A thorough review of the scientific literature revealed empirical evidence that more than 58% of human diseases caused by pathogens, such as dengue, hepatitis, pneumonia, malaria, Zika, and others, have been exacerbated by climatic hazards at some point. That startling and eye-opening discovery is the subject of a research paper published in Nature Climate Change on August 8 by a team of researchers from the University of Hawaii at Manoa.
The researchers conducted a systematic search for empirical evidence on the effects of ten climatic hazards sensitive to greenhouse gas (GHG) emissions on each known human pathogenic disease. Warming, drought, heatwaves, wildfires, extreme precipitation, floods, storms, sea level rise, ocean biogeochemical change, and land cover change were among the hazards.
Researchers reviewed more than 70,000 scientific papers for empirical examples of each possible combination of a climatic hazard impacting each of the known diseases after combining two authoritative lists of all known infections and pathogenic diseases that have affected humanity in recorded history.
Warming, precipitation, floods, drought, storms, land cover change, ocean climate change, fires, heatwaves, and sea level changes were all found to influence diseases caused by viruses, bacteria, animals, fungi, protozoans, plants, and chromists, according to the study. Pathogenic diseases were primarily transmitted by vectors, though cases of waterborne, airborne, direct contact, and foodborne transmission were also found. Ultimately, the research found that more than 58%, or 218 out of 375, of known human pathogenic diseases had been affected at some point, by at least one climatic hazard, via 1,006 unique pathways.
Given the extensive and pervasive consequences of the COVID 19 pandemic, it was truly frightening to discover the massive health vulnerability resulting from greenhouse gas emissions. There are simply too many diseases and disease transmission pathways for us to believe that we can truly adapt to climate change.
Camilo Mora
“Given the extensive and pervasive consequences of the COVID 19 pandemic, it was truly frightening to discover the massive health vulnerability resulting from greenhouse gas emissions,” said Camilo Mora, geography professor in the College of Social Sciences (CSS) and lead author of the study. “There are simply too many diseases and disease transmission pathways for us to believe that we can truly adapt to climate change. It emphasizes the critical need for global reductions in greenhouse gas emissions.”
The research team created an interactive web page that depicts each link between a climatic hazard and a disease case. The tool allows users to search for specific hazards, pathways, and disease groups and view the available evidence.
The UH Manoa research team included experts from CSS, Department of Earth Sciences in the School of Ocean and Earth Science and Technology, Marine Biology Graduate Program in the School of Life Sciences, Department of Natural Resources and Environmental Management in the College of Tropical Agriculture and Human Resources, and Hawaii Institute of Marine Biology in SOEST.
Other key findings include:
- Climate change is bringing pathogens closer to humans. Numerous climatic hazards are expanding the area and duration of environmental suitability, allowing vectors and pathogens to spread spatially and temporally. Climate change, for example, has been linked to the range expansion of vectors such as mosquitoes, ticks, fleas, birds, and several mammals implicated in outbreaks of viruses, bacteria, animals, and protozoans such as dengue, chikungunya, plague, Lyme disease, West Nile virus, Zika, trypanosomiasis, echinococcosis, and malaria, to name a few.
- Climate change is bringing pathogens closer to people. Climate change was also linked to forced displacement and migration of people, which resulted in or increased new pathogen contacts. Heatwaves, for example, have been linked to an increase in the number of cases of several waterborne diseases, including Vibrio (a type of bacteria)-associated infections, primary amoebic meningoencephalitis, and gastroenteritis. Human displacements caused by storms, floods, and sea level rise have been linked to cases of leptospirosis, cryptosporidiosis, Lassa fever, giardiasis, gastroenteritis, Legionnaires’ diseases, cholera, salmonellosis, shigellosis, pneumonia, typhoid, hepatitis, respiratory disease, and skin diseases, among other things.
- Climatic hazards have enhanced specific aspects of pathogens, including improved climate suitability for reproduction, acceleration of the life cycle, increasing seasons/length of likely exposure, enhancing pathogen vector interactions (for example, by shortening incubations) and increased virulence. For instance, storms, heavy rainfall and floods created stagnant water, increasing breeding and growing grounds for mosquitoes and the array of pathogens that they transmit (for example, leishmaniasis, malaria, Rift Valley fever, yellow fever, St. Louis encephalitis, dengue and West Nile fever). Climatic hazards were also implicated in the increasing capacity of pathogens to cause more severe illness. For example, heatwaves were suggested as a natural selective pressure toward “heat resistant” viruses, whose spillover into human populations results in increased virulence as viruses can better cope with the human body’s main defense, which is fever.
- Climatic hazards have also diminished human capacity to cope with pathogens by altering body condition; adding stress from exposure to hazardous conditions; forcing people into unsafe conditions; and damaging infrastructure, forcing exposure to pathogens and/or reducing access to medical care. Drought, for instance, was conducive to poor sanitation responsible for cases of trachoma, chlamydia, cholera, conjunctivitis, Cryptosporidium, diarrheal diseases, dysentery, Escherichia coli, Giardia, Salmonella, scabies and typhoid fever.
Researchers also discovered that, while climate change exacerbated the vast majority of diseases, it also reduced the severity of others (63 out of 286 diseases). Warming, for example, appears to have slowed the spread of viral diseases, most likely due to unsuitable conditions for the virus or a stronger immune system in warmer conditions. However, most diseases that were reduced by at least one risk factor were sometimes exacerbated by another, and sometimes even the same risk factor.
