Editing IPCC:AR6/SRCCL/Chapter-2 (section)

== CCB4 Climate change and urbanisation ==

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Nathalie de Noblet-Ducoudré (France), Peng Cai (China), Sarah Connors (France/United Kingdom), Martin Dallimer (United Kingdom), Jason Evans (Australia), Rafiq Hamdi (Belgium), Gensuo Jia (China), Kaoru Kitajima (Japan), Christopher Lennard (South Africa), Shuaib Lwasa (Uganda), Carlos Fernando Mena (Ecuador), Soojeong Myeong (Republic of Korea), Lennart Olsson (Sweden), Prajal Pradhan (Nepal/Germany), Lindsay Stringer (United Kingdom)

Cities extent, population, and expected growth

Despite only covering 0.4–0.9% of the global land surface (Esch et al. 2017 <sup>[[#fn:r1352|1352]]</sup> ; Zhou et al. 2015 <sup>[[#fn:r1353|1353]]</sup> ), over half the world’s population live in towns and cities (United Nations, 2017 <sup>[[#fn:r1354|1354]]</sup> ) generating around three-quarters of the global total carbon emissions from energy use (Creutzig et al. 2015b <sup>[[#fn:r1355|1355]]</sup> ; Seto et al. 2014 <sup>[[#fn:r1356|1356]]</sup> ). Urban food consumption is a large source of these anthropogenic GHG emissions (Goldstein et al. 2017 <sup>[[#fn:r1357|1357]]</sup> ). In developed countries, per capita emissions are larger in small cities than bigger ones, while the opposite is found in developing countries (Gudipudi et al. 2019 <sup>[[#fn:r1358|1358]]</sup> ). Climate change is expected to increase the energy demand of people living in urban areas (Santamouris et al. 2015 <sup>[[#fn:r1359|1359]]</sup> ; Wenz et al. 2017 <sup>[[#fn:r1360|1360]]</sup> ).

In addition to being a driver of emissions, urbanisation contributes to forest degradation, converts neighbouring agricultural, forested or otherwise undeveloped land to urban use, altering natural or semi-natural ecosystems both within and outside of urban areas (Du and Huang 2017 <sup>[[#fn:r1361|1361]]</sup> ). It has been identified as a major driver of land degradation, as illustrated in Chapters 3, 4 and 5. Highly productive lands are experiencing the highest rate of conversion to urbanised landscapes (Nizeyimana et al. 2001 <sup>[[#fn:r1362|1362]]</sup> ; Pandey et al. 2018 <sup>[[#fn:r1363|1363]]</sup> ), affecting food security. Loss of agricultural land and increased pollution and waste are some of key challenges arising from urbanisation and urban growth (Chen 2007 <sup>[[#fn:r1364|1364]]</sup> ). The proportion of urban population is predicted to reach about 70% by the middle of the century (United Nations 2017 <sup>[[#fn:r1365|1365]]</sup> ) with growth especially taking place in the developing world (Angel et al. 2011 <sup>[[#fn:r1366|1366]]</sup> ; Dahiya 2012 <sup>[[#fn:r1367|1367]]</sup> ). Urban sprawl is projected to consume 1.8–2.4% and 5% of the current cultivated land by 2030 and 2050, respectively (Pradhan et al. 2014 <sup>[[#fn:r1368|1368]]</sup> ; Bren d’Amour et al. 2016 <sup>[[#fn:r1369|1369]]</sup> ), driven by both general population increase and immigration from rural areas (Adger et al. 2015 <sup>[[#fn:r1370|1370]]</sup> ; Seto et al. 2011 <sup>[[#fn:r1371|1371]]</sup> ; Geddes et al. 2012 <sup>[[#fn:r1373|1373]]</sup> ). New city dwellers in developing countries will require land for housing to be converted from non-urban to urban land (Barbero-Sierra et al. 2013 <sup>[[#fn:r1373|1373]]</sup> ), indicating future degradation. These growing urban areas will experience direct and indirect climate change impacts, such as sea level rise and storm surges (Boettle et al. 2016 <sup>[[#fn:r1374|1374]]</sup> ; Revi et al. 2014 <sup>[[#fn:r1375|1375]]</sup> ), increasing soil salinity and landslides from precipitation extremes. Furthermore, poorly planned urbanisation can increase people’s risk to climate hazards as informal settlements and poorly built infrastructure are often the most exposed to hazards from fire, flooding and landslides (Adger et al. 2015 <sup>[[#fn:r1376|1376]]</sup> ; Geddes et al. 2012 <sup>[[#fn:r1377|1377]]</sup> ; Revi et al. 2014 <sup>[[#fn:r1378|1378]]</sup> ). Currently, avoiding land degradation and maintaining/enhancing ecosystem services are rarely considered in planning processes (Kuang et al. 2017 <sup>[[#fn:r1379|1379]]</sup> ).

Climate change, urban heat island and threats specific to urban populations

Cities alter the local atmospheric conditions as well as those of the surrounding areas (Wang et al. 2016b <sup>[[#fn:r1380|1380]]</sup> ; Zhong et al. 2017 <sup>[[#fn:r1381|1381]]</sup> ). There is ''high confidence'' that urbanisation increases mean annual surface air temperature in cities and in their surroundings, with increases ranging from 0.19–2.60°C (Torres-Valcárcel et al. 2015 <sup>[[#fn:r1382|1382]]</sup> ; Li et al. 2018a <sup>[[#fn:r1383|1383]]</sup> ; Doan et al. 2016 <sup>[[#fn:r1384|1384]]</sup> ) (Cross-Chapter Box 4; Figure 1). This phenomenon is referred to as the urban heat island (UHI) effect (Oke et al. 2017 <sup>[[#fn:r1385|1385]]</sup> ; Bader et al. 2018 <sup>[[#fn:r1386|1386]]</sup> ). The magnitude and diurnal amplitude of the UHI varies from one city to another and depends on the local background climate (Wienert and Kuttler 2005 <sup>[[#fn:r1387|1387]]</sup> ; Zhao et al. 2014 <sup>[[#fn:r1388|1388]]</sup> ; Ward et al. 2016 <sup>[[#fn:r1389|1389]]</sup> ). There is nevertheless ''high confidence'' that urbanisation affects night-time temperatures more substantially than daytime ones (Argüeso et al. 2014 <sup>[[#fn:r1390|1390]]</sup> ; Alghamdi and Moore 2015 <sup>[[#fn:r1391|1391]]</sup> ; Alizadeh-Choobari et al. 2016 <sup>[[#fn:r1392|1392]]</sup> ; Fujibe, 2009 <sup>[[#fn:r1393|1393]]</sup> ; Hausfather et al. 2013 <sup>[[#fn:r1394|1394]]</sup> ; Liao et al. 2017 <sup>[[#fn:r1395|1395]]</sup> ; Sachindra et al. 2016 <sup>[[#fn:r1396|1396]]</sup> ; Camilloni and Barrucand 2012 <sup>[[#fn:r1397|1397]]</sup> ; Wang et al. 2017a <sup>[[#fn:r1398|1398]]</sup> ; Hamdi, 2010 <sup>[[#fn:r1399|1399]]</sup> ; Arsiso et al. 2018 <sup>[[#fn:r1400|1400]]</sup> ; Elagib 2011 <sup>[[#fn:r1401|1401]]</sup> ; Lokoshchenko 2017 <sup>[[#fn:r1402|1402]]</sup> ; Robaa 2013 <sup>[[#fn:r1403|1403]]</sup> ). In addition, there is ''high confidence'' that the UHI effect makes heatwaves more intense in cities by 1.22–4°C, particularly at night (Li and Bou-Zeid 2013 <sup>[[#fn:r1404|1404]]</sup> ; Li et al. 2017b <sup>[[#fn:r1405|1405]]</sup> ; Hamdi et al. 2016 <sup>[[#fn:r1406|1406]]</sup> ; Founda and Santamouris 2017 <sup>[[#fn:r1407|1407]]</sup> ; Wang et al. 2017a <sup>[[#fn:r1408|1408]]</sup> ). As there is a well-established relationship between extremely high temperatures and morbidity, mortality (Watts et al. 2015 <sup>[[#fn:r1409|1409]]</sup> ) and labour productivity (Costa et al. 2016 <sup>[[#fn:r1410|1410]]</sup> ), an expected increase in extreme heat events with future climate change will worsen the conditions in cities.

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'''Cross Chapter Box 4 Figure 1'''

<span id="change-in-annual-mean-surface-air-temperature-resulting-from-urbanisation-ºc.-the-colour-and-size-of-the-circles-refer-to-the-magnitude-of-the-change.-this-map-has-been-compiled-using-the-following-studies-kim-et-al.-2016-sun-et-al.-2016-chen-et-al.-2016a-founda-et-al.-2015-rafael-et-al.-2017-hinkel"></span>
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'''Change in annual mean surface air temperature resulting from urbanisation (ºC). The colour and size of the circles refer to the magnitude of the change. (This map has been compiled using the following studies: Kim et al. (2016), Sun et al. (2016), Chen et al. (2016a), Founda et al. (2015), Rafael et al. (2017), Hinkel […]'''
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[[File:7d1b06576b4a11c280889e770ee51413 Cross-Chapter-Box-4-Figure-1-1024x528.jpg]]

Change in annual mean surface air temperature resulting from urbanisation (ºC). The colour and size of the circles refer to the magnitude of the change. (This map has been compiled using the following studies: Kim et al. (2016) <sup>[[#fn:r1411|1411]]</sup> , Sun et al. (2016) <sup>[[#fn:r1412|1412]]</sup> , Chen et al. (2016a) <sup>[[#fn:r1413|1413]]</sup> , Founda et al. (2015) <sup>[[#fn:r1414|1414]]</sup> , Rafael et al. (2017) <sup>[[#fn:r1415|1415]]</sup> , Hinkel and Nelson (2007) <sup>[[#fn:r1416|1416]]</sup> , Chrysanthou et al. (2014) <sup>[[#fn:r1417|1417]]</sup> , Dou et al. (2014) <sup>[[#fn:r1418|1418]]</sup> , Zhou et al. (2016) <sup>[[#fn:r1419|1419]]</sup> , (2017) <sup>[[#fn:r1420|1420]]</sup> , Polydoros et al. (2018) <sup>[[#fn:r1421|1421]]</sup> , Li et al. (2018a) <sup>[[#fn:r1422|1422]]</sup> , Bader et al. (2018) <sup>[[#fn:r1423|1423]]</sup> , Alizadeh-Choobari et al. (2016) <sup>[[#fn:r1424|1424]]</sup> , Fujibe (2009) <sup>[[#fn:r1425|1425]]</sup> , Lokoshchenko (2017) <sup>[[#fn:r1426|1426]]</sup> , Torres-Valcárcel et al. (2015) <sup>[[#fn:r1427|1427]]</sup> , Doan et al. (2016) <sup>[[#fn:r1428|1428]]</sup> , Elagib (2011) <sup>[[#fn:r1429|1429]]</sup> , Liao et al. (2017) <sup>[[#fn:r1430|1430]]</sup> ).

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Individual city case studies show that precipitation mean and extremes are increased over and downwind of urban areas, especially in the afternoon and early evening when convective rise of the atmosphere is the strongest ( ''medium confidence'' ). The case studies covered: different inland and coastal US cities (Haberlie et al. 2014 <sup>[[#fn:r1431|1431]]</sup> ; McLeod et al. 2017 <sup>[[#fn:r1432|1432]]</sup> ; Ganeshan and Murtugudde 2015 <sup>[[#fn:r1433|1433]]</sup> ), Dutch coastal cities (Daniels et al. 2016 <sup>[[#fn:r1434|1434]]</sup> ), Hamburg (Schlünzen et al. 2010 <sup>[[#fn:r1435|1435]]</sup> ), Shanghai (Liang and Ding 2017 <sup>[[#fn:r1436|1436]]</sup> ), Beijing (Dou et al. 2014 <sup>[[#fn:r1437|1437]]</sup> ), and Jakarta and Kuala Lumpur (Lorenz et al. 2016 <sup>[[#fn:r1438|1438]]</sup> ). Increased aerosol concentrations, however, can interrupt the precipitation formation process and thereby reduce heavy rainfall (Daniels et al. 2016 <sup>[[#fn:r1439|1439]]</sup> ; Zhong et al. 2017 <sup>[[#fn:r1440|1440]]</sup> ). Urban areas also experience altered water cycle in other aspects: the evaporative demand for plants in cities are increased by as much as 10% (Zipper et al. 2017 <sup>[[#fn:r1441|1441]]</sup> ), while the high proportion of paving in cities means that surface runoff of water is high (Hamdi et al. 2011 <sup>[[#fn:r1442|1442]]</sup> ; Pataki et al. 2011 <sup>[[#fn:r1443|1443]]</sup> ). In addition, water retention is lower in degraded, sealed soils beneath urban surfaces compared to intact soils. Increased surface water runoff, especially when and where the rainfall intensity is likely to intensify (IPCC 2013a <sup>[[#fn:r1444|1444]]</sup> ), leads to a greater likelihood of flooding in urban areas without implementation of adaptation measures (Shade and Kremer 2019 <sup>[[#fn:r1445|1445]]</sup> ; Wang et al. 2013 <sup>[[#fn:r1446|1446]]</sup> ; EPA 2015 <sup>[[#fn:r1447|1447]]</sup> ).

Urbanisation alters the stock size of soil organic carbon (SOC) and its stability. The conversion of vegetated land to urban land results in a loss of carbon stored in plants, while stresses associated with the urban environment (e.g., heat, limited water availability, pollution) reduce plant growth and survival in cities (Xu et al. 2016b <sup>[[#fn:r1448|1448]]</sup> ). Overall, carbon densities or stocks decrease from natural land areas to the urban core along the rural-urban gradient (Tao et al. 2015 <sup>[[#fn:r1449|1449]]</sup> ; Zhang et al. 2015 <sup>[[#fn:r1450|1450]]</sup> ). For example, the Seoul Forest Park, an urban park, shows a tenfold difference in SOC stocks across its land cover types (Bae and Ryu 2015 <sup>[[#fn:r1451|1451]]</sup> ). In Changchun in Northeast China, however, SOC density is higher in recreational forests within urban areas compared to a production forest (Zhang et al. 2015 <sup>[[#fn:r1452|1452]]</sup> ).

Urban air pollution as an environmental risk increases with climate change. Increased air temperatures can lead to reduced air quality by enhancing the formation of photochemical oxidants and increasing the concentration of air pollutants such as ozone, with corresponding threats to human health (Sharma et al. 2013 <sup>[[#fn:r1453|1453]]</sup> ). The occurrence of bronchial asthma and allergic respiratory diseases is increasing worldwide, and urban residents are experiencing poor air quality conditions more frequently than rural residents (D’Amato et al. 2010 <sup>[[#fn:r1454|1454]]</sup> ). Excess morbidity and mortality related to extremely poor air quality are found in many cities worldwide (Harlan and Ruddell 2011 <sup>[[#fn:r1455|1455]]</sup> ). Some emissions that lead to reduced air quality are also contributors to climate change (Shindell et al. 2018 <sup>[[#fn:r1456|1456]]</sup> ; de Coninck et al. 2018 <sup>[[#fn:r1457|1457]]</sup> ).

Urban response options for climate change, desertification, land degradation and food security

Urban green infrastructure (UGI) has been proposed as a solution to mitigate climate change directly through carbon sequestration (Davies et al. 2011 <sup>[[#fn:r1458|1458]]</sup> ; Edmondson et al. 2014 <sup>[[#fn:r1459|1459]]</sup> ). However, compared to overall carbon emissions from cities, its mitigation effects are likely to be small ( ''medium confidence'' ). UGI nevertheless has an important role in adapting cities to climate change (Demuzere et al. 2014 <sup>[[#fn:r1460|1460]]</sup> ; Sussams et al. 2015 <sup>[[#fn:r1461|1461]]</sup> ; Elmqvist et al. 2016 <sup>[[#fn:r1462|1462]]</sup> ; Gill et al. 2007 <sup>[[#fn:r1463|1463]]</sup> ; Revi et al. 2014 <sup>[[#fn:r1464|1464]]</sup> ). Adaptation through UGIs is achieved through, for example, (i) reduction in air temperature (Cavan et al. 2014 <sup>[[#fn:r1465|1465]]</sup> ; Di Leo et al. 2016 <sup>[[#fn:r1467|1467]]</sup> ; Feyisa et al. 2014 <sup>[[#fn:r1468|1468]]</sup> ; Zölch et al. 2016 <sup>[[#fn:r1469|1469]]</sup> ; Li et al. 2019 <sup>[[#fn:r1470|1470]]</sup> ) which can help improve human health and comfort (e.g., Brown and Nicholls 2015 <sup>[[#fn:r1471|1471]]</sup> ; Klemm et al. 2015 <sup>[[#fn:r1472|1472]]</sup> ), (ii) reduction in the energy demands of buildings through the use of green roofs and walls (e.g., Coma et al. 2017 <sup>[[#fn:r1473|1473]]</sup> ), and (iii) reduction in surface water runoff and flood risk (Zeleňáková et al. 2017 <sup>[[#fn:r1474|1474]]</sup> ). Given that UGI necessarily involves the retention and management of non-sealed surfaces, co-benefits for land degradation will also be apparent (limited evidence, high agreement) (Murata and Kawai 2018 <sup>[[#fn:r1475|1475]]</sup> ; Scalenghe and Marsan 2009 <sup>[[#fn:r1476|1476]]</sup> ).

Urban agriculture is one aspect of UGI that has the potential to both meet some of the food needs of cities and reduce land degradation pressures in rural areas ( ''low confidence'' ) (e.g., Wilhelm and Smith (2018) <sup>[[#fn:r1477|1477]]</sup> ). Urban agriculture has many forms, such as backyard gardening, allotments, plants on rooftops or balconies, urban-fringe/peri-urban agriculture, hydroponics, aquaponics, livestock grazing in open spaces and vertical farming (Gerster-Bentaya 2013 <sup>[[#fn:r1478|1478]]</sup> ) (Section 5.6.5).

Consuming locally produced food and enhancing the efficiency of food processing and transportation can minimise food losses, contribute to food security and, in some circumstances, reduce GHG emissions (Brodt et al. 2013 <sup>[[#fn:r1479|1479]]</sup> ; Michalský and Hooda 2015 <sup>[[#fn:r1480|1480]]</sup> ; Tobarra et al. 2018 <sup>[[#fn:r1481|1481]]</sup> ) (Section 5.5.2.3). Furthermore, urban agriculture has the potential to counteract the separation of urban populations from food production. This separation is one driver of the transition towards more homogeneous, high-protein diets, which are associated with increased GHG emissions (Goldstein et al. 2017 <sup>[[#fn:r1482|1482]]</sup> ; Moragues-Faus and Marceau 2018 <sup>[[#fn:r1483|1483]]</sup> ; Magarini and Calori 2015 <sup>[[#fn:r1484|1484]]</sup> ). Barriers to the uptake of urban agriculture as a climate change mitigation option include the need for efficient distribution systems to ensure lowered carbon emissions (Newman et al. 2012 <sup>[[#fn:r1485|1485]]</sup> ) and the concern that urban agriculture may harbour pathogenic diseases, or that its products be contaminated by soil or air pollution (Hamilton et al. 2014 <sup>[[#fn:r1486|1486]]</sup> ; Ercilla-Montserrat et al. 2018 <sup>[[#fn:r1487|1487]]</sup> ).

In summary

Climate change is already affecting the health and energy demand of large numbers of people living in urban areas ( ''high confidence'' ) (Section 2.2). Future changes to both climate and urbanisation will enhance warming in cities and their surroundings, especially during heatwaves ( ''high confidence'' ). Urban and peri-urban agriculture and, more generally, the implementation of urban green infrastructure, can contribute to climate change mitigation ( ''medium confidence'' ) as well as to adaptation ( ''high confidence'' ), including co-benefits for food security and reduced soil-water-air pollution.

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