Editing IPCC:AR6/WGIII/Chapter-8 (section)

==== 8.3.4.1 Urban Land Expansion and Greenhouse Gas Emissions ====

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The uncertainties across urban land expansion forecasts, and associated SSPs, highlight an opportunity to pursue compact, low or net-zero GHG emissions development that minimises land-use competition, avoids carbon lock-in, and preserves carbon-sequestering areas like forests and grasslands (Sections 8.4. and 8.6, and Figure 8.21). Among the forecasts available are six global-scale spatially explicit studies of urban land expansion that have been published since AR5; four of the six, which present forecasts for each of the five SSPs, are considered in Table 8.1 and Figure 8.10 ( [[#Huang--2019|Huang et al. 2019]] ; [[#Li--2019b|Li et al. 2019b]] ; [[#Chen--2020a|Chen et al. 2020a]] ; [[#Gao--2020|Gao and O’Neill 2020]] ). All four have forecasts to 2050 but only three to 2100. One of the two not included here ( [[#van%20Vliet--2017|van Vliet et al. 2017]] ) also forecasts land displacement due to urban land expansion.

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'''Figure 8.10: Forecasts of urban land expansion in 2050 and 2100 according to each SSP and AR6 WGIII 6-region aggregation, by study, where A: [[#Gao--2020|Gao and O’Neill (2020)]] , B: Chen''' '''et al''' '''.''' '''(2020a), C: Li''' '''et al.''' '''(2019), D: Huang''' '''et al.''' '''(2019), E: mean across studies, and F: median across all studies.''' Three studies ( [[#Li--2019b|Li et al. 2019b]] ; [[#Chen--2020a|Chen et al. 2020a]] ; [[#Gao--2020|Gao and O’Neill 2020]] ) report forecasts of urban land expansion to both 2050 and 2100. One study ( [[#Huang--2019|Huang et al. 2019]] ) reports the forecast only to 2050. Global current urban extents and the respective initial years vary slightly among the four studies. Years for values of current urban extent range from 2010 to 2020. See Table 8.1 for the range of data across the four studies and across SSPs. Source: data compiled form [[#Huang--2019|Huang et al. (2019)]] , Li et al. (2019), Chen et al. (2020), and [[#Gao--2020|Gao and O’Neill (2020)]] .

'''Table 8.1: Forecasts of total urban land per AR6 WGIII regio''' '''n (6-region aggregation) in 2050 for each SSP, with the median and range of estimates from four studies:''' '''Huang''' '''et al.''' '''(2019), Li''' '''et al.''' '''(2019), Chen''' '''et al.''' '''(2020), and [[#Gao--2020|Gao and O’Neill (2020)]] .''' Median estimates for the 2015 urban extent are based on the mean/median of estimates in [[#Huang--2019|Huang et al. (2019)]] and Chen et al. (2020). Median and range of estimates for each SSP in 2050 are based on values derived from the four studies: [[#Huang--2019|Huang et al. (2019)]] , Li et al. (2019), Chen et al. (2020), and [[#Gao--2020|Gao and O’Neill (2020)]] . While each study and SSP forecast increases in urban land in each region, the range and magnitude vary. Source: data compiled from [[#Huang--2019|Huang et al. (2019)]] , Li et al. (2019), Chen et al. (2020), and [[#Gao--2020|Gao and O’Neill (2020)]] .

{| class="wikitable"
|-
!
! '''2015'''

'''median'''

(km 2 ; range)

! '''SSP1'''

'''median'''

(km 2 ; range)

! '''SSP2'''

'''median'''

(km 2 ; range)

! '''SSP3'''

'''median'''

(km 2 ; range)

! '''SSP4'''

'''median'''

(km 2 ; range)

! '''SSP5'''

'''median'''

(km 2 ; range)

|-
| rowspan="2"| '''Africa'''

| '''64,423'''

| '''97,718'''

| '''116,486'''

| '''96,571'''

| '''119,971'''

| '''138,604'''

|-
| (41,472–87,373)

| (67,488–303,457)

| (59,638–274,683)

| (56,071–235,922)

| (54,633–344,645)

| (79,612–309,532)

|-
| rowspan="2"| '''Asia and Pacific'''

| '''241,430'''

| '''293,647'''

| '''355,445'''

| '''296,431'''

| '''329,485'''

| '''419,781'''

|-
| (167,548–315,312)

| (244,575–732,303)

| (236,677–624,659)

| (224,520–483,335)

| (240,639–632,678)

| (250,670–787,257)

|-
| rowspan="2"| '''Developed Countries'''

| '''260,167'''

| '''459,624'''

| '''506,301'''

| '''414,661'''

| '''496,526'''

| '''616,847'''

|-
| (188,660–331,674)

| (407,483–648,023)

| (431,592–614,592)

| (362,063–479,584)

| (411,320–586,058)

| (510,468–761,275)

|-
| rowspan="2"| '''Eastern Europe and West-Central Asia'''

| '''35,970'''

| '''63,625'''

| '''65,251'''

| '''59,779'''

| '''64,434'''

| '''76,994'''

|-
| (27,121–44,819)

| (42,990–91,612)

| (52,397–91,108)

| (44,129–90,794)

| (50,806–86,546)

| (54,039–93,008)

|-
| rowspan="2"| '''Latin America and Caribbean'''

| '''62,613'''

| '''86,236'''

| '''88,793'''

| '''93,804'''

| '''85,369'''

| '''102,343'''

|-
| (60,511–64,716)

| (63,507–163,329)

| (86,411–162,526)

| (65,286–162,669)

| (82,148–144,940)

| (82,961–167,102)

|-
| rowspan="2"| '''Middle East'''

| '''21,192'''

| '''51,351'''

| '''51,221'''

| '''48,032'''

| '''49,331'''

| '''55,032'''

|-
| (19,017–23,366)

| (187,68–69,266)

| (25,486–69,716)

| (19,412–63,236)

| (25,415–71,720)

| (33,033–75,757)

|-
| rowspan="2"| '''World'''

| '''685,795'''

| '''1,023,220'''

| '''1,174,742'''

| '''980,719'''

| '''1,123,900'''

| '''1,412,390'''

|-
| (669,246–702,343)

| (919,185– 1,991,579)

| (927,820–1,819,174)

| (850,681–1,493,454)

| (922,539–1,851,438)

| (1,018,321–2,180,816)

|}

Four overarching findings can be gleaned from these studies.

First, urban land areas will expand significantly by 2050 – by as much as 211% (see SSP5 forecast in [[#Huang--2019|Huang et al. 2019]] ), but likely within a large potential range of about 43–106% over the 2015 extent by 2050 – to accommodate the growing urban population (Table 8.1). Globally, there are large uncertainties and variations among the studies – and between the SSPs – about the rates and extent of future urban expansion, owing to uncertainties about economic development and population growth (ranges of estimates are provided in Table 8.1). Overall, the largest urban extents are forecasted under SSP5 (fossil fuel-intensive development) for both 2050 and 2100, whereas the smallest forecasted urban extents are under SSP3 (‘regional rivalry’). Forecasted global urban extents could reach between 1 and 2.2 million km 2 (median of 1.4 million km 2 , a 106% increase) in 2050 under SSP5, and between 0.85 and 1.5 million km 2 (median of 1 million km 2 , a 43% increase) in 2050 under SSP3. Under SSP1, which is characterised by a focus on sustainability with more compact, low-emissions development, urban extents could reach 1 million km 2 (range of 0.9 to 2 million km 2 , a 49% increase) in 2050. By 2100, the forecasted urban extents reach between 1.4 and 3.6 million km 2 (median 2.5 million km 2 ) under SSP5 and between 1 and 1.5 million km 2 (median 1.3 million km 2 ) under SSP3. Across the studies, substantially larger amounts of urban land expansion are expected after 2050 under SSP5 compared to other SSPs.

Second, there is a wide variation in estimates of urban land expansion across regions (using the AR6 WGIII 6-region aggregation). Across all four sets of forecasts, current urban land (circa 2015) is the largest in Developed Countries and in the Asia and Pacific region, with approximately two-thirds of the current urban extent occurring in those two regions (Table 8.1 and Figure 8.10). The largest increases in urban land by 2050 are expected in the Asia and Pacific and Developed Countries regions, across all the SSPs. However, the rate of increase in urban land in Eastern Europe and West-Central Asia, Latin America and Caribbean, and the Middle East is significant and urban land could more than double by 2050. One-third of the studies conclude that the United States, China, and India will experience continued urban land expansion at least until 2050 ( [[#Huang--2019|Huang et al. 2019]] ; [[#Li--2019b|Li et al. 2019b]] ). However, Li et al. (2019) report that, after 2050, China could experience a decrease in the rate of urban land expansion, while growth will continue for India. This is not surprising since India’s urban demographic transition will only get underway after the middle of the century, when the urban population is expected to exceed the rural population. In contrast, China’s urban demographic transition could be nearly complete by 2050.

Third, in spite of these general trends, there are differences in forecasted urban expansion in each region across the SSPs and studies, with [[#Huang--2019|Huang et al. (2019)]] forecasting the most future urban land expansion between 2015 and 2050. The range across studies is significant. Under SSP1, urban land areas could increase by between 69,000 and 459,000 km 2 in Developed Countries, 77,000–417,000 km 2 in Asia and Pacific, and 28,000–216,000 km 2 in Africa. Under SSP3, where urban land expansion is forecasted to be the lowest, urban land areas could increase by between 23,000 and 291,000 km 2 in Developed Countries, 57,000–168,000 km 2 in Asia and Pacific, and 16,000–149,000 km 2 in Africa. Under SSP5, where urban land expansion is forecasted to be the highest, urban land area could increase by between 129,000 and 573,000 km 2 in Developed Countries, 83,000–472,000 km 2 in Asia and Pacific, and 40,000–222,000 km 2 in Africa ( [[#Huang--2019|Huang et al. 2019]] ; [[#Li--2019b|Li et al. 2019b]] ; [[#Chen--2020a|Chen et al. 2020a]] ; [[#Gao--2020|Gao and O’Neill 2020]] ). By 2100, however, the Developed Countries region is expected to have the most urban expansion only in SSP5. In SSP2 and SSP4, the Developed Countries and Asia and Pacific regions have about equal amounts of new urban land; in SSP3, Asia and Pacific has more new urban land forecasted.

Fourth, both the range of estimates and their implications on land-use competition and urban life point to an opportunity for urban areas to consider their urban form when developing. Under the current urbanisation trajectory, 50–63% of newly expanded urban areas are expected to occur on current croplands ( [[#Chen--2020a|Chen et al. 2020a]] ). However, there is significant regional variation; between 2000 and 2040, 12.5% of cropland in China and 7.5% of cropland in the Middle East and North Africa could potentially be displaced due to urban expansion, compared to the world average of 3.7% ( [[#van%20Vliet--2017|van Vliet et al. 2017]] ). As urban clusters increase in size and greenspace is converted, future urban land expansion is expected to intensify UHIs and exacerbate night-time extreme temperatures. An urban footprint increase of 78–171% by 2050 over the urban footprint in 2015 is expected to result in average summer daytime and night-time warming in air temperature of 0.5°C–0.7°C, even up to about 3°C in certain locations ( [[#Huang--2019|Huang et al. 2019]] ). Furthermore, this urban expansion-induced warming is on average about half – and in certain locations nearly twice – as strong as warming that will be caused by GHG emissions based on the multi-model ensemble average forecasts in RCP4.5. In short, future urban expansion will amplify the background warming caused by GHG emissions, with extreme warming most pronounced during night-time ( ''very high confidence'' ) ( [[#Huang--2019|Huang et al. 2019]] ). These findings corroborate those in the Technical Summary of AR6 WGI ( [[#Arias--2021|Arias et al. 2021]] ).

The forecasted amounts and patterns of urban expansion presented here bear significant uncertainty due to underlying factors beyond mere methodological differences between the studies. These factors include potential changes in the social, economic, and institutional dynamics that drive urban land development across the world ( [[#Güneralp--2013|Güneralp and Seto 2013]] ). Some of these changes may come in the form of sudden shocks such as another global economic crisis or pandemic. The forecasts presented here do not take such factors into account.

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