Editing IPCC:AR6/WGII/Chapter-7 (section)

==== 7.3.1.2 Projected Changes in Heat- and Cold-Related Exposure and Related Health Outcomes ====

<div id="h3-31-siblings" class="h3-siblings"></div>

This section considers the broad impacts of projected changes in heat- and cold-related exposure and related outcomes including mortality and work productivity. Several of the most common heat- and cold-related specific health outcomes (e.g., CVD) are assessed individually in later sections of this chapter.

''Population heat exposure will increase under climate change'' ( ''very high confidence'' ) ''.'' Since AR5 there has been considerable progress with quantifying future human exposure to extreme heat ( [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ), especially as determined by different combinations of SSPs and RCPs ( [[#Chambers--2020|Chambers, 2020]] ; [[#Cheng--2020|Cheng et al., 2020]] ; [[#Jones--2018|Jones et al., 2018]] ; [[#Liu--2017|Liu et al., 2017]] ; [[#Ma--2021|Ma and Yuan, 2021]] ; [[#Russo--2019|Russo et al., 2019]] ). For example, Table 7.1 shows projections of population exposure to heatwaves, as expressed by the number of person-days, for the 2061–2080 period aggregated by geographical region and SSP/RCP. At the global level, projected future exposure increases from approximately 15 million person-days for the current period to 535 billion person-days for high population growth under the high GHG emission SSP3-RCP8.5 scenario, while for the low population growth/high urbanisation and business as usual SSP5-RCP4.5 scenario, the exposure is substantially lower at 170 billion person-days. Spatial variations in future heatwave frequency and population growth play out in the form of significant geographical contrasts in exposure, with the largest increases projected for low latitude regions such as India and significant portions of sub-Saharan Africa, where increases in heatwave frequency and population are expected. Over East Asia and especially eastern China, exposures are projected to rise, with the effect of increases in heatwave frequency exceeding the countering effect of projected reductions in population, especially in non-urban areas. Further, for North America and Europe, where rural depopulation is projected, the predominant driver of increases in exposure is urban growth ( [[#Jones--2018|Jones et al., 2018]] ).

'''Table 7.1 |''' Projected exposure to heatwaves in millions of person-days by region under different SSP/RCP combinations.

{| class="wikitable"
|-
! rowspan="2"| '''Region'''

! colspan="5"| '''Exposure in millions of person-days'''

|-
! '''Current'''

! '''SSP3-4.5'''

! '''SSP5-4.5'''

! '''SSP3-8.5'''

! '''SSP5-8.5'''

|-
| Global

USA

North America

Europe

Latin America and Caribbean

North Africa and Middle East

Sub-Saharan Africa

Russia and Central Asia

South Asia

East Asia

Southeast Asia

Oceania

| 14,811

375

376

191

803

1,335

1,427

272

7,194

977

711

37

| 244,807

4,769

4,821

2,967

17,287

34,721

67,442

3,074

84,044

12,176

12, 452

247

| 168,488

8,671

8,778

3,775

10,856

23,160

41,339

1,951

53,655

10,855

9,146

492

| 534,848

10,802

10,990

7,326

45,612

65,072

158,290

6,554

146,709

35,381

60,909

822

| 374,269

19,646

20,153

9,969

28,435

43,648

96,054

4,360

94,288

31,918

47,141

1,158

|}

''Comparisons of heatwave exposure for 1.5°C and 2.0°C warming for different SSPs indicate strong geographical contrasts in potential heatwave risk'' ( ''high confidence'' ) ''.'' One global level assessment for a 1.5°C warming projects that low human development index countries will experience exposure levels equal to or greater than the exposure levels for very high human development index countries under a 2°C warming (Russo, 2019). The same assessment also finds that holding global warming below 1.5°C in tandem with achieving sustainable socioeconomic development (e.g., SSP1 as opposed to SSP4) yields reduced levels of heatwave exposure, especially for low human development index countries, particularly across sub-Saharan Africa. Similar findings were found in other global level assessments. Global exposure to extreme heat increases almost 30 times under a SSP3-8.5 combination, with the average exposure for Africa 118 times greater than historical levels, in stark contrast to the four-fold increase projected for Europe. Compared to a SSP3-8.5 scenario, exposure was reduced by 65% and 85% under the SSP2-4.5 and SSP1-2.6 scenarios, respectively ( [[#Liu--2017|Liu et al., 2017]] ).

''Regional level assessments of changes in population heat exposure for Africa, Europe, the USA, China and India corroborate general findings at the global level, that the impact of warming is amplified under divergent regional development pathways (e.g., SSP4 – inequality) compared to those fostering sustainable development (e.g., SSP1 – sustainability)'' ( ''high confidence'' ) ''( [[#Rohat--2019a|Rohat et al., 2019a]] ; [[#Weber--2020|Weber et al., 2020]] ; [[#Broadbent--2020|Broadbent et al., 2020]] ; [[#Dahl--2019|Dahl et al., 2019]] ; [[#Harrington--2018|Harrington and Otto, 2018]] ; [[#Rohat--2019b|Rohat et al., 2019b]] ; [[#Vahmani--2019|Vahmani et al., 2019]] ; Huang and et al., 2018; [[#Zhang--2020a|Zhang et al., 2020a]] ; [[#Liu--2017|Liu et al., 2017]] )'' . For some regions, such as Europe, changes in exposure are projected to be largely a consequence of climate change, while for others, such as Africa and to a lesser extent Asia, Oceania, North America and South America, the interactive effects of demographic and climate change are projected to be important ( [[#Jones--2018|Jones et al., 2018]] ; [[#Liu--2017|Liu et al., 2017]] ; [[#Russo--2016|Russo et al., 2016]] ; [[#Ma--2021|Ma and Yuan, 2021]] ) ( ''medium confidence'' ).

Compared to research that estimates the temperature only impacts of climate change on heat-related mortality (see below), the number of studies that explicitly model mortality responses considering various combinations of SSPs and RCPs is small and mostly restricted to the country or regional level. These studies point to increases in heat-related mortality especially amongst the elderly across a range of SSPs, with the greatest increases under SSP5 and RCP8.5 ( [[#Rail--2019|Rail et al., 2019]] ; [[#Yang--2021|Yang et al., 2021]] ).

''Estimates of heat-related mortality based solely on changes in temperature point to elevated levels of global and regional level mortality compared to the present, with the magnitude of this increasing from RCP4.5 through to RCP8.5'' ( ''high confidence'' ) ''( [[#Ahmadalipour--2018|Ahmadalipour and Moradkhani, 2018]] ; [[#Cheng--2019|Cheng et al., 2019]] ; [[#Kendrovski--2017|Kendrovski et al., 2017]] ; [[#Lee--2020|Lee et al., 2020]] ; [[#Limaye--2018|Limaye et al., 2018]] ; [[#Morefield--2018|Morefield et al., 2018]] )'' . Further support comes from the projection that heat-related health impacts for a 2°C increase in global temperatures will be greater than those for 1.5°C warming ( ''very high confidence'' ) ( [[#Dosio--2018|Dosio et al., 2018]] ; [[#Mitchell--2018|Mitchell et al., 2018]] ; [[#King--2017|King and Karoly, 2017]] ; [[#Vicedo-Cabrera--2018a|Vicedo-Cabrera et al., 2018a]] ).

''Estimates of future mortality that incorporate adaptation in addition to temperature change point to increases in heat-related mortality under global warming, albeit at lower levels than the case of no adaptation'' ( ''high confidence'' ) ''( [[#Anderson--2018|Anderson et al., 2018]] ; [[#Gosling--2017|Gosling et al., 2017]] ; [[#Guo--2018|Guo et al., 2018]] ; [[#Honda--2020|Honda and Onozuka, 2020]] ; [[#Vicedo-Cabrera--2018b|Vicedo-Cabrera et al., 2018b]] ; [[#Wang--2018b|Wang et al., 2018b]] )'' . Whether adaptation is considered or not, the consensus is Central and South America, southern Europe, southern and Southeast Asia and Africa will be the most affected by climate change in terms of heat-related mortality ( ''high confidence'' ). Similarly, projections of the impacts of future heat on occupational health, worker productivity and workability point to these regions as problematic under climate change ( ''high confidence'' ) ( [[#Andrews--2018|Andrews et al., 2018]] ; [[#de%20Lima--2021|de Lima et al., 2021]] ; [[#Dillender--2021|Dillender, 2021]] ; [[#Kjellstrom--2018|Kjellstrom et al., 2018]] ; [[#Orlov--2020|Orlov et al., 2020]] ; [[#Rao--2020|Rao et al., 2020]] ; [[#Tigchelaar--2020|Tigchelaar et al., 2020]] ), especially for occupations with high exposure to heat, such as agriculture and construction. This accords with the findings from independent projections of population heat exposure as outlined above ( ''high confidence'' ).

''The effect of climate change on productivity is projected to reduce GDP at a range of geographical scales'' ( ''high confidence'' ) ''( [[#Borg--2021|Borg et al., 2021]] ; [[#Oppermann--2021|Oppermann et al., 2021]] ; [[#Orlov--2020|Orlov et al., 2020]] )'' . For example, measuring economic costs using occupational health and safety recommendations, it was estimated that RCP8.5 would result in a 2.4% reduction in global GDP compared to a 0.5% reduction under RCP2.6 ( [[#Orlov--2020|Orlov et al., 2020]] ). For the USA, it was estimated that the total hours of labour supplied declined ∼ 0.11% (±0.004%) per degree Celsius increase in global mean surface temperature for low-risk workers and 0.53% (±0.01%) per degree Celsius increase for high-risk workers exposed to outdoor temperatures ( [[#Hsiang--2017|Hsiang et al., 2017]] ). Further, a systematic review of the literature indicates that extreme heat exacts a substantial economic burden on health systems, which bears implications for future heat-attributable healthcare costs ( [[#Wondmagegn--2019|Wondmagegn et al., 2019]] ).

''Since AR5, there has been an increase in the understanding of the extent to which a warming world is'' likely ''to affect cold- or winter-related health impacts. Future increases in heat-related deaths are expected to outweigh those related to cold'' ( ''high confidence'' ) ''( [[#Aboubakri--2020|Aboubakri et al., 2020]] ; [[#Achebak--2020|Achebak et al., 2020]] ; [[#Burkart--2021|Burkart et al., 2021]] ; [[#Huber--2020b|Huber et al., 2020b]] ; [[#Martinez--2018|Martinez et al., 2018]] ; [[#Rodrigues--2020|Rodrigues et al., 2020]] ; [[#Vardoulakis--2014|Vardoulakis et al., 2014]] ; [[#Weinberger--2017|Weinberger et al., 2017]] ; [[#Weinberger--2018a|Weinberger et al., 2018a]] ; [[#Weitensfelder--2020|Weitensfelder and Moshammer, 2020]] )'' . However, strong regional contrasts in heat- and cold-related mortality trends are ''likely'' under a RCP8.5 scenario, with countries in the Global North experiencing minimal to moderate decreases in cold-related mortality while warm climate countries in the Global South are projected to experience increases in heat-attributable deaths by the end of the century ( [[#Gasparrini--2017|Gasparrini et al., 2017]] ; [[#Burkart--2021|Burkart et al., 2021]] ). Projections of the magnitude of change in the temperature-related burden of disease do, however, demonstrate great variability, due to the application of a wide range of climate change, adaptation and demographic scenarios ( [[#Cheng--2019|Cheng et al., 2019]] ).

''A particular focus since AR5 has been the impact of climate change on cities (see AR6 Chapter 6). Heat risks are expected to be greater in urban areas due to changes in regional heat exacerbated by ‘heat island’ effects'' ( ''high confidence'' ) ''( [[#Doan--2018|Doan and Kusaka, 2018]] ; [[#Heaviside--2016|Heaviside et al., 2016]] ; [[#Li--2021|Li et al., 2021]] ; [[#Rohat--2019a|Rohat et al., 2019a]] ; [[#Rohat--2019c|Rohat et al., 2019c]] ; [[#Varquez--2020|Varquez et al., 2020]] ; [[#Wouters--2017|Wouters et al., 2017]] ; [[#Zhao--2021|Zhao et al., 2021]] ), with intra-urban scale variations in heat exposure attributable to land cover contrasts and urban form and function ( [[#Avashia--2021|Avashia et al., 2021]] ; [[#Jang--2020|Jang et al., 2020]] ; [[#Macintyre--2018|Macintyre et al., 2018]] ; [[#Schinasi--2018|Schinasi et al., 2018]] ).'' However, further research is required to establish the health implications of increasing chronic slow-onset extreme heat ( [[#Oppermann--2021|Oppermann et al., 2021]] ) in addition to the acute health outcomes of UHI–heatwave synergies under climate change. The latter is particularly important as studies that address UHI–heatwave interactions have mainly focused on changes in UHI intensity (e.g., [[#Ramamurthy--2017|Ramamurthy and Bou-Zeid (2017)]] ; Scott et al. (2018)). Whether significant urban mortality anomalies arise from the interplay of heatwaves and UHIs largely remains an open question although at least one study demonstrated higher urban compared to rural mortality rates during heatwaves ( [[#Ruuhela--2021|Ruuhela et al., 2021]] ). The benefits of the winter UHI effect for cold-related mortality remain largely unexplored, but one study for Birmingham, UK, indicates the winter UHI will continue to have a protective effect in future climate ( [[#Macintyre--2021|Macintyre et al., 2021]] ).

<div id="7.3.1.3" class="h3-container"></div>

<span id="projected-impacts-on-vector-borne-diseases"></span>