COVID-19 is a slowly unfolding catastrophe that poses a serious threat to humanity worldwide. Unlike a tornado, hurricane, or even flood, it is hard to say when the event will end. Unlike heat waves and droughts, it is likely not being enhanced by climate change just yet. But, this latest pandemic has prompted me to think about how climate, if not climate change, may affect the outcome and future events.
Historically, climate has certainly influenced the formation and spread of disease. A couple of significant pandemics come to mind.
When Indonesia’s Mt. Tambora erupted in 1816, the quantity of volcanic ash and sulfate aerosols injected into the stratosphere was so great that it caused a global veil around the entire planet for the next two years that lowered temperatures by as much as 3oC. The global drop in temperatures caused abrupt changes to weather patterns worldwide and even changed the Indian Monsoon.
Drought followed by unseasonal flooding altered the microbial ecology of the Bay of Bengal. The cholera bacterium, which has an unusually adaptive genetic structure highly sensitive to changes in its aquatic environment, mutated into a new strain. This was met with no resistance among the local population and spread across Asia.
Unlike COVID-19, Vibrio Cholerea is spread from contaminated food and water. Seafood shipped long distances as well as increasing migration and commerce around the world helped to transmit the disease globally. By 1824, the death toll from Bengal cholera was easily several hundred thousand. Other outbreaks followed later in the century, but better sanitation practices and the use of penicillin have truly minimized modern day cholera epidemics.
1918 Influenza Pandemic
In 1918 an influenza pandemic broke out, sometimes popularly called “the Spanish Flu.” Despite what it was dubbed, the disease may have originated in Kansas, as the virus jumped from infected fowl to farmers who soon found themselves training for war with more than 50,000 others at Camp Funston, Kansas. The winter of 1917-18 was the coldest on record east of the Rockies and as troops were training and preparing to be shipped overseas to Europe, the U.S. Army ordered the men to huddle together near stoves and sleep packed in tents to stay warm.
Thousands were infected within weeks and then sent overseas where harsh wartime conditions, continued close proximity, and further relocation accelerated the spread. Tens of millions died; in Spain alone the death toll was more than 8 million in May.
Then a second wave began in the U.S. in early autumn, sooner than a typical flu season. Unusually cold conditions, especially in the Northeast, did not help the situation in Philadelphia, where a parade was held on September 28 to promote the sale of War Bonds. Within days the city was on the verge of functional collapse.
Nearly 12,000 Philadelphians died in October alone, with 759 in one day. Across the Northeast, nearly 200,000 succumbed that month, and ultimately tens of millions perished.
Climate was not the cause of the 1816-1823 cholera pandemic or the 1918 influenza pandemic, but it did not help. Today, with better public health practices such as social distancing, frequent hand washing, and faster development of vaccines we can certainly reduce the spread of future diseases. But climate change and growing extremes will likely increase the threat.
From changing bird migration patterns to expanding mosquito domains to flood-induced increased food supply for rodents, the spread of new and potentially increasingly lethal viruses will likely continue to be influenced by climate.
Interannual temperature impacts on flu are important too—historically, and maybe more so in the future. It turns out that warm winters followed by cold winters lead to more significant flu outbreaks. The connection is that a flu strain does not spread as readily during mild winters, which means we may not get exposed at all and build immunity to it; during the following winter, when that strain returns, we are more likely to get infected. Such was the case during the 2012-13 flu season, which coincided with a very cold winter and followed a very mild one in 2011-12. This study looked at the effect in some detail.
The 2019-20 winter is in the top five warmest in many places around the world, but thanks to an increasingly weakening polar vortex, next year may be a cold one for some regions in the Northern Hemisphere. With the COVID-19 virus likely to still be lurking, it is possible that it will surge early in locations with the first Arctic wave that descends. And, in years to come as climate change continues and more winters are generally mild but interspersed with record cold ones, our immune systems may be caught off guard.