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Mapping current and future thermal limits to suitability for malaria transmission by the invasive mosquito Anopheles stephensi



Anopheles stephensi is a malaria-transmitting mosquito that has recently expanded from its primary range in Asia and the Middle East, to locations in Africa. This species is a competent vector of both Plasmodium falciparum and Plasmodium vivax malaria. Perhaps most alarming, the characteristics of An. stephensi, such as container breeding and anthropophily, make it particularly adept at exploiting built environments in areas with no prior history of malaria risk.


In this paper, global maps of thermal transmission suitability and people at risk (PAR) for malaria transmission by An. stephensi were created, under current and future climate. Temperature-dependent transmission suitability thresholds derived from recently published species-specific thermal curves were used to threshold gridded, monthly mean temperatures under current and future climatic conditions. These temperature driven transmission models were coupled with gridded population data for 2020 and 2050, under climate-matched scenarios for future outcomes, to compare with baseline predictions for 2020 populations.


Using the Global Burden of Disease regions approach revealed that heterogenous regional increases and decreases in risk did not mask the overall pattern of massive increases of PAR for malaria transmission suitability with An. stephensi presence. General patterns of poleward expansion for thermal suitability were seen for both P. falciparum and P. vivax transmission potential.


Understanding the potential suitability for An. stephensi transmission in a changing climate provides a key tool for planning, given an ongoing invasion and expansion of the vector. Anticipating the potential impact of onward expansion to transmission suitable areas, and the size of population at risk under future climate scenarios, and where they occur, can serve as a large-scale call for attention, planning, and monitoring.


Malaria remains a critical global health challenge, with 241 million cases reported by the World Health Organization (WHO) in 2020 alone [1]. Although the current distribution of malaria is largely pantropical, the overwhelming majority of cases and deaths occur in non-arid regions of Africa, where the WHO estimated approximately 600,000 deaths occurred in 2020 [1,2,3]. Malarial transmission throughout sub-Saharan Africa is historically attributable to a few key mosquito vectors, most notably those in the Anopheles gambiae species complex [1, 4]. However, in 2019 the WHO issued a notice to alert public health authorities to the recent expansion of invasive Anopheles stephensi into the Horn of Africa, identifying this new vector species as a major potential threat to malaria control in the region [5,6,7,8].

The expansion of An. stephensi represents a new critical threat, not only to communities in Africa, but also to global public health. A competent vector of both Plasmodium falciparum and Plasmodium vivax malaria, An. stephensi has been implicated in malaria transmission throughout much of its native range in Asia and the Middle East, including India, Iran, and Pakistan [9,10,11,12,13]. In contrast with other Anopheline species, An. stephensi is capable of exploiting containers of standing water for ovipositional habitat, similar to container-breeding mosquitoes in the genus Aedes, including Aedes aegypti and Aedes albopictus [14]. This notable difference in life history has enabled the incursion of An. stephensi into built environments, fueling urban outbreaks of malaria and facilitating invasions into new geographic areas. In the past decade, An. stephensi has successfully expanded its range into the African continent, with established populations in Djibouti, Ethiopia, and Sudan [15]. Alarmingly, the arrival of this new vector has precipitated epidemics in populations centrally located in urban areas, where rates of malaria have historically been significantly lower compared to rural and peri-urban areas [16]. This shift in underlying risk was exemplified by a notable malaria outbreak in Djibouti City in 2012, where such outbreaks have since become increasingly severe, and are now an annual occurrence [17, 18]. Other physiological adaptations of An. stephensi, such as acquired insecticide resistance [19] and greater range of thermal tolerance compared to An. gambiae [20], raise further concerns regarding the continued success of this mosquito as an invasive species, and its ability to potentially undermine existing vector control strategies [21]. Perhaps unsurprisingly, An. stephensi has been identified as a major risk to malaria elimination targets, with global public health organizations calling for increased entomological surveillance and vector control in areas at imminent risk of invasion [5].

Mapping geographic estimates of transmission suitability can provide essential tools for assessing the current and future risk of An. stephensi invasions, and subsequent malaria transmission. A great deal of research has been conducted to delineate the extent of temperature-dependent malaria suitability in Africa [22, 23] and beyond [24,25,26,27,28,29]. In previous work, malaria transmission suitability for Africa was mapped, using a model comprised of an array of Anopheles spp. input parameters, primarily for P. falciparum malaria transmitted by An. gambiae [30, 31]. In a recently updated model, An. gambiae and An. stephensi, and the two main malarial parasites they transmit (P. falciparum and P. vivax), were separately modelled to produce thermal suitability curves for transmission [20, 30]. While the potential geographic dispersal of An. gambiae is functionally limited by arid conditions, An. stephensi is a container breeder that is resilient to habitat extremes [5]. Therefore, An. stephensi is able to thrive in close association with people, and thus potentially able to establish itself everywhere that temperature is not limiting. Thus, understanding the potential areas for suitability for transmission by this invasive mosquito now, and in the future, is important for capacity building and planning control efforts.

In this study, the global suitability of malaria transmission by An. stephensi was mapped using modelled thermal limits under current and future climate scenarios. Unlike previous studies to map the distribution of malaria, the projected distributions are not limited to non-arid regions, given the life history of An. stephensi, and instead make similar assumptions to those for mapping Aedes spp. transmitted diseases. Additionally, An. stephensi thermal suitability maps were combined with projected human population density estimates, enabling us to assess not only the areas that are vulnerable to malaria transmission through An. stephensi expansions, but also the magnitude of threat in terms of people at risk (PAR).


Thermal suitability model

In a previous study (Villena et al. [20]), used mechanistic modelling to establish thermal suitability curves for transmission of P. falciparum and P. vivax by An. stephensi. Briefly, thermal response data for vector and parasite pairings were synthesized from published data. These data were used to parameterize a formulation for \(R_{0}\), the basic reproductive number, that explicitly incorporated temperature-dependent traits for both mosquito vectors and malarial parasites,, building on a model for P. falciparum malaria transmission initially described in Mordecai et al. [32]. The temperature-dependent components of the \(R_{0}\) formulation were used to define a suitability metric \(S\left(T\right)\), defined as:

$$S(T) = \left( {\frac{{a(T)^{2} bc(T)e^{{ - \frac{\mu (T)}{{PDR(T)}}}} EFD(T)P_{EA} (T)MDR(T)}}{{\mu (T)^{3} }}} \right)^{\frac{1}{2}} ,$$

where \(a\) is mosquito biting rate; \(bc\) is vector competence; μ is the mosquito mortality rate; \(PDR\) is the parasite development rate; EFD is mosquito fecundity expressed as the number of eggs per female per day; \({P}_{EA}\) is the proportion of eggs surviving to adulthood; and \(MDR\) is the mosquito development rate.

A Bayesian approach was used in Villena et al. 2022 to fit unimodal thermal response curves of traits for each mosquito or parasite species [20]. Samples from the resulting joint posterior distribution of the suitability metric were used to calculate overall thermal response, in addition to critical temperature thresholds for pathogen transmission by species [20].

In this study, the thermal boundaries from Villena et al. [20] were used as the basis for mapping thermal suitability, taking values where the malaria transmission suitability metric for An. stephensi was greater than zero (S(T) > 0), with a posterior probability greater than 0.975 [20]. The resulting thermal limits for malaria transmitted by An. stephensi are temperatures of 16.0–36.5 °C for P. falciparum and 16.6–31.7 °C for P. vivax [20].

Climate data

In this paper, baseline and future scenarios for An. stephensi transmitted P. falciparum and P. vivax suitability are described. Outputs are presented for a baseline climate scenario, and future potential climate driven outputs for four General Circulation Models (GCMs), following the methodology used in Ryan et al. [33, 34] to describe climate impacts on the global distribution of Aedes spp. transmitted diseases.

Baseline and future scenario climate model output data were acquired from the research programme on Climate Change, Agriculture, and Food Security (CCAFS) web portal (, part of the Consultative Group for International Agricultural Research (CGIAR). The baseline climate model from which these are projected is the WorldClim v1.4 baseline [35], and thus it serves as the baseline for these models. The CCAFS future model outputs were created using the delta downscaling method, from the IPCC AR5. The GCMs used in this study are the Beijing Climate Center Climate System Model (BCC-CSM1.1); the Hadley GCM (HadGEM2-AO and HadGEM2-ES); and the National Center for Atmospheric Research's Community Climate System Model (CCSM4). The datasets were obtained at a resolution of 5-arc minutes, matching the spatial resolution of baseline data.

For visualizations, one of the General Circulation Models (GCMs) was used: the Hadley Centre Global Environment Model version 2, Earth-System configuration (HADGEM2-ES) under two scenarios for greenhouse gas emissions, or representative concentration pathways (RCPs): RCP 4.5 and RCP 8.5. The Intergovernmental Panel on Climate Change (IPCC) describes RCP 4.5 as a moderate scenario in which emissions peak around 2040 and then decline, while RCP 8.5 is the highest baseline emissions scenario in which emissions continue to rise throughout the twenty-first century. Mechanistic transmission models were projected onto climate data in R (v. 4.1.2) with the package ‘raster’ [36]. Monthly mean temperatures were thresholded according to the thermal suitability limits for each malaria species, and the number of suitable months of transmission summed (0–12) in a pixel-wise analysis for the globe.

Population data

In order to establish a population baseline, the 2020 Gridded Population of the World (GPW4, ver 4.11) was used [37]. The decision about how to best match climate baselines with population is complex, as baselines represent climate normal periods around the start of the twenty-first century, rather than a ‘current’ climate baseline. However, in this study, the nearest decade to current conditions for population baseline was chosen. For the future population, 2050 projections for two Shared Socioeconomic Pathways (SSPs) [38, 39] were chosen, best matched to the chosen RCPs. As the combinations of RCPs and SSPs are not all realistic, CMIP5 RCP 4.5 and RCP 8.5 for SSP2 and SSP5, respectively, were modelled. SSP2 represents a “middle of the road” scenario, assuming patterns of social, economic, and technological growth that do not appreciably deviate from historical trends. SSP5 assumes significant investments in health, technology, economic, and social development, coupled with simultaneous exploitation of fossil fuel resources and the adoption of resource-intensive lifestyles. All geographic layers in the analyses were aggregated to a 0.25° grid cell for consistency.

Suitability mapping

Monthly suitability maps were produced for baseline and future climate scenarios (i.e., RCP 4.5 and RCP 8.5), using one GCM (HADGEM-ES) at a near-future time horizon (i.e., 2050), for illustration (Fig. 1). Following the approach of Ryan et al. [34, 40] Maps for figures were created using ArcGIS (ver. 10.1)[41].

Fig. 1
figure 1

Thermal suitability for transmission of P. falciparum and P. vivax malaria by An. stephensi. Transmission suitability is shown under current climate conditions, and for the year 2050 at RCP 4.5 and RCP 8.5. The number of months of suitable temperatures are given as shaded areas, where the posterior probability of S(T) > 0 is 0.975

To describe the impact to populations in current and future landscapes, the Global Burden of Disease (GBD) regions [42] were used to summarize the population at risk (PAR). Additional file 1: Tables S1–S4 summarize the top 10 regions, and global gain overall, in terms of increased (difference between current and projected) PAR for both year-round (12 month, endemic), and for ‘any’ (one or more months), for each of transmission suitability of P. falciparum and P. vivax by An. stephensi, under the two future RCP x SSP scenarios, averaged across the 4 GCMs (also summarized at a global level in Table 1).

Table 1 People at Risk (PAR) for thermal suitability of transmission of malaria (P. falciparum or P. vivax) by Anopheles stephensi, under a baseline climate, and under two representative concentration pathways (RCP 4.5 and 8.5), across four global circulation model output projections for 2050 (BC: Beijing Climate Center Climate System Model (BCC-CSM1.1); CC: National Center for Atmospheric Research's Community Climate System Model (CCSM4); HD: Hadley GCM HadGEM2-AO; HE: Hadley GCM HADGEM2-ES), paired with shared socioeconomic pathway projections of population (RCP 4.5 × SSP2; RCP 8.5 × SSP5), for 2050. These are given for Year-round transmission suitability (12 months), and for one or more months of suitability


Baseline suitability and duration of transmission season

Maps of months of An. stephensi malaria transmission suitability for P. falciparum and P. vivax are shown in Fig. 1. Much of Africa is projected to be suitable at baseline for nearly year-round transmission for both malaria parasites. Beyond Africa, the predicted baseline thermal transmission suitability for both P. falciparum and P. vivax extends throughout the global tropics, throughout the known existing range of the Middle East, extending throughout Asia, Central America, South America, and marginally in North America. The predicted potential range for year-round transmission suitability of P. falciparum extends further North and South than P. vivax, with notable extension of the transmission season in northern Africa, the Middle East, India, and central Australia. The narrower thermal suitability bounds for P. vivax constrains the baseline potential extent, compared to that for P. falciparum. Seasonal transmission suitability at baseline climate conditions is projected to be globally widespread for both P. falciparum and P. vivax, extending well into temperate regions in North America, Europe, and Asia.

Predicted future suitability

Mapped transmission suitability in 2050, for RCP 4.5 and RCP 8.5 scenarios is shown in Fig. 1. An expansion of the transmission suitability season for P. falciparum is seen in both RCPs. Notably, the potential for any transmission (i.e., one or more months) is predicted to expand at northern latitudes, where areas with no current malaria suitability will have the potential for transmission, at least for one month every year. This includes portions of Alaska in the US, northern Canada, Scandinavia, and Russia. The length of the P. falciparum transmission season is expected to increase in temperate regions of North America and Europe, southern Africa, and in central Australia. Yet, changing climate conditions will shorten the length of the P. falciparum transmission season in some areas, most notably in northern Africa, the Middle East, and northern India. The transmission suitability for P. vivax is also predicted to extend further North in the future, encroaching on areas that do not currently experience malaria transmission. The length of the P. vivax transmission season is expected to increase in southern Africa and in parts of North America, including Mexico and along the Gulf Coast in the US. There are also marked decreases in the length of the P. vivax transmission season, most notably throughout northern Africa, the Middle East, India, Asia, northern Australia, and South America. Raster output for additional scenarios of future climate change from this study is available on the Harvard Dataverse.

Population at risk

The projections of thermal suitability for transmission of P. falciparum and P. vivax by AS for the two future scenarios of RCP 4.5 and RCP 8.5, in combination with matched population projections SSP2 and SSP5, respectively, revealed that in many regions of the world, increases in people at risk of transmission suitability will occur (Additional file 1: Tables S1-S4). The prediction for SSP2 population for the globe is larger than SSP5 in 2050 (9.17 billion vs 8.56 billion [43]), this reflects the combination of potential geographic shifts of suitability and the underlying population changes. Perhaps counterintuitively, the RCP 4.5 scenario predicts a larger net increase than RCP 8.5, for PAR in both ‘any’ (one or more) transmission, and year-round (endemic) transmission scenarios (Table 1). The baseline and net global future population at risk (PAR) for transmission suitability across the 4 GCMs are given in Table 1, comparing P. falciparum and P. vivax suitability.

At 2020 population baseline, 7.45 billion people are predicted to be at risk for transmission suitability for one or more months for P. falciparum in An. stephensi, and 7.38 billion for P. vivax in AS. Under RCP 4.5, the net PAR for P. falciparum suitability increases to a range of 8.75–8.77 billion, and for P. vivax 8.73–8.76 billion, across the 4 GCMs; and under RCP 8.5, the estimated PAR for P. falciparum suitability is 8.18–8.19 billion, and for P. vivax 8.16–8.18 billion (Table 1).

At baseline, the year-round PAR for P. vivax is 2.13 billion people, while it is 3.77 billion for P. falciparum, emphasizing the difference in risk imposed by the broader temperature range of suitability for P. falciparum. Under RCP 4.5, the net PAR for P. falciparum suitability increases to a range of 3.73–4.05 billion, and 1.98–2.25 billion for P. vivax. Under RCP 8.5 conditions, net par for P. falciparum suitability increases to 3.16–3.36 billion, and 1.53–1.81 billion for P. vivax.

The top 10 largest regional increases in PAR for each of P. falciparum, P. vivax, and for year-round and ‘any’ transmission are given in Additional file 1: Tables S1-4, including the global gains in increases (in contrast to net changes). For P. falciparum, for endemic (year-round) transmission PAR increases, East and West Sub-Saharan Africa regions are the top two affected, under both the RCP 4.5 and RCP 8.5 scenario (Additional file 1: Table S1); for P. vivax, while East sub-Saharan Africa is also the top affected region, the second place is Central African Region, indicating a shifted geographic impact with the narrower thermal bounds (Additional file 1: Table S2). For ‘any’ (one or more months) transmission suitability, South Asia region is the top affected for both P. falciparum and P. vivax, ahead of East sub-Saharan Africa region, suggesting a shift of seasonal, sub-endemic risk into high density population areas in both of the future scenarios explored here (Additional file 1: Tables S3, S4).


Assessing the future risk of An. stephensi expansion against the backdrop of changing climate is imperative for public health planning and risk mitigation. Its propensity to spread and establish outside of its current range is already underway, drawing the attention of the global health community [5, 7, 18, 44]. As a malaria transmitting Anopheline capable of exploiting a similar niche to the urban adapted Aedes spp. mosquitoes, anticipating where the bounds of thermal limits to persistence and transmission exist is a step towards understanding where it can invade and establish, both now and in the future.

Using a recently published thermal suitability model for transmission of both P. falciparum and P. vivax malaria by An. stephensi [20], mapped months of suitability demonstrated that a large part of the world is already suitable for one or more months of the year, putting an estimated 7.38–7.45 billion people at risk of that potential. While the actual arrival, establishment, and onwards transmission of malaria may be less risky for areas with a low number of months of suitability, this approach indicates that a baseline of 2.13–3.77 billion people are currently living in places with endemic risk—not simply in the known existing range for transmission by An. stephensi. The potential for future shifts in the range of suitable areas was explored, as a function of a changing climate and shifting population projections, reflective of those potential climate scenarios. The mapped number of months of transmission suitability showed a poleward expansion of areas becoming suitable, and a shift from some lower latitude locations to becoming hotter than suitable for transmission during parts of the year, shortening the season. While this shift away from suitability results in predicted declines in risk to populations, as transmission suitability shifts out of hotter regions, other health crises are exacerbated at overly high temperatures [45,46,47,48], and this is thus not cause for less alarm, nor is it mitigation for malaria.

This exploration of potential future climate impacts on a vector currently expanding its range is based on current vector-pathogen biology and thermal limits to the life-history of An. stephensi and the malaria parasites. The conditions under which parameters in the underlying thermal suitability model were established were idealized laboratory conditions, and are not yet established for An. stephensi undergoing the climate changes modelled here. The environment experienced in a changed climate in 2050 may induce different interactions between vectors and their pathogens, but the plastic responses of the vector (e.g. urban adaptation, behavioral avoidance of environmental extremes) and the pathogen (e.g. rapid evolution under novel environmental pressures, or fluctuating temperatures), and how that will impact the vector microbiome [49], potentially altering vector competence, will lead to broader potential temperature limits to suitability, making the estimates presented here conservative. Further, these projections only consider malaria transmission in terms of extrinsic incubation (i.e., development in the mosquito) and transmission to the host. This does not include the human stages of malaria development, and some malarial parasites have adaptations, such as hypnozoites for P. vivax [50], which can leverage human reservoirs beyond the environmental bounds of transmission suitability in adult mosquitoes. Conversely, these estimates may represent a “worst case” scenario. Model output at the global scale may oversimplify local effects that protect against mosquito invasion, and likewise this study cannot account for the potentially mitigating effects of rapid interventions and successful malaria control initiatives. This underscores the importance of expanding surveillance capacity for the early detection and rapid elimination of expanding An. stephensi populations to reduce PAR in the future, particularly in countries neighboring current areas of expansion.

A knock-on effect of the potential expansion of a novel, urban-adapted, malaria vector into, for example, the Americas, is that adding a competent malaria vector to areas with existing competent malaria vectors expands the competent vector community. This compounds the risk in a changing world for facilitating spillover from a novel invader experiencing perhaps only a shortened suitability season to established Anopheline species (e.g. Anopheles quadrimaculatus in parts of N. America).

The ongoing expansion of An. stephensi is troubling, given its implication in the shift from primarily rural to urban malaria transmission. The mapped risk projections in this study, though global in extent, will be useful to local governments and agencies for planning broadscale vector control and resource allocation efforts. Potential vulnerability to invasion is useful information for decision making and policy formation, particularly for countries at the forefront of An. stephensi expansion, neighbouring current locations where the vector has been documented. The successful expansion and establishment of invasive urban mosquitoes is mediated by many factors beyond the scope of these models, such as introduction pathway, frequency of travel, and individual water storage practices [51, 52]. Thus, these results demonstrate the need for increased surveillance activities in areas at risk of transmission, but importantly, underscore the need for timely sharing and dissemination of known distribution data. Although urban malaria transmission represents a new threat for many existing vector control programmes to manage, there may be opportunities for the formation of successful mitigation efforts, given that agencies are aware of potential expansion. With enough lead time, mosquito control agencies may be able to successfully leverage knowledge, experience, and tools for controlling other urban container-breeding mosquitoes to suppress the proliferation of invasive An. stephensi [6]. For example, dengue fever surveillance and control programmes that target Ae. aegypti may have the capacity to expand efforts to include An. stephensi without a major investment in novel resources. Though there are still challenges in programme adaptation, such as the need to address insecticide resistance, it is likely that effective control measures will not have to be designed from the ground up.


Mapping thermal suitability for malaria transmission for the invasive urban-adapted An. stephensi for baseline and future climate and population projection scenarios shows that much of the world is suited to continued range expansion now and into the future. While this work demonstrates that around a third of the world’s population lives in areas of potential risk, understanding where range expansion is plausible, and how that may shift in the future, provides broad scale tools for motivating surveillance and opportunities for preemptive interventions. Of key importance, the similarity between An. stephensi and Aedes spp, and their management as urban container breeders may provide an opportunity to leverage existing vector management and control for An. stephensi.

Availability of data and materials

All model output rasters are available through Harvard Dataverse (, all climate layers used are freely available online and described within the paper. R Code for creating model outputs is available publicly on GitHub (



Consultative Group for International Agricultural Research


Climate Model Intercomparison Project


General Circulation Model


Gridded Population of the World


Representative Concentration Pathway


Shared Socioeconomic Pathway


World Health Organization


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The authors would like to acknowledge the thoughtful insights of our anonymous reviewers.


SJR, CAL, and LRJ were supported by CIBR: VectorByte: A Global Informatics Platform for studying the Ecology of Vector-Borne Diseases NSF DBI 2016265; LRJ was additionally supported by NSF DMS/DEB 1750113 and NIH R01AI122284.

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SJR, CAL and AS conducted analyses; SJR and CAL drafted the paper, figures and tables. All authors contributed to the final version. All authors read and approved the final manuscript.

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Correspondence to Sadie J. Ryan.

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Supplementary Information

Additional file 1. Table S1.

Top 10 Global Burden of Disease defined regional increase in people at risk (PAR) for year-round transmission suitability of P. falciparum by An. stephensi in 2050, under RCP 4.5 (SSP2 population projection) and RCP 8.5 (SSP5 population projection) future climate scenarios, averaged across four general circulation models (GCMs) as described in the main methods. Global increase is the sum of all gains in PAR increases across all GBD regions. Table S2.Top 10 Global Burden of Disease defined regional increase in people at risk (PAR) for year-round transmission suitability of P. vivax by An. stephensi in 2050, under RCP 4.5 (SSP2 population projection) and RCP 8.5 (SSP5 population projection) future climate scenarios, averaged across four general circulation models (GCMs) as described in the main methods. Global increase is the sum of all gains in PAR increases across all GBD regions. Table S3.Top 10 Global Burden of Disease defined regional increase in people at risk (PAR) for one or months transmission suitability of P. falciparum by An. stephensi in 2050, under RCP 4.5 (SSP2 population projection) and RCP 8.5 (SSP5 population projection) future climate scenarios, averaged across four general circulation models (GCMs) as described in the main methods. Global increase is the sum of all gains in PAR increases across all GBD regions. Table S4.Top 10 Global Burden of Disease defined regional increase in people at risk (PAR) for one or months transmission suitability of P. vivax by An. stephensi in 2050, under RCP 4.5 (SSP2 population projection) and RCP 8.5 (SSP5 population projection) future climate scenarios, averaged across four general circulation models (GCMs) as described in the main methods. Global increase is the sum of all gains in PAR increases across all GBD regions.

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Ryan, S.J., Lippi, C.A., Villena, O.C. et al. Mapping current and future thermal limits to suitability for malaria transmission by the invasive mosquito Anopheles stephensi. Malar J 22, 104 (2023).

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