Editing IPCC:AR6/WGII/Chapter-16 (section)

===== 16.5.2.3.5 Risk to human health (RKR-E) =====

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This RKR includes (i) mortality from heat, and morbidity and mortality from (ii) vector-borne diseases and (iii) waterborne diseases. It builds on KRs identified primarily in [[IPCC:Wg2:Chapter:Chapter-7|Chapter 7]] and health risks in regional chapters.

A severe risk to health is the potential for a widespread, substantial worsening of health conditions due to climate change. We measure severity in terms of the magnitude of mortality and morbidity. We consider a severe mortality impact to be a sustained increase in the crude mortality rate (CMR) of more than about 2–4 deaths per 10,000 people yr –1 . This range of increase is consistent with current mortality impacts with substantial global effects, including traffic fatalities (CMR of 1.6/10,000 yr −1 ; [[#IHME--2019|IHME, 2019]] ) and the COVID-19 pandemic (CMR of 4/10,000 yr −1 , as of April 2021, expressed as an annualised rate; [[#Ritchie--2021|Ritchie et al., 2021]] ). We use these global rates as thresholds in all cases, recognising that they reflect substantial variation across regions and sub-populations (for other points of comparison, see [[#IHME--2019|IHME, 2019]] ). Morbidity impacts are measured in numbers of disease cases or hospital admissions. We find that severe health impacts are projected to occur for particular sub-populations and regions where vulnerability is currently high and is assumed to persist into the future; we focus our assessment on these cases. In other cases, literature is either inadequate or does not support severe outcomes.

# Risks of heat-related mortality would become severe at global and regional scales with high levels of warming and vulnerability ( ''medium confidence'' ). Under these conditions (SSP3–8.5), accounting for adaptation, heat mortality would increase the global CMR by up to 7/10,000 yr −1 by 2100 ( [[#Carleton--2020|Carleton et al., 2020]] ). For example, the USA would experience a CMR increase of 2–4/10,000 yr −1 by the end of the century (medium vulnerability without adaptation, and recent vulnerability with adaptation, respectively) ( [[#Weinberger--2017|Weinberger et al., 2017]] ; [[#Shindell--2020|Shindell et al., 2020]] ). Also assuming no adaptation and recent vulnerability, most populations of the world would experience an increase of 2–10 percentage points in the percentage of deaths attributable to heat by the end of the century (RCP8.5) (Vicedo-Cabrera, 2018a; Gasparrini, 2017). Harmful conditions for health are expected to increase in frequency and intensity over all land areas along with the rising temperatures in the coming decades ( [[#Pal--2016|Pal and Eltahir, 2016]] ; [[#Russo--2017|Russo et al., 2017]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Saeed--2021|Saeed et al., 2021]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ). Projections of exposure are an incomplete measure of risk but suggest the potential for severe impacts. For example, the percent of global population exposed to deadly heat stress would increase from today’s 30% to 48–74% by the end of the century depending on level of warming and population distribution ( [[#Mora--2017|Mora et al., 2017]] ). Projected impacts are larger if exposure and/or vulnerability increases due to ageing of the population or increased inequality ( [[#Weinberger--2017|Weinberger et al., 2017]] ; [[#Chen--2020a|Chen et al., 2020a]] ; [[#IPCC--2021|IPCC, 2021]] ) and with limited adaptation capacity (e.g., poor infrastructure, limited air conditioning, few medical and public health resources) (SM16.7.4) ( [[#Carleton--2020|Carleton et al., 2020]] ). Higher risks are also expected in urban areas owing to hazard amplification (i.e., urban heat island effect) and in highly dense settlements with other environmental hazards such as air pollution ( [[#Zhao--2018|Zhao et al., 2018]] ; [[#Sera--2019|Sera et al., 2019]] ).
# Risks of vector-borne disease would become severe with high warming and current vulnerability, concentrated in children and in sensitive regions ( ''medium confidence'' ). Severity is defined by regionally substantial numbers of additional malaria deaths, disease cases and episodic hospitalisation demands (for dengue).

With high warming, the CMR for malaria among children under the age of 1 year could increase by 5.2–10.1/10,000 yr −1 in Africa under current vulnerability levels. This estimate assumes a net increase of 70–130 million more people exposed to potential disease transmission due to climate change in a high-warming scenario (RCP8.5, end of century) ( [[#Caminade--2014|Caminade et al., 2014]] ; Colón- [[#González--2021|González et al., 2021]] ; [[#Ryan--2020|Ryan et al., 2020]] ), representing a 14–27% increase in the current population at risk ( [[#Ryan--2020|Ryan et al., 2020]] ), and assumes children under 1 year of age are facing the same crude mortality in the future as for the African region today ( [[#IHME--2019|IHME, 2019]] ). The largest increase is observed in Eastern Africa, where the population exposed could nearly double by 2080 ( [[#Ryan--2020|Ryan et al., 2020]] ) without accounting for population growth, driven mainly by changes among previously unexposed populations at higher altitude areas (Colón- [[#González--2021|González et al., 2021]] ). Actual future disease burden of malaria will be highly sensitive to regional socioeconomic development and the effectiveness of malaria intervention programs.

For dengue, with high warming and current levels of vulnerability there could be as much as a doubling of cases and hospital admissions per year globally, relative to today, driven by both warming and population growth. These estimates are derived by assuming similar relative incidence rates as today ( [[#Shepard--2016|Shepard et al., 2016]] ) combined with projections of a more than doubling of the population exposed to potential disease transmission by the end of the century in a high-warming scenario (RCP8.5), although much of this increase is driven by population growth ( [[#Colón-González--2018|Colón-González et al., 2018]] ; [[#Monaghan--2018|Monaghan et al., 2018]] ; [[#Messina--2019|Messina et al., 2019]] ). There are around 3 billion people exposed to dengue today.

# Climate change would lead to severe risks of morbidity and mortality caused by waterborne diseases, particularly for diarrhoea in children in many lower- and middle-income countries (LMICs) and where vulnerability remains high ( ''medium confidence'' ). The global CMR for diarrhoea is 1.98 for all ages, but varies by region and age group, reaching as high as 53 for &lt;1-year-olds in Africa ( [[#IHME--2019|IHME, 2019]] ). In these vulnerable populations, even a small percentage increase can lead to substantial additional morbidity and mortality. For example, assuming no change in vulnerability or population, an increase in diarrhoea mortality of only 5% over 2019 baseline rates would create a severe risk (CMR of 2.0) for children under the age of 1 in the World Health Organization (WHO) Africa (AFRO) region. This percent increase due to climate change is plausible since diarrhoea incidence increases of 7% (95% confidence interval 3–10%) are associated with a 1°C increase in ambient temperature ( [[#WHO--2014|WHO, 2014]] ; [[#Carlton--2016|Carlton et al., 2016]] ), and diarrhoea is positively associated with heavy rainfall and flooding events ( [[#Levy--2016|Levy et al., 2016]] ), expected in some regions (WGI). Assuming vulnerability remains the same as today, mortality and morbidity rates would increase equivalently.

However, risks will be highly dependent on development trajectories, given that waterborne disease transmission is exacerbated by lack of clean drinking water and sanitation systems, inadequate food safety and hygiene conditions, lack of flood and drought protections, and interactions with other risks such as cholera outbreaks, food insecurity and infrastructure damage. Climate change threatens the progress that has been made towards reducing the burden of diarrhoea. For example, in Sub-Saharan Africa, while overall diarrhoea rates are expected to continue to decline (GBD 2016 Diarrhoeal Disease Collaborators, 2018), warming in 2030 (relative to the late 20th century) is projected to lead to diarrhoeal deaths in children under 15 equivalent to a CMR increase of 0.56/10,000 yr −1 (based on population projections for the region and age group; UN, 2020; [[#WHO--2014|WHO, 2014]] ). In China, by 2030, climate change could delay progress towards reducing waterborne disease burden by 8–85 months ( [[#Hodges--2014|Hodges et al., 2014]] ).

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