A mosquito-borne disease needs two organisms to each finish their own life cycle inside the same temperature window: the mosquito must complete larval development and the pathogen must complete its incubation inside the mosquito before it dies of old age. Below a minimum temperature that window never closes in time; above a maximum, heat itself becomes lethal or disrupts the pathogen. That leaves a narrow band of suitable latitudes, shown here as the green strip on the landscape.
T(lat, year) = T_equator − lapse·lat + warming(year)
suitable when T_min ≤ T(lat, year) ≤ T_max
warming(year) = severity · (year − 1990) / 110
- Warming severity — total projected warming by 2100. Raising it shifts the whole temperature curve upward, so the same suitable band appears at higher, previously-cold latitudes.
- Simulated year — scrub or press Play to watch the band's poleward edge creep outward year by year — and, at high severity, watch a strip near the equator eventually get too hot and drop out of the band.
- Newly-suitable city — sits just beyond the historical range. Once the expanding band reaches it, local transmission risk switches on for the first time — no prior exposure, no accumulated immunity, no existing surveillance network, so case counts climb fast once seeded.
- Endemic city — sits deep inside the historical range at every severity level. Generations of exposure and public-health infrastructure keep its case count comparatively flat and managed.
Real-world relevance: this is the mechanism behind malaria and dengue appearing at higher elevations in East Africa and Aedes mosquitoes establishing in southern Europe — climate warming does not create new pathogens, it re-draws the temperature map that decides where existing ones can persist.