Global expansion and redistribution of Aedes-borne virus transmission risk with climate change

Where will dengue, chikungunya, and Zika go next? It's not about which countries are unprepared or which vaccines are coming, but rather where, geographically, the mosquitoes that carry these viruses will be able to survive and transmit, and who will be living there when they arrive. Ryan and colleagues set out to answer that question with a mechanistic, temperature-driven model. Their headline finding: close to one billion people face new exposure by the end of this century. However, the details are more complicated — and more interesting — than that number alone suggests. Let’s start with the biology. Temperature is not a simple on-off switch for mosquito-borne virus transmission; it's a dial that traces a nonlinear curve. Transmission rises as temperatures warm, peaks, then falls as conditions become too hot. What makes this study powerful is that Ryan and colleagues do not just apply that logic generically — they apply it separately to two different mosquito species, Aedes aegypti and Aedes albopictus, whose thermal niches differ in ways that turn out to matter enormously. Aedes aegypti, the primary dengue vector, has a thermal window for transmission between twenty-one point three and thirty-four degrees Celsius, with an optimum around twenty-nine degrees. Aedes albopictus — the tiger mosquito, which is common across Europe and Asia — runs cooler, with a window between nineteen point nine and twenty-nine point four degrees and an optimum near twenty-six. Those six-degree differences in upper limits may sound modest, but they are not. As the tropics warm, those limits start to matter in ways that challenge your intuitions about which regions face increasing risk. To get those numbers, Ryan and colleagues built a mechanistic model rooted in the classic Ross-MacDonald framework — the standard mathematical architecture for vector-borne disease. The basic reproduction number, R0, represents the average number of new infections generated by a single case in a susceptible population. If R0 is above one, an outbreak can sustain itself. Below one, it fades. The key aspect here is that every biological trait feeding into R0 — biting rate, mosquito lifespan, egg-to-adult survival, the time it takes the virus to develop inside the mosquito, and several others — is itself a function of temperature. Ryan and colleagues fitted each of those trait curves independently using a Bayesian statistical framework, drawing on experimental data from Johnson and colleagues and prior mechanistic work by Mordecai and colleagues. They then combined those curves to produce a full posterior distribution of R0 as a function of temperature. Because large-scale human population size and recovery rate are uncertain, they rescaled R0 to a zero-to-one range and defined thermal suitability conservatively: a location qualifies only if there is at least a ninety-seven point five percent probability that transmission is not excluded by temperature alone. For climate projections, they ran this model against four general circulation models and four representative concentration pathways — RCPs two point six, four point five, six point zero, and eight point five — where two point six represents strong mitigation and eight point five is the worst-case business-as-usual scenario. The numbers themselves refer to radiative forcing in watts per square meter by twenty-one hundred. They projected forward to twenty fifty and twenty eighty, using current population grids to count who falls inside newly suitable climate envelopes. Now for the results. The Aedes aegypti story is the more straightforward one, and it’s alarming. Right now, roughly six billion people live in places with at least one month per year of thermally suitable conditions for Aedes aegypti transmission. Under the worst-case scenario by twenty eighty, nearly one billion additional people — nine hundred fifty-one million — would be newly exposed. The relationship is monotonic: more severe warming consistently produces larger exposed populations. There is no plateau or counterintuitive dip. Just a steady increase in risk as the climate pushes temperatures into Aedes aegypti's broad thermal window across higher latitudes and elevations. The geography of that expansion is specific. Europe is the single largest source of newly at-risk people in these projections. By twenty eighty under RCP eight point five, Western Europe alone contributes two hundred twenty-four million newly exposed people, Eastern Europe adds one hundred fifty-six million, and Central Europe another ninety-one million. East sub-Saharan Africa adds roughly ninety-three million. These are not marginal shifts at the edge of suitability — these are regions where climate change is projected to create conditions suitable for sustained vector transmission within decades. Even in the shorter term, by twenty fifty, the models show substantial European exposure increases across nearly all warming scenarios. Now here's where the story gets complicated. Aedes albopictus behaves differently, and the difference carries real policy weight. At present, six point thirty-three billion people live in areas suitable for Aedes albopictus transmission for at least one month per year — slightly more than for Aedes aegypti. Under moderate warming, that number grows as the species expands poleward into temperate Eurasia and North America. However, in some of its current strongholds — Southeast Asia, West Africa, and South Asia — Aedes albopictus is projected to lose suitability as temperatures exceed its upper thermal limit of twenty-nine point four degrees Celsius. The model estimates that by twenty eighty under RCP eight point five, roughly seven hundred million fewer people than at present would be at risk from Aedes albopictus. In South and Southeast Asia alone, more than four hundred million people could lose year-round transmission risk as conditions become too hot for this species. The most striking finding is about intermediate warming. The largest increases in Aedes albopictus transmission suitability do not occur under the worst-case scenario — they occur under intermediate pathways, RCP four point five and six point zero. If you switch from RCP six point zero to eight point five, you lose roughly fifty million people at risk in West Africa and about one hundred million in Southeast Asia. The species gets pushed past its thermal optimum before the worst-case scenario fully plays out. Ryan and colleagues note that this contrast "highlights just how significant the degree of mitigation will be for regional health pathways." That’s not just a rhetorical flourish — it's pointing to a genuine nonlinearity where the policy calculus is inverted from what you might expect. Taken together, these two species tell a story about a global shift from year-round to seasonal risk. As tropical regions warm past Aedes albopictus's upper limit, year-round suitability for that species contracts toward high-elevation refugia, southern Africa, and Brazil's Atlantic coast. Meanwhile, temperate regions gain new seasonal windows. The global map of Aedes-borne disease doesn't just expand outward — it reshapes. There’s one more thing worth naming clearly: what this model can and cannot do. Ryan and colleagues are explicit that it maps thermal suitability, not actual outbreak risk. A region crossing the temperature threshold doesn't automatically develop a mosquito population, doesn't guarantee viral introduction, and doesn't account for human movement, land use, urbanization, vector control, or the evolutionary trajectories of the viruses themselves. These projections are the first filter — necessary but not sufficient. What they do show is that climate mitigation is not a secondary concern for infectious disease preparedness. The difference between aggressive mitigation and business-as-usual, in this model, is measured in hundreds of millions of people. Year-round transmission risk from Aedes aegypti is likely to expand in South Asia and sub-Saharan Africa. Seasonal risk will emerge across Europe and North America. In the tropics, the picture is genuinely mixed — some regions gaining months of Aedes aegypti suitability while simultaneously losing Aedes albopictus suitability as temperatures push past that species' ceiling. The scale of potential first exposures matters because naïve populations — people who have never encountered these viruses — are especially vulnerable to explosive outbreaks if the vectors establish and a virus arrives. Knowing where temperatures will become thermally suitable for transmission is the first step toward positioning surveillance, vector control, and health systems ahead of the curve rather than behind it. That is the work this model enables. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.