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After the deluge — cascading effects of extreme weather on human health » Yale Climate Connections


News coverage of this summer’s devastating flood in Pakistan has peaked, but the deluge left behind hasn’t subsided: Experts predict the floodwaters could take six months to fully recede.

An arial view of Shahdadkot, Khairpur Nathan Shah, Mado, Faridabad, Mehar and other cities of Sindh, Pakistan covered with flood water in 2022. (Photo credit: Ali Hyder Junejo / CC BY 2.0)

The initial damage was devastating. More than 1,500 people died — about half of them children — when record rainfalls and melting glaciers caused catastrophic flooding during the 2022 monsoon season.

But the flooding’s human impacts will be far more long-lasting. Eight million people are still displaced, and Pakistan now faces ongoing threats to lives and livelihoods — the floods affected 15% of the country’s rice crop and 40% of its cotton crop.

Climate scientists agree a rapidly warming atmosphere will generate more intense and frequent weather disasters. These disasters cause immediate death, injury, or homelessness, but their effects on human health and well-being often persist long after the skies clear, floods recede, or fires are extinguished.

A new study from climate and health researchers Jan C. Semenza, Joacim Rocklov, and Kristi L. Ebi in the journal Infectious Diseases and Therapy explains how climate events often can have “cascading effects” on human health, stemming from altered environmental conditions or disruptions to infrastructure.

Consider an extreme storm, which first generates heavy rainfall and can produce runoff contaminated by harmful toxins or sewage.Once settled, the runoff can contaminate croplands causing food shortages; or produce standing water, which in turn serves as a breeding ground for mosquitoes that can be vectors of disease. Disruptions to pest control programs may also lead to more spreading of diseases. These events compound particular harms from the initial devastation.

The risks increase for those living in low-income countries already lacking adequate food, water, or hygiene systems prior to experiencing a climate disaster. But many communities in wealthy countries like the United States are also particularly at risk because of a history of racial discrimination or underinvestment.

As the study authors point out, decision-makers from local emergency response teams to federal agency officials must understand how these risks are interrelated so they can invest in comprehensive intervention and adaptation plans to reduce human suffering and save lives.

Disrupted water and food systems can trigger longer-lasting threats

To identify how climate-related events can trigger a series of cascading risks, the study researchers reviewed more than 100 studies of extreme weather impacts on global health from the past five years. Their findings demonstrate many ways a single event can interact with existing infrastructure and social systems, generating ripple effects having widespread impacts on health. Recent climate disasters also illustrate how these “cascading effects” can directly and indirectly cause illness.

Figure : Extreme weather events can exacerbate existing hazards and societal vulnerabilities, leading to cascading impacts on human health due to contamination of water and food, and the rise of vector-borne diseases. (Source: Semenza et al. 2022.)

As they explain in their study, heavy rainfall can create land runoff that then overwhelms water treatment facilities, or flood waters can carry sediment or pathogens that contaminate drinking water wells. These risks were evident in early October after Hurricane Ian led Florida health officials to warn affected residents to boil water from the tap. Those warnings came assome 760,000 residents of hardest-hit Lee Country had no access to water at all after the storm badly damaged the water system.

Historic flooding this past summer overwhelmed the water system in the majority Black city of Jackson, Mississippi, leaving residents without clean water for more than a month. Decades of neglect and underinvestment made the area’s water treatment facilities especially vulnerable to this kind of extreme event. Mississippi already has the lowest life expectancy in the U.S., and area physicians have noted their patients with chronic illnesses are at higher risk of death from unsafe water.

Contaminated floodwaters can also trigger outbreaks of deadly cholera or other bacterial illnesses, especially in areas where the deadly bacteria is already present. Nigeria recently experienced just this situation after a flood hit some of the country’s poorest states in the northeastern part of the country.  Contaminated waters then led to a cholera outbreak that killed 64 people.

Drought can also set off cascading, and sometimes totally unforeseen, effects on health. The Semenza study points to the 2016 Zika outbreaks in Latin America that are thought to have come from people storing water in open containers in their homes during a period of drought, serving as suitable breeding sites for mosquitoes carrying the disease.

Compounding the challenges, extreme events can impede access to hygienic facilities, or what public health professionals label “WASH” – Water, Sanitation, And Hygiene –  helping to spread illnesses. In 2005, after Hurricane Katrina, a norovirus outbreak occurred after evacuees were living in close contact at evacuation centers without access to proper hygiene. In the case of a cholera or other bacterial outbreaks from contaminated water, the lack of WASH can perpetuate the cycle of infection.

Climate events also threaten access to healthy food supplies. Short-term effects include food spoilage from power outages or supply chain interruptions, but they can be overshadowed by much longer lasting vulnerabilities.

Inspecting damage to corn crops caused by fall armyworm, a pest that can devastate staple crops. Changing climate conditions are causing increased outbreaks of the pest around the world. (Image source: Buteme et al. 2020)

Crops can be contaminated by bacteria or parasites, for instance, making fruits or vegetables unsuitable for human consumption. And entire crops might fail because of  drought or flood, as is occurring in Pakistan. Though an event may be local, crop failures can lead to global food shortages and price spikes, with widespread effects on food security

Underlying climate change conditions aggravate all these challenges by providing more hospitable conditions for bacteria, decreasing the nutritional value of crops, allowing crop pests to proliferate, and increasing the spread of disease-spreading insects into new parts of the world. As the above examples illustrate, myriad other alterations affect risk, and climate change’s health effects do not affect people around the world equally — those who have historically been subject to racially-motivated discrimination or colonialism are already bearing a disproportionate impact.

Breaking through silos can lessen threats, foster resilience

So what can be done? Authors of the cascading risk study say identifying vulnerabilities and weaknesses in public health or other social systems is a necessary first step for developing beneficial adaptation and intervention strategies. Anticipating the potential cascading failures before extreme weather events can help public health and disaster response agencies better plan form prevent, and reduce harm.

The authors urge collaboration across various agencies and planning departments to avoid the kinds of silos that lead to isolated solutions that can increase risk. For instance, a traditional mosquito control program includes treatment, bed nets, and vector control. But in an era of climate change, such a program should also engage land-use managers to consider how storms or floods that affect wetlands or standing water could contribute to an outbreak, and housing authorities and city planners can be involved to design more resilient residences.

The World Health Organization recently created guidance for how communities can develop water safety plans that account for a range of flood risks. This guidance showcases tactics water managers can use to diminish water quality impacts directly or indirectly responsible for illness from contaminated waters. For example, managers can adapt storage systems to keep water cooler or adjust disinfection protocols to reduce pathogens that cause disease. Adaptation planners can also collaborate with farmers, providing incentives for them to change how fertilizers are applied to minimize runoff near water collection areas.

Climate impacts vary by region, just as health systems vary dramatically depending on local populations, politics, and governance, so interventions will need to be designed to reflect local cultural and socioeconomic conditions. International coordination is also important for identifying when threats may affect regional and global food or water systems and planning collective responses.

Getting the solutions right will rely in part on addressing current information gaps, since climate and public health data are disproportionately collected in high-income countries rather than in low- and middle-income countries, which experience more harmful climate and health impacts.

Meanwhile, long-standing injustices must be rectified both within borders and beyond. The World Bank estimates resilient reconstruction in Pakistan will cost at least $16 billion dollars, not including new investments to support climate adaptation to future flooding events, though such calls for international aid have fallen on deaf ears. In Jackson, Mississippi, the U.S. Environmental Protection Agency has launched a civil rights investigation into the water crisis.

As the largest historical emitters, the wealthiest countries have a responsibility to help lower-income countries adapt to climate change with financial support for resilient and equitable health, water, food, and housing infrastructure. These countries must also ensure that their own domestic climate investments confront and address decades of underinvestment in their communities most impacted by pollution and climate change.


Emily Jack-Scott is Program Director, Aspen Global Change Institute, and Sarah Spengeman is Deputy Director of Communications, Energy Innovation. Both organizations are content-sharing partners with Yale Climate Connections.



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