A comprehensive assessment of a vast body of scientific research led by the University of Hawaii at Manoa has found empirical evidence that more than 58% of human pathogenic diseases have been exacerbated at some point by climate hazards, including global warming, droughts, heatwaves, heavy rainfall, floods, storms, sea level rise, ocean biochemical changes and land cover changes. By examining the influence of these climatic factors on all known pathogenic diseases, the experts found that 218 out of 375 of these diseases were affected at some point by at least one climatic factor, through 1006 unique pathways.
“Given the wide-ranging and widespread effects of the Covid-19 pandemic, it was truly terrifying to discover the enormous health vulnerabilities resulting from greenhouse gas emissions,” said study lead author Camilo Mora, professor of geography at the University of Hawaii. “There are too many diseases and transmission routes for us to think we can really adapt to climate change. This highlights the urgent need to reduce greenhouse gas emissions worldwide”
The analysis showed that climate hazards bring pathogens closer to humans, facilitating the spatial and temporal spread of vectors and pathogens. For example, global warming and changes in rainfall are associated with an expansion of disease vectors such as mosquitoes, ticks, fleas, birds, and some mammals that are involved in outbreaks of viruses, bacteria, or protozoa, including malaria, dengue, Lyme disease, West Nile virus, or plague bacteria.
On the other hand, climate threats also often bring people closer to pathogens. Forced movements and migrations of people after hurricanes, floods or rising sea levels have led to massive outbreaks of infectious diseases such as cholera, typhoid and Lassa fever, legionnaires’ disease, salmonellosis, gastroenteritis or a huge number of respiratory and skin diseases.
Moreover, climate threats also enhance specific aspects of certain pathogens, such as improved climatic conditions for reproduction, accelerated life cycle, increased virulence, or increased duration of likely exposure. For example, heat waves can act as natural selective forces, resulting in heat-tolerant pathogens that are better able to cope with the human body’s primary defense mechanism, fever. In addition, severe storms and floods often create stagnant pools of water, which are ideal breeding grounds for mosquitoes and the many pathogens they carry.
Finally, climate hazards also reduce people’s ability to resist pathogens by changing the state of the body, increasing the stress of exposure to extreme weather events, forcing people to live in unhygienic conditions, and damaging protective infrastructure. For example, prolonged droughts are known to contribute to poor sanitation, often resulting in diseases such as cholera, dysentery, scabies, chlamydia, E. coli or salmonella.
Although the majority of pathogenic diseases have been exacerbated by climatic factors, some of them (63 out of 286) have decreased. For example, global warming seems to have reduced the spread of some viruses that thrive in colder climates. However, most diseases that have been reduced by one climatic factor have sometimes been exacerbated by another, or even by the same factor.
“We knew that climate change could affect human pathogenic diseases,” said study co-author Kira Webster, a doctoral student in the Department of Geography at the University of Hawaii. “However, as our database has grown, we have been both fascinated and dismayed by the overwhelming number of case studies available that have already shown how vulnerable we are becoming due to the continued rise in greenhouse gas emissions,” she concluded.
The study was published in the journal Nature Climate Change. In addition, the researchers also created an interactive webpage showing links between specific hazards and diseases.