More than half of all human infectious diseases in recorded history—Lyme, West Nile, hantavirus, typhoid, HIV, and influenza, to name a few—have been exacerbated by the increasing impact of greenhouse gas-driven climate change.
That’s the sobering conclusion of a new, first-of-its-kind paper that combed through more than 70,000 scientific studies to determine how a range of climate hazards affected 375 pathogenic diseases known to affect humans. A team of 11 researchers from the University of Hawaii at Manoa conducted the analysis, which was published Monday in the peer-reviewed scientific journal Nature Climate Change.
“I have to tell you, when this database started growing, I started to get scared, man,” Camilo Mora, a climate scientist at UH Manoa and lead author of the paper, told HuffPost. “We’re just beginning to realize that this one thing we’re doing – greenhouse gas emissions – can affect 58% of all the diseases that have affected humanity. You realize the scale of the vulnerability we are under. I went from excited to terrified.
Scientists have long known and warned that climate collapse is making infectious diseases more common and dangerous. But the new paper quantifies the extent of this growing threat, concluding that a staggering 58% of all documented infectious diseases — 218 of a total of 375 — have been exacerbated in some way by one or more climate hazards linked to greenhouse gas emissions, including warming temperatures, drought, wildfires, sea level rise and extreme rainfall.
Mora emphasized that this estimate, as alarming as it is, is conservative. The findings are based exclusively on cases with evidence linking climate hazards to infectious diseases, he said.
The research team examined the existing scientific literature on a myriad of pathogens—viral, bacterial, fungal, animal-borne, and more—and found that warming temperatures negatively affected 160 unique diseases, the most of any climate impact analyzed. Extreme rainfall affected 122 diseases, followed by floods (121), drought (81), storms (71), land cover change (61), ocean climate change (43), fires (21), heat waves (20) and sea level rise (10).
On the other hand, the analysis identified 63 diseases that were reduced in some way by climate hazards; However, 54 of them were aggravated by one or more other climate impacts.
The study comes as the world remains in the grip of an ever-evolving COVID-19 pandemic that has so far killed 6.4 million people worldwide and infected more than half a billion, according to World Health Organization figures. And as the new paper highlights, there is evidence that climate impacts, particularly changes in rainfall and temperatures, have had mixed effects on disease transmission.
A 2020 study “suggests that heavy rainfall may exogenously induce social isolation, which helps explain lower cases of COVID-19 following heavy rainfall; however, increased cases of COVID-19 were associated with increased rainfall in Indonesia, possibly reflecting different behavioral responses to extreme rain,” the paper summarizing the available research said. “In some cases, warmer temperatures have been linked to increased cases of COVID-19, and although no mechanism has been outlined, it is possible that extreme heat may force people to stay indoors, which may increase the risk of transmission of the virus, especially when combined with poor or reduced ventilation.”
Volunteers wearing personal protective equipment spray disinfectant at an exam venue ahead of China’s national college entrance exam on June 5, 2022 in Bozhou, China’s Anhui Province. (Photo by Zhang Yanlin/VCG via Getty Images)
In their paper, the UH researchers break down the ways in which one crisis fueled another. Climate change has brought humans and pathogens closer together. Warming temperatures and changes in rainfall have allowed mosquitoes, ticks, birds and other disease vectors to expand their range, while human displacement and migration from rising sea levels and extreme weather have led to new contacts with dangerous pathogens, the analysis notes. Warmer temperatures on land lead to a surge in mosquito-borne viruses such as dengue fever, while warming oceans are linked to a significant increase in vibriosis, bacterial infections caused by eating contaminated seafood or swimming in contaminated water. In addition, climate impacts have allowed pathogens to reproduce more successfully and become more virulent, while blunting our own ability to avoid and fight disease.
Many infectious diseases are negatively affected by multiple climate hazards. For example, leptospirosis, a bacterial disease transmitted through contact with the urine of infected animals, has been worsened by eight separate climate impacts, including warming, flooding, extreme rainfall and even drought, according to the findings.
But the problem is much more complex than how each climate stressor can interact to exacerbate each infectious disease. This is not a 1-to-1 relationship; many pathogens can be transmitted to humans in many different ways. The paper identifies more than 1,000 unique pathways between climate hazards and disease outbreaks.
Mora said the dynamics present monumental challenges.
“It’s so naive to think we’re going to be able to adapt to it,” he said. “There’s no way, with so many diseases and so many different pathways, we can fully adapt. It made it abundantly clear to me that if we really want to avoid this problem, the best way to avoid it is to tackle greenhouse gas emissions. The last thing we want to do is unleash the power of one of these diseases that can be affected by greenhouse gases.
Patients infected with dengue are treated in a special ward of a hospital in Dhaka, Bangladesh, on June 13, 2022. (Photo by Kazi Salahuddin Razu/NurPhoto via Getty Images)
NurPhoto via Getty Images
One particularly alarming example of how warming can spread disease occurred in 2016, when anthrax, a rare bacterial disease, broke out in a remote village in Siberia. One child died and dozens of people were hospitalized. Scientists eventually attributed the outbreak to a summer heat wave that thawed the permafrost and exposed the carcass of a 75-year-old infected reindeer, releasing spores of the bacteria that cause anthrax. Thousands of reindeer eventually died from the outbreak.
“You can imagine how many diseases have built up over time in these ice caps, and now that they’re starting to melt, all these diseases are starting to come out,” Mora said.
Louis Ostrosky, chief of infectious diseases at UTHealth Houston’s McGovern School of Medicine, now spends most of his time studying COVID-19 and monkeypox. But one of his specialties is mycology, the study of fungi. He jokes that it has now become his “night job”.
In recent years, mycologists have documented significant geographic shifts in fungi that for centuries were found only in certain regions, he said. Histoplasmosis, for example, is an infection caused by inhaling fungal spores found in bird and bat droppings. Although historically found only in the eastern half of the United States, it is now beginning to appear in the western states. Likewise, coccidioidomycosis, a fungal disease better known as “valley fever,” is increasingly appearing outside its usual range in the Southwest.
“This is thought to be related to climate change and bird migration, both deeply interconnected,” Ostroski said of the changes.
Ostroski was not involved in the UH study, but applauds the authors for their comprehensive effort to quantify the clear changes scientists are seeing around the world.
“If anything, it really brings the data together very elegantly and points to the fact that indeed, with climate change, we’re going to see dramatic changes in the patterns of infectious disease spread and human infection.”
When it comes to humanity’s ability to adapt, Ostrosky says we don’t have much of a choice.
“I think we’re very resilient as a species. We’re going to have to adapt to a lot of things, one of which is pathogens,” Ostroski said. “But it’s very worrying.”
Mora has a personal connection to the results of the study. He is from a rural area outside of Cali, Colombia. During a visit home several years ago, he contracted chikungunya, a mosquito-borne virus that causes fever and severe joint pain. Global warming, extreme rainfall and flooding are contributing to disease outbreaks, a new analysis has found.
Mora called her encounter with chikungunya one of the most brutal and painful experiences of her life.
“I started studying this thing and found out it’s transmitted by mosquitoes that populate like nobody’s business with heat and excess rain – two things that are becoming so common in my country,” he said, speaking via Zoom from his family farm in Columbia. “I couldn’t help thinking to what extent even I myself was affected.”
Along with the paper, the UH Manoa team released an interactive tool that allows users to filter the data by climate hazards, transmission types and individual diseases.
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