Increasing temperatures may bring unprecedented risks to human health. Image credit: Jerry Zhang, via Unsplash.
On World Health Day 2008, the World Health Organisation (WHO) declared that ‘protecting human health is the “bottom line” of climate change strategies.’
In the wake of severe flooding, heatwaves, and droughts of the past few years, the world is starting to realise that climate change may be the most significant global health threat of the 21st century. Much evidence also points to the rising risk of global pandemics as the world warms. Those who are not motivated by the environmental consequences of anthropogenic warming should be moved to action by the increasing wealth of research linking climate change to increased disease risk across the globe.
Spreading disease
Communicable diseases are those which are passed from one organism to another—common examples include the flu, measles, and chickenpox. These diseases can be passed directly between infected organisms or via vectors, such as mosquitos and ticks. Vector-borne diseases currently have the biggest burden in tropical and subtropical countries due to environmental, social, and economic factors, including the warmer and wetter climate found in these areas. Under predicted climate change scenarios, the poleward migration of organisms which act as vectors and an increased number of contacts with mammals carrying novel diseases are the two main concerns regarding disease transmission.
A study in 2019 showed that by 2080, nearly one billion more people will be exposed to diseases such as malaria and dengue fever under the ‘business-as-usual’ emissions scenario.
A study in 2019 focusing on yellow fever and Asian tiger mosquitos (two species that carry common tropical viruses) showed that by 2080, nearly one billion more people will be exposed to mosquito-borne diseases, such as malaria, Zika virus, and dengue fever, under the ‘business-as-usual’ emissions scenario (i.e., the modelled situation if no concerted efforts are taken to curb our emissions). The most affected countries will be those in Europe, including the UK—where these diseases feel like a far-away threat—as average annual temperatures soar and the habitability range of mosquitos moves away from the tropics. Although the study does not consider all the factors important to mosquito survival, such as the availability of breeding sites, it demonstrates that we are heading into a future where many more people will be exposed to viruses to which they currently are not.
It is not only tropical diseases that are likely to expand their ranges in a warming world. Those familiar with the perils of walking through long grass or sitting in a field near livestock will know the horror of finding a tick buried in their skin and the urgency of removing the unwelcome guest as quickly as possible. Lyme disease is the main concern in the UK. Still, ticks in other countries can carry a multitude of other diseases, such as Crimean-Congo haemorrhagic fever (CCHF) or tick-borne encephalitis (TBE), the latter of which is associated with over a 50% chance of resulting in death or long-term neurological complications.
Although there is limited data on the long-term distribution and abundance of tick populations in most parts of the world, some research suggests that American, Canadian, and European tick species are spreading north and to higher altitudes, while exotic species are becoming established in the US and Southern Europe. Further studies also indicate that increases in ambient temperature may reduce the transmission processing time for the pathogen in infected ticks. As a result, the urgency of removing ticks as soon as they bite is even more significant, as pathogens need a lot shorter time to transfer from the tick into the bloodstream, and the risk of infection is much greater.
Pandemics
No one wants to be reminded of the state of the world in 2020 at the height of the COVID-19 pandemic. Despite predictions of an upcoming pandemic, healthcare systems and societies were not prepared for such an event. The virus was thought to have been transferred from bats to humans, perhaps by a pangolin as an intermediatory. A similar sequence of events is thought to have caused the Middle East Respiratory Syndrome outbreak in Saudi Arabia in 2012 (from dromedary camels) and the SARS-CoV epidemic in Guangdong in 2002, which spilled over from palm civets (a fairly innocuous-looking and quite cute relative of the mongoose).
Zoonotic diseases—those passed from a vertebrate animal to a human—account for two-thirds of infectious diseases and 75% of newly emerging diseases in humans.
Zoonotic diseases—those passed from a vertebrate animal to a human—account for two-thirds of infectious diseases and 75% of newly emerging diseases in humans. Spillovers occur when harmful pathogens are transferred from animals to humans, often during first encounters, acting as a platform for viruses to evolve. When the disease in question has severe symptoms and high mortality, there is less chance of the disease spreading, limiting outbreaks to local areas. However, as in the case of COVID-19, less severe symptoms, such as a slight cough or headache, allow people to continue with their daily lives, transferring the virus to those they come in contact with and turning a localised epidemic into a global pandemic.
An estimated 1.7 million viruses from mammalian and avian populations have yet to spill over to humans. Zoonoses—diseases transferred from animals to humans—will rise as species migrate to follow shifts in climate, increasing the risk of another pandemic. A 2022 study of viral transfer between mammals and humans showed that even if we meet the targets of the Paris Agreement, which aims to keep global warming under 1.5°C, over 300,000 first encounters will likely occur, with at least 15,000 cross-species transmission events. Accelerating our action to tackle climate change would do little to mitigate the risk. The damage has already been done. The only way to prevent the next pandemic is to step up the global monitoring of emerging viruses to stop the spread of disease in its tracks.
Nature’s defence
Climate change and biodiversity loss may be destroying the protection that nature itself gives us against disease. So-called “ecosystem services” are direct and indirect contributions that ecosystems provide for human wellbeing and are often used to evaluate the value of nature. Many of these services are linked to protecting human health; biodiversity has a role in both infectious disease transmission and biopharmaceuticals, and many organisms naturally filter microbes out of the air and water we breathe and drink.
Loss of biodiversity can amplify disease transmission. Although unintuitive, the loss disproportionality affects non-vector species. Thus, decreasing biodiversity can increase host species’ density and disease transmission. This pattern has been found across passerine birds, which act as a host for the mosquito-transmitted West Nile virus, and for hantavirus pulmonary syndrome, a high-mortality virus carried by rodents. As ecosystems become more dominated by vector species, more infected individuals can interact, thereby promoting the spread of viruses.
Climate change and biodiversity loss may be destroying the protection that nature itself gives us against disease.
Another study carried out across four islands of the Line Islands, which stretch across the Equator just over 1000 miles southwest of Honolulu, highlights the impact that human populations have on the ability of the natural ecosystem to minimise disease risk. The four islands, Kingman, Teraina, Tabuaeran, and Kiritimati, share oceanographic and climate conditions, as well as flora and fauna. The only difference between them is human habitation and, therefore, the amount of fishing around the island. Kingman is completely unoccupied, with relatively undisturbed reefs dominated by sharks and corals. The human population on the islands increases to around 5,100 people on Kiritimati. The microbe abundance relative to human presence increases 10-fold from Kingman to Kiritimati.
As the abundance of these microbes changes, so too do their roles. Photosynthetic organisms such as Prochlorococcus dominate the waters of Kingman, while pathogens such as Staphylococcus and, more worryingly, Vibrio (a group of bacteria that cause diseases such as cholera and septicaemia), become more dominant in the turbid waters of the more populated islands. Vibrio is also responsible for disease in corals, and its increase may be linked to the observed shift from coral-dominated to seaweed-dominated reefs.
Alongside the decline in corals is a reduction in the giant clam population, which experiments show actively filters Vibrio from the water. Without the natural filtering capacities of a healthy ecosystem, the risk to human health grows. As Enric Sala, a conservationist leading the National Geographic expedition to the Line Islands, puts it, ‘even if it is just for selfish reasons—for our own survival—now more than ever, we need the wild’.
The increasing toll of extreme weather
It is not only communicable diseases that are a health concern as the climate changes. Increased food insecurity and extreme weather events will lead to a rise in non-communicable diseases, which comprise all illnesses not spread from person to person, such as mental health disorders, diabetes, and cardiovascular disease.
Extreme weather events are at the heart of many of the problems associated with climate change and human health. We have seen the death toll from heatwaves in Europe throughout the summer of 2022, floods in Pakistan in the same year, and the loss of livelihoods worldwide due to wildfires. Many studies have shown that these events also had mental health impacts—in the wake of such disasters, women are more likely to face domestic violence and sexual assault, while the risk of suicide increases for men.
Events like these are devastating regarding food security, especially in developing countries where climate change may outweigh any positive effects of economic development. This has profound implications for child malnourishment. Research in sub-Saharan Africa showed that children born in years of lower crop yield are significantly less likely to survive until their fifth birthday than children born in years of higher crop yield. Even if farmers can adjust to a changing climate by altering the crops they grow, research has found that there may be longer-term impacts of climate change on the food system. Shifts in temperature can affect the physiology of crops and animals, and some studies have suggested that higher atmospheric CO2 may change the micronutrient content in crops.
Two birds with one stone—tying together climate & health targets
In developed countries, tackling climate change can go hand-in-hand with reducing risks to human health. Think of, for example, conditions linked with both air pollution and obesity, such as asthma, lung disease, diabetes, and high cholesterol. The Intergovernmental Panel on Climate Change (IPCC) report estimates that 21–37% of total greenhouse gas emissions come from the food system, of which a significant proportion comes from the meat and dairy industries. By encouraging populations to reduce their consumption of these products, especially red meat, we can reduce emissions while also mitigating diet-related risk factors associated with the overconsumption of these goods.
By encouraging populations to reduce their consumption of meat and dairy, we can reduce emissions while also mitigating diet-related risk factors.
Air pollution is another significant worldwide health concern, accounting for around seven million deaths yearly. Efforts to phase out fossil fuels, and encourage active travel (such as walking and cycling) or the use of public transport, can reduce air pollution and the risk to human health. A model considering the nine countries which represent more than 70% of global emissions and half of the worldwide population shows that aligning public health and climate change agendas could save around 10 million lives across the countries under consideration by 2040.
Keeping global warming within the 1.5°C target and protecting biodiversity is, albeit selfishly, in our best interest. Ten million is not a number to be scoffed at. When the lives taken by the COVID-19 pandemic—just under 7 million as of 14 June 2023—are also taken into consideration, along with the lives lost in epidemics and extreme weather events across the globe, it is clear that taking action on climate change is not just to the benefit of our planet, but also to humankind.
