Editing Megacity

{{Short description|Metropolitan area with a total population in excess of ten million people}}
{{Redirect|Mega city||Megacity (disambiguation)}}
{{pp-protected|small=yes}}

{{World city population tables}}

A '''megacity''' is a very large [[city]], typically with a [[population]] of more than 10 million people.<ref>{{cite web|title=Definition of megacity in English|url=https://en.oxforddictionaries.com/definition/megacity|archive-url=https://web.archive.org/web/20180327212555/https://en.oxforddictionaries.com/definition/megacity|url-status=dead|archive-date=March 27, 2018|publisher=Oxford Dictionaries|access-date=27 March 2018}}</ref><ref>{{cite web|title=megacity meaning |url=https://dictionary.cambridge.org/dictionary/english/megacity|website=Cambridge English Dictionary |access-date=27 March 2018|language=en}}</ref><ref>{{cite journal |last1=Safarik |first1=Daniel |last2=Ursini |first2=Shawn |last3=Wood |first3=Antony |title=Megacities and tall buildings: symbiosis |bibcode-access=free |journal=E3S Web of Conferences |date=2018 |volume=33 |pages=01001 |doi=10.1051/e3sconf/20183301001 |bibcode=2018E3SWC..3301001S |doi-access=free}}</ref><ref>{{cite journal |last1=Yeung |first1=Y. -M. |last2=Shen |first2=Jianfa |last3=Kee |first3=Gordon |title=Megacities |journal=International Encyclopedia of Human Geography (Second Edition) |date=2020 |pages=31–38 |doi=10.1016/B978-0-08-102295-5.10284-7|isbn=9780081022962 |s2cid=241912815 }}</ref> The [[United Nations Department of Economic and Social Affairs]] (UN DESA) in its 2018 "World Urbanization Prospects" report defines megacities as [[urban agglomeration]]s with over 10 million inhabitants.<ref name = "UrbanizationProspects2018"/> A [[University of Bonn]] report holds that they are "usually defined as [[metropolitan area]]s with a total population of 10 million or more people".<ref>{{cite web|last1=Kötter|first1=Theo|last2=Friesecke|first2=Frank|title=Developing urban Indicators for Managing Mega Cities|url=http://siteresources.worldbank.org/INTIE/Resources/T_Koetter.doc|website=[[World Bank]]|publisher=[[University of Bonn]]|date=1 March 2009}}</ref> Elsewhere in other sources, from five to eight million is considered the minimum threshold, along with a [[population density]] of at least 2,000 per square kilometre.<ref name=seos>{{cite web|url=http://www.seos-project.eu/modules/landuse/landuse-c02-s03-p01.html |title=Land Use and Land Use Change |website=seos-project.eu|access-date=March 26, 2018}}</ref> The terms [[conurbation]], [[metropolis]], and metroplex are also applied to the latter.<ref name=seos/>

The total number of megacities in the world varies between different sources and their publication dates. The world had 32 according to EU [[Global Human Settlement Layer]] (in 2024), 33 according to UN DESA (in 2018), 39 according to the OECD, 44 according to ''[[Demographia]]'' (in 2023), and 45 according to ''CityPopulation.de'' (in 2023). In total, 53 unique places are mentioned as megacities across these sources. A good percentage of these urban agglomerations are in [[China]] and [[India]]. The other four countries with more than one megacity are [[Brazil]], [[Japan]], [[Pakistan]], and the [[United States]]. African megacities are present in [[Nigeria]], [[Egypt]], [[South Africa]], [[Angola]] and [[the DRC]]; [[Europe]]an megacities are present in [[Russia]], [[France]], the [[United Kingdom]], and [[Turkey]] (also in [[Asia]]); megacities can be found in [[Latin America]] in the countries of Brazil, [[Mexico]], [[Colombia]], [[Peru]], and [[Argentina]].

Some sources identify the [[Greater Tokyo Area]] as the largest megacity in the world,<ref name = "UrbanizationProspects2018"/><ref name="Demographia"/> while some others give the title to the [[Pearl River Delta]] in China.<ref name="citypopulation"/><ref>{{cite web|url=https://www.theguardian.com/cities/2015/jan/28/china-pearl-river-delta-overtake-tokyo-world-largest-megacity-urban-area|title=China's Pearl River Delta overtakes Tokyo as world's largest megacity|work=[[The Guardian]]|first=Nick|last=Van Mead|date=January 28, 2015|access-date=April 3, 2020}}</ref><ref>{{cite web|url=https://www.businessinsider.com/chinese-mega-city-has-more-people-than-canada-argentina-or-australia-2015-7?IR=T|title=The world's largest megacity already has more people than Canada, Argentina, or Australia|first=Chris|last=Weller|date=July 8, 2015|work=[[Business Insider]]|access-date=April 3, 2020}}</ref>

==Urban Metric System==
{{main|Settlement hierarchy}}
{{rewrite section|date=February 2025}}
Since, presently, urban data are based on arbitrary definitions that vary from country to country and from year or census to the next, making them difficult to compare, an Urban Metric System (UMS) has been conceived that could correct the problem,<ref>{{cite web |url=https://authors.elsevier.com/sd/article/S1757-7802(24)00342-1|title=Estimating urban sprawl standards by means of the Urban Metric System |last1=Tellier |first1=Luc-Normand |last2=Quesnel |first2=Frédéric |last3=Bur |first3=Justin |publisher=Regional Science Policy and Planning |date=November 2024 }}</ref> since it allows computing the urban area limits and central points, and it can be applied in the same way to all past, present and future population and job distributions. 

It is based on vector field calculations obtained by assuming that, in a given space, all inhabitants and jobs exert the same attractive force ''A'' and repulsive force ''R''. The net force (''A'' - ''R'') exerted by each inhabitant or job is given by [1/(1 + ''d'')] - [1/( ''β'' + ''d''/2)], where ''d'' = distance and ''β'' is the only parameter. 

UMS distinguishes the following types of urban areas(including "patropolises" that are tantamount to "megacities"), each type corresponding to a given value of ''β'':
{| class="wikitable"
|+
!
!Urban area
!Distance at which the attractive force = the repulsive force 
!Value of ''β''
|-
!1
!Central city
!10 km
!6
|-
!2
!Agglomeration
!20 km
!11
|-
!3
!Metropolis
!40 km
!21
|-
!4
!Patropolis
!80 km
!41
|-
!5
!Megalopolis
!160 km
!81
|-
!6
!Urban system
!320 km
!161
|-
!7
!Urban macrosystem
!640 km
!321
|-
!8
!Continental system
!1,280 km
!641
|-
!9
!Intercontinental system
!2,560 km
!1,281
|-
!10
!World system
!5,120 km
!2,561
|}

UMS has been applied to some Canadian cases since 2018, but the data presented in this article are still based on the various existing national definitions, which are disparate.

==List of megacities==
{{sticky header}}{{static row numbers}}
{| class="wikitable sortable plainrowheaders sticky-header-multi static-row-numbers" style="margin:auto; margin:auto; text-align: center;"
|-
!scope="col" rowspan=2| Megacity
!scope="col" rowspan=2 class="unsortable"| Image
!scope="col" rowspan=2| Country
!scope="col" rowspan=2| Region
! colspan="5" scope="col" | Estimated population
|- class="static-row-numbers-norank"
!scope="col"| Citypopulation.de<br />(2025)<ref name="citypopulation">{{cite web |title=Major Agglomerations of the World - Population Statistics and Maps |url=http://www.citypopulation.de/en/world/agglomerations/ |website=City Population |access-date=16 May 2024}}</ref>
!scope="col"| [[Wendell Cox|Demographia]]<br />(2023)<ref name="Demographia">{{cite web|title=Demographia World Urban Areas, 19th Annual Edition|url=http://demographia.com/db-worldua.pdf|publisher=[[Demographia]]|date=August 2023|access-date=22 October 2023}}</ref>
![[Global Human Settlement Layer|GHSL]]<br />(2024)<ref name="GHS">{{cite web | url=https://human-settlement.emergency.copernicus.eu/ghs_ucdb_2024.php | title=Global Human Settlement - Urban Centre database R2024A - European Commission }}</ref>
! scope="col" | [[United Nations Department of Economic and Social Affairs|UN DESA]]<br />(2018)<ref name = "UrbanizationProspects2018">{{cite web|title=World Urbanization Prospects, The 2018 Revision|url=https://population.un.org/wup/Publications/Files/WUP2018-Report.pdf|publisher=[[United Nations Department of Economic and Social Affairs|UN DESA]]|page=77|date=7 August 2019|access-date=15 February 2023|archive-url=https://web.archive.org/web/20200318222514/https://population.un.org/wup/Publications/Files/WUP2018-Report.pdf|archive-date=18 March 2020|url-status=live}}</ref>
!scope="col"| [[OECD]]<br />(2020)<ref name = "OECD-FUA">{{cite web|title=OECD: FUAs and Cities|url=https://regions-cities-atlas.oecd.org/EFUA/x/x/T_T/2020|publisher=[[OECD]]|page=77|date=|access-date=10 October 2024}}</ref>
|-
!scope="row"| [[Bangalore]]
| [[File:UB City.jpg|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|IND}}
| [[South Asia]]
| 14,700,000
| 15,257,000
| 15,178,533
| 11,440,000
| 14,253,019
|-
!scope="row"| [[Bangkok]]
| [[File:0008871 - Krung Thep Bridge 001.jpg|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|THA}}
| [[Southeast Asia]]
| 21,800,000
| 18,884,000
|19,048,032
| 10,156,000
| 18,601,400
|-
!scope="row"| [[Beijing]]
| [[File:Skyline of Beijing CBD with B-5906 approaching (20211016171955).jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 21,500,000
| 18,883,000
|18,150,576
| 19,618,000
| 20,738,738
|-
!scope="row"| [[Bogotá]]
| [[File:Bogota z veže Colpatria (34432642862).jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|COL}}
| [[South America]]
| 10,600,000
| 10,252,000
|10,419,361
| 10,574,000
| 10,544,590
|-
!scope="row"| [[Buenos Aires]]
| [[File:High-rises of Puerto Madero (40022145164).jpg|120px]] 
| {{flagg|pspe|al=c|pref=List of cities in|ARG}}
| [[South America]]
| 16,800,000
| 15,748,000
|14,179,912
| 14,967,000
| 14,590,526
|-
!scope="row"| [[Cairo]]
| [[File:Cropped Cairo.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|EGY}}
| {{sort|Africa|[[North Africa]]}}
| 22,800,000
| 22,679,000
|25,230,325
| 20,076,000
| 27,925,433
|-
!scope="row"| [[Changsha]]
| [[File:Skyline with Xiang River.png|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 11,500,000
| {{red|5,065,000}}
|{{red|3,246,971}}
| {{red|4,345,000}}
| {{red|4,009,195}}
|-
!scope="row"| [[Chengdu]]
| [[File:雪山下的成都市天际线 Chengdu skyline with snow capped mountains.jpg|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 18,100,000
| 15,016,000
|{{red|5,609,627}}
| {{red|8,813,000}}
| {{red|9,768,500}}
|-
!scope="row"| [[Chennai]]
| [[File:Chennai skyline.JPG|120px]] 
| {{flagg|pspe|al=c|pref=List of cities in|IND}}
| [[South Asia]]
| 12,900,000
| 11,570,000
|11,466,400
| 10,456,000
| 11,528,915
|-
!scope="row"| [[Chongqing]]
| [[File:重庆市渝中区半岛.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 12,900,000
| 12,653,000
|{{red|8,449,690}}
| 14,838,000
| {{red|8,913,804}}
|-
!scope="row"| [[Delhi]]
| [[File:Skyline at Rajiv Chowk.JPG|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|IND}}
| [[South Asia]]
| 35,700,000
| 31,190,000
|31,422,508
| 28,514,000
| 33,495,554
|-
!scope="row"| [[Dhaka]]
|[[File:Dhaka 14th March (32624769393).jpg|alt=|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|BAN}}
| [[South Asia]]
| 23,100,000
| 19,134,000
|37,307,160
| 19,578,000
| 22,762,988
|-
!scope="row"| [[Dongguan]]
| [[File:东莞市环城路 东莞水道特大桥 航拍DJI 0744.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| {{n/a|Combined with<br />[[Guangzhou]]}}
| 10,753,000
| {{n/a|Combined with<br />[[Guangzhou]]}}
| {{red|7,360,000}}
| {{n/a|Combined with<br />[[Guangzhou]]}}
|-
!scope="row"| [[Guangzhou]]
| [[File:Guangzhou Twin Towers.jpg|alt=|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 72,700,000
| 27,119,000
|42,987,704
| 12,638,000
| 16,650,322
|-
!scope="row"| [[Hangzhou]]
| [[File:20201012从钱塘江江面上空观看钱江新城.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 14,600,000
| {{red|9,618,000}}
|{{red|6,387,064}}
| {{red|7,236,000}}
| {{red|9,013,951}}
|-
!scope="row"| [[Ho Chi Minh City]]
| [[File:Saigon skyline night view.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|Vietnam}}
| [[Southeast Asia]]
| 14,300,000
| 14,953,000
|14,557,830
| {{red|8,145,000}}
| 14,247,593
|-
!scope="row"| [[Hyderabad]]
| [[File:Manjeera Trinity corporate building, KPHB.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|India}}
| [[South Asia]]
| 11,700,000
| {{red|9,797,000}}
|{{red|9,455,230}}
| {{red|9,482,000}}
| {{red|9,706,886}}
|-
!scope="row"| [[Istanbul]]
| [[File:View of Levent financial district from Istanbul Sapphire.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|TUR}}
| [[Europe]]<hr>[[West Asia]]
| 16,000,000
| 14,441,000
|14,210,222
| 14,751,000
| 14,693,269
|-
!scope="row"| [[Jakarta]]
| [[File:SCBD, Jakarta.jpg|120px]]
|{{flagg|pspe|al=c|pref=List of cities in|INA}}
| [[Southeast Asia]]
| 29,500,000
| 35,386,000
|40,545,126
| 10,517,000
| 32,513,588
|-
!scope="row"| [[Jieyang]]
| [[File:Puning Urban Night View from Baierqiutian.jpg|120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| {{n/a|{{red|Combined with<br />[[Shantou]]}}}}
| {{red|2,516,000}}
|10,579,303
| -
| 13,891,202
|-
!scope="row"| [[Johannesburg]]
| [[File:Johannesburg CBD.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|South Africa}}
| {{sort|Africa|[[Southern Africa]]}}
| 14,800,000
| 15,551,000
|{{red|8,592,843}}
| {{red|5,486,000}}
| {{red|9,497,036}}
|-
!scope="row"| [[Karachi]]
| [[File:Skyline view in Karachi after lockdown.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|PAK}}
| [[South Asia]]
| 21,000,000
| 20,249,000
|21,031,703
| 15,400,000
| 18,916,709
|-
!scope="row"| [[Kinshasa]]
| [[File:Boulevard du 30 juin, Kinshasa.jpg|120x120px]] 
| {{flagg|pspe|al=c|pref=List of cities in the|COD}}
| {{sort|AAfrica|[[Central Africa]]}}
| 16,300,000
| 13,493,000
|12,945,683
| 13,171,000
| 10,077,694
|-
!scope="row"| [[Kolkata]]
| [[File:EM Bypass Kolkata.jpg|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|IND}}
| [[South Asia]]
| 17,900,000
| 21,747,000
|23,314,585
| 14,681,000
| 24,106,859
|-
!scope="row"| [[Lagos]]
| [[File:Lagos skyline.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|NGR}}
| {{sort|Africa|[[West Africa]]}}
| 21,300,000
| 14,540,000
|12,486,045
| 13,463,000
| 12,642,198
|-
!scope="row"| [[Lahore]]
| [[File:Badshahi Mosquee, Lahore.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|PAK}}
| [[South Asia]]
| 14,600,000
| 13,504,000
|14,305,060
| 11,738,000
| 15,696,939
|-
!scope="row"| [[Lima]]
|  [[File:City of Lima, Peru.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|PER}}
| [[South America]]
| 12,000,000
| 10,556,000
|10,828,104
| 10,391,000
| 10,496,389
|-
!scope="row"| [[London]]
| [[File:City of London skyline from London City Hall - Sept 2015 - Crop Aligned.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|UK}}
| [[Europe]]
| 15,100,000
| 10,803,000
|10,408,333
| {{red|9,046,000}}
| 13,475,297
|-
!scope="row"| [[Los Angeles]]
| [[File:Los Angeles with Mount Baldy.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in the|USA}}
| [[North America]]
| 17,100,000
| 15,587,000
|13,474,333
| 12,458,000
| 16,206,529
|-
!scope="row"| [[Luanda]]
| [[File:Marginal de Luanda HD Dji Mavic 3 Classic - By Délcio Geovany Borges.jpg|120px]]
|{{flagg|pspe|al=c|pref=List of cities in|ANG}}
| {{sort|AAfrica|[[Central Africa]]}}
| {{red|9,650,000}}
| 10,914,000
|11,672,134
| {{red|7,774,000}}
| 10,212,263
|-
!scope="row"| [[Metro Manila]]
| [[File:Makati Guadalupe-Poblacion skyline with Paco, Quirino (Manila; 12-23-2023).jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|PHI}}
| [[Southeast Asia]]
| 27,800,000
| 24,156,000
|25,921,189
| 13,482,000
| 27,327,889
|-
!scope="row"| [[Mexico City]]
| [[File:Ciudad.de.Mexico.City.- Paseo.Reforma.Skyline CDMX 2016 (cropped).jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|MEX}}
| [[North America]]
| 25,400,000
| 21,905,000
|17,639,164
| 21,581,000
| 19,229,491
|-
!scope="row"| [[Moscow]]
| [[File:Business Centre of Moscow 2.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|RUS}}
| [[Europe]]
| 18,800,000
| 17,878,000
|14,384,082
| 12,410,000
| 17,217,606
|-
!scope="row"| [[Mumbai]]
| [[File:ওরলির গগনরৈখিক দৃশ্য.jpg|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|IND}}
| [[South Asia]]
| 27,600,000
| 25,189,000
|20,453,270
| 19,980,000
| 23,435,141
|-
!scope="row"| [[Nagoya]]
| [[File:Meieki from Heiwa Park Aqua Tower.jpg|120x120px|]] 
| {{flagg|pspe|al=c|pref=List of cities in|JPN}}
| [[East Asia]]
| 10,500,000
| {{red|9,439,000}}
|{{red|7,721,742}}
| {{red|9,507,000}}
| {{red|9,853,994}}
|-
!scope="row"| [[New York City]]
| [[File:A view of New York City with the Empire State Building and One World Trade Center from the Rockefeller Center.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in the|USA}}
| [[North America]]
| 21,800,000
| 21,396,000
|14,197,659
| 18,819,000
| 20,106,617
|-
!scope="row"| [[Osaka]]
| [[File:Nakanoshima Skyscrapers in 201504 001.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|JPN}}
| [[East Asia]]
| 17,700,000
| 14,916,000
|12,653,994
| 19,281,000
| 16,866,788
|-
!scope="row"| [[Paris]]
| [[File:Eiffel Tower from the Tour Montparnasse 3, Paris May 2014.jpg|120px]]
| {{flagg|pspe|al=c|pref=List of cities in|FRA}}
| [[Europe]]
| 11,500,000
| 11,108,000
|{{red|9,328,385}}
| 10,901,000
| 11,249,025
|-
!scope="row"| [[Rhine-Ruhr]]
| [[File:Aerial view of Essen.jpg|120px]]
|{{flagg|pspe|al=c|pref=List of cities in|GER}}
|[[Europe]]
|10,900,000
| {{red|6,769,000}}
| -
| -
| -
|-
!scope="row"| [[Rio de Janeiro]]
| [[File:Aerial View of Flamengo 1.jpg|120px]] 
| {{flagg|pspe|al=c|pref=List of cities in|BRA}}
| [[South America]]
| 13,600,000
| 12,306,000
|{{red|9,853,693}}
| 13,293,000
| 11,068,999
|-
!scope="row"| [[São Paulo]]
| [[File:SP from Altino Arantes Building.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|BRA}}
| [[South America]]
| 22,600,000
| 21,486,000
|19,485,158
| 21,650,000
| 21,671,857
|-
!scope="row"| [[Seoul]]
| [[File:Seoul (South Korea).jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|KOR}}
| [[East Asia]]
| 25,200,000
| 23,225,000
|22,261,692
| {{red|9,963,000}}
| 25,199,125
|-
!scope="row"| [[Shanghai]]
| [[File:Shanghai skyline waterfront pudong 5166168 69 70.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 41,600,000
| 24,042,000
|30,678,616
| 25,582,000
| 30,504,083
|-
!scope="row"| [[Shenzhen]]
| [[File:Shenzhen Skyline from Nanshan.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}} 
| [[East Asia]]
| {{n/a|Combined with<br />[[Guangzhou]]}}
| 17,778,000
| {{n/a|Combined with<br />[[Guangzhou]]}}
| 11,908,000
| {{n/a|Combined with<br />[[Guangzhou]]}}
|-
!scope="row"| [[Surabaya]]
| [[File:Central Surabaya view taken from JW Marriott Surabaya.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|INA}}
| [[Southeast Asia]]
| {{red|5,950,000}}
| {{red|6,556,000}}
|{{red|6,856,993}}
| -
| 10,695,358
|-
!scope="row"| [[Suzhou, Jiangsu|Suzhou]]
| [[File:东方之门1.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}} 
| [[East Asia]]
| {{n/a|Combined with<br />[[Shanghai]]}}
| {{red|6,091,000}}
|11,540,430
| {{red|6,339,000}}
| 13,458,006
|-
!scope="row"| [[Taipei]]
| [[File:Taipei Skyline 2022.06.29.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|TWN}} 
| [[East Asia]]
| 10,100,000
| {{red|9,662,000}}
|{{red|9,686,521}}
| -
| 10,048,037
|-
!scope="row"| [[Tehran]]
| [[File:North of Tehran Skyline view.jpg|120px]] 
| {{flagg|pspe|al=c|pref=List of cities in|IRN}}
| [[West Asia]]
| 16,800,000
| 13,382,000
|{{red|9,363,124}}
| {{red|8,896,000}}
| 13,563,316
|-
!scope="row"| [[Tianjin]]
| [[File:Tianjin Skyline 2009 Sep 11 by Nangua 1.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 11,700,000
| 10,047,000
|{{red|7,330,648}}
| 13,215,000
| {{red|8,963,397}}
|-
!scope="row"| [[Tokyo]]
| [[File:Minato City, Tokyo, Japan.jpg|alt=|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|JPN}}
| [[East Asia]]
| 41,200,000
| 37,785,000
|33,155,907
| 37,468,000
| 36,697,549
|-
!scope="row"| [[Wuhan]]
| [[File:Wuhan Yangtze River Bridge in 2020.jpg|alt=|120x120px]]
| {{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 12,600,000
| 10,353,000
|{{red|8,079,484}}
| {{red|8,176,000}}
| {{red|8,947,812}}
|-
!scope="row"| [[Xiamen]]
| [[File:Amoy Skyscrapers 2018.jpg|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 15,400,000
| {{red|5,253,000}}
|{{red|1,676,987}}
| {{red|3,585,000}}
| {{red|4,261,898}}
|-
!scope="row"| [[Xi'an]]
| [[File:Xi'an_erhuan_southeast.JPG|alt=|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 13,400,000
| 12,211,000
|{{red|5,298,991}}
| {{red|7,444,000}}
| {{red|6,818,858}}
|-
!scope="row"| [[Zhengzhou]]
| [[File:20211224 CBD of Zhengdong New Area.jpg|120x120px]]
|{{flagg|pspe|al=c|pref=List of cities in|CHN}}
| [[East Asia]]
| 10,300,000
| 11,068,000
|{{red|5,126,112}}
| {{red|4,940,000}}
| {{red|6,381,637}}
|}

==History==
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 at Austin|University of Texas]] in 1904.<ref>{{cite journal |date=1994 |title=Perspectives on Political and Economic Trends in the Americas |url=https://books.google.com/books?id=-1ALAAAAYAAJ&q=megacity |journal=Hemisfile |publisher=Institute of the Americas |volume=5-8 |page=12 |access-date=16 July 2015}}</ref> Initially the [[United Nations]] used the term to describe cities of 8 million or more inhabitants, but now uses the threshold of 10 million.<ref>{{cite journal |date=1981 |title=Special topics |url=https://books.google.com/books?id=pVxQAQAAIAAJ&q=Megacity+%228+million%22 |journal=Population Reports |location=Baltimore |publisher=Johns Hopkins University |issue=15–19 |page=38}}</ref> In the mid 1970s the term was coined by urbanist Janice Perlman referring to the phenomenon of very large urban agglomerations.<ref>{{Cite book|title=Encyclopedia of the City|last=Caves|first=R. W.|publisher=Routledge|year=2004|isbn=9780415252256|pages=454}}</ref> 
[[File:2020 1million cities.jpg|thumb|260px|right|Map showing urban areas with at least one million inhabitants in 2020]]
In 1800, only 3% of the [[world population|world's population]] lived in cities, a figure that rose to 47% by the end of the twentieth century. In 1950, there were 83 cities with populations exceeding one million; by 2007, this number had risen to 468,<ref>{{cite web|url=http://www.citypopulation.de/World.html |title=Principal Agglomerations of the World |publisher=Citypopulation.de |access-date=2010-09-01}}</ref> with 153 of them located in Asia. Among the 27 megacities with populations over 10 million globally, 15 were situated in Asia.<ref name=":0">{{Cite journal |last1=Li |first1=Deren |last2=Ma |first2=Jun |last3=Cheng |first3=Tao |last4=van Genderen |first4=J. L. |last5=Shao |first5=Zhenfeng |date=2019-12-02 |title=Challenges and opportunities for the development of megacities |url=https://www.tandfonline.com/doi/full/10.1080/17538947.2018.1512662 |journal=[[International Journal of Digital Earth]] |volume=12 |issue=12 |pages=1382–1395 |doi=10.1080/17538947.2018.1512662 |bibcode=2019IJDE...12.1382L |issn=1753-8947|url-access=subscription }}</ref>

In 2010, UN forecasted that urban population of 3.2 billion would rise to nearly 5 billion by 2030, when three out of five, or 60%, of people would live in cities.<ref>{{cite magazine|url=https://www.forbes.com/2007/06/11/megacities-population-urbanization-biz-cx_21cities_ml_0611megacities.html|title=Megacities Of The Future|magazine=Forbes.com|date=2007-06-11|access-date=2010-09-01}}</ref> This increase will be most dramatic on the least-urbanized continents, [[Asia]] and [[Africa]]. Surveys and projections indicate that all urban growth over the next 25 years will be in [[developing countries]].<ref>{{cite web|url=http://www.energypublisher.com/article.asp?id=5307 |title=Nigeria: Lagos, the mega-city of slums and plums |publisher=Energypublisher.com |access-date=2010-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20110218170316/http://www.energypublisher.com/article.asp?id=5307 |archive-date=February 18, 2011 }}</ref> One billion people, almost one-seventh of the world's population, now live in [[shanty towns]].<ref>{{cite news|last=Whitehouse |first=David |url=http://news.bbc.co.uk/2/hi/science/nature/4561183.stm |title=Half of humanity set to go urban |work=BBC News |date=2005-05-19 |access-date=2010-09-01}}</ref> In many poor countries, [[overcrowding|overcrowded]] slums exhibit high rates of [[tropical diseases|disease]] due to unsanitary conditions, malnutrition, and lack of basic health care.<ref>{{cite web|url=http://www.blackcommentator.com/88/88_reprint_planet_slums.html |title=Planet of Slums – The Third World's Megacities |publisher=Blackcommentator.com |access-date=2010-09-01}}</ref> By 2030, over 2 billion people in the world will be living in [[slum]]s.<ref>{{cite web|url=http://www.unfpa.org/swp/2007/english/chapter_2/slums.html |title=State of World Population 2007 |publisher=Unfpa.org |access-date=2010-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20090122105358/http://www.unfpa.org/swp/2007/english/chapter_2/slums.html |archive-date=2009-01-22 |df=ymd-all}}</ref> Over 90% of the urban population of [[Ethiopia]], [[Malawi]] and [[Uganda]], three of the world's most rural countries, already live in slums.

By 2025, Asia alone will have at least 30 megacities, including [[Mumbai]], India (2015 population of 20.75 million people), [[Shanghai]], China (2015 population of 35.5 million people), [[Delhi]], India (2015 population of 21.8 million people), [[Tokyo]], [[Japan]] (2015 population of 38.8 million people) and [[Seoul]], South Korea (2015 population of 25.6 million people). The top eight provincial capital cities in China with urban areas exceeding 400 km²—Beijing, Shanghai, Tianjin, Guangzhou, Chongqing, Hangzhou, Wuhan, and Xi'an—accounted for 54.8% of the total urban area of all provincial capital cities in the country in 2015.<ref name=":0" />

In Africa, [[Lagos]], Nigeria has grown from 300,000 in 1950 to an estimated 21 million today.

===Growth===
[[File:Détail de la maquette de Rome à lépoque de Constantin (5839479770).jpg|thumb|[[Italo Gismondi|Gismondi]]'s model of Rome in the time of [[Constantine the Great|Constantine]]]]
For almost five hundred years, during the period of the [[Roman Republic|Republic]] and later of the [[Roman Empire|Empire]], [[Rome]] was the [[Historical urban community sizes|largest]], wealthiest, and most politically important city of the ancient world, rulling over Europe, Western Asia and Northern Africa.<ref>{{cite web|url=http://www.unrv.com/empire/roman-population.php |title=Roman Empire Population |publisher=Unrv.com |access-date=2010-09-01}}</ref><ref>{{Cite web |title=Pax Romana: Rome's Golden Age |url=https://www.historyhit.com/pax-romana-romes-golden-age/ |access-date=2024-09-18 |website=History Hit |language=en-GB}}</ref> It is often stated that its population passed one million people by the end of the 1st century BC, however, it is debated about whether the population actually reached such a large size.<ref>{{cite web|url=http://home.vicnet.net.au/~ozideas/poprus.htm |title=Population crises and cycles in history |publisher=Home.vicnet.net.au |access-date=2010-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20110405081151/http://home.vicnet.net.au/~ozideas/poprus.htm |archive-date=April 5, 2011 }}</ref><ref>{{cite journal|last=Storey|first=Glenn R.|year=1997|title=The population of ancient Rome|journal=Antiquity|volume=71|issue=274|pages=966–978|doi=10.1017/S0003598X00085859|url=https://www.cambridge.org/core/services/aop-cambridge-core/content/view/BACD7DF32B0B77609CD6713B8AF88882/S0003598X00085859a.pdf/div-class-title-the-population-of-ancient-rome-div.pdf|access-date=30 May 2024|archive-date=23 July 2018|archive-url=https://web.archive.org/web/20180723072210/https://www.cambridge.org/core/services/aop-cambridge-core/content/view/BACD7DF32B0B77609CD6713B8AF88882/S0003598X00085859a.pdf/div-class-title-the-population-of-ancient-rome-div.pdf|url-status=bot: unknown}}</ref> Rome's population started declining in 402 AD when [[Honorius (emperor)|Flavius Honorius]], [[Western Roman Empire|Western Roman Emperor]] from 395 to 423, moved the government to [[Ravenna]] and Rome's population declined to a mere 20,000 during the [[Early Middle Ages]], reducing the sprawling city to groups of inhabited buildings interspersed among large areas of ruins and vegetation.

[[Baghdad]] was likely the largest city in the world from shortly after its foundation in 762 AD until the 930s, with some estimates putting its population at over one million.<ref>{{cite web |url=http://geography.about.com/library/weekly/aa011201a.htm |title=Largest Cities Through History |publisher=Geography.about.com |date=2010-06-16 |access-date=2010-09-01 |archive-date=2005-05-27 |archive-url=https://web.archive.org/web/20050527095609/http://geography.about.com/library/weekly/aa011201a.htm |url-status=dead }}</ref> Chinese capital cities [[Chang'an]] and [[Kaifeng]] also experienced huge population booms during prosperous empires. According to the census in the year 742 recorded in the ''[[New Book of Tang]]'', 362,921 families with 1,960,188 persons were counted in [[Jingzhao Fu]] (京兆府), the [[metropolitan area]] including small cities in the vicinity of Chang'an.<ref>''New Book of Tang'', vol. 41 (Zhi vol. 27) Geography 1.</ref> The medieval settlement surrounding [[Angkor]], the one-time capital of the [[Khmer Empire]] which flourished between the 9th and 15th centuries, could have supported a population of up to one million people.<ref>[https://www.independent.co.uk/news/world/asia/metropolis-angkor-the-worlds-first-megacity-461623.html Metropolis: Angkor, the world's first mega-city], The Independent, August 15, 2007</ref>
[[File:Trafalgar Square by James Pollard.jpg|thumb|During the 19th century, [[London]] was transformed into the world's largest city and capital of the [[British Empire]].]]

From around 1825 to 1918 [[London]] was the largest city in the world, with the population growing rapidly; it was the first city to reach a population of over 5 million in 1900. In 1950, [[New York City]] was the only urban area with a population of over 10 million.<ref>{{cite web|url=http://geography.about.com/library/weekly/aa011201g.htm|title=Top 10 Cities of the Year 1950|work=Four Thousand Years of Urban Growth: An Historical Census|author=Tertius Chandler, 1987, St. David's University Press|access-date=2007-03-24|archive-date=2016-04-15|archive-url=https://web.archive.org/web/20160415044639/http://geography.about.com/library/weekly/aa011201g.htm|url-status=dead}}</ref> Geographers had identified 25 such areas as of October 2005,<ref>{{cite web|url=http://www.citypopulation.de/World.html |title=Population statistics |publisher=Citypopulation.de |access-date=2010-09-01}}</ref> as compared with 19 megacities in 2004 and only nine in 1985. This increase has happened as the world's population moves towards the high (75–85%) urbanization levels of [[North America]] and [[Western Europe]].

Since the 2000s, the largest megacity has been the [[Greater Tokyo Area]]. The population of this [[urban agglomeration]] includes areas such as [[Yokohama]] and [[Kawasaki, Kanagawa|Kawasaki]], and is estimated to be between 37 and 38 million. This variation in estimates can be accounted for by different definitions of what the area encompasses. While the prefectures of [[Tokyo]], [[Chiba Prefecture|Chiba]], [[Kanagawa Prefecture|Kanagawa]], and [[Saitama Prefecture|Saitama]] are commonly included in statistical information, the Japan Statistics Bureau only includes the area within 50 kilometers of the [[Tokyo Metropolitan Government Building|Tokyo Metropolitan Government Offices]] in [[Shinjuku, Tokyo|Shinjuku]], thus arriving at a smaller population estimate.<ref>{{cite web|url=http://www.stat.go.jp/english/data/handbook/c02cont.htm |title=Greater Tokyo population statistics |publisher=Stat.go.jp |date=2008-10-01 |access-date=2010-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20080411094023/http://www.stat.go.jp/English/data/handbook/c02cont.htm |archive-date=April 11, 2008 }}</ref><ref>{{cite web|url=http://www.citypopulation.de/World.html |title=Tokyo metropolitan area population statistics |publisher=Citypopulation.de |access-date=2010-09-01}}</ref> A characteristic issue of megacities is the difficulty in defining their outer limits and accurately estimating the populations.

Another list defines megacities as [[urban agglomeration]]s instead of metropolitan areas.<ref>{{Cite web |title=World Megacities - Urban Areas with More than 10,000,000 Population (2015) |url=http://www.demographia.com/db-megacity.pdf |website=Demographia}}</ref> As of 2021, there are 28 megacities by this definition, like Tokyo.<ref>{{Cite web |last=Society |first=National Geographic |date=2018-08-28 |title=The Age of Megacities |url=http://www.nationalgeographic.org/interactive/age-megacities/ |access-date=2022-04-19 |website=National Geographic Society |language=en}}</ref> Other sources list [[Nagoya]]<ref name="citypopulation"/> and the [[Rhein-Ruhr]]<ref>{{cite web |url=http://www.espon.eu/export/sites/default/Documents/Projects/ESPON2006Projects/StudiesScientificSupportProjects/UrbanFunctions/fr-1.4.3_April2007-final.pdf |title=ESPON project 1.4.3 Study on Urban Functions Final Report |access-date=2013-08-07 |url-status=dead |archive-url=https://web.archive.org/web/20150924002318/http://www.espon.eu/export/sites/default/Documents/Projects/ESPON2006Projects/StudiesScientificSupportProjects/UrbanFunctions/fr-1.4.3_April2007-final.pdf |archive-date=2015-09-24 }}</ref> as megacities.

==Challenges==
{{Cleanup|section|reason=A lot of vague statements without up-to-date / worldwide-applicable supporting examples or data. Can also be further expanded.|date=May 2014}}

===Slums===
[[File:Mumbai (5356346073).jpg|thumb|[[Mumbai]]'s [[Dharavi]] slum is home to 1 million residents]]
According to the United Nations, the proportion of urban dwellers living in [[slum]]s or informal settlements decreased from 47 percent to 37 percent in the [[developing world]] between 1990 and 2005.<ref>{{cite web|url=http://www.un.org/millenniumgoals/pdf/mdg2007.pdf |title=p. 26 |access-date=2010-09-01}}</ref> However, due to rising population, the absolute number of slum dwellers is rising and passed 1 billion in 2018.<ref>{{cite web | url=https://unstats.un.org/sdgs/report/2019/goal-11/ | title=— SDG Indicators }}</ref> The increase in informal settlement population has been caused by massive migration, both internal and transnational, into cities, which has caused growth rates of urban populations and spatial concentrations not seen before in history.{{Citation needed|date=May 2014}} The majority of these are located in informal settlements which often lack sufficient quality housing, sanitation, drainage, water access, and officially recognized addresses. These issues raise problems in the political, social, and economic arenas.<ref>{{cite web | url=https://www.weforum.org/stories/2023/08/informal-settlements-are-growing-heres-how-we-provide-everyone-a/ | title=Informal settlements are growing everywhere — here's what we do }}</ref> People who live in slums or informal settlements often have minimal or no access to education, healthcare, or the urban economy.

===Crime===
[[File:View over Complexo do Alemao (Favela) - From Igreja da Penha - Rio de Janeiro - Brazil - 03 (17556920491).jpg|thumb|Most murders in [[Rio de Janeiro]], Brazil are [[gang]] related and happen in the [[favela]]s]]
As with any large concentration of people, there is usually crime.<ref>{{cite journal |last1=Glaeser |first1=Edward L. |last2=Sacerdote |first2=Bruce |title=Why is There More Crime in Cities? |journal=[[Journal of Political Economy]] |date=1999 |volume=107 |issue=S6 |pages=S225–S258 |doi=10.1086/250109|s2cid=56443047 |url=http://papers.nber.org/papers/w5430.pdf }}</ref><ref>{{cite journal |last1=Yang |first1=V. Chuqiao |last2=Papachristos |first2=Andrew V. |last3=Abrams |first3=Daniel M. |title=Modeling the origin of urban-output scaling laws |journal=[[Physical Review E]] |date=2019 |volume=100 |issue=3 |pages=032306 |doi=10.1103/PhysRevE.100.032306|pmid=31639910 |arxiv=1712.00476 |bibcode=2019PhRvE.100c2306Y |s2cid=201646296 }}</ref> High population densities often result in higher crime rates, as visibly seen in growing megacities such as [[Organised crime in Pakistan|Karachi]], [[Crime in India|Delhi]], [[Crime in Egypt|Cairo]], [[Crime in Brazil|Rio de Janeiro]], and [[Crime in Nigeria|Lagos]].<ref>{{cite book|url=https://books.google.com/books?id=F9mHmudq2scC |title=The Real Population Bomb: Megacities, Global Security & the Map of the Future |author1=P. H. Liotta |author2=James F. Miskel |publisher=Potomac Books |date= 2012-02-01|access-date=2014-05-03|isbn=9781597975513 }}</ref>

===Homelessness===

Megacities often have significant numbers of [[Homelessness|homeless]] people. The actual legal definition of homelessness varies from country to country, or among different entities or institutions in the same country or region.<ref>{{cite web|url=http://www.homeless.org.au/glossary.htm |title=Glossary defining homelessness |publisher=Homeless.org.au |access-date=2010-09-01}}</ref>

In 2002, research showed that children and families were the largest growing segment of the homeless population in the United States,<ref>FACS, "Homeless Children, Poverty, Faith and Community: Understanding and Reporting the Local Story", March 26, 2002
Akron, Ohio. {{cite web |url=http://www.facsnet.org/edu/progs/family_03-26-02.php3 |title=Homeless Children, Poverty, Faith and Community: Understanding and Reporting the Local Story |access-date=2006-10-24 |url-status=dead |archive-url=https://web.archive.org/web/20070928072235/http://www.facsnet.org/edu/progs/family_03-26-02.php3 |archive-date=2007-09-28 }}</ref><ref>National Coalition for the Homeless, "Homeless Youth" 2005 {{cite web|url=http://www.nationalhomeless.org/publications/facts/youth.pdf|title=Homeless Youth|access-date=2013-04-15|archive-date=2018-08-22|archive-url=https://web.archive.org/web/20180822050017/http://www.nationalhomeless.org/publications/facts/youth.pdf|url-status=dead}}&nbsp;{{small|(164&nbsp;KB)}}</ref> and this has presented new challenges, especially in services, to agencies.<!-- Too specific? Applies worldwide. --> In the US, the government asked many major cities to come up with a ten-year plan to end homelessness. One of the results of this was a "[[Housing first]]" solution, rather than to have a homeless person remain in an emergency homeless shelter it was thought to be better to quickly get the person permanent housing of some sort and the necessary support services to sustain a new home. But there are many complications with this kind of program and these must be dealt with to make such an initiative work successfully in the middle to long term.<ref name="HF1">Abel, David, [http://www.boston.com/news/local/articles/2008/02/24/for_the_homeless_keys_to_a_home/ "For the homeless, keys to a home: Large-scale effort to keep many off street faces hurdles"], Boston Globe, February 24, 2008.</ref><ref name="HF2">[[Public Broadcasting Service|PBS]], [https://www.pbs.org/now/shows/305/index.html "Home at Last?&nbsp;– A radical new approach to helping the homeless"], ''[[NOW on PBS|NOW]]'' TV program, December 21, 2007.</ref>

===Traffic congestion===
[[File:Ratchadamri to Pratunam.jpg|thumb|[[Bangkok]] is notorious for its traffic congestion.]]

[[Traffic congestion]] is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, increased pollution, and increased vehicular [[Queueing theory|queueing]]. The [[Texas Transportation Institute]] estimated that, in 2000, the 75 largest metropolitan areas experienced 3.6 billion vehicle-hours of delay, resulting in 5.7 billion U.S. gallons (21.6 billion liters) in wasted fuel and $67.5 billion in lost productivity, or about 0.7% of the nation's [[Gross domestic product|GDP]]. It also estimated that the annual cost of congestion for each driver was approximately $1,000 in very large cities and $200 in small cities.<ref>{{Cite book |last=Sabry |first=Fouad |url=https://books.google.com/books?id=5r3-EAAAQBAJ&dq=congestion+$1,000++$200&pg=PT145 |title=Induced Demand: Unlocking the Mysteries of Induced Demand, Navigating the Roads to Sustainable Cities |date=2024-03-27 |publisher=One Billion Knowledgeable |language=en}}</ref> Traffic congestion is increasing in major cities and delays are becoming more frequent in smaller cities and rural areas. It also can result in various issues, including economic losses, energy waste, air and noise pollution, and more.<ref name=":0" />

===Urban sprawl===
[[File:South-Los-Angeles-110-and-105-freeways-Aerial-view-from-north-August-2014.jpg|thumb|right|A flat land area in the [[Greater Los Angeles Area]] in the U.S. state of California with houses, buildings, roads, and freeways. Areas constructed to capacity contribute to [[urban expansion]].]]

[[Urban sprawl]], also known as suburban sprawl, is a multifaceted concept, which includes the spreading outwards of a city and its [[suburb]]s to its outskirts to low-density, auto-dependent development on rural land, with associated design features that encourage [[car dependency]].<ref name="sprawlcity">{{usurped|1=[https://web.archive.org/web/20100105123334/http://www.sprawlcity.org/hbis/wis.html What is Sprawl?]}}. ''SprawlCity.org''. Retrieved on 2008-02-07.</ref> As a result, some critics argue that sprawl has certain disadvantages including longer transport distances to work, high [[Automobile dependency|car dependence]], inadequate facilities (e.g. health, cultural. etc.) and higher per-person infrastructure costs. Discussions and debates about sprawl are often obfuscated by the ambiguity associated with the phrase. For example, some commentators measure sprawl only with the average number of residential units per acre in a given area. But others associate it with decentralization (spread of population without a well-defined center), discontinuity (leapfrog development), segregation of uses, etc.<ref>{{Cite book |last=Jain |first=Shri V. K. |url=https://books.google.com/books?id=qRYsEAAAQBAJ&dq=sprawl+decentralization+discontinuity++segregation&pg=PA352 |title=Applied Ecology and Sustainable Environment |date=2021-04-30 |publisher=BFC Publications |isbn=978-93-90880-19-5 |language=en}}</ref>

===Gentrification===
[[Gentrification]] and urban gentrification are terms for the socio-cultural changes in an area as a result of wealthier people buying property in a less prosperous community.<ref name="PBS">{{cite web|url=https://www.pbs.org/pov/flagwars/special_gentrification.php|title=PBS Documentaries with a point of view: What is Gentrification?|author=Benjamin Grant|publisher=Public Broadcasting Service|date=June 17, 2003|access-date=September 4, 2017|archive-date=December 6, 2015|archive-url=https://web.archive.org/web/20151206002239/http://www.pbs.org/pov/flagwars/special_gentrification.php|url-status=dead}}</ref> As living costs rise, lower-income residents are forced to move out of the community leading to an increase in average income, which in turn makes the area more desirable to other wealthier property or business owners, further pushing the living costs up. This process also tends to lead to a decrease in average family size in the area. This type of population change reduces industrial [[land use]] when it is redeveloped for commerce and housing.

===Air pollution===
[[File:Shanghai Smog.JPG|thumb|Air pollution in [[Shanghai]], China]]
[[Air pollution]] is the introduction into the [[Earth's atmosphere|atmosphere]] of [[chemical]]s, [[Atmospheric particulate matter|particulate matter]], or [[biotic material|biological materials]] that cause harm or discomfort to humans or other living organisms, or damages the [[natural environment]].<ref>{{cite journal |last1=Decker |first1=Ethan H. |last2=Elliott |first2=Scott |last3=Smith |first3=Felisa A. |title=Megacities and the Environment |journal=[[The Scientific World Journal]] |date=2002 |volume=2 |pages=374–386 |doi=10.1100/tsw.2002.103|pmid=12806023 |pmc=6009397 |doi-access=free }}</ref><ref>{{cite journal |last1=Folberth |first1=Gerd A. |last2=Butler |first2=Timothy M. |last3=Collins |first3=William J. |last4=Rumbold |first4=Steven T. |title=Megacities and climate change – A brief overview |journal=[[Environmental Pollution (journal)|Environmental Pollution]] |date=2015 |volume=203 |pages=235–242 |doi=10.1016/j.envpol.2014.09.004 |pmid=25300966 |doi-access=free|bibcode=2015EPoll.203..235F }}</ref> This issue is particularly prevalent in developing nations. As part of the Global Environment Monitoring System, [[WHO]] and [[UNEP]] established an air pollution monitoring network that oversees 50 cities.<ref>{{Cite journal |last1=Mage |first1=David |last2=Ozolins |first2=Guntis |last3=Peterson |first3=Peter |last4=Webster |first4=Anthony |last5=Orthofer |first5=Rudi |last6=Vandeweerd |first6=Veerle |last7=Gwynne |first7=Michael |date=1996-03-01 |title=Urban air pollution in megacities of the world |url=https://linkinghub.elsevier.com/retrieve/pii/1352231095002197 |journal=Atmospheric Environment |series=Supercities: Environment Quality and Sustainable Development |volume=30 |issue=5 |pages=681–686 |doi=10.1016/1352-2310(95)00219-7 |bibcode=1996AtmEn..30..681M |issn=1352-2310|url-access=subscription }}</ref> Many urban areas have significant problems with [[smog]], a type of [[air pollution]] derived from [[Motor vehicle emissions|vehicle emissions]] from [[internal combustion engine]]s and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form [[photochemical smog]].<ref name=":0" />

===Energy and material resources===
The sheer size and complexity of megacities gives rise to enormous social and environmental challenges. Whether megacities can develop sustainably depends to a large extent on how they obtain, share, and manage their energy and material resources. There are correlations between [[Electric energy consumption|electricity consumption]], heating and industrial fuel use, [[ground transportation]] energy use, [[Drinking water|water consumption]], [[Waste|waste generation]], and [[Steel making|steel production]] in terms of level of consumption and how efficiently they use resources.<ref>{{cite journal |last1=Kennedy |first1=Christopher A. |last2=Stewart |first2=Iain |last3=Facchini |first3=Angelo |last4=Cersosimo |first4=Igor |last5=Mele |first5=Renata |last6=Chen |first6=Bin |last7=Uda |first7=Mariko |last8=Kansal |first8=Arun |last9=Chiu |first9=Anthony |last10=Kim |first10=Kwi-gon |last11=Dubeux |first11=Carolina |last12=Lebre La Rovere |first12=Emilio |last13=Cunha |first13=Bruno |last14=Pincetl |first14=Stephanie |last15=Keirstead |first15=James |last16=Barles |first16=Sabine |last17=Pusaka |first17=Semerdanta |last18=Gunawan |first18=Juniati |last19=Adegbile |first19=Michael |last20=Nazariha |first20=Mehrdad |last21=Hoque |first21=Shamsul |last22=Marcotullio |first22=Peter J. |last23=González Otharán |first23=Florencia |last24=Genena |first24=Tarek |last25=Ibrahim |first25=Nadine |last26=Farooqui |first26=Rizwan |last27=Cervantes |first27=Gemma |last28=Sahin |first28=Ahmet Duran |title=Energy and material flows of megacities |journal=Proceedings of the National Academy of Sciences |date=2015 |volume=112 |issue=19 |pages=5985–5990 |doi=10.1073/pnas.1504315112 |pmid=25918371 |pmc=4434724 |bibcode=2015PNAS..112.5985K |doi-access=free}}</ref>

==In fiction==

Megacities are a common backdrop in [[dystopia]]n [[science fiction]], with examples such as the Sprawl in [[William Gibson]]'s ''[[Neuromancer]]'',<ref name="sharp">{{cite book |title=Popular Contemporary Writers |last=Sharp |first=Michael D. |year=2005 |publisher=Marshall Cavendish |isbn= 978-0-7614-7601-6 }}</ref> and [[Mega-City One]], a megalopolis of between 50 and 800 million people (fluctuations due to war and disaster) across the east coast of the [[United States]], in the ''[[Judge Dredd]]'' comic.<ref name="legal">{{cite book |title=The Legal Companion |last=Powell |first=Vincent |year=2005 |publisher=Robson |isbn= 978-1-86105-838-6 |page=54 }}</ref> In ''[[Demolition Man (film)|Demolition Man]]'' a megacity called "[[San Angeles]]" was formed from the joining of [[Los Angeles]], [[Santa Barbara, California|Santa Barbara]], [[San Diego]] and the surrounding metropolitan regions following a massive earthquake in 2010.<ref name="blackspace">{{cite book |title=Black space: imagining race in science fiction film |last=Namu |first= Adilifu |year=2008 |publisher=University of Texas Press |isbn= 978-0-292-71745-9 }}</ref> Fictional planet-wide megacities ([[ecumenopoleis]]) include [[Trantor]] in [[Isaac Asimov]]'s [[Foundation (book series)|''Foundation'' series]] of books and [[Coruscant]] (population two trillion) in the ''[[Star Wars]]'' universe.<ref name="greenwood">{{cite book |title=The Greenwood encyclopedia of science fiction and fantasy: themes, works, and wonders, Volume 2 |last=Westfahl |first=Gary |year=2005 |publisher=Greenwood Publishing Group |isbn= 978-0-313-32952-4 }}</ref>

== See also ==
{{portal|border=no|Cities|World}}
*[[Economies of agglomeration]]
*[[Global city]]
*[[List of largest cities]]
*[[List of largest cities throughout history]]
*[[Megalopolis]]
*[[Urban sprawl]]

== References ==
{{Reflist|30em}}

{{Urban pop list}}
{{Cities}}
{{Megacities}}
{{Authority control}}

[[Category:Urban studies and planning terminology]]
[[Category:Human habitats]]
[[Category:Population]]
[[Category:Types of cities]]
[[Category:Cyberpunk themes]]