A cyclone hit Peru in 2023 and 22,000 extra dengue cases followed - climate change made it three times more likely

Peru's coastal desert does not usually breed mosquitoes. Then Cyclone Yaku arrived in March 2023, dumping extreme rainfall on a region accustomed to aridity. Flooding followed. Standing water pooled in streets, drainage ditches, and improvised settlements. Temperatures stayed warm. And within weeks, Peru was experiencing a dengue fever outbreak ten times larger than normal.

A team of researchers from Stanford, the Peruvian Ministry of Health, and the Latin American Center of Excellence in Climate Change and Health has now put precise numbers on what happened - and what climate change had to do with it. Their analysis, published in One Earth, represents the first time scientists have been able to measure the specific contribution of an extreme weather event to a dengue outbreak.

60% of cases in the hardest-hit areas were storm-caused

Using a statistical technique borrowed from economics - a method designed to estimate what would have happened in the absence of an intervention - the researchers modeled dengue incidence in northwestern Peru with and without the 2023 cyclone and coastal El Nino.

In the hardest-hit districts, 60% of dengue cases were directly attributable to the extreme rainfall and warm temperatures associated with the storm. That translates to approximately 22,000 additional people who fell ill with dengue fever and would not have otherwise.

The mechanism is straightforward. Heavy rain floods low-lying areas and overwhelms water and sanitation infrastructure, creating pools of stagnant water. Aedes aegypti and Aedes albopictus mosquitoes - the species that carry dengue - breed in standing water. Warm temperatures accelerate mosquito reproduction, shorten the virus incubation period inside the mosquito, and increase biting rates. The combination of water and heat is a force multiplier for transmission.

A critical control comparison supported the causal interpretation: cooler areas hit by the same cyclone showed no significant increase in dengue. Heat, not just rain, was the essential ingredient.

Climate change tripled the odds

Stanford climate modelers Jared Trok and Noah Diffenbaugh then asked how much more likely these weather conditions have become because of climate change. Comparing precipitation simulations across 1965-2014 against a pre-industrial baseline, they found that extreme precipitation events like the one in 2023 are now 31% more likely in northwestern Peru than before industrialization.

When combined with warming temperatures, the probability of climate conditions similar to those that fueled the 2023 outbreak has nearly tripled. What was once a rare meteorological event - extreme coastal rainfall in a desert region paired with elevated temperatures - is becoming increasingly routine.

Dengue's global surge

Peru's outbreak is not an isolated event. Dengue incidence has surged more than tenfold globally since 2000, according to the World Health Organization, with tens of millions of cases annually. Diseases historically absent from the United States have been appearing in Florida, Texas, and California. The mosquitoes that carry dengue are expanding their geographic range as temperatures warm, and extreme precipitation events are providing them with breeding habitat in places they could not previously survive.

"Health impacts of climate change aren't something we're waiting for," said lead author Mallory Harris, a postdoctoral scholar at the University of Maryland who conducted the research during her PhD at Stanford. "They're happening now."

Prevention before the next storm

The researchers argue that their analytical approach - quantifying the attributable fraction of disease cases from a specific weather event - could be applied to hurricanes, monsoons, and other extreme events worldwide. This would give governments a tool for estimating the health costs of future storms and directing resources before outbreaks take hold rather than after.

For Peru specifically, the study provides the Ministry of Health with a quantitative basis for advocating greater public health investment. Targeted mosquito control, vaccination in high-risk urban districts, better drainage infrastructure, sturdier housing, and more reliable water systems could all reduce vulnerability to the next climate-amplified outbreak.

Limits of the analysis

The counterfactual modeling approach is powerful but relies on assumptions about what dengue incidence would have been in the absence of the storm. These assumptions are only as good as the historical data they are built on. The study also focused on one outbreak in one region; while the methods are portable, each new application requires local calibration.

The study does not account for population-level immunity, which fluctuates between dengue outbreaks and can modulate the size of subsequent epidemics. And while the climate attribution component is robust, it addresses the probability of weather conditions, not the probability of outbreaks per se - the link between weather and disease depends on local factors like mosquito control infrastructure, urbanization patterns, and healthcare capacity.

The study was funded by the National Institutes of Health, the National Science Foundation, the Stanford Center for Innovation in Global Health, the Stanford Woods Institute for the Environment, and other supporters.