Climate change is recognized by WHO as one of the most important health challenges of the 21st century, posing multiple threats to human survival, health and well-being. Exacerbating climate change is further unlocking a Pandora's box of viruses, which are expected to lead to the spread of thousands of new viruses among animal species by 2070, dramatically increasing the risk of emerging zoonotic diseases. WHO Director-General Tedros Adhanom Ghebreyesus has warned that "the risks posed by climate change could dwarf the risks of any single disease." "In the face of a potential public health emergency, there is an urgent need for concerted climate action to mitigate global warming, mitigate the adverse effects of climate change, and work together to save the future of humanity.
1. Climate change drives the interaction of epidemiological triangular components
About 58% of infectious diseases facing humanity, such as dengue, hepatitis, pneumonia, malaria, Zika virus and anthrax, will be exacerbated by climate change. According to the Epidemiologic Triangle model, as shown in Figure 1, infectious disease events occur when the right host, pathogen, and environment interact. Currently, climate change is affecting all three factors at the same time. With changes in host susceptibility, pathogen survival, and environmental structure, there will be more infectious diseases and global pandemics in the future.
Fig.1 Disease triangle model
图源：The Aggie Transcript
From a host perspective, climate change can lead to the virus acquiring multiple new hosts at the same time, thereby increasing its spread. More than 15,000 cross-species transmission events involving more than 3,000 mammals are expected in the next 50 years. From the perspective of pathogens, climate warming and water warming are more suitable for the survival and reproduction of pathogenic microorganisms, and the gradual melting of glaciers and permafrost may lead to the reappearance of ancient and unknown viruses and bacteria frozen by ice and snow underground, causing infectious diseases. From an environmental perspective, extreme weather events disrupt the balance of ecosystems, disrupt seasonal changes, increase the severity and frequency of natural disasters such as hurricanes, droughts, and floods, and provide a more suitable environment for the breeding and spread of infectious pathogens. The Intergovernmental Panel on Climate Change (IPCC) warned in its report Climate Change 2022: Impacts, Adaptation and Vulnerability that humanity faces the threat of escalating infectious diseases. The pathogen will spread to new areas and may re-emerge in previously controlled areas, leading to a surge in infections, while emerging diseases that have never previously infected humans may "spill over" from animals and into the human world.
2. Climate change exacerbates the threat of infectious diseases to human health
In March 2024, the U.S. Department of State's International Security Advisory Board (ISAB) released its Report on New Security Challenges, which identified several new security challenges, including the emergence and spread of human, animal and plant diseases. Climate change/extreme weather may affect the frequency and severity of biological events, such as changing rainfall patterns and temperatures that alter the range of insects capable of transmitting diseases such as malaria and dengue, contributing to their growth and persistence and potentially causing outbreaks, the report said. At the same time, resource scarcity is helping to drive changes in land-use patterns and habitats, creating new opportunities for zoonotic diseases to "spill over" from wildlife populations to humans and their domesticated animals.
First, there is an increased risk of cross-species transmission of infectious agents. On the one hand, the number of disease-carrying insects has proliferated, expanding the geographical range of insect-borne diseases that may be affected. Rising temperatures have led to a surge in the number of vectors such as ticks and mosquitoes that carry the disease, and their breeding areas and ranges have expanded significantly, leading to potentially life-threatening insect-borne diseases such as dengue, Lyme disease, malaria, Zika virus, West Nile virus and Rocky Mountain spotted fever entering new areas where such viruses have not been previously affected, such as some high-latitude areas, where the local population has little resistance to the diseases they carry and have never seen before, and disease prevention and treatment will also be limited by inexperience, resulting in increased morbidity and case fatality rates. In November 2022, the European Union pointed out in the report "Climate Change Threats to Health and Well-being in Europe: Focusing on Heat-Induced Diseases and Infectious Diseases" that climate change has led to longer transmission seasons and wider distribution of vector mosquitoes, coupled with large cross-border movements, and the increasing number of imported cases, which have exacerbated the possibility of local outbreaks of infectious diseases. In addition, rising temperatures have increased the risk of West Nile fever outbreaks in Central, Eastern and Southern Europe and extended the risk of transmission to previously unaffected regions of Northern and Western Europe. On the other hand, closer contact between humans and disease-causing animals increases the risk of zoonotic disease outbreaks. Climate change is a key driver of biodiversity loss, destroying the natural habitats and forcing large-scale or transboundary migrations of species such as rodents, primates, bats and artiodactyls, many of which are reservoirs for zoonotic diseases. In the process of finding new habitats, animals are highly likely to enter the range of human life or encounter humans, which brings wildlife carrying pathogens such as coronavirus, avian influenza virus, and Ebola virus closer to humans, and increases the risk of zoonotic disease outbreaks due to virus spillover. Studies have shown that about 60 per cent of known human infectious diseases and 75 per cent of emerging infectious diseases are caused by increased animal-human contact, and that species migration caused by climate change could now lead to viruses acquiring thousands of new hosts. Bats are the animals with the highest proportion of zoonotic diseases among mammals and are therefore important vectors for zoonotic events. In April 2022, a study in the journal Nature predicted that in the future, 88% of new species encounters between mammals would occur between bats and other species, or cause 110 transmissions of the virus between new species, meaning more diseases will spread from animals to humans. South-East Asia, Africa and Europe are zoonotic hotspots in the next 50 years, and infectious diseases are likely to spread from these three regions to the rest of the world.
Second, the environment is more suitable for the transmission of infectious agents. First, the thawing of permafrost will lead to the resurgence of ancient infectious diseases or the emergence of unknown pathogenic microorganisms. The permafrost is cold, oxygen-deprived and dark, creating a natural reservoir of microorganisms and germs, some of which survive hundreds, thousands, or even millions of years of freezing, and early diseases preserved under the permafrost can even date back to the last ice age. In November 2022, the University of Marseille in France resurrected 7 species of giant viruses infected with amoebae that had been frozen in the Siberian permafrost for tens of thousands of years, the oldest of which was frozen for 48,500 years, making it the oldest virus to be resurrected so far. Studies have shown that these viruses can still infect cells. In recent decades, the Arctic has warmed much faster than the rest of the world. In 2022, the Finnish Meteorological Institute confirmed that the Arctic is warming at about four times the average rate of global warming. The melting of glaciers and permafrost and the retreat of the polar cryosphere will "thaw" a large number of ancient or unknown pathogenic microorganisms that have been frozen underground for a long time. For example, in 2016, the anthrax outbreak in Siberia was caused by a heat wave that caused the permafrost to melt, exposing the carcasses of reindeer buried underground and once infected with Bacillus anthracis. It is still unclear what diseases are hidden in all the permafrost on Earth, and millions of unclassified bacteria are found in corpses and minerals that are "protected" by the permafrost. In 2020, researchers at The Ohio State University in the United States discovered 28 unknown new viruses about 15,000 years old in glacier samples taken from 50 meters below the surface of the Tibetan Plateau, and in October 2022, scientists at the University of Ottawa in Canada conducted a survey of Lake Hussen, the largest freshwater lake in the Arctic Hazen) sediments from the bottom of the lake were sampled and sequenced and found that the risk of virus spillover is highest in and around the melting glaciers, and that climate warming may expose viruses in the Arctic to new environments and hosts; International research teams from multiple institutions, including the United Kingdom and Canada, have taken samples of glacier runoff from eight glaciers in North America and Europe and two ice sheets in Greenland and found tens of thousands of microorganisms in a few millimeters of water, estimating that the number of bacteria and algae released by glaciers in the Northern Hemisphere is currently around 650,000 tonnes per year. This is expected to last for about 80 years, when the glaciers will disappear. In addition, the bacteria in the water samples have pigments that absorb sunlight, which will further contribute to the warming trend in the northern latitudes. Second, the warming of seawater provides a good breeding environment for infectious pathogens. Warming oceans have become fertile waters for bacteria to grow, which is conducive to the growth of many dangerous pathogens such as Vibrio spp., which can lead to an increase in infectious diseases such as cholera. In recent decades, the incidence of vibriosis has increased rapidly, and increased sea temperature is considered one of the main causes of the disease, especially at high latitudes. The 2021 Lancet Countdown to Health and Climate Change report notes that the number of vibrio bacteria thriving on coastlines has increased by 25% in the northeastern United States and 4% in the Pacific Northwest. In 2022, the European Union also pointed out that warmer waters are becoming more suitable for the survival of pathogenic Vibrio bacteria, especially in the Baltic Sea coast, where the risk of Vibrio infection is the highest. Third, the spread of bacteria and microorganisms has become wider or their drug resistance has been exacerbated. Bacterial infections are a significant cause of global health loss and are becoming the second leading cause of death worldwide, after ischemic heart disease. The warmer environment is ideal for bacteria to grow and multiply faster, exchanging genes with each other (also known as horizontal gene transfer), and the faster this happens, the more likely it is that bacteria will develop resistance. Over the past 50 years, Plasmodium falciparum has become resistant to all available antimalarial drugs, and as the climate crisis worsens, it will become even more difficult to control. In addition, floods and droughts have devastated human homes, displaced people, poor sanitation, lack of clean water and reduced access to treatment in temporary settlements, which can lead to an increase in the number of bacterial infections that can predispose bacteria to drug resistance. In addition, elevated rainfall, heat and humidity increase the spread of malaria, scarcity of fresh water supplies makes the spread of bacteria associated with mild cholera more common, and permafrost thaws or releases resistant strains of bacteria, thereby increasing the risk of infectious diseases.
Third, it indirectly weakens human resistance to infectious diseases and other diseases. Climate change is worsening global hunger. Rising temperatures, changing rainfall patterns, and the frequency of extreme weather events have severely impacted the four pillars of food security: accessibility, affordability, quality and sustainability, affecting farmland productivity and crop yields, leading to a sharp decline in food supply, food insecurity and malnutrition, and serious human health. Nutrition is a key determinant of susceptibility to infectious diseases and disease development, and obtaining essential nutrients is one of the important conditions for ensuring the normal functioning of immune function, while chronically malnourished people have little immune reserves to fight bacteria, viruses and other pathogens, and are susceptible to infectious diseases. In 2019, research from the University of Tokyo in Japan showed that heat waves and food insecurity caused by climate change can weaken the body's immune system response to the flu.
3. Prevent and prepare for the increased risk of infectious diseases caused by the climate crisis
As global warming continues to be triggered by climate change, there will be more extreme weather events and greater changes in rainfall and temperature in the future, bringing the threat of hundreds of known and unknown infectious diseases and increasing the burden of epidemic prevention and control. Still, the world is not ready, and governments and international agencies need to step up their efforts to be actively engaged in the prevention of the next global health emergency.
First, strengthen monitoring and research. Monitoring climate-related threats is the most commonly mentioned measure in national health or climate adaptation strategies, and the development of early warning mechanisms is essential to reduce the risk of infectious disease outbreaks. Therefore, the key to addressing both climate change and infectious disease threats is to accurately monitor and predict climate change and its impact on infectious diseases. This requires the strengthening of surveillance systems combined with technologies to detect human and animal diseases to provide early information on emerging pathogenic microorganisms, as well as cross-border collaboration to identify and develop public health responses to outbreaks and epidemics. In October 2022, the National Science Foundation (NSF) awarded $1 million to research teams at the University of Chicago and Argonne National Laboratory as part of its Predictive Intelligence for Pandemic Prevention (PIPP) program to support its planning and design of future PIPP centers to develop tools such as predictive models, environmental sensors, and multi-scenario and multi-objective decision analytics to help monitor, predict, and respond to future infectious diseases.
Second, the development of vaccines and therapeutics. Vaccines are one of the most cost-effective medical interventions for the prevention and control of infectious diseases. At present, vaccines for less than 30 diseases have been developed globally, and effective vaccines have not yet been developed for many deadly diseases, and some existing vaccines have not achieved the expected results. The rapid development of effective vaccines is the best opportunity to limit the spread and damage of emerging infectious threats. The Medicines for Neglected Diseases initiative (DNDi) states that new drug/vaccine development must be an integral part of climate adaptation efforts. To better address the threat of infectious diseases, all countries should provide a robust policy environment for vaccine development and deployment. In 2022, the United States launched the National Biodefense Strategy and Implementation Plan, which will invest $88 billion in the next five years to fund research on infectious diseases and other projects, improve innovative vaccines and therapeutics, and so on. Japan established the Strategic Center for Advanced Vaccine R&D in Biomedicine (SCARDA) and committed to invest US$2 billion in pandemic vaccine research, with broad-spectrum universal vaccines being the current R&D focus. In 2021, the Coalition for Epidemic Preparedness Innovations (CEPI) launched a $3.5 billion plan to reduce or eliminate the risk of future infectious diseases and epidemics, setting six strategic goals for vaccine research and development, of which $200 million will be spent on developing universal vaccines against multiple coronavirus strains such as SARS, MERS, and SARS-CoV-2. In addition, governments should scale up public education and vaccination campaigns, especially in historically marginalized countries and regions, to help combat misinformation and vaccine hesitancy around vaccines.
Third, promote equal health. The international community and governments should play a greater role in working to ensure equitable access to health care, especially in areas where infectious disease outbreaks are hotspots. The impact of climate change on countries is unequal, with low- and middle-income countries often being more vulnerable to climate change and natural disasters, while their role in advancing the climate crisis is much lower than that of high-income countries. The WHO Global Survey on Health and Climate Change 2021 notes that more and more countries are developing national health and climate change plans or strategies, but the biggest obstacle to implementing health and climate plans in about 70% of countries is insufficient funding. With support from institutions such as WHO, the G20 and the World Bank, the Lancet Commission on COVID-19 recommends increasing effective investments in communicable disease preparedness and health systems in developing countries, with a focus on primary care, achieving universal health coverage and broader disease control. In addition, WHO's Global Vaccine Market Report 2022 shows that limited supply and uneven distribution of vaccines are widening health disparities in high- and lower-middle-income countries. The inequitable distribution of vaccines is reflected in many key vaccines for communicable and non-communicable diseases. The free market is depriving some of the world's poorest and most vulnerable people of their right to vaccines, so WHO is calling on governments, manufacturers and partners to take steps to reform the global vaccine market to ensure equitable access to vaccines for all.
Fourth, reform global cooperation. Global cooperation is an important part of the response to pandemics and infectious diseases, and the integration of sectors and research disciplines such as agriculture and livestock, veterinary medicine, and environmental pollution is necessary to address future global health threats. Mitigating the impact of future health threats, addressing global health inequalities and achieving long-term sustainable development all depend on strengthening multilateralism, with WHO at the centre and strengthening national pandemic preparedness and health systems, with a special focus on vulnerable populations. Reassessing and strengthening global institutions and multilateralism will help not only to address current and future pandemics, but also to address any crisis with global implications. In recent years, the One Health approach to the sustainable development of human, animal and ecosystem health has received a lot of attention. In October 2022, the Food and Agriculture Organization of the United Nations (FAO), the United Nations Environment Programme (UNEP), the World Health Organization (WHO) and the World Organisation for Animal Health (OIE) officially launched the One Health Joint Action Plan, which focuses on support and capacity building in six areas: One Health capacity of health systems, emerging and re-emerging zoonotic diseases, neglected tropical diseases and insect-borne diseases, food security risks, antibiotic resistance and the environment.
Fifth, reduce emissions. Agriculture is one of the main drivers of climate issues, currently accounting for 19%-29% of total greenhouse gas emissions. Under current policies, the global average temperature is expected to rise by 2.5°C above pre-industrial levels by 2100, well above the 1.5°C target set by the Paris Agreement. Therefore, tackling agricultural carbon emissions and food waste is critical to helping meet climate goals and reduce environmental pressures. In March 2024, the Food and Agriculture Organization of the United Nations (FAO) highlighted the urgency of the transition to climate-resilient food and agricultural systems in its UN State of the Global Climate report. Climate-smart agriculture (CSA) can address the interconnected challenges of food security and climate change, reduce agriculture's ecological footprint, better support sustainable agricultural production, reduce the risk of crop failure, and protect the global food supply, while significantly reducing agriculture's environmental impact and mitigating climate change. In recent years, the World Bank Group (WBG) has been promoting climate-smart agriculture (CSA), and in its Climate Change Action Plan (2021-2025), it emphasizes working with countries to develop climate-smart agriculture to achieve a win-win situation of increased productivity, resilience, and emissions reduction, which can help achieve the Sustainable Development Goals (SDGs) in terms of climate action, poverty, and hunger, thereby reducing the risk of climate change-induced infectious disease outbreaks.
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About the Author
Zhang Ruiqing, Research Office III, Institute of International Technology and Economics, Development Research Center of the State Council
Research interests: tracking the situation in the field of biology and research on key core technologies and cutting-edge technologies
Editor丨Zheng Shi
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