Climate change and rising sea levels to worsen urban health risks in Mumbai

Rainfall and flooding frequently disrupt the lives of city dwellers around the world, creating significant public health risks. Mumbai, India, known for its ferocious monsoon season, is a prime example of the loss of life that extreme urban flooding can cause. But despite the growing recognition and urgency of these hazards, the health impacts of precipitation remain poorly understood and the impact of sea level rise is entirely unquantifiable.

A recent study by Princeton University and the University of Chicago takes a closer look at the relationship between climate change, hazards and public health in Mumbai and finds that the number of deaths caused by rainfall and rising sea levels is almost ten times higher than official statistics suggest.

Conclusions published in the magazine Nature.

Monsoon season in Mumbai

Mumbai is famous for its heavy rain during the monsoon season from June to September. Situated between the Arabian Sea coast and the foothills of the Western Ghats mountain range, Mumbai's climate results in the city receiving significant amounts of rainfall, with some days receiving more than 300 mm (11.8 in) of rainfall.

One of the most notorious examples is the 2005 Maharashtra flood, which caused catastrophic flooding in Mumbai. This flood brought a record 944 mm (37.2 in) of rainfall to the city, resulting in more than 1,000 deaths, hundreds of millions of dollars in damage, and forced evacuations of hundreds of people.

“Mumbai is one of the largest and fastest growing cities in the world,” says co-author and STEP Ph.D. student Tom Bearpark. “Every year, heavy monsoon rains cause widespread destruction, closing roads, damaging infrastructure and putting lives at risk.”

How researchers linked rainfall to mortality

Drawing on a unique combination of high-resolution data sets, the researchers linked individual death records with sub-daily rainfall data from the Municipal Corporation of Greater Mumbai (MCGM), covering the city's monsoon seasons from 2006 to 2015. Each death was matched to highly localized rainfall measurements using residential addresses and classified as “slum” or “non-slum” residents based on census listings.

The team then used an econometric model that captured both the immediate and delayed effects of rainfall on mortality in each of Mumbai's 89 PIN (i.e. postal) codes, allowing the impact to vary by age, gender and residential status. They also developed a new method that estimates the impact of hourly rainfall on mortality taking into account the height of the tide, a crucial factor for a coastal city like Mumbai.

“We developed a model that differentiated the impact of precipitation depending on the tide level during the hour it rained,” Bearpark explains. “This model allows us to quantify how rising sea levels could increase the health impacts of precipitation in the future.”

Key Findings Reveal Deep Inequalities

The results are sobering. The results showed that precipitation accounts for more than 8% of deaths during the season. rainy season in Mumbai – this estimate is almost 10 times higher than official statistics.

Slum dwellers were overwhelmingly represented in monsoon-related deaths, accounting for more than 80% of the deaths caused by rainfall. Among children under five, 18% of deaths were caused by monsoon rains. The study also found that women are at higher risk than men.

“Our findings illustrate deep inequalities in the impact of extreme weather and future climate change, with poorer areas facing disproportionate damage,” Bearpark says. “Differences in drainage, sanitation, and access to healthcare and resources vary widely within cities, resulting in highly uneven impacts.”

Researchers warn that climate change is likely to worsen this crisis. Even a modest rise in sea level of five centimeters (a change that is already likely by 2030) could increase the proportion of deaths caused by rainfall from 8.5% to 9.1% of total deaths, an increase of 7%. With a sea level rise of 15 centimeters, rainfall-related deaths could reach 10% of total deaths, representing an increase of 21%.

These sea level changes are small compared to projections for Mumbai by 2050, highlighting that even modest sea level rise could significantly increase rainfall-induced health risks in low-lying, densely populated cities.

Lessons for cities around the world

Beyond the implications for Mumbai, this study offers vital lessons for cities around the world. The authors emphasize the importance of investing in water infrastructuresanitation, waste management and drainage systems in the fight against rain-related mortality.

The authors also call for the urgent expansion of climate impact assessments that go beyond temperature-related mortality and consider the health risks posed by rainfall and sea ​​level rise.

“Although our research shows that rainfall and flooding are severe public health problems, it also highlights the value of targeted investments in sanitation, clean water, waste managementand drainage systems save lives and reduce inequality,” explains study co-author Ashwin Road, director of research at the Energy Policy Institute at the University of Chicago.

“Climate change has already led to increased rainfall and rising sea levels around the world. It's time to make those investments.”