Stop the Spread, Curb Climate Change

During the height of the COVID-19 pandemic in June 2020, many began to speculate whether the pandemic would end during the summer. Higher temperatures and stronger ultraviolet radiation were purported to kill the virus and therefore limit its transmissibility. While COVID-19 cases have been shown to decrease with proximity to the equator, cases remained high enough after the summer of 2020 that many countries remained in different levels of lockdown.

Evidently, heat did not end the COVID-19 pandemic. In fact, while heat may limit the spread of some viruses, the notion that heat is an adverse factor in all disease transmission dismisses the many ways that pathogens, and their vectors (host organisms), can benefit from higher temperatures.

In August of this year, a group of researchers in the United States found that warming temperatures, and the environmental consequences of such climate change, could aggravate 58 percent of infectious diseases affecting humans, with the greatest impact on viral and bacterial diseases. Higher temperatures were directly implicated with just a portion of these pathologies, while events resulting from global warming, such as floods and droughts, aggravated many more diseases. In contrast, only 16 percent of human pathologies were found to exhibit lower transmission with warming temperatures.

One way that rising temperatures can benefit diseases is by causing climate events that allow pathogens to emerge, or reemerge, and come into contact with humans. Habitat destruction caused by droughts, wildfires, or floods could cause vectors hosting novel pathogens to migrate to areas that are in closer proximity to human populations. These events can directly affect pathogens, rather than only their vectors, as well. University of London honorary research fellow Michael Gross reported in January of 2019 that a 2016 permafrost thaw in the Arctic Yamal Peninsula may have unfrozen strains of Bacillus anthracis, the bacteria responsible for causing anthrax. The resulting anthrax outbreak killed hundreds of reindeer and infected dozens of humans in the region.

Global warming could also allow diseases that already infect humans to spread to more regions of the world and for longer periods of time by providing more locations and seasons with optimal temperatures for diseases and vectors. Diseases that were previously confined to the tropics, for example, could spread much further from the equator if other regions also reach temperatures that are high enough to harbor the diseases and/or their vectors. Rising temperatures can also increase the population and activity level of vectors by increasing their metabolic needs, decreasing their pathogenic incubation period, and accelerating their development. This heat-induced population growth and greater activity are demonstrated in mosquitoes, which transmit diseases to over 700 million people every year. In January of 2021, researchers found that 1.3 billion new people, particularly populations in North America and Europe, could be exposed to the mosquito-borne Zika virus by the year 2050. Populations in tropical regions that are already experiencing warmer temperatures and higher frequencies of disease outbreaks will also be the ones most economically affected by climate change due to their dependence on agriculture, thus impacting these populations’ abilities to build hospital infrastructure and mitigate outbreaks. Many diseases that currently exhibit heightened transmission during the summertime may also be able to spread with such severity year-long if temperatures in the wintertime are high enough.

So while COVID-19 is limited by warming temperatures, the majority of diseases, many of which are as severe or as transmissible as COVID-19, are not. We cannot simply hope that pandemics more extreme than COVID-19 will not arise, especially as we make the planet warm enough to harbor them.