Spring through summer 2023 has been a challenging time for Peru. As reported in Reuters Health Information on 9th June, the country has been experiencing a combination of unusually wet weather and the largest outbreak on record of Dengue Fever. Peruvian President Dina Boluarte has signed a decree declaring a state of emergency for much of the country.
This combination of misfortunes is likely no coincidence. You see, dengue fever is a disease known to be spread by mosquitoes, which breed more rapidly when there is an excess of rainfall. What’s more, the high rainfall very likely has its origins in the waters to the west of Peru, where sea surface temperatures have been unusually high in recent months.
It’s a trait associated with a phenomenon called El Niño and correlates with not just increased rainfall into Peru (via more evaporation of the sea surface), but higher mean temperatures there too. That’s another condition that hastens the mosquito breeding cycle.
El Niño, which is associated with anomalously warm waters in the central and eastern tropical Pacific, has a long history of driving highly unusual months, even whole seasons, of weather in various parts of the world.
For example, one such event during 1987-88 has been connected to unusually warm and wet weather in Rwanda, central-eastern Africa, during late 1988. As with Peru this year, it provided ideal conditions for mosquitos to breed, so it’s likely no coincidence that a more than threefold increase in malaria infection rate was observed shortly afterward (female mosquitoes can transmit diseases when they drink blood).
The 2023 event has been unusually focused in the eastern tropics, where temperatures have often been near record highs during recent months.
So, we have ideal conditions for mosquitos to multiply, increasing the rate of transmission to humans. Sadly, the oceanic warmth is unlikely to subside more than very gradually over the coming months.
Historically, disease outbreaks corresponding to extreme weather events have been observed all over the world, even in some of the wealthiest nations.
There has been a great deal of research on the relationship between high-end weather events and large scale disease outbreaks. Reviewing the literature in a PubMed article published in 2015, Anthony J. McMichael of The Australian National University stated that ‘Both recent and historical experiences indicate that infectious disease outbreaks very often follow extreme weather events, as microbes, vectors and reservoir animal hosts exploit the disrupted social and environmental conditions of extreme weather events’.
An earlier 2009 article by Juan Quin and Jinliang Zhang concluded that infectious disease most obviously responds to floods, warm winters, extreme heat events, and droughts.
To get a better sense of the connection, let’s delve into a few prominent historical examples. We begin hundreds of years ago, with one of the most fearsome outbreaks in recorded history.
This world-traversing pandemic has its origins in both climatic changes and extreme weather events. First, disease-carrying marmots (a type of ground-burrowing rodent) shifted population distribution toward China from Kazakhstan. There, they infected countless black rats. Then, immense river flooding struck, forcing vast numbers of rats & humans to drier locations where they crowded together.
Rat-human transfer of the disease escalated and so began one of humanity’s most devastating pandemics. Before long, it had spread along trades routes by both human & rodent hosts (as stowaways, particularly on boats), reaching Europe via the Mediterranean in 1347. It would be several centuries before the last significant flareup was recorded.
This extraordinary event inundated one fifth of the nation, as the expansive Indus River Basin burst its banks in the wake of unusually intense monsoon rains in late July. It was the worst single flooding event in Pakistan’s history, with adverse effects on over 20 million people, including near 2,000 fatalities and approx. 3,000 injuries.
With millions left homeless amidst widely damaged or destroyed infrastructure including health services, the environment was ripe for the spread of infectious diseases.
Taking refuge in what were often crowded and unhygienic settings, cholera outbreaks readily occur, often via faecal contamination of drinking water. In wet climates, it’s evident that flooding is likely the most common culprit for weather-driven outbreaks. By contrast, drought may well have the highest attribution frequency in dry climates, due to populations relying on limited water resources that tend to have a more concentrated cholera-bacterial contamination.
Sure enough, the year following the ‘mega-flood’ saw more than 37 million medical consultations reported, of which acute respiratory infection, acute diarrhoea, skin diseases, and suspected malaria, were the most common ailments.
A record-setting outbreak of 45,111 local dengue cases were recorded during May-Nov 2014 in the Guangdong province of China. During this time, both temperatures and rainfall averaged above the seasonal norm. Importantly, there was an unusually high frequency of extreme days for both hot weather and excessive rainfall.
A 2021 study by J. Cheng et al. found with high confidence (95% interval) that both extremes are associated with increased dengue incidence. The strongest correlation was found for rainfall.
As you may have inferred reading these examples, of the extreme events known to underpin historical disease outbreaks, high rainfall, with its associated flooding, is by far the most common. However, it’s apparent that high temperatures can amplify the effect via effect on insect populations.
The question now is, what lies ahead?
In his article referenced earlier, Anthony J. McMichael also neatly summarised the connection between an increasingly warm climate and a raised frequency of extreme weather events: ‘The greater the amount of heat trapped within the lower atmosphere, the greater the energy flux within the atmosphere, and the more energetic and variable the weather patterns become.’
Although to be pedantic, ‘amount of heat trapped within’ should be replaced with ‘mean temperature of’, since heat is the process of energy transferring from a higher to a lower temperature entity. Compared to decades ago, there is simply more energy in the ocean-atmosphere system, hence more moving around on average.
The consequence of this is most prominent when internal variability (e.g. weather systems) concentrates that energy into a relatively small area. A slight increase in atmospheric moisture within, say, 50 miles of a location, can become a larger localised increase if wind patterns cause the moisture to converge there. Such can happen due various phenomena, such as a low pressure system.
It follows that in a warming climate, the intensity ‘ceiling’ for extreme events will increase. On top of that, more events will reach sufficient magnitude to be deemed ‘extreme’ in the first place. This implies that, unfortunately, weather-driven infectious disease outbreaks will become more frequent.
James Peacock MSc
Head Meteorologist at MetSwift