After the emergence of the COVID-19 pandemic in early 2020, causes triggering zoonotic disease outbreaks have been widely discussed. While the definite answer on the origin of the COVID-19 pandemic remains unconfirmed to date, close contact between wild animals and humans has likely enabled the pathogen to make the leap from animal vector to human host. In recent decades, large-scale changes in land use, urbanization, and a growing global human population have narrowed the natural habitat of many wild animals, among those some that can be vectors for pathogenic diseases. As a result, conflicts of space and exposure arise between humans and wildlife. Additionally, climatic hazards can impact the emergence patterns of pathogenic diseases. Rising greenhouse gas emissions trigger climatic and environmental changes, including warming of the atmosphere and the oceans, changes in precipitation, floods, droughts, and heatwaves, among others. Many of these climate hazards pose immediate threats to human health¹. Apart from the direct effects these climate hazards can have on humanity, they also influence the dynamics and pathways between pathogens, vectors, and humans. Pathogenic disease outbreaks are being reported from areas where diseases have not historically occurred, pathogens and vectors are encroaching on larger cities which increases risk of transmission, or are changing in seasonality.

Recently, the COVID-19 pandemic changed many aspects of life as we knew it. As a graduate student in environmental sciences, such highly topical issues can awaken curiosity for research into the underlying dynamics of these events. When the seminar team of graduate students and faculty led by Prof. Camilo Mora (University of Hawai’i at Manoa) was discussing possible topics for our project in early 2020, the relationship of humans and pathogenic diseases was becoming especially relevant. This seminar has run annually since 2013 and led to seminal works on the impacts of climate change on human society and natural ecosystems^1-4. Our seminar included participants of various scientific backgrounds, many with a focus on climate related studies, which led us to examine the relationship between climate hazards and human pathogenic diseases. Being part of a generation that is shaped by climate change awareness with an uncertain future lying ahead, investigating interactions between climate hazards and diseases is truly a cause with a purpose.

An initial literature review conducted by seminar participants revealed a wide variety of publications connecting individual climatic hazards to pathogenic diseases. Apart from case studies reporting isolated disease outbreaks in relation to climatic events, there were several review papers on the topic, but we didn’t encounter a comprehensive review considering all major climatic hazards and their connection to a full catalogue of pathogenic diseases affecting humanity. We were also struggling to find a complete list of human pathogenic diseases to base our search on. These are the challenges we chose to address. By merging pathogenic disease lists of databases from the CDC (United States Centers for Disease Control) and GIDEON (Global Infectious Diseases and Epidemiology Network), we compiled a comprehensive list of human pathogenic diseases. After developing a search index for publications addressing pathogenic diseases and climatic hazards, the seminar team spent countless hours scrutinizing thousands of publications in a joint effort to produce this ‘complete picture’ that was previously missing.

While we expected to find some effects of climate change on diseases, the magnitude we found in our results was startling: over 58% of pathogenic diseases affecting humanity can be aggravated by climate change.

We found that climatic hazards contribute to the emergence of  diseases mainly through four mechanisms: bringing pathogens closer to people (e.g., climate induced range shifts of vector organisms such as mosquitos), bringing people closer to pathogens (e.g., heatwaves promoting recreational water-related activities increasing case numbers of water-borne diseases), strengthening pathogens (e.g., improved climate suitability for reproduction of certain vectors or pathogens), and impairing human capacity to cope with pathogens (e.g., added stress from exposure to hazardous conditions).

Visualizing this large amount of data in a comprehensive and accessible way posed its own challenges. We worked with a web developer to produce an interactive website in which users can navigate among a Sankey plot connecting climatic hazards, transmission types, and pathogens (https://camilo-mora.github.io/Diseases/). Clicking on distinct pathways and nodes directs the user to individual entries and source publications. We hope that the website will provide a valuable resource for the community of practitioners working in the intersection of human disease and climate change.

Our study adds to the overwhelming amount of evidence about the urgent need to mitigate greenhouse gases. The large number of diseases that climatic hazards can exacerbate reveals the magnitude of this looming human health crisis, which poses a significant threat to human lives and socioeconomic wellbeing across the globe. It would take adaptation efforts in innumerable ways to sufficiently prepare the global community for these effects, the cost of which would be staggering, leaving the poorest people at the most vulnerable end of the spectrum yet again. With global populations projected to increase to an estimated 9-10 billion by mid-century⁵, the need to mitigate emissions becomes even more urgent. We hope that our alarming results will be used to inspire climate action and generate increased preparedness for these upcoming challenges to humanity.

1. Mora, C. et al. Broad threat to humanity from cumulative climate hazards intensified by greenhouse gas emissions. Nat. Clim. Chang. 8, 1062–1071 (2018).
2. Mora C. et al. The projected timing of climate departure from recent variability. Nature 502: 183-187 (2013)
3. Mora C. et al. Global risk of deadly heat. Nature Climate Change 7, 501-506 (2017)
4. Mora C. et al. Bitcoin emissions alone could push global warming above 2°C. Nature Climate Change 8, 931-933 (2018)
5. Vollset, S. E. et al. Fertility, mortality, migration, and population scenarios for 195 countries and territories from 2017 to 2100. Lancet 396, 1285–1306 (2020).

Poster image: Tasnim News Agency, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons