Climate change is fueling disease outbreaks
Stanford University
Diseases historically absent from the United States have been showing up in Florida, Texas, California and other U.S. states in recent years. To understand why, look to Peru. That’s where researchers from Stanford and other institutions analyzed the connection between a cyclone and a massive outbreak of dengue fever, a mosquito-borne viral disease that can cause fever, rash, and life-threatening symptoms like hemorrhage and shock. Their findings, published March 17 in One Earth, reveal that warmer, wetter weather linked to climate change is making disease epidemics more likely.
"Health impacts of climate change aren't something we're waiting for,” said study lead author Mallory Harris, a postdoctoral scholar at the University of Maryland who conducted the research as a PhD student in biology at Stanford. “They're happening now."
Standing water + heat = sick people
Dengue fever, transmitted by Aedes aegypti and Aedes albopictus mosquitoes, sickens an estimated tens of millions of people worldwide each year, according to the World Health Organization, and has surged more than 10-fold globally since 2000. A 2023 cyclone and coastal El Niño in a normally dry region of Peru was followed by a dengue fever outbreak 10 times larger than normal.
Using a statistical technique developed in economics, the researchers asked what share of this historic outbreak was due to the unusual 2023 weather, by modeling what would have happened without the storm. In collaboration with scientists at the Peruvian Ministry of Health and the Latin American Center of Excellence in Climate Change and Health, the team estimated that 60% of dengue cases in the hardest hit districts were directly caused by extreme rainfall and warm temperatures during the cyclone. That translates to roughly 22,000 additional people falling ill who otherwise would not have.
The link goes like this: heavy rains flood low-lying areas, knock out water and sanitation infrastructure, and create pools of water ideal for breeding Aedes aegypti and Aedes albopictus mosquitoes. Warm weather turbocharges mosquito breeding and disease transmission processes. By comparison, cooler areas hit by the cyclone saw no significant effect of extreme precipitation on dengue incidence.
“While we often observe large dengue outbreaks following extreme weather events, this is the first time scientists have been able to pinpoint the role of climate change and precisely measure the impact of a particular storm on dengue—one of the most rapidly-growing infectious diseases,” said study senior author Erin Mordecai, an associate professor of biology in the Stanford School of Humanities and Sciences and co-lead of the Disease Ecology in a Changing World program based at the Center for Human and Planetary Health.
Stanford climate modelers Jared Trok and Noah Diffenbaugh then analyzed simulations comparing precipitation in March across 1965-2014 to a pre-industrial baseline. The result: extreme precipitation conditions like those seen in 2023 are now 31 percent more likely in northwestern Peru than they were before industrialization. When combined with warming temperatures, the probability of climate conditions like those that fueled the 2023 dengue epidemic has nearly tripled.
Fighting a tiny enemy
The findings are both a warning and the seed of a possible solution. Targeted mosquito control and vaccination in high-risk urban districts could all blunt the impact of a mosquito-borne disease surge, according to the researchers. Investments in urban flood resilience, such as better drainage, sturdier housing, and more reliable water infrastructure could also help stave off the threat.
"This research provides Peru's Ministry of Health an initial estimate to quantify the specific health impacts of extreme climatic events,” said study coauthor Andrés Lescano of the Latin American Center of Excellence for Climate Change and Health. “That can be used as a reference to advocate for greater public health investments in preparation and response."
Similar analyses could be applied to hurricanes, monsoons, and other extreme events around the world. They could help governments prepare before mosquito-borne outbreaks take hold, and better understand the impact climate change is already having on human health.
“As extreme weather events become more frequent with climate change, we need to think strategically and act decisively to prevent mosquito borne epidemics,” Harris said.
Mordecai is also a senior fellow at the Stanford Woods Institute for the Environment; a faculty fellow in the Center for Innovation in Global Health, the Center for Human and Planetary Health, and the King Center on Global Development; and a member of Bio-X. Trok is a Ph.D. student in Earth system science at Stanford. Diffenbaugh is the William Wrigley Professor and Kimmelman Family Senior Fellow in the Stanford Doerr School of Sustainability, and the Olivier Nomellini Family University Fellow in Undergraduate Education
Coauthors of "Extreme precipitation, exacerbated by anthropogenic climate change, drove Peru's record-breaking 2023 dengue outbreak,” also include Kevin Martel and César Munayco of the Peruvian Ministry of Health and the Latin American Center of Excellence for Climate Change and Health; Mercy Borbor Cordova of the Escuela Superior Politécnica del Litoral in Ecuador.
The study was funded by the Achievement Rewards for College Scientists Scholarship; the National Institutes of Health; Montgomery County, Maryland; the National Science Foundation; the Stanford Center for Innovation in Global Health; the King Center on Global Development; the Stanford Woods Institute for the Environment; the Fogarty International Center; the National Institute on Aging, and the National Institute of Environmental Health Sciences.
Journal
One Earth
Article Title
Extreme precipitation, exacerbated by anthropogenic climate change, drove Peru’s record-breaking 2023 dengue outbreak
Article Publication Date
17-Mar-2026
Climate lessons from the last interglacial for today’s climate change
By integrating ancient geological archives with high-tech climate simulations, researchers identified that the Levant experienced a 20% increase in rainfall during the Last Interglacial peak. The study reveals that this wetting was driven by a "thermodynamic" shift, where a warmer atmosphere held more moisture that was then dumped into the desert by intensified Red Sea Troughs. These findings suggest that such localized, high-intensity weather patterns transformed the arid southern Levant into a viable migration path for early humans moving out of Africa.
For modern residents of the Levant, the "Red Sea Trough" usually brings a brief, dusty transition between seasons. But 127,000 years ago, this same weather pattern may have been the literal key to human history.
A new study led by PhD student Efraim Bril, Prof. Adi Torfstein and Dr. Assaf Hochman from the Institute of Earth Sciences at the Hebrew University of Jerusalem, published in Climate of the Past, reveals that during the Last Interglacial (LIG) peak, the Levant wasn’t just a dry bridge between continents, it was a dynamic more relatively wet conditions fuelled by intense, localized rain. This shift in ancient weather likely provided the water sources necessary for early humans to successfully migrate "out of Africa".
The Last Interglacial (roughly 129,000 to 116,000 years ago) was a period of global warmth, with higher sea levels and temperatures than today. While the region was generally arid, geological "clues", ranging from Dead Sea sediment cores to ancient cave formations in the Negev, show evidence of brief, extremely wet phases.
"Proxy-based reconstructions indicate that during the peak of the LIG, the southern Levant experienced relatively wetter conditions," the researchers note. But how did a desert suddenly get enough water to support a human migration?
To solve this, Bril et al. used advanced climate models (PMIP4) to simulate how rain-bearing weather systems behaved 127,000 years ago. They focused on the two primary systems that still govern our rain today:
- Cyprus Lows: The winter storms that bring most of Israel’s annual rainfall from the Mediterranean.
- Red Sea Troughs: Systems that usually peak in autumn and can pull moisture from the tropics.
The study found that during the LIG peak, these systems were roughly 20% more productive than they are in modern times.
The most striking finding involves the southern Levant. While northern Israel and Lebanon saw more winter rain from Cyprus Lows, the arid south (areas like Eilat and the Negev) relied on a "turbo-charged" Red Sea Trough.
The researchers discovered that this wasn't necessarily because these storms happened more often, but because they were physically different. Essentially, the ancient Levant became wetter because a warmer atmosphere acts like a larger sponge. During the peak of the Last Interglacial, significantly higher temperatures, especially in the summer, increased the air's capacity to hold water vapor.
When a Red Sea Trough moved through the region during the year, it had access to a much larger reservoir of atmospheric moisture than it does today. This physical change in the air, rather than just a change in wind patterns, was the primary reason for the intense rainfall that transformed the southern desert into a more viable landscape.
Beyond historical curiosity, this research provides a vital "mirror" for our future. As we face modern global warming, understanding how natural variability once transformed the Levant’s water balance is crucial for predicting future climate impacts.
The study highlights that even in a generally dry region, specific weather types can become significantly more intense due to rising temperatures, a pattern we may already be starting to see in 21st-century projections. By integrating ancient geological "proxies" with high-tech modelling, Bril et al have mapped the path of our ancestors and, perhaps, the weather challenges of our descendants.
Journal
Climate of the Past
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Hydroclimatic variability and weather type characteristics in the Levant during the last interglacial
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