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 mounting impacts of greenhouse gas-driven climate change.
That is the sobering conclusion of a new, first-of-its-kind paper that combed through more than 70,000 scientific studies to pinpoint how an array of climate hazards have impacted 375 pathogenic diseases known to have impacted humans. A team of 11 researchers at 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 that as this database started to grow, I started to get scared, man,” Camilo Mora, a climate scientist at UH Manoa and the paper’s lead author, told HuffPost. “We just started realizing that this one single thing that we do — the emission of greenhouse gasses — can influence 58% of all of the diseases that have impacted humanity. You realize the magnitude of the vulnerability that we are under. I went from excited to terrified.”
Scientists have long known and warned that climate breakdown is supercharging infectious diseases, making them more frequent and dangerous. But the new paper quantifies the extent of that growing threat, concluding that a stunning 58% of all documented infectious diseases — 218 of the total 375 — have been aggravated in some way by one or more climate hazards associated with greenhouse gas emissions, including warming temperatures, drought, wildfires, sea-level rise and extreme precipitation.
Mora stressed that that estimate, as alarming as it is, is conservative. The findings exclusively draw on cases with evidence linking climate hazards to infectious disease, he said.
The research team dug through existing scientific literature on myriad pathogens — viral, bacterial, fungal, animal-borne and more — and found that warming temperatures negatively impacted 160 unique diseases, the highest of any climate impact analyzed. Extreme precipitation 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 flip side, the analysis identified 63 diseases that were diminished in some way by climate hazards; however, 54 of those were also aggravated by one or more other climate impacts.
The study comes as the world remains in the grips of an ever-evolving COVID-19 pandemic that has so far killed 6.4 million people globally and infected more than half a billion, according to data from the World Health Organization. And as the new paper highlights, there is evidence that climate impacts, specifically changes in precipitation and temperatures, have had mixed effects on the transmission of the disease.
One 2020 study “suggested that heavy rainfall could exogenously induce social isolation, helping to explain lower COVID-19 cases after heavy rainfall; however, increased cases of COVID-19 were associated with increases in precipitation in Indonesia, perhaps reflecting different behavioral responses to extreme rain,” the paper says, summarizing available research. “Higher temperatures have been associated with increased COVID-19 cases in some instances, and although a mechanism was not outlined, it is possible that extreme heat forces people indoors, which can increase the risk of virus transmission, especially when combined with poor or reduced ventilation.”
In their paper, UH researchers break down the ways one crisis has helped fuel another. Climate change has brought people and pathogens in closer proximity. Warming temperatures and precipitation changes have allowed for mosquitoes, ticks, birds and other disease vectors to expand their range, while human displacement and migration from sea-level rise and extreme weather has resulted in new contacts with dangerous pathogens, the analysis notes. Hotter land temperatures are driving a surge in mosquito-borne viruses like dengue fever, while warming oceans have been linked to major increases in vibriosis, bacterial infections caused by eating contaminated seafood or swimming in tainted water. Additionally, climate impacts have allowed for pathogens to more successfully reproduce and become more virulent, while simultaneously blunting our own ability to avoid and fight off disease.
Many infectious diseases have been negatively influenced by multiple climate hazards. For example, leptospirosis, a bacterial disease transmitted through contact with the urine of infected animals, has been made worse by eight separate climate impacts, including warming, flooding, extreme precipitation and even drought, according to the findings.
But the problem is far more complex than how any single climate stressor might interact with and exacerbate each infectious disease. It’s not a 1-to-1 connection; many pathogens can be transmitted to humans in multiple different ways. The paper identified more than 1,000 unique pathways between climate hazards and disease outbreaks.
Mora said that dynamic presents monumental challenges.
“It is so naive for us to think that we are going to be able to adapt to this,” he said. “There is no way, with so many diseases and so many different pathways, that we can fully adapt. For me, that made it super clear that if we really want to avoid this problem, the best way to avoid it is to deal with the emission of greenhouse gasses. The last thing that we want to do is unleash the power of one of these diseases that can be impacted by greenhouse gasses.”
One particularly alarming example of how warming can let diseases loose occurred in 2016, when anthrax, a rare bacterial illness, broke out in a remote village in Siberia. One child died and dozens of people were hospitalized. Scientists ultimately attributed the outbreak to a summer heat wave that thawed permafrost and exposed the carcass of a 75-year-old infected reindeer, releasing spores of the bacteria that cause anthrax. Thousands of reindeer ultimately died from the outbreak.
“You can imagine how many diseases that have accumulated over time in these ice caps, and now as they start melting all of these diseases start being exposed,” Mora said.
Luis Ostrosky, chief of infectious diseases at UTHealth Houston’s McGovern Medical School, 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 to fungi that for centuries were only found in certain regions, he said. Histoplasmosis, for example, is an infection caused by inhaling the spores of a fungus found in bird and bat feces. While it historically was found only in the eastern half of the United States, it is now starting to pop up in western states. Similarly, coccidioidomycosis, a fungal disease better known as “valley fever,” is increasingly turning up outside its common range in the Southwest.
“This is thought to be related to climate change and bird migration, both deeply tied to each other,” Ostrosky said of the shifts.
Ostrosky was not involved in the UH study but applauded the authors for their comprehensive effort to quantify the clear changes scientists are observing around the globe.
“If nothing else, it really puts together data very elegantly and it points to the fact that, indeed, with climate change we are going to be seeing dramatic changes in the patterns infectious diseases spread and infect humans.”
As for humanity’s ability to adapt, Ostrosky says we don’t have much choice.
“I think we are very resilient as a species. We are going to have to adapt to many things, one of them being pathogens,” Ostrosky said. “But it is very concerning.”
Mora has a personal connection to the study’s findings. He is from a rural area outside Cali, Colombia. During a visit home several years ago, he was infected with chikungunya, a mosquito-borne virus that causes fever and severe joint pain. Planetary warming, extreme precipitation and flooding are all contributing to outbreaks of the disease, the new analysis found.
Mora called his bout with chikungunya one of the most brutal, painful experiences of his life.
“I started studying this thing and I realized that it’s transmitted by mosquitoes, which 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 Colombi. “I couldn’t resist thinking to what degree even myself was affected.”
Along with the paper, the team from UH Manoa released an interactive tool that allows users to filter the data by climate hazards, types of transmission and individual diseases.