As global temperatures have risen in recent decades, so have the number of outbreaks of infectious diseases. SARS, MERS, Zika, West Nile, COVID-19, and now clusters of monkeypox and polio have all recently threatened public health.
That’s no coincidence. In a study published in August in Nature Climate Change, researchers tried to understand the relationship between major environmental changes related to higher greenhouse gas emissions—including global warming, rising sea levels, storms, floods, drought, and heat waves—and the outbreaks of 375 human infectious diseases caused by viruses, bacteria, and other pathogens. They found that 58% of these public-health threats were fueled by climate change.
“The health impacts of climate change are here,” says Dr. Vishnu Laalitha Surapaneni, assistant professor of medicine at the University of Minnesota. “And they are affecting us right here, right now.”
Viruses and other pathogens aren’t becoming better at living in higher environmental temperatures, scientists say. Instead, it’s more likely that the host animals they infect are affected by changing climates. Increasing global temperatures, for example, mean that the geographic range for many pathogen-carrying animals—including insects like mosquitoes—is expanding rapidly. “As they move around to find better climates, there are more opportunities for viruses to spill over among other mammals, and then from some of those mammals to humans,” says Gigi Gronvall, senior scholar at the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health. In the same way that highways, planes, and trains connect remote parts of the world, these animals are transporting their microbial payloads into new places.
Tickborne diseases are one example of climate’s impact on human health. With warming temperatures, the habitable environment for ticks is stretching further north, into Canada and even Nova Scotia. Neither place had reported cases of Lyme disease, which is carried by ticks, until 2002.
Warming ocean waters are also allowing dangerous pathogens to thrive, including flesh-eating bacteria from the vibrio genus. Infections from these bacteria have recently increased—not only when natural climate disasters such as hurricanes sweep over land and bring contaminated waters in closer contact to people, but also when people with exposed cuts or wounds venture into oceans, which have become fertile waters for bacterial growth. In a report published by the Lancet in 2021, a group of health and climate experts from 43 academic institutes and U.N. agencies analyzed metrics of environmental impact on human health, including the role of pathogens. The amount of coastline in which vibrio now thrive has increased by 25% in the U.S. northeast and by 4% in the Pacific northwest.
With global warming, mosquitoes are also encroaching further north than they have ever ventured before. Diseases they carry, such as dengue, have been reported far outside of their typical tropical areas in places like New York City—and disturbingly, the cases aren’t always from travelers, but often from infected mosquitoes circulating in the region. Other pathogens these insects transport, including the ones responsible for Zika and yellow fever, are also being reported in parts of the world where they haven’t previously been common. The Lancet report showed that in some lower income countries in which mosquitoes are known to thrive, the period of time during which the insects are now active has increased 39% between 1950-1959 and 2010-2019.
While mosquitoes generally thrive in wetter, humid climes, hotter temperatures resulting in droughts may also promote certain mosquito-borne illnesses, such as West Nile disease. Under drier drought conditions, birds—one of the animal vectors for mosquitoes—tend to congregate in the scarce areas where water is available, creating the ideal conditions for mosquitoes to find new hosts.
Bats, too, are expanding into new territories as they respond to the stress of climate change. Warming temperatures send them searching for more preferable climes, and their increased movement raises the chances of interacting with people and spreading diseases, such as coronaviruses. “There is a lot more mixing of populations of animals that haven’t been mixed before,” says Gronvall.
Understanding these shifting behaviors and changing habitats requires appreciating that the health of one species is intimately tied to the health of all species. Scientists say devoting more resources to understanding how viruses, bacteria, and other pathogens circulate among different animal species is crucial in order to better understand how their behaviors can impact human health. “It’s really hard to build bridges between different communities,” such as the U.S. Centers for Disease Control and Prevention and the Environmental Protection Agency or the Department of Agriculture, Gronvall says. “But we are going to need a much more integrated approach to tackling research questions that everyone agrees are important.”