Characteristics of reviewed articles
Our searches yielded a total of 532 articles, after the removal of 136 duplicate records (Fig. 1). We removed 202 studies upon screening of article titles and abstracts, which excluded those that did not specifically address either climate change impacts or policy responses and adaptation measures related to either malaria or dengue. Upon full-text review, we removed 162 articles, the majority (n = 127) of which did not mention or use a systematic search strategy.
Characteristics of the articles included in the review are presented in Table 1. Of the 32 included reviews, 63% (n = 20) were systematic reviews, one of which included a meta-analysis; in addition, 16% (n = 5) were scoping reviews, 9% (n = 3) were narrative reviews, 9% (n = 3) were critical reviews, and 3% (n = 1) was a realist review (Table 1) [4, 14,15,16,17, 23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49].
The number of review articles increased over time, with the greatest increase noted from 2015 onwards (Fig. 2). This timing follows the release of the IPCC Fifth Assessment Report in 2014  and publication of the 2015 Lancet Commission on Climate and Health .
94% (n = 30) of the 32 review articles addressed climate change impacts on malaria and/or dengue, while 34% (n = 11) addressed policy responses or adaptation measures related to dengue and/or malaria. The majority of review articles (41%, n = 13) included studies from all geographic regions, while 13% (n = 4) included studies from the Southeast Asian region and 13% (n = 4) from the African region. A smaller number of articles reviewed the evidence from the Western Pacific (9%, n = 3) and European regions (9%, n = 3), while the Eastern Mediterranean and Americas accounted for 6% (n = 2) each. One article, accounting for 3% of all articles, reviewed studies from the Asia-Pacific region and was classified as Western Pacific & Southeast Asian (Fig. 3).
The majority of review articles (47%, n = 15) addressed both malaria and dengue, typically amongst a broader range of climate-sensitive infectious diseases. Twenty-eight percent (n = 9) of the eligible review articles had a focus on dengue, while 25% (n = 8) had a focus on malaria. Both malaria and dengue were addressed to some extent in reviews in each of the geographic regions, however malaria was the main focus in the African region and dengue was the main focus in the Southeast Asian region (Fig. 4).
The evidence for climate change impacts and adaptation measures related to dengue and malaria transmission and spread in different geographic regions is summarized in Table 2, with further detail on the results from individual review articles in the Supplemental Materials (Table S1). A narrative synthesis of results is presented below.
Positive associations between short-term variations in meteorological factors and dengue incidence were frequently reported, although the nature of effects often varied at different regional and sub-national scales . Precipitation, temperature and humidity were all associated with dengue incidence in the Americas, including an effect of El Niño on Aedes mosquito populations . In Puerto Rico, the effect of temperature on dengue incidence was highest in the country’s mountainous area, while the effect of precipitation was greatest in the hot and dry coastal region . Similarly, in Pakistan, the seasonal transmission pattern of dengue occurs after the monsoon when higher rainfall, combined with optimum temperature and humidity, provides a conducive environment for Aedes mosquitoes [29,30,31]. In China and other countries in the Asia-Pacific, temperature, precipitation, humidity and air pressure were considered as major weather factors for dengue fever (DF) transmission by many studies, including in Zhongshan City, Guangdong Province, and in Guangzhou City .
However, it is important to note that increases in temperature and precipitation do not necessarily translate to higher disease incidence – regions that are currently within optimal temperature ranges for dengue transmission may experience a decrease in disease incidence with increasing temperature , while intense rainfall may reduce dengue incidence through elimination of larvae from overflowing containers and other breeding sites, as observed in South America, Thailand, Indonesia and Taiwan [26, 40]. Indeed, in Malaysia a positive relationship between minimum temperature and dengue cases was observed with the highest risk of dengue cases observed from 21 to 26 °C at a lag of 1–8 weeks . Furthermore, several non-climatic factors, such environmental and socioeconomic changes, population movements, and population immunity, may potentially confound assessments of the climate-dengue relationship [35, 40], leading to inconsistencies in the empirical evidence linking dengue fever to climate change across different geographical locations.
Climate change impacts on dengue
According to Watts et al., from 1950 to 2018, the global climate suitability for the transmission of dengue increased by almost 10% for Aedes aegypti and 15% for Ae. albopictus, the primary vectors of dengue virus . Ongoing climate change is anticipated to further extend the latitudinal range Ae. aegypti mosquitoes, increasing the population at risk of dengue in several African countries in Southern and Central Africa . Similar increases in climatically suitable areas for the establishment of Ae. albopictus are anticipated in western, central and eastern Europe, including in southeast England, with increasing risk for dengue transmission around Mediterranean and Adriatic coasts towards the end of twenty-first century [32, 33]. In China, there has been a trend of expanded geographical region for dengue infections, from South to North China in line with warming temperatures  and expansion of the geographic range of Ae. albopictus . In Nepal, Dhimal et al. conclude that climate change can intensify the risk of dengue epidemics in the mountain regions of the country, where Ae. aegypti vectors have been rapidly expanding, if other non-climatic drivers remain constant .
Malaria prevalence and epidemic resurgence has been significantly associated with temperature and rainfall, and to El Niño-Southern Oscillation (ENSO) events in South Africa, the East African highlands, and Madagascar [23,24,25]. Similar to dengue, the role of epidemiological, socio-economic and environmental factors in driving malaria transmission has been noted in many geographical areas . Precipitation and/or temperature have been positively associated with malaria incidence and vector population in Americas , Europe , Iran , Pakistan [30, 31], China  and Nepal .
Climate change impacts on malaria
There is strong consensus among the reviewed studies that climate change is expected to increase malaria transmission at higher altitudes in the highlands of Africa, parts of Latin America and Southeast Asia, and in other regions at the margin of current distributions depending on demographic, socio-economic and ecological factors [4, 17, 24, 44, 45, 48]. In China, in the absence of preventive measures, climate change is anticipated to increase the geographical range of local malaria vectors and the incidence of malaria in some regions . In Europe, studies have predicted a northward spread of Anopheles mosquitoes and an extension of seasonality, enabling malaria transmission for up to 6 months per year in the years 2051–2080, particularly in Southern and South-Eastern European , while in the UK, southern Great Britain is predicted to be climatically suitable for Plasmodium vivax malaria transmission 2 months of the year by 2030 and for 4 months in parts of southeast England; by 2080, southern Scotland will be climatically suitable for malaria transmission for 2 months per years, with 4 months of the year conducive to malaria transmission in southern Great Britain . In some areas that currently sustain year-round malaria transmission, climate change may result in a contraction of the malaria transmission season or geographic range. For example, in the Philippines, model projections show an overall reduction in the climate suitability for Anopheles due to heat stress, causing large areas to exceed thresholds for of malaria vectors (> 40 °C) .
Adaptation strategies to address climate-driven malaria and dengue transmission and spread
The majority of reviewed studies recommend further emphasis on developing predictive models and early warning systems (EWS) to enhance outbreak preparedness and response, in addition to strengthening the capacity of surveillance and control systems. Enhanced epidemic prediction capability can allow for early intervention and more effective resource allocation. Importantly, predictive models and EWS should aim to integrate climate and meteorological factors alongside non-climate factors, such as socioeconomic variations, land use changes (including urbanization) and population growth, to more accurately predict disease emergence and/or spread [4, 14, 16, 17, 25, 32, 35,36,37, 39, 44, 46]. As noted by Hii et al., the development of models will require that governments make human case data publicly available for research purposes and that they support synchronized efforts across national disease surveillance systems and meteorological departments to develop climate-based disease forecasting systems . As noted by several studies, vector-borne disease monitoring, surveillance and research should be strengthened, including in areas where risk of vector-borne diseases is not yet determined, to assess the impacts of climate change on the observed transmission and distribution of vector-borne diseases in new areas. Well-designed long-term local studies are needed to provide the relevant information to develop locally-relevant models and responses [35, 37]. For example, Li et al. highlights the need to promote more advanced research on the relationship between extreme weather events and dengue fever to develop regional-specific models for the high-risk regions of dengue fever in south China. Ultimately, enhancing interdisciplinary collaboration between climate studies and health services, and enhancing public health education, are both future priorities. Bai et al. points to the additional need to focus adaptation strategies and policies on vulnerable communities while strengthening the capacity of public health system to adapt to climate change . As noted by several studies, new vector control strategies, such as wetland management and integrated vector management (IVM), will be needed, despite the challenges of funding and inter-sectoral cooperation [17, 30, 33].
Importantly, Bardosh et al. highlight the need to recognize that the myriad global changes, including climate change, land use, agriculture, dams, irrigation, urbanization, economic development, population movement, conflict, socio-political shifts, biological change, drug resistance, etc., do not occur in isolation . As such, predictions of expanded disease transmission should take into consideration current control initiatives, economic development trends and the future adaptation measures implemented by local populations and public health agencies. Importantly, successful adaptation and response will require interdisciplinary collaboration between meteorologists, biologists, climate scientists, social scientists, and epidemiologists, as well as partnerships with local communities to integrate local knowledge [17, 41].
As noted by Watts et al., adaptation planning and risk management is essential across all levels of government, with national strategies linked to subnational and local implementation . According to the 2020 Lancet Countdown Report, 50% of 101 countries surveyed had developed national health and climate change strategies or plans while 48% had assessed national vulnerability and adaptation for health . However, funding was highlighted as a key barrier to implementation of these strategies, with only 9% of countries reporting to have the funds to fully implement their plans. Encouragingly, the number of countries reporting that their meteorological services provide climate information to the health sector has grown in recent years.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.