As seasonal patterns continue to shift swiftly across the globe, scientists have found that warmer summers and moderate winters are making it easier for carriers of pathogenic diseases to spread to new areas and sharply increase the number of people at risk.
The scale of the issue makes it a matter of deep concern. One analysis, published last year in Nature Climate Change, has found that nearly half of known pathogenic diseases in humans may see an increase due to climate-related reasons. The researchers have identified several possible causes. Firstly, disease-carrying species, such as mosquitos, ticks, fleas, birds, and some mammals, are expanding their geographical range as a result of global warming and changes in rainfall patterns, which will increase the transmission of dengue fever, Lyme disease, plague, West Nile virus, Zika, and malaria, among others.
Secondly, drastic weather conditions are also bringing people closer to the very sources of pathogens; for instance, heatwaves may lead a greater number of people to partake in water-related activities, which has been linked with the increasing prevalence of waterborne diseases. Moreover, changes in the use of land, often due to rising temperatures and unfavorable rainfall patterns, is also facilitating human and livestock encroachment into wild areas, bringing them into greater contact with pathogens.
Thirdly, in addition to making it easier for pathogens to spread, increasing global temperatures have also created favorable conditions for various pathogens to reproduce faster and heightened their ability to cause severe disease. For instance, it was observed that mosquitos (responsible for the transmission of a variety of pathogenic diseases globally) experienced enhanced population growth and survival, greater replication of pathogenic viruses, and higher biting rates as a result of increasing temperatures.
Fourthly, climate change, via its impacts on the food supply, infrastructure, and major changes in our environment, is impairing our immune systems, making us more susceptible to pathogenic diseases. Higher CO2 concentration in the atmosphere reduces the concentration of nutrition in crops, leading to malnutrition. The weakening of our immune systems is tied to a variety of reasons, such as drinking from unsafe water sources due to floods and droughts, higher stress levels as a result of loss suffered due to climatic events, as well as damage to critical sanitation and healthcare infrastructure.
According to the US Centers for Disease Control and Prevention, fungal diseases such as Valley Fever are now able to spread to regions that were previously too cold for them to survive. Other fungi, which previously could only survive at cooler temperatures, seem to be adapting to higher temperatures. Moreover, various fungi have also been developing resistance to fungicides as a consequence of their widespread use. All of this means that we are likely to see a much larger range of fungi-related infections which will be more resistant to antifungal medications in the future.
Harmful algal blooms (HABs) are caused by huge growths of algae in water bodies. They produce toxins and sharply reduce the oxygen content of the water, killing fish and other marine life, and also causing human illnesses. According to the US Environmental Protection Agency, HABs are likely to occur in summers when water temperatures are warmer, which makes it easier for them to bloom, grow, and move. They also absorb heat and make the surrounding water even warmer. Cycles of floods and droughts and change in rainfall patterns causes the water to have a higher salt and nutrient content than usual, and increasing CO2 content in both the air and water also leads to the rapid growth of HABs.
Malaria, dengue fever, Zika, and numerous other diseases cause millions of deaths annually and inflict enormous human suffering and economic losses. For reference, the COVID-19 pandemic has been estimated to have cost nearly $16 trillion in the US alone. According to another study published last year in the reputed medical journal The Lancet, 33 bacterial pathogens alone led to an estimated 7.7 million deaths globally in 2019. Warming temperatures, changes in rainfall patterns, and more frequent floods and droughts have the potential to magnify these diseases to a catastrophic level.
Outside the medical and research communities, there seems to be little understanding or concern for these infectious diseases. Yet, this issue may prove to be devastating to densely populated regions of the developing world, which are predicted to be the most severely affected by climate change, and where people are already at significantly higher risks of poor nutrition, high pollution, lack of adequate infrastructure and funds for sanitation and healthcare.