In the summer of 2022, one of the worst monsoons on record turned swaths of Bangladesh, a low-lying country in South Asia, into huge, muddy lakes. When the brunt of the flooding finally eased, at least 141 people had died and millions of others throughout the region had been injured, impoverished, or displaced. The sheer scale of the destruction made 2022 an outlier year, but data from the past few decades signals that the historic monsoon was part of a larger trend: Climate change is making South Asia’s rainy season more intense and inconsistent. Unusually fierce floods have plagued the region earlier in the year and more often than they used to — a pattern that research shows will continue, and worsen, as the planet warms in the years ahead. 

A study published last week shows Bangladesh’s intensifying monsoons come with a staggering death toll, both in the immediate aftermath of the flooding itself and, more significantly, in the months that follow. The true scale of the toll has not been fully captured by local officials, aid organizations, or the international research community. 

The same is likely true for other parts of the world that experience recurrent climate disasters. “In the climate and health field, we often evaluate the health effects of specific acute events, because it’s easier to account for all the other potential factors that could be confounding the association,” said Lara Schwarz, an epidemiologist at University of California, San Diego, who was not involved in the study. But a focus on the short-term obscures the larger picture. “Most climate events don’t occur only once and are likely to harm vulnerable populations over and over, through years, decades, and generations,” she said. 

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A young girl gets treatment for dengue fever, a mosquito-borne illness, at Mugda Medical College and Hospital in Bangladesh in October. Munir Uz Zaman / AFP via Getty Images

In the new study, researchers from the University of California in San Diego and in San Francisco found that flooding contributed to the deaths of 152,753 infants — defined as children 11 months and younger — in Bangladesh in the three decades between 1988 and 2017. The researchers used health surveys conducted by the United States Agency for International Development to collect data on more than 150,000 births over the course of the 30 years. They compared that data against high-resolution maps of major floods over that time span and found a stark difference in mortality risk: There were 5.3 more infant deaths per 1,000 births in flood-prone areas than in non-flood-prone areas. The authors extrapolated from this finding to estimate how many infant deaths, overall, were attributable to flooding in Bangladesh over the time period they studied. 

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Infants are an especially vulnerable subset of the population, and changes in infant health can reflect the prevalence of health issues in the wider population. “Death is the most severe health outcome,” said Schwarz. “The increased risk of infant mortality suggests that populations living in a flood-prone region may also be at higher risk of other adverse health problems such as improper nutrition, water-borne diseases, and poor mental health.”

The majority of the deaths were likely linked to three flooding-related conditions. The first, diarrheal disease, often spreads when flooding overwhelms local sanitation infrastructure and causes drinking-water supplies to be contaminated. Cholera, one of the most common and deadliest water-borne bacterial diseases, is a particular concern in poor countries where sanitation infrastructure is underdeveloped. Flooding also contributes to outbreaks of mosquito-borne diseases like dengue, because standing water creates ample breeding ground for mosquitoes. Finally, flooding turns agricultural fields into bogs and can lead to massive crop losses, which contribute to existing food insecurity in Bangladesh. Babies are extremely vulnerable to hunger. The Lancet, a leading medical journal that publishes an annual analysis of the impacts of climate change on human health around the world, has identified bacterial and vector-borne diseases and malnutrition as top areas of concern. 

Drownings and other injuries from the flooding also led to a small percentage of the deaths, the study’s authors told Grist. All of the health-related risks posed by flooding, from the first drowning to the last case of dengue, were exacerbated by socioeconomic factors like food security, family income, vaccination history, access to medical care, and the condition of local infrastructure such as sewage systems and drinking-water treatment facilities.

Houses are damaged after a heavy storm in Khulna, Bangladesh, on December 6. Mushfiqul Alam / NurPhoto / Getty Images

The authors of the study told Grist that their results indicate that the risks of environmental health hazards are shifting as climate change worsens. Government health agencies and researchers often collect information on the immediate public health impacts of a single extreme weather event. But, because a warmer world also means a world plagued by more frequent and intense disasters, communities are being affected by extreme weather repeatedly. The long-term, cumulative health consequences of events that occur on a yearly or sometimes even more frequent basis are not well understood by the scientific community. And as such, the world has a flawed understanding of the true human cost of extreme weather.

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“We need to understand this kind of long-term impact in the context of climate change, because communities are going to be repeatedly and systematically exposed to these hazards,” said Tarik Benmahria, an environmental health researcher at University of California, San Diego, and one of three authors of the Bangladesh study. “These types of issues used to be exceptional by definition,” he added. “They’re not anymore.”

The method used by the researchers to determine the burden of flooding on communities in Bangladesh over multiple years, Schwarz said, “has the potential to be applied to evaluate the long-term effects of other climate exposures.” Extreme heat, hurricanes, and drought, to name a few of the environmental disasters being exacerbated by climate change, can also have compounding health effects that occur weeks, months, even years after the event takes place. If future research pinpoints how and when these effects occur, it could potentially save lives. “The approach is very relevant to other areas of the world that are vulnerable to recurrent climate hazards,” Schwarz said.