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How will climate change impact human health?

By: Peter Sousounis

Climate change has emerged into a defining issue of the 21st century. But it’s no secret that extreme weather and climate events have been responsible for a staggering number of fatalities and disruptions for quite some time. In 1931, one flood in China alone claimed almost 4 million lives.1 A 1970 cyclone in Bangladesh completely submerged low-lying islands that were just offshore, leading to approximately 500,000 deaths.2

These are just a couple of the calamitous weather events that have directly contributed to the deaths of millions and upended the lives of so many more. Sadly, major storms and disruptive droughts may just be the tip of the iceberg when it comes to how climate change can threaten our society. Indirectly, shifts in climate patterns can also cause other processes that can be a detriment to human health.

If the fears of many in the scientific community are realized, more droughts and floods will very likely have a catastrophic impact. We’ll touch on some of those risks but also steer attention to how climate can serve as a trigger for the transmission of disease, among other health impacts.

Direct effects

Drought: No rain means no food

Drought is a surreptitious and effective killer. According to The International Disaster Database (EM-DAT), drought is the leading cause of climate-related fatalities worldwide over the last five decades, averaging 14,000+ deaths/year, resulting primarily from starvation and dehydration.3 Although these numbers have declined during the last couple of decades, this dip may be temporary. The Intergovernmental Panel on Climate Change (IPCC) indicates that areas of the world currently prone to droughts now will likely experience worse and longer droughts in the years ahead.4

What could be the cause? The sub-tropical belts of high pressure that shoulder the equatorial region in both hemispheres are expected to expand poleward into areas that still get some rainfall, which could create desertification. The danger is not that this will happen slowly and steadily, but that it will happen sporadically—from extreme drought episodes possibly spawned from increasingly strong El Niños.5 6

Heat waves: Nighttime high temperatures are killers

The World Meteorological Organization (WMO) defines a heatwave as at least five consecutive days with daily high temperatures 41o F/5o C above normal or more.7 The daily high temperatures are not the problem per se; what plays a big role is apparent temperature (heat index), which factors in relative humidity. Hence, the ability for the human body to cool by evaporation of perspiration also plays an important role in a heatwave.

In addition, overnight temperatures are an important factor, as low overnight temperatures allow the body to recover from the day’s heat stress. Because overnight low temperatures have been increasing even more than daytime temperatures have in many locations, it has reduced the ability to cool down and recover overnight.8 The warmer nights result from an increase in humidity, which inhibits the radiating of heat to space. Populations in locations without appropriate infrastructure, such as adequate air conditioning or proper ventilation, often suffer most.

High-temperature extremes have obvious health impacts; based on EM-DAT data from the last 50 years, an estimated 3,600+ people die each year due to excess heat.9 Although excess heat death totals lag behind those caused by floods, storms, and droughts, the number can be considerably larger in any given year. This was the case in 2003 when more than 70,000 people died from a heatwave in Europe.10 Attribution studies immediately following that event concluded that anthropogenic global warming had doubled the likelihood of such an extreme event. In 2010, more than 50,000 people died in Russia from excess heat.11

Floods: Extreme events will likely be wetter

The scientific community has high confidence that climate change will likely lead to more heavy precipitation events, triggering property damage, deaths, and upheavals of life. As air temperature increases, so will the amount of water vapor in the atmosphere. More precipitation will likely fall just from the added warmth —about 7 percent for each degree Celsius the global temperature increases.12

This increase in rainfall is being compounded by the emerging trend of storms getting stronger and moving more slowly. Some of the strongest tropical cyclones ever recorded have occurred during the 2010-2019 decade—Haiyan 2013, Patricia 2014, Maria 2017, Michael 2018, Dorian 2019, for example. Recent studies also show a global slowdown in forward speed, as Harvey 2017 and Florence 2018 demonstrated.13

Coastal flooding, or storm surge, will also likely increase in the future from increases in cyclone intensity as well as from increases in sea level. Thermal expansion of ocean water, as well as melting polar ice, may contribute an additional meter of sea-level rise by the end of the 21st century,14 which may all but eliminate islands like Micronesia, The Maldives, and the Solomon Islands, and certainly extend flooding across many already susceptible coastal regions like Southern Florida, Bangladesh, and the Pearl River Delta in China.

The direct effects of climate change should be clear as day. But climate change also poses indirect threats that may be much easier to overlook.

Indirect effects

Worsening air quality may increase respiratory problems

While there is a clear link between how oppressive heat and more severe flooding will likely lead to significant property damage and fatalities, a less obvious–but perhaps just as significant–downstream effect could be a greater frequency of severe respiratory illnesses.

Rising atmospheric temperatures and related meteorological effects can worsen ground-level pollution—most notably ozone—leading to diminished lung function, increased health care utilization, and premature death.15 Individuals with COPD, CVD, diabetes, and chronic ozone exposures are at particular risk.

Increased pollution is expected to also emerge as a result of more wildfires that stem from more intense droughts and heatwaves. The 2019-20 bushfires across portions of Australia, and particularly near Sydney, created a significant air pollution episode, for example. Smoke from the bushfires was 12 times more deadly than the fires themselves.16

Longer growing seasons resulting from a more temperate climate in many locations will increase aero-allergenic plant pollen (tree, grass, weed) production and distribution as well, leading to more allergies and asthma attacks.17

Finally, increased occurrences of heavy rainfall will likely lead to more floods and persistent dampness in homes, which can promote microbial growth, particularly molds. High outdoor mold ratios were observed in the months following Hurricanes Rita and Katrina in 2005, for example, indicating the potential for high indoor exposures.18 Mold levels may also increase due to the increased temperatures and elevated carbon dioxide concentrations from climate change that encourage growth.

Water quality will likely be compromised

Citizens of developed countries often take clean water for granted, but nearly 2 billion people (about one-third of the world’s population) can not.19 Flooding rains can compromise sewage systems, leading to increased cholera outbreaks. Diseases from contaminated water are the second leading cause of death in children under five worldwide.20

The 1816 eruption of Mt. Tambora in Indonesia provides an example of how climate can affect the spread of disease. The explosion injected so much volcanic ash and sulfate aerosols into the stratosphere that it changed global weather patterns, including the Indian Monsoon, for the next two years. 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, which first affected the local population then spread across Asia. 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 this strain of cholera was easily several hundred thousand. Other outbreaks followed later in the century, but better sanitation practices and the use of penicillin have somewhat contained modern-day cholera epidemics.21

Cholera, a bacterium, continues to evolve; we are still under the threat of El Tor, a particularly lethal strain of Vibrio cholerae for the last 50 years.22 The next 50 years could increase the threat from other strains as temperatures continue to rise, and heavy precipitation and coastal flooding events interspersed with droughts provide favourable conditions for spread.

Mosquito populations will likely increase

Increasing temperatures, moisture, and heavy rainfall events will likely all contribute to environmental conditions that are expected to spur the growth of mosquito populations and other disease-carrying vectors. The diseases that mosquitos carry (both viral and bacterial) and transfer to humans results in millions of deaths each year.23 A recent study noted that almost all of the world's human population could be exposed to disease-carrying mosquitos at some point in the next 50 years, given expanded areas of year-round transmission in some areas and seasonal infections in almost all others.24 And the diseases mosquitos may carry will likely not be just the vector-borne diseases we know about today, such as malaria, West Nile virus, yellow fever, dengue fever, eastern equine encephalitis (EEE), and Zika. There could be more of these diseases—from mutations of the ones we know about; from mosquitoes biting new animal species that can also migrate poleward because of climate change; and from more mosquitos biting more people. In addition, other insects such as ticks (carriers of Lyme disease) will also likely spread over larger geographical regions.

Disturbing nature’s balance: Reduced biodiversity and changing migratory patterns

The poleward expansion of a tropical climate is also causing the migration of larger animal species. Although there is limited data on observed impacts, a deep understanding of why these migrations will likely occur does exist. Other anthropogenic activities, such as deforestation, are expected to disrupt ecosystems by exacerbating the displacement of existing disease-carrying wildlife potentially towards more urban locations. Disrupted ecosystems tend to lose their biggest predators first—reducing biodiversity, which facilitates disease transmission from animal to animal and from animal to human. What these predators often leave behind are smaller creatures that have short lifespans, reproduce early and in large numbers, and have immune systems more capable of carrying disease without dying from it, and that thrive near people.25

A changing climate could also influence bird migration patterns and potentially, the locations where viruses get transmitted. The El-Niño Southern Oscillation that affects precipitation and temperature patterns also disrupts wind patterns that make the tropical Pacific a two-way street for disease transmission. Studies have shown that for each of the last four significant pre-COVID-19 pandemics: The 1918 Flu, the 1957 Flu, the 1968 Flu, and the 2009 H1N1, a significant La Niña was present beforehand, which typically evolved into a moderate-to-strong El Niño that coincided with the pandemic’s outbreak. Some of the El Niño events also coincided with anomalous temperature and/or precipitation that could explain the enhanced transmission. Still, most of the studies that discuss pandemics and the El Niño/Southern Oscillation (ENSO) phenomenon note the connection to the onset of the pandemic.26

These studies explain a possible causality in terms of changes in bird health and or migration habits and patterns that facilitate viral reassortment, essentially making a new virus. It is worth noting that two very strong ENSO events (1998-99 and 2015-16) did not coincide with pandemics, although the order was flipped (e.g., El Niño first, La Niña second).27

Viruses coming out of the cold

Another way that a climate change–disrupted environment can impact virus transmission may come from the thawing of a live virus from the deep past. The Arctic is warming two-to-three times as fast as the tropics—a phenomenon known as Arctic amplification.28 As the polar ice melts, not only is Earth’s reflectivity changing so that even more heat can be absorbed in the polar regions, but this melting will also likely lead to the release of glacial microbes and viruses. In 2015, a group of scientists worked on samples of ice from the Tibetan Plateau of China to study ice core microbiological communities and how to use them to understand past climatic and environmental conditions archived in the glaciers. They found that for the past 15,000 years, the glacier has hosted 33 groups of virus genera in the ice cores. Of those groups, 28 are unknown to modern science, so their potential threat is unquantifiable.29

Future studies will likely provide a better understanding of microbial and viral evolution and interactions and contribute to establishing predictive ecological models of past climate changes from such “frozen archive” environments. Ultimately, the most proactive and effective solution remains protecting the integrity of glaciers and snow cover.

A warning that must not be ignored

Rising sea levels, increasingly powerful and slow-moving storms, more pervasive heat, and extended droughts are some of the major climate change risks that tend to stand out. These events produce poignant images broadcast around the world, as well as unmistakable economic and societal damages that are clearly illustrated in the aftermath of the event.

Less obvious risks – but perhaps equally as threatening – can emerge through the interconnected web of interactions that occur on a daily basis. While a Category 5 hurricane typically wreaks havoc in a concentrated area, the overall damage it inflicts has proven to pale in comparison to an encounter at an obscure wet market, an event that may have jumpstarted the COVID-19 pandemic.

Worsened air quality, disrupted ecosystems, exacerbation of an already growing water issue, and other subtle impacts produced by a gradually changing climate must not be ignored and should serve as another motivating factor to strive for societal changes that can minimize climate change.

Peter Sousounis, Ph.D., is vice president, director of climate change research, research and modeling division, AIR Worldwide at Verisk. Peter can be reached at

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