Editing IPCC:AR6/WGII/Chapter-9 (section)

==== 9.10.2.1 Vector-borne Diseases ====

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===== 9.10.2.1.1 Malaria =====

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Observed impacts

Higher temperatures and shifting patterns of rainfall influence the distribution and incidence of malaria in sub-Saharan Africa ( ''high confidence'' ) ( [[#Agusto--2015|Agusto et al., 2015]] ; [[#Beck-Johnson--2017|Beck-Johnson et al., 2017]] ). Up to 10.9 million km 2 of sub-Saharan Africa is optimally suitable for year-round malaria transmission ( [[#Mordecai--2013|Mordecai et al., 2013]] ; [[#Ryan--2015|Ryan et al., 2015]] ). Current climate suitability for endemic malaria transmission is concentrated in the central African region, some areas along the southern coast of west Africa and the east African coast ( [[#Ryan--2020|Ryan et al., 2020]] ).

In east Africa, there has been an expansion of the ''Anopheles'' vector into higher altitudes ( [[#Gone--2014|Gone et al., 2014]] ; [[#Carlson--2019|Carlson et al., 2019]] ) and increasing incidence of infection with ''Plasmodium falciparum'' with higher temperatures ( ''high confidence'' ) ( [[#Alemu--2014|Alemu et al., 2014]] ; [[#Lyon--2017|Lyon et al., 2017]] ). Over southern Africa, changes in temperature and rainfall are increasing malaria transmission ( [[#Abiodun--2018|Abiodun et al., 2018]] ). In west Africa, studies show both positive ( [[#Adu-Prah--2015|Adu-Prah and Kofi Tetteh, 2015]] ; [[#Darkoh--2017|Darkoh et al., 2017]] ) and negative ( [[#M’Bra--2018|M’Bra et al., 2018]] ) correlations of malaria incidence with increases in mean monthly temperatures, and an abundance of ''Anopheles gambiae'' s.s. associated with mean diurnal temperature ( [[#Akpan--2018|Akpan et al., 2018]] ).

Malaria incidence and outbreaks in east Africa were linked with both moderate monthly rainfall and extreme flooding ( [[#Boyce--2016|Boyce et al., 2016]] ; [[#Amadi--2018|Amadi et al., 2018]] ; [[#Simple--2018|Simple et al., 2018]] ), and increase 1–2 months after periods of rainfall in southern and west Africa ( [[#Diouf--2017|Diouf et al., 2017]] ; [[#Ferrão--2017|Ferrão et al., 2017]] ; [[#Adeola--2019|Adeola et al., 2019]] ). The years following La Niña events (southern Africa) ( [[#Adeola--2017|Adeola et al., 2017]] )) and high relative humidity (west Africa) ( [[#Adu-Prah--2015|Adu-Prah and Kofi Tetteh, 2015]] ; [[#Darkoh--2017|Darkoh et al., 2017]] ) have been positively linked with malaria incidence.

Projected risks

Since AR5, significant progress has been made in understanding how changes in climate influence the seasonal and geographical range of malaria vectors, transmission intensity and burden of disease of malaria across Africa. Yet projecting changes remains challenging given the range of factors that influence transmission and disease patterns, and model outputs contain high degrees of uncertainty ( [[#Zermoglio--2019|Zermoglio et al., 2019]] ; [[#Giesen--2020|Giesen et al., 2020]] ). Models have limited ability to account for population changes and development trends ( [[#Kibret--2015|Kibret et al., 2015]] ; 2017), investments in health sectors and interventions ( [[#McCord--2016|McCord, 2016]] ; [[#Colborn--2018|Colborn et al., 2018]] ; [[#Caminade--2019|Caminade et al., 2019]] ), and confounders such as age, socioeconomic status, employment, labour migration and climate variability ( [[#Bennett--2016|Bennett et al., 2016]] ; [[#Karuri--2016|Karuri and Snow, 2016]] ; [[#Byass--2017|Byass et al., 2017]] ; [[#Chuang--2017|Chuang et al., 2017]] ; [[#Colborn--2018|Colborn et al., 2018]] ). Nevertheless, available models do allow for projections of malaria transmission under different climate change scenarios to be made with high levels of certainty.

In east and southern Africa and the Sahel, malaria vector hotspots and prevalence are projected to increase under RCP4.5 and RCP8.5 by 2030 (1.5°C–1.7°C global warming) ( ''high confidence'' ) ( [[#Leedale--2016|Leedale et al., 2016]] ; [[#Semakula--2017b|Semakula et al., 2017b]] ; [[#Zermoglio--2019|Zermoglio et al., 2019]] ), becoming more pronounced later in the century (2.4°C–3.9°C global warming) ( [[#Ryan--2020|Ryan et al., 2020]] ). Under RCP4.5, 50.6–62.1 million people in east and southern Africa will be at risk of malaria by the 2030s (1.5°C global warming), and 196–198 million by the 2080s (2.4°C global warming) ( [[#Ryan--2020|Ryan et al., 2020]] ). Northern Angola, southern DRC, western Tanzania and central Uganda are predicted to be worst impacted in 2030, extending to western Angola, upper Zambezi River basin, northeastern Zambia and the east African Highlands by 2080 ( [[#Ryan--2020|Ryan et al., 2020]] ). Under rising temperatures, by the 2050s, the greatest shifts in suitability for malaria transmission will be seen in east, southern and central Africa (2°C global warming) ( [[#Tonnang--2014|Tonnang et al., 2014]] ; [[#Zermoglio--2019|Zermoglio et al., 2019]] ; [[#Ryan--2020|Ryan et al., 2020]] ).

Conversely, in some regions, changing climatic conditions are projected to reduce malaria hotspots and prevalence. With continued GHG emissions, these include: west Africa by 2030 (1.7°C global warming) ( ''high confidence'' ) ( [[#Yamana--2016|Yamana et al., 2016]] ; [[#Semakula--2017b|Semakula et al., 2017b]] ; [[#Ryan--2020|Ryan et al., 2020]] ), parts of southern central Africa and dryland regions in east Africa by 2050 (2.5°C global warming) ( ''high confidence'' ) ( [[#Semakula--2017b|Semakula et al., 2017b]] ; [[#Ryan--2020|Ryan et al., 2020]] ) and large areas of southern central Africa and the western Sahel by 2100 (&gt;4°C global warming) ( [[#Yu--2015|Yu et al., 2015]] ; [[#Tourre--2019|Tourre et al., 2019]] ). These reductions in transmission correspond with decreasing environmental suitability for the malaria vector and parasite in these regions ( [[#Ryan--2015|Ryan et al., 2015]] ; [[#Mordecai--2020|Mordecai et al., 2020]] ). Most areas in Burkina Faso, Cameroon, Ivory Coast, Ghana, Niger, Nigeria, Sierra Leone, Zambia and Zimbabwe will have almost zero malaria transmission under RCP8.5 ( [[#Semakula--2017b|Semakula et al., 2017b]] ; [[#Tourre--2019|Tourre et al., 2019]] ).

The ENSO cycle currently contributes to seasonal epidemic malaria in epidemic-prone areas ( ''high confidence'' ), and is projected to shift the malaria epidemic fringe southward and into higher altitudes by mid- to end-century ( ''high confidence'' ) ( [[#Bouma--2016|Bouma et al., 2016]] ; [[#Semakula--2017b|Semakula et al., 2017b]] ; [[#Caminade--2019|Caminade et al., 2019]] ). More evidence is needed, however, of climate variability impacts through ENSO cycles in future risk projections, as well as a deeper understanding of how climate change will impact the length of transmission season for mosquitoes, particularly in areas where increases in spring and autumn temperatures may increase suitability for the reproduction of malaria vectors ( [[#Ryan--2020|Ryan et al., 2020]] ). Other gaps in knowledge include a better understanding of mosquito thermal biology and thermal limits for a variety of species, potential adaptations to extreme temperatures and how landscape changes contribute to malaria transmission ( [[#Tompkins--2016|Tompkins and Caporaso, 2016]] ).

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===== 9.10.2.1.2 Mosquito-borne viruses =====

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Observed impacts

Climate variability has driven a global intensification of mosquito-borne viruses (e.g., dengue, Zika and RVF), including expansion into areas with higher altitudes ( [[#Leedale--2016|Leedale et al., 2016]] ; [[#Mweya--2016|Mweya et al., 2016]] ; [[#Messina--2019|Messina et al., 2019]] ). Concerns centre on diseases vectored by the yellow fever mosquito ( ''Aedes aegypti'' ), common throughout most of sub-Saharan Africa, and the tiger mosquito ( ''Aedes albopictus'' ), currently largely confined to western central Africa ( [[#Kraemer--2019|Kraemer et al., 2019]] ; [[#Mordecai--2020|Mordecai et al., 2020]] ).

Although warming temperatures are largely responsible for increasing environmental suitability for mosquito vectors ( [[#Mordecai--2019|Mordecai et al., 2019]] ), droughts can augment transmission when open water storage provides breeding sites near human settlements, and when flooding enables mosquitoes to proliferate and spread viruses further ( [[#Mweya--2017|Mweya et al., 2017]] ; [[#Bashir--2019|Bashir and Hassan, 2019]] ). Within Africa’s rapidly growing cities, diseases vectored by urban-adapted ''Aedes'' mosquitoes pose a major threat, especially in west Africa ( [[#Zahouli--2017|Zahouli et al., 2017]] ; [[#Weetman--2018|Weetman et al., 2018]] ; [[#Messina--2019|Messina et al., 2019]] ). Dengue virus expansion may cause explosive outbreaks but the burden of dengue haemorrhagic fever and associated mortality is higher in areas where transmission is already endemic ( [[#Murray--2013|Murray et al., 2013]] ).

Projected risks

Populations of ''Aedes aegypti'' and ''Aedes albopictus'' mosquitoes and epidemics of dengue and yellow fever and other ''Aedes'' -borne viruses are expected to increase, including at high altitudes ( [[#Weetman--2018|Weetman et al., 2018]] ; [[#Messina--2019|Messina et al., 2019]] ; [[#Ryan--2019|Ryan et al., 2019]] ; [[#Gaythorpe--2020|Gaythorpe et al., 2020]] ; [[#Mordecai--2020|Mordecai et al., 2020]] ). ''Aedes albopictus'' may expand beyond western central Africa into Chad, Mali and Burkina Faso by mid-century at &gt;2°C global warming ( [[#Kraemer--2019|Kraemer et al., 2019]] ). Shifts projected in ''Aedes'' range due to changing environmental suitability, combined with rapid urbanisation and population growth, suggest that by 2050 populations exposed to these vectors in Africa may double, and by 2080 nearly triple at &gt;2°C global warming ( [[#Kraemer--2019|Kraemer et al., 2019]] ). Southern limits of dengue transmission in Namibia and Botswana, and the western Sahel, may show the greatest expansions in environmental suitability under 1.8°C–2.6°C global warming ( [[#Messina--2019|Messina et al., 2019]] ). In the warmest scenarios (RCP8.5), however, some parts of central Africa may become too hot for mosquitoes to transmit dengue, and thus at-risk populations may peak at intermediate warming levels ( [[#Ryan--2019|Ryan et al., 2019]] ). Climatic conditions favourable for mosquitoes, combined with the increase of animal trade, may result in the expansion of the geographic range of zoonotic diseases like RVF ( [[#Martin--2008|Martin et al., 2008]] ), a threat for human and animal health with strong socioeconomic impacts ( [[#Peyre--2015|Peyre et al., 2015]] ).

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