2,14]. Named "One Bangkok", it will house more than 60,000 people and integrate a variety of parks and plazas, which will total eight hectares. Overall, in the first two decades of the 21st Century, Bangkok will add 95 skyscrapers to the 27 skyscrapers it built previously [2]. The new buildings are located in close proximity to older ones, which reinforce the image of the  Bangkok, the capital of Thailand, is also an emerging skyscraper city in Asia. Recently, Bangkok has completed its tallest building, which is the 314-m (1031-ft) MahaNakhon. The Magnolias Waterfront Residences Tower 1, however, will snatch the tallest building title from MahaNakhon soon by rising higher by merely one meter. Other important towers under construction include Capella Residences (300 m (983 ft)), Magnolias Waterfront Residences Tower 2 (280 m (919 ft)), and Canapaya Residences I (253 m (830 ft)). Lately, SOM has unveiled a vision for a 16-hectare "skyscraper village" in the city [2,14]. Named "One Bangkok", it will house more than 60,000 people and integrate a variety of parks and plazas, which will total eight hectares. Overall, in the first two decades of the 21st Century, Bangkok will add 95 skyscrapers to the 27 skyscrapers it built previously [2]. The new buildings are located in close proximity to older ones, which reinforce the image of the city's downtown ( Figure 6). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 205 m to 274 m, equaling a 134% increase (Table 16). city's downtown ( Figure 6). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 205 m to 274 m, equaling a 134% increase (Table 16).

London
As a global financial city, London has been facing an increased demand for commercial office space. Consequently, it has been building significant skyscrapers [41]. Despite the recent Brexit fears and continuing objection by local communities, tall building construction in London continues with full force. In his article "London's Future Skyline: the 455 New Skyscrapers Turning our Capital into Manhattan," Patrick Scott (2017) explains, "More tall buildings were constructed in London last year than ever before with 26 opening their doors and 455 more in the pipeline, according to new industry research" [42].  [8,43]. Overall, in the first two decades of the 21st century, London will add 80 skyscrapers to the 23 skyscrapers it built previously. Furthermore, 21 of

London
As a global financial city, London has been facing an increased demand for commercial office space. Consequently, it has been building significant skyscrapers [41]. Despite the recent Brexit fears and continuing objection by local communities, tall building construction in London continues with full force. In his article "London's Future Skyline: the 455 New Skyscrapers Turning our Capital into Manhattan," Patrick Scott (2017) explains, "More tall buildings were constructed in London last year than ever before with 26 opening their doors and 455 more in the pipeline, according to new industry research" [42].  (Table 17).  (Table 17).  Among all Russian cities, Moscow has been very active in constructing tall buildings. New buildings were spread out in multiple centers, which reinforce the polycentric nature of the city. Remarkably, Moscow International Business Center (also known as Moscow City) has some of Europe's tallest buildings with the following ranks: first, second, third, fifth, sixth, and seventh [3]. When completed in 2016, Federation Towers-Vostok Tower (374 m/1227 feet) became the tallest building in Russia and Europe. Overall, in the first two decades of the 21st Century, Moscow will add 151 skyscrapers to the 31 skyscrapers it built previously (Figure 8). The average height of the 10

Moscow
Among all Russian cities, Moscow has been very active in constructing tall buildings. New buildings were spread out in multiple centers, which reinforce the polycentric nature of the city. Remarkably, Moscow International Business Center (also known as Moscow City) has some of Europe's tallest buildings with the following ranks: first, second, third, fifth, sixth, and seventh [3]. When completed in 2016, Federation Towers-Vostok Tower (374 m/1227 feet) became the tallest building in Russia and Europe. Overall, in the first two decades of the 21st Century, Moscow will add 151 skyscrapers to the 31 skyscrapers it built previously (Figure 8). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 164 m to 314 m, equaling a 191% increase (Table 18). tallest skyscrapers in 2000 and 2020 will increase from 164 m to 314 m, equaling a 191% increase (Table 18).

New York City
Notably, New York City experienced a rapid pace of building exceptional high-rises. From 1930 to 2015, the city completed 56 200 m+ (656 ft+) buildings while, in the third quarter of 2015, the city had 25 200 m+ buildings under construction, which is almost half the figure that it took 85 years to complete [44,45]. Similarly, between 1930 and 2015, the city built eight supertalls (300 m+). There are currently 14 such buildings under construction or proposed to rise by 2020. Particularly, Manhattan is likely to build more than 30 200 m+ buildings in the next five to 10 years, as compared to merely four since 2010 [43]. Currently, NYC holds 32% of all North America's 200 m+ buildings (63/196) and will have 35% (90/255) by 2020. Furthermore, NYC holds six out of the 10 tallest skyscrapers in North America and it will sustain this rank in 2020. Overall, NYC "remains the world's skyscraper capital

New York City
Notably, New York City experienced a rapid pace of building exceptional high-rises. From 1930 to 2015, the city completed 56 200 m+ (656 ft+) buildings while, in the third quarter of 2015, the city had 25 200 m+ buildings under construction, which is almost half the figure that it took 85 years to complete [44,45]. Similarly, between 1930 and 2015, the city built eight supertalls (300 m+). There are currently 14 such buildings under construction or proposed to rise by 2020. Particularly, Manhattan is likely to build more than 30 200 m+ buildings in the next five to 10 years, as compared to merely four since 2010 [43]. Currently, NYC holds 32% of all North America's 200 m+ buildings (63/196) and will have 35% (90/255) by 2020. Furthermore, NYC holds six out of the 10 tallest skyscrapers in North America and it will sustain this rank in 2020. Overall, NYC "remains the world's skyscraper capital and a long-time established center of global business," as Richard Barkham and colleagues explain in their recent article (2017) titled "Reaching for the Sky: The Determinants of Tall Office Development in Global Gateway Cities" [44] (p. 24). Overall, in the first two decades of the 21st Century, New York City will add 304 skyscrapers to the 590 skyscrapers it built previously. Most of these buildings reinforce existing clusters in Midtown, Lower Manhattan and along Central Park [20]. The city's new 38 200 m+ skyscrapers reinforce existing clusters in which 20 are located in Midtown and 16 in Lower Manhattan. Similarly, all new eight 300 m+ skyscrapers reinforce these two clusters, six in Midtown and two in Lower Manhattan (Figure 9). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 314 m to 398 m, equaling a 127% increase (Table 19).  (Figure 9). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 314 m to 398 m, equaling a 127% increase (Table 19).

Chicago
Although Chicago and the State of Illinois at large have been losing their population rates in the past two decades, downtown Chicago has been gaining population and building new skyscrapers. City planners focus on reviving the Loop Community (Chicago's CBD) because they view it to be critical for the city's long-term sustainable economic growth. Salient examples of skyscrapers (under   [3]. Overall, in the first two decades of the 21st Century, Chicago will add 117 skyscrapers to the 207 skyscrapers it built previously. Most of these buildings reinforce existing clusters such as the Chicago Loop ( Figure 10). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 308 m to 326 m, equaling a 106% increase (Table 20).  [3]. Overall, in the first two decades of the 21st Century, Chicago will add 117 skyscrapers to the 207 skyscrapers it built previously. Most of these buildings reinforce existing clusters such as the Chicago Loop ( Figure 10). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 308 m to 326 m, equaling a 106% increase (Table 20).

Miami
Miami has been experiencing a boom in high-rise construction. Among the significant towers rising in the city are Panorama Tower, Brickell Flatiron, One Thousand Museum, Miami Worldcenter, and Grove at Grand Bay. Soaring to 252 m (828 ft), Panorama Tower has recently snatched the city's tallest building title from the 240 m (789 ft) Four Seasons Hotel & Tower. Overall, in the first two decades of the 21st Century, Miami will add 93 skyscrapers to the 27 skyscrapers it built previously. All new skyscrapers reinforce existing skyscraper clusters in the downtown and along the Atlantic shore ( Figure 11). Many of the new skyscrapers fill spatial gaps, which creates a stronger continuous

Miami
Miami has been experiencing a boom in high-rise construction. Among the significant towers rising in the city are Panorama Tower, Brickell Flatiron, One Thousand Museum, Miami Worldcenter, and Grove at Grand Bay. Soaring to 252 m (828 ft), Panorama Tower has recently snatched the city's tallest building title from the 240 m (789 ft) Four Seasons Hotel & Tower. Overall, in the first two decades of the 21st Century, Miami will add 93 skyscrapers to the 27 skyscrapers it built previously. All new skyscrapers reinforce existing skyscraper clusters in the downtown and along the Atlantic shore ( Figure 11). Many of the new skyscrapers fill spatial gaps, which creates a stronger continuous skyline. Three of the five 200 m+ skyscrapers are located in the southern part of the downtown and two are in the upper part. Before and after 2000, the city has no supertall buildings [2]. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 154 m to 219 m, equaling a 142% increase (Table 21). skyline. Three of the five 200 m+ skyscrapers are located in the southern part of the downtown and two are in the upper part. Before and after 2000, the city has no supertall buildings [2]. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 154 m to 219 m, equaling a 142% increase (Table 21).   (1070 ft)) has become the city's tallest building. Overall, in the first two decades of the 21st Century, San Francisco will add 29 skyscrapers to the 67 skyscrapers it built previously [2]. All these buildings reinforce the current skyscraper cluster in the downtown area and they are located within one square mile ( Figure 12). They are also in close proximity to major mass transit services. In addition, the city's only two 200 m+ skyscrapers including the 300 m+ Salesforce Tower are located at the epicenters of these transit areas.   (1070 ft)) has become the city's tallest building. Overall, in the first two decades of the 21st Century, San Francisco will add 29 skyscrapers to the 67 skyscrapers it built previously [2]. All these buildings reinforce the current skyscraper cluster in the downtown area and they are located within one square mile ( Figure 12). They are also in close proximity to major mass transit services. In addition, the city's only two 200 m+ skyscrapers including the 300 m+ Salesforce Tower are located at the epicenters of these transit areas. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 194 m to 221 m, equaling a 114% increase (Table 22). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 194 m to 221 m, equaling a 114% increase (Table 22).

Melbourne
Melbourne, which is the coastal capital of the southeastern Australian state of Victoria, has been very active in constructing tall buildings. It has begun constructing the 317 m (1040 ft), 100-story Australia 108, which will be the tallest building in Oceana when completed in 2020 [6]. . Melbourne will build more distinct skyscrapers including the Collins Arch and Premier Tower soon [2]. Overall, in the first two decades of the 21st century, Melbourne will add 116 skyscrapers to the 36 skyscrapers it built previously. The overwhelming

Melbourne
Melbourne, which is the coastal capital of the southeastern Australian state of Victoria, has been very active in constructing tall buildings. It has begun constructing the 317 m (1040 ft), 100-story Australia 108, which will be the tallest building in Oceana when completed in 2020 [6]. Other notable towers under construction include the 88-story Aurora Melbourne Central (2019), the 78-story Premier Tower (2020), the 75-story Victoria One (2018), and the 71-story Swanston Central (2019). Recently, Melbourne completed the 69-story Vision Apartments (2016), the 68-story 568 Collins Street (2015), and the 59-story Abode318 (2015). Melbourne will build more distinct skyscrapers including the Collins Arch and Premier Tower soon [2]. Overall, in the first two decades of the 21st century, Melbourne will add 116 skyscrapers to the 36 skyscrapers it built previously. The overwhelming majority of these skyscrapers are located in the city's downtown and along the Yarrar River. Fourteen of the 200 m+ are located near the upper and lower edges of the downtown including one supertall. Melbourne has currently 48 tall buildings under construction. Overall, this massive addition in the heart of the downtown strengthens the image of Melbourne's city center (Figure 13). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 216 m to 266 m, equaling a 123% increase (Table 23). majority of these skyscrapers are located in the city's downtown and along the Yarrar River. Fourteen of the 200 m+ are located near the upper and lower edges of the downtown including one supertall. Melbourne has currently 48 tall buildings under construction. Overall, this massive addition in the heart of the downtown strengthens the image of Melbourne's city center ( Figure 13). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 216 m to 266 m, equaling a 123% increase (Table 23).

Sydney
Sydney continues to experience urban and economic growth. However, it is physically constrained by the harbor, which has reinforced building skyward. Sydney has started on constructing the 271 m  [16,21]. Overall, in the first two decades of the 21st Century, Sydney will add 62 skyscrapers to the 83 skyscrapers it built previously. Forty of the new skyscrapers are located in the downtown near the older ones. Additionally, all nine 200 m+ are located in the same area as well. The rest of the added skyscrapers (22) are dispersed throughout the city. Sydney has no supertalls. Consequently, the added 40 skyscrapers in the downtown area strengthen its image as the city's center ( Figure 14). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 208 m to 236 m, equaling a 114% increase (Table 24) [2,15]. story International Towers Tower 3 (2016), and the 40-story 200 George Street (2016) [16,21]. Overall, in the first two decades of the 21st Century, Sydney will add 62 skyscrapers to the 83 skyscrapers it built previously. Forty of the new skyscrapers are located in the downtown near the older ones. Additionally, all nine 200 m+ are located in the same area as well. The rest of the added skyscrapers (22) are dispersed throughout the city. Sydney has no supertalls. Consequently, the added 40 skyscrapers in the downtown area strengthen its image as the city's center ( Figure 14). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 208 m to 236 m, equaling a 114% increase (Table 24) [2,15].

Dubai
Among all Middle Eastern cities, Dubai has been the most conspicuous "instant" skyscraper city. Not long ago, Dubai was an unnoticeable, small village with no tall buildings at all. Today, it is a home to the world's tallest building known as the 828 m (2716 ft) Burj Khalifa as well as other  3.5. The Middle East

Dubai
Among all Middle Eastern cities, Dubai has been the most conspicuous "instant" skyscraper city. Not long ago, Dubai was an unnoticeable, small village with no tall buildings at all. Today, it is a home to the world's tallest building known as the 828 m (2716 ft) Burj Khalifa as well as other remarkable skyscrapers [6,20,21]. Today, Dubai is home to the greatest number of 200 m+ and 300 m+ buildings, which surpasses major skyscraper cities in the world including Hong Kong, New York City, Chicago, and Shanghai. Among the remarkable towers under construction are the 111-story Entisar Tower (2020), the 104-story Marina 106 (2019), the 80-story S Residence (2020), 74-story The Address Jumeirah Resort and Spa at Jumeirah Beach Residence (2020), the 74-story Al Habtoor City Tower 1 (2017), the 74-story Al Habtoor City Tower 2 (2017), and the 20-story Opus (2018). Overall, in the first two decades of the 21st Century, Dubai will add 319 skyscrapers to the 28 skyscrapers it built previously. 144 of these skyscrapers are built along Shaikh Zayed road by stretching southwest and connecting to the new "downtown" near Burj Khalifa ( Figure 15). Significantly, 174 skyscrapers were built away from the downtown (about 10 miles southwest) in new districts such as Dubai Marina [2]. Fifty-seven 200 m+ skyscrapers (including 17 supertalls) are placed near Burj Khalifa while 34 skyscrapers (including 10 supertalls) are located in Dubai Marina. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 198 m to 424 m, equaling a 213% increase (Table 25). remarkable skyscrapers [6,20,21]. Today, Dubai is home to the greatest number of 200 m+ and 300 m+ buildings, which surpasses major skyscraper cities in the world including Hong Kong, New York City, Chicago, and Shanghai. Among the remarkable towers under construction are the 111-story Entisar Tower (2020), the 104-story Marina 106 (2019), the 80-story S Residence (2020), 74-story The Address Jumeirah Resort and Spa at Jumeirah Beach Residence (2020), the 74-story Al Habtoor City Tower 1 (2017), the 74-story Al Habtoor City Tower 2 (2017), and the 20-story Opus (2018). Overall, in the first two decades of the 21st Century, Dubai will add 319 skyscrapers to the 28 skyscrapers it built previously. 144 of these skyscrapers are built along Shaikh Zayed road by stretching southwest and connecting to the new "downtown" near Burj Khalifa ( Figure 15). Significantly, 174 skyscrapers were built away from the downtown (about 10 miles southwest) in new districts such as Dubai Marina [2]. Fifty-seven 200 m+ skyscrapers (including 17 supertalls) are placed near Burj Khalifa while 34 skyscrapers (including 10 supertalls) are located in Dubai Marina. The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 198 m to 424 m, equaling a 213% increase (Table 25).

Doha
Doha, the capital of Qatar, is also among the most active cities in building significant skyscrapers in the Middle East. In the first two decades of the 21st Century, Doha will add 48 skyscrapers to the only one it built previously. Overall, all these new skyscrapers concentrate in the downtown region ( Figure 16). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 115 m to 243 m, equaling a 211% increase (Table 26). Doha, the capital of Qatar, is also among the most active cities in building significant skyscrapers in the Middle East. In the first two decades of the 21st Century, Doha will add 48 skyscrapers to the only one it built previously. Overall, all these new skyscrapers concentrate in the downtown region ( Figure 16). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 115 m to 243 m, equaling a 211% increase (Table 26).  Riyadh, the Capital of Saudi Arabia, is also experiencing an unprecedented pace of building significant skyscrapers. Being the seat of government, this "conservative" city has recently broken its long tradition of maintaining a low-rise city profile. In the first two decades of the 21st Century, Riyadh will add 43 buildings to the only three skyscrapers it built previously. Interestingly, all new skyscrapers attain a linear stretch along a major downtown boulevard ( Figure 17). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 156 m to 278 m, equaling a 178% increase (Table 27).

Riyadh
Riyadh, the Capital of Saudi Arabia, is also experiencing an unprecedented pace of building significant skyscrapers. Being the seat of government, this "conservative" city has recently broken its long tradition of maintaining a low-rise city profile. In the first two decades of the 21st Century, Riyadh will add 43 buildings to the only three skyscrapers it built previously. Interestingly, all new skyscrapers attain a linear stretch along a major downtown boulevard ( Figure 17). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 156 m to 278 m, equaling a 178% increase (Table 27).    (525 ft)). Overall, in the first two decades of the 21st Century, Tel Aviv will add 42 skyscrapers to the 11 skyscrapers it built previously. By then, the city will have only two 200 m+ skyscrapers called the Azrieli Sarona (2017) and ToHA Tower 1 (2020) [2]. The new skyscrapers follow a linear stretch ( Figure 18). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 136 m to 192 m, equaling a 142% increase (Table 28).   (525 ft)). Overall, in the first two decades of the 21st Century, Tel Aviv will add 42 skyscrapers to the 11 skyscrapers it built previously. By then, the city will have only two 200 m+ skyscrapers called the Azrieli Sarona (2017) and ToHA Tower 1 (2020) [2]. The new skyscrapers follow a linear stretch ( Figure 18). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 136 m to 192 m, equaling a 142% increase (Table 28).

São Paulo
Overall, in the first two decades of the 21st Century, São Paulo, Brazil, will add 70 skyscrapers to the 17 skyscrapers it built previously. While some of the new skyscrapers reinforce existing "downtown" clusters, others have created a new significant linear stretch ( Figure 19). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 148 m to 160 m, equaling a 108% increase (Table 29).

São Paulo
Overall, in the first two decades of the 21st Century, São Paulo, Brazil, will add 70 skyscrapers to the 17 skyscrapers it built previously. While some of the new skyscrapers reinforce existing "downtown" clusters, others have created a new significant linear stretch ( Figure 19). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 148 m to 160 m, equaling a 108% increase (Table 29).   [2]. Overall, in the first two decades of the 21st Century, Panama City will add 63 skyscrapers to the seven skyscrapers it built previously. While some of the new skyscrapers reinforce the existing center, others form a new one, thereby initiating a polycentric city ( Figure 20). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 156 m to 253 m, equaling a 162% increase (Table 30).   [2]. Overall, in the first two decades of the 21st Century, Panama City will add 63 skyscrapers to the seven skyscrapers it built previously. While some of the new skyscrapers reinforce the existing center, others form a new one, thereby initiating a polycentric city ( Figure 20). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 156 m to 253 m, equaling a 162% increase (Table 30).  Mexico City, which is the capital of Mexico and the most populous city in North America, also experienced a rapid pace of building tall towers. In the first two decades of the 21st Century, Mexico City will add 53 skyscrapers to the 27 skyscrapers that it built previously. Most of the new skyscrapers reinforce existing clusters ( Figure 21). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 160 m to 213 m, equaling a 133% increase (Table 31).  Mexico City, which is the capital of Mexico and the most populous city in North America, also experienced a rapid pace of building tall towers. In the first two decades of the 21st Century, Mexico City will add 53 skyscrapers to the 27 skyscrapers that it built previously. Most of the new skyscrapers reinforce existing clusters ( Figure 21). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 160 m to 213 m, equaling a 133% increase (Table 31).

Africa
Nairobi Nairobi, which is the capital and the largest city of Kenya, experienced a moderate pace of building skyscrapers. In the first two decades of the 21st century, Nairobi will add six skyscrapers to the four skyscrapers it built previously. Most of these skyscrapers reinforce existing locations of tall buildings, which improve the image of the downtown (Figure 22). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 117 m to 159 m, equaling a 159% increase (Table 32).

Africa
Nairobi Nairobi, which is the capital and the largest city of Kenya, experienced a moderate pace of building skyscrapers. In the first two decades of the 21st century, Nairobi will add six skyscrapers to the four skyscrapers it built previously. Most of these skyscrapers reinforce existing locations of tall buildings, which improve the image of the downtown (Figure 22). The average height of the 10 tallest skyscrapers in 2000 and 2020 will increase from 117 m to 159 m, equaling a 159% increase (Table 32).

Counts
Upon reviewing the examined cities, we find that in the 21st Century all cities have been adding significant numbers of skyscrapers. Cities that added the greatest number of skyscrapers follow this order: Dubai, New York City, Shenzhen, Shanghai, Moscow, Chicago, Melbourne, Bangkok, Miami, London, São Paulo, Panama City, Sydney, Beijing, Mexico City, Doha, Riyadh, Tel Aviv, San Francisco, and Nairobi (Table 33). When we calculate the percentage of increase of skyscrapers, we find that:

Counts
Upon reviewing the examined cities, we find that in the 21st Century all cities have been adding significant numbers of skyscrapers. Cities that added the greatest number of skyscrapers follow this order: Dubai, New York City, Shenzhen, Shanghai, Moscow, Chicago, Melbourne, Bangkok, Miami, London, São Paulo, Panama City, Sydney, Beijing, Mexico City, Doha, Riyadh, Tel Aviv, San Francisco, and Nairobi (Table 33). When we calculate the percentage of increase of skyscrapers, we find that: These findings confirm the views that the Chinese and Middle Eastern cities are the "truest" emerging skyscraper cities in the 21st century. The percentages of increase (550%-4800%) are very remarkable to the extent that one may question the soundness of these activities. The urban transition is so quick and immense and, therefore, some unintended consequences are inevitable. For example, recent research indicates that millions of homes in new high-rise buildings in China are vacant, which highlights the problem of building too many high-rises in a very short period of time [46].
Certainly, the Chinese government has enormously promoted constructing high-rise cities to house new massive urban population flocking from rural areas. However, Chinese people have largely shunned these developments because they disliked the design, layout, architectural styles, schools, and amenities. Certainly, these cities were developed in a rushed manner, attempting to house hundreds of thousands of people in a few years and, consequently, city planners disengaged residents from the design process and deprived them from voicing their preferences. The design process did not take into consideration that many of the intended inhabitants were villagers who were accustomed to low-rise living instead of high-rise living [46].
Ordos Kangbashi in China is one of several new high-rise cities sitting almost vacant because of these reasons. Although Ordos Kangbashi offers beautiful architecture, attractive plazas, and state-of the-art recreational spaces, this stillborn city lacks the essential ingredients of success including affordability, socio-economic vitality, and employment opportunities [46]. Furthermore, Ordos Kangbashi's location is detrimental for being remote from any settlements. It is located in the Inner Mongolia's vast steppe that features a harsh climate and lacks basic life essentials such as water and vegetation. Consequently, while planners have designed this new city to house over a million of inhabitants, barely a few thousand people ever settled.
Chinese people have nicknamed these cities "modern ghost cities" because they evoke an eerie sensation promoted by silent streets, vacant high-rises, empty parks, and dead public spaces. In turn, this prevailing negative image has further discouraged people from considering moving into these new high-rise cities [46]. This local phenomenon has promoted global consciousness and a negative image about high-rise developments. David Nicholson-Cole concisely captures these notions in an article Rise of the Glass Giants: How Modern Cities are Forcing Skyscrapers to Evolve. He explains: "Mixed-use towers make the best use of land and are more resilient to economic shocks because the rental income comes from lots of different sources-and the flows of people are balanced instead of peaking twice daily" [48]. Furthermore, mixed-use towers have the potential to use resources and waste efficiently. For example, the water system can capture graywater from residential spaces (which generate a larger amount of graywater) and transfer recovered water to the cooling system of office spaces where water consumption is high and potable water use is low. This type of system drastically reduces the use of potable water in office spaces (which is generally used for the cooling system); consequently, it results in significant savings.
Furthermore, as cities build taller buildings, the smaller floor plates at the upper floors fit functional use that requires fewer tenants such as in hotel rooms and small apartments. Smaller floor plates at upper floors are the results of structural requirements to withstand powerful wind at higher altitudes. In addition to residential and hotel spaces, upper floors can host amenities and services such as restaurants, café, sky gardens, and observation decks. For instance, The Shard in London integrates a viewing gallery at the upper floors. The Marina Bay Sands in Singapore incorporates a SkyPark that offers a wide range of amenities and the Wilshire Grand Center in Los Angeles, CA (completed recently) features a sky-deck with café and al fresco dining. Remarkably, Lotte World Tower dedicates its top 10 floors for extensive public use and entertainment facilities. Observation decks are a lucrative business and increasingly are coupled with amenities such as bars, restaurants, café, and interactive media displays, glass floors, and the like. "These unique spaces quench humanity's innate desire to see what the world looks like from the tops of these buildings and they, in turn, give viewers a new perspective on our cities" [49] (p. 13).
As such, over the past two decades, the mixed-use tower concept has been further developed and refined. Additional examples of mixed-use towers include Shanghai Tower, Singapore's PS100, Hong Kong's Hysan Place, Shenzhen's Hanking Center Tower (under construction), and the proposed redevelopment of the Khalil Gibran International Academy in New York City. Overall, the mixed-use tower represents a new architectural/planning paradigm that aims to create a sustainable "vertical neighborhood" or "vertical village" characterized by being self-contained, self-sufficient, and resilient [50].

Residential Supertalls
When we review functional change against height, we discover an important trend of building residential supertalls. In the past century, the tallest residential towers prevailed in pioneering vertical cities such as Hong Kong, New York City, and Chicago. Today, however, new places such as Dubai, Jeddah, and Abu Dhabi (Middle East), Shanghai and Beijing (China), and Mumbai (India) have joined the race of building the highest residential towers. Other cities that are building significant residential towers include Busan (South Korea), Moscow (Russia), Sydney, Gold Coast, and Melbourne (Australia), Bangkok (Thailand), Shanghai and Shenzhen (China), Istanbul (Turkey), and Toronto (Canada) among others [2,15].
This phenomenon started around the turn of the new century, but it slowed down after the 9/11 catastrophic event and the economic recession between 2007 and 2011. Later, taller residential

Spatial Patterns
Upon reviewing the spatial patterns of skyscrapers, we find that, before the turn of the century, many cities featured fragmented arrangement of skyscrapers. Examples of these cities include Nairobi, Tel Aviv, Beijing, Bangkok, Doha, Panama City, São Paulo, Riyadh, London, Dubai, and Shenzhen. However, in 2020, the spatial distributions of skyscrapers in these cities vary since they became semi monocentric, monocentric, super monocentric, semi polycentric, polycentric, and super polycentric. Other cities change from monocentric to super monocentric, e.g., Miami, San Francisco, Sydney, and Melbourne while some cities such as Shanghai, Moscow, Chicago, and New York change from polycentric to super polycentric (Table 36).
However, a common theme among these various spatial patterns stresses the principle of clustering. As stated earlier, clustering of tall buildings fosters urban synergy among the provided diverse activities and specialized services. The high concentration of activities creates "knowledge spillovers" between firms in the same sector and across sectors that lead to increased innovation. Clustering also enhances the image of an existing downtown area or a new one. A group of tall buildings accentuates their height effect. In other words, the idea that the whole can be greater than the sum of the parts applies here. The great image of a modern downtown often comes from the cumulative total of its tall buildings fitting together instead of just from scattered buildings. Ideally, every tall building should be a new masterpiece of the evolving metropolis, enriching the ever-changing urban collage. This collective work of art should form an unflinching record of our scientific and artistic development. Many of the newly constructed tall buildings offer a fresh, modern style made possible by advancements in computer design and improvements in building materials.
Furthermore, the agglomeration of tall, mixed-use buildings in conjunction with mass-transit systems can provide a defined spatial structure for the city, which improves its urban functionality, boosts its commercial synergy, and enhances neighborhood place making. When we cluster tall buildings near mass-transit nodes, these buildings visually define the locations of these transit centers, which highlights their functional significance. In this way, taller buildings can provide reference points that make navigating the city more intuitive while drawing attention to mass-transit nodes. Because of their prominent height and physical distinction, clustered nodes create contrasts with low-rise buildings and streetscapes. These nodes have the potential to break the visual monotony established by buildings of a similar height and architectural design [51]. The Hudson Yards development in New York (under construction) clusters mixed-use towers around mass transit. This massive 18-million-square-foot complex will create a vibrant socio-economic magnet and form an important focal point in Manhattan's West Side. At a cost of $20 billion, it will contain a cluster of skyscraper masterpieces including: 10 Hudson Yards, 15 Hudson Yards, 30 Hudson Yards, 35 Hudson Yards, 50 Hudson Yards, 55 Hudson Yards, 15 Penn Plaza, 3 Manhattan West, 3 Hudson Boulevard, and Hudson Rise Hotel. In addition to marvelous engineering (required to construct skyscrapers over active large rail yards), the new complex will integrate state-of-the-art technologies (e.g., building management tools, security technology, and elevator systems). Similarly, in Beijing, 20 tall buildings, ranging from 150 m to 350 m in height, will cluster around Zun Tower, which will rise to 528 m (1732 ft) and become the tallest building in Beijing and the eighth tallest building in China.
In the same vein, London has embraced a policy of clustering tall buildings around key rail stations and creating mixed-use "spatial magnets" [30]. London's neighborhoods have been attempting to create social life and vibrancy after office hours and during weekends by integrating residential developments, retails, amenities, services, and transit centers. In this regard, David Nicholson-Cole explains, "To cope with the pressures of dynamic mass-transit systems and rising land values, urban citizens must grow accustomed to living-as well as working-in high-rise developments, clustered around key transport nodes" [48]. Union Square in Hong Kong has clustered distinct high-rises including ICC-the city's tallest building-within a small area of about 30 acres (12 hectares). Internally, the iconic transit station gives Union Square a unique identity. Similarly, in Dubai, planners have clustered the world's tallest building (Burj Khalifa), world's biggest mall, and largest "dancing" fountain to give this section of Dubai's downtown a special character.

Conclusions
This paper elucidated the massive construction boom of tall buildings globally. All major cities in all the world's continents are actively constructing tall buildings. As such, planners, architects, and politicians must carefully study these developments because they are profoundly reshaping these cities. Tall buildings are urban planning game changers that alter the spatial patterns and physical character of cities. Planners must engage the public in examining these developments and work together to minimize their negative impacts and maximize their environmental, social, and economic benefits [52].
While it is impossible to provide a definitive scientific proof, this research, based on the provided review, predicts that tall building activities will continue in the near future for multiple reasons. Due to the influx of large numbers of people into cities from rural and suburban areas, numerous cities around the world will need to plan for rapid urban growth and many of them already do. If growth cannot be stopped, then it must be managed. The places that will thrive are those prepared to grow to improve a city's quality of life. The number of megacities with a population of 10 million or more has climbed from 1 in 1950 to 5 in 1975 to 14 in 1995, and to 23 in 2015. In addition, the number is expected to reach 30 by 2020 [39]. Megacities will continue to develop and grow, which is a phenomenon requiring more skyscrapers to avoid urban sprawl and other associated problems [53,54].
In the advent of rapid urbanization, urban theorist Carol Willis explained that cities will have three options: (1) horizontal overcrowding, (2) urban sprawl, and (3) vertical expansion. She views vertical expansion as the prevailing choice [21]. This view is shared by other urban theorists, architects, and urban designers such as Ken Yeang, Roger Trancik, Richard Rogers, and Trevor Boddy [28]. In addition, in this regard, Heller et al. wrote, "The forces which are shaping cities worldwide, related to the effects of population growth and urbanization, are compelling and unavoidable. In the near term, we should continue to see an expanding role for tall buildings in our urban fabric" [55] (p. 379).
Sustainability or sustainable development is increasingly making its way to the top of the agenda of planning and urban design policies and regulations. Sustainability promotes dense and mixed-use living that reduces travel time and carbon emission. Although vertical density is not the only way to achieve dense living, it is a viable option especially in areas where land is in demand and expensive [56,57].
Concerning the near future (10-15 years), numerous tall and supertall buildings are under construction and in the planning stage worldwide. As this review reveals, in China, numerous first-tier cities including Beijing, Shanghai, Shenzhen, and Nanjing are building high-rises and second-tier and third-tier cities are following suit. The same is happening but to a less extent in India and Japan. Furthermore, major U.S. cities such as Chicago, New York City, Miami, and Houston continue to build and plan to build significant high-rises [58].
Regarding the distant future, no one has a crystal ball to see what will happen 50 to 100 years from today, but it can be predicted that urban population will increase as explained earlier. Therefore, we may ask an important question: Can cities expand laterally without sprawling and destroying valuable agricultural land? The migration of rural population to cities in developing countries will continue unabated. People want to move to where the din and bustle of other people, a flurry of activities, social and recreational services, and above all, employment opportunities are. Because of this, once people become comfortable with urban living, they will likely embrace it for a long time.
The tall building and the city are intertwined and their prospects are tied together. When a tall building is constructed, it must work well with the city both at present and in the future. Both tall buildings and cities are multi-faceted and multi-disciplinary. They both have a common goal of realizing a sustainable future for their posterity. Therefore, the integration of their respective systems and professional collaboration among planners, architects, engineers, developers, and contractors as well as cooperation and support of the city officials and the public at large are all essential. For the 21st century, it can be predicted that skyscrapers will have a significant international role in the global village [59,60].

Future Research
This paper offers the groundwork for many future studies on cities and tall buildings nexus. Each examined city requires deeper analysis and investigation of the various socio-economic, population, and demographic variables. Comparative studies of cities within or across continents