Climate change is expected to weaken humans, overwhelm pathogens, and push both into closer quarters.
A new study shows that extreme weather, changing oceans and land disturbance have already helped spread more than 200 pathogens.
Three graphs show how extreme weather and environmental changes help spread disease.
The first outbreak of Nipah virus, a deadly bat-borne disease, may have started with a wildfire.
In 1997 and 1998, devastating wildfires raged across Indonesia, marking a long drought and, many scientists suspect, driving fruit bats from their forest homes in Malaysia. There, the story goes, the increased bat population was attracted to fruit trees in pig farms.
Soon the pig farmers started experiencing fever, headaches, sore throats, vomiting and brain swelling. People in Singapore who ate Malaysian pork also developed symptoms. In total, more than 300 people became ill and more than 100 died. To contain the disease, more than 1 million pigs were killed and pig farming was permanently banned in some areas.
Nipah virus is just one of 218 infectious diseases that have become more widespread in humans due to climate extremes – floods, droughts, heat waves, hurricanes, ocean chemistry, sea level rise or other climate-sensitive environmental conditions. change – according to a study published in the journal Nature Climate Change on August 8.
“I can tell you this story about bats, I can tell you this story about birds, I can tell you this story about rats, mice, deer. And I can tell you about viruses and bacteria. And I can tell you through heatwaves, floods, fires, even hurricanes, things that really force these species to move,” Camilo Mora, a data scientist at the University of Hawaiʻi Manoa who led the study, told Insider.
By evaluating historical records of infectious diseases dating back to the Roman Empire, Mora’s team cataloged instances of extreme climatic conditions facilitating the spread of 58% of known human pathogens.
“I didn’t expect such a high number,” Mora said.
It is likely an undercount, he added, as it only includes cases documented in published papers. Although not all of these cases can be attributed to current human-induced climate change, 80% of the papers are relatively recent, published within the last 20 years. They rely on a mountain of evidence that extreme changes caused by rising global temperatures are helping to spread infectious diseases through three main pathways.
Pathway 1: Extreme weather and land disturbance spread disease by pushing animals and humans closer together
In Siberia in 2016, an anthrax outbreak was traced to a decades-old reindeer carcass unearthed by melting permafrost. This is an extreme case of climate change creating new contact between humans and infectious diseases, but the phenomenon is widespread.
Extreme weather events, which are becoming more frequent and severe with climate change, can displace populations of animals and birds, driving them closer to humans. The Nature study found that vector-borne diseases—those carried by animals and insects—were exacerbated by extreme climates.
Changes in land use—such as deforestation—can drive animal populations into places where people live or bring people into the animals’ range. In the eastern US, studies suggest that clearing forested areas for development has led to increased overlap between humans and ticks, facilitating the spread of Lyme disease.
Extreme weather can also bring people into close proximity to each other. Hurricanes and cyclones often lead to outbreaks of cholera, norovirus and other deadly diseases. Such cases were well documented in the aftermath of Hurricane Katrina. Flooding can expose people to water-borne diseases such as shivering. A 1995 analysis even found that the spread of leprosy in Malawi was not linked to population density, but to rainfall.
Pathway 2: Excessive heat and rain can overload pathogens
Mosquitoes thrive in high temperatures and heavy rainfall, which creates standing water where they can lay their eggs. The diseases they transmit, such as malaria, West Nile virus and chikungunya, are thriving as climate change increases temperatures and heavy rainfall events in many parts of the world.
Pathogens themselves can grow stronger in extreme conditions. Warming oceans create fertile breeding grounds for vibrio bacteria, which show signs of increased virulence in the heat, allowing them to cause more serious diseases.
Extreme heat waves, for example, can kill many infectious viruses, bacteria, fungi and the creatures that transmit them. What survives, however, adapts to the extreme heat—including the fever our bodies produce to kill pathogens.
“The ones that survive will survive at 42 degrees Celsius, which means that when they come and infect us, one of the main mechanisms to fight these diseases and these pathogens is not effective at all,” Mora said.
Pathway 3: Extreme weather weakens infrastructure and makes people susceptible to disease
People and their infrastructure are more vulnerable to the devastating effects of disease when they are hit by extreme weather events. Fire smoke, for example, can irritate the lining of the lungs, cause inflammation, suppress the immune system, and make people more vulnerable to respiratory illnesses like COVID-19.
Extreme weather events such as heat waves can affect access to health care, making it difficult or dangerous for people to leave their homes or destroying essential infrastructure. Just this summer, the heat melted roads and airport tarmac, bent railroads and caused power outages.
People affected by rapidly changing weather or extreme events such as hurricanes or wildfires may be stressed, leading to elevated cortisol levels that weaken their immune systems. Malnutrition, expected to become more prevalent as a changing climate affects the world’s breadbaskets, has a serious negative impact on the immune system.
Adapting infrastructure, emergency plans and health care to these new extremes can reduce the spread of disease. But the Nature study concludes that pathogens boosted by climate threats “are too numerous for comprehensive societal adaptations.” Instead, the authors write, their findings underscore “the urgent need to work at the source of the problem: the reduction [greenhouse gas] shows”.
“Keep in mind that this is not some weird alien causing climate change,” Mora said, adding, “It’s the contribution of small things that you and I do, multiplied by almost 8 billion people.”
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