Monsoon flooding in Mumbai this month. HIMANSHU BHATT/NURPHOTO VIA GETTY IMAGES

For centuries, Indians have rejoiced at the arrival of the monsoon to break summer’s fever. From June to September, rains water the crops, revive rivers and wells, and cool the air.

Increasingly, however, the season’s sweet relief is laced with apprehension. The torrential rains that submerged parts of India this year are the latest in a string of major floods in the past decade, some caused by record rainfall — a scenario that many worry could become the “new normal” as climate change increases the frequency of extreme weather. This year, for instance, the monsoon arrived late and on the heels of a drought. Rains sputtered through June, then came on with a fierce intensity. Since early July, spells of heavy rain have led to flooding in 11 states, taking 1,200 lives and displacing millions. Many farmers desperate for rain saw their crops washed away.

India’s summer monsoon has always been variable and has often precipitated floods, especially in the basins of the great Himalayan rivers. But experts say that a combination of global warming, unplanned urban growth, and environmental degradation is increasing flood risk in India.

New studies show that extreme precipitation events are on the rise in large parts of India, especially multi-day deluges that lead to large-scale floods. Warmer temperatures are also speeding up glacier melt in the Himalayas, which is projected to increase flow rates in the Ganges and Brahmaputra Rivers. As importantly, destruction of mountains and hills, as well as development on floodplains and marshes, are exacerbating risk, as was seen in last year’s historic floods in the southern state of Kerala. Those floods were caused by extreme rainfall and mismanagement of dam reservoirs, but mining and construction in the Western Ghats, a major hill range, contributed to damaging landslides. The floods in August 2018 took 483 lives, affected 5.4 million people, and temporarily shut down the state’s new airport, which was built on a floodplain.

One study showed a three-fold rise in extreme precipitation events across central India from 1950 to 2015.

“Hydro-meteorological events are on the rise,” says Muralee Thummarukudy, operations manager at the crisis management branch of the United Nations Environment Program (UNEP). “But in India, especially, more people are also living in vulnerable areas and leading more affluent lifestyles than before. So more individual and community resources are at risk.”

The number of floods in India rose to 90 in the 10-year period from 2006 to 2015, up from 67 in the 10 years between 1996 to 2005, according to the UN Office for Disaster Risk Reduction. Globally, floods made up 47 percent of all climate-related disasters from 2006 to 2015, up from 40 percent in the 1996-2005 period, says the agency’s 2015 report. Overall flood mortality fell in those two decades, except in a handful of countries, including India, where death tolls continued to rise, from 13,660 from 1996 to 2005, to 15,860 from 2006 to 2015. Experts say this is a sign that despite improved warnings and response, India needs to do more to reduce risk. Chief among those steps is regulation of housing and infrastructure in floodplains — a trend that has intensified in recent years as India’s urban population and economy have grown.

A 2017 global analysis by the World Resources Institute said that India has the most GDP exposed to river flooding ($14.3 billion), a number that could rise 10-fold by 2030 as the economy continues to grow. “However good the emergency response,” says Thummarukudy, “the damages will keep coming unless we tackle the problem head on.”

One key to preventing or reducing flood damage is understanding the shifting contours of the summer monsoon, which brings about 35 inches of rainfall to India every summer. A complex weather system influenced by both global atmospheric circulation and regional meteorological forces, the monsoon is an important piece of the climate puzzle — any change in the system affects the food and water security of billions in the Indian subcontinent, many of them extremely poor.

A water line in the middle of this wall shows how high floodwaters reached in one Badlapur home in July. VAISHNAVI CHANDRASHEKHAR / YALE E360

Those changes are not fully understood. Observations have shown a decline in mean monsoon rainfall since the 1950s partly due to the rapid warming of the Indian Ocean, which weakens monsoon circulation. The conversion of forests to farmland is also reducing evapotranspiration, which contributes significantly to rainfall, especially in the latter half of the monsoon season. But this century, an increase in land warming appears to be pulling in more moisture from the ocean and reviving the overall monsoon — at least for now.

As important as total rainfall is the distribution of that precipitation. Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, said that in parts of India, “what we are seeing is that there are more frequent heavy rainfall events as well as dry spells, and fewer moderate rainfall events.”

A 2017 study by Koll showed a three-fold rise in extreme precipitation events across central India from 1950 to 2015. The study also appeared to explain why extreme precipitation increased even as monsoon circulation weakened: The heavy rain events are caused by surges of moisture from a warming Arabian Sea.

Most studies project that extreme rainfall will continue to rise with temperatures. “In terms of total rainfall, the models don’t all agree,” said Roxy. “But we have high confidence that heavy rainfall events are going to increase further.”

At one point last month, an estimated 1.2 million people were living in government relief camps after widespread flooding.

In new research simulating rainfall and floods under different climate scenarios in 18 Indian river basins, scientists from the Indian Institute of Technology in Gandhinagar projected that multi-day rainfall events would increase across these basins. That’s important, said associate professor Vimal Mishra, because heavy rainfall over several days is what causes large-scale basin flooding.

“You have to evacuate more people and more poor people are affected,” said Mishra. Responding to floods in different areas at the same time — as happened this year — also strains emergency response efforts, he notes. Last month, the Indian army, air force, and National Disaster Response Force were deployed to rescue people across six states in northern and western India. At one point, an estimated 1.2 million people were living in government relief camps.

Mishra’s study did find that the frequency of floods in the low-carbon emissions scenario, in which global temperature increases stabilize at 1 degree C, was half that of the high-emission one, in which temperatures would rise by 3.7 degrees Celsius (6.6 degrees Fahrenheit). However, with global CO2 emissions continuing to steadily increase, the chances of holding temperature increases to under 2 degrees C (3.6 degrees F) seem increasingly remote. Still, the study shows climate change mitigation is important, said Mishra.

For the rivers fed by the Himalayas, glacier melt is an added worry. A study published in June that analyzed satellite data on 650 glaciers across the Himalayas found that the average ice loss rate had doubled during the 2000-2016 period, compared to the previous 25 years. More glacier melt means increased runoff, said Vikrant Jain, another scientist at IIT Gandhinagar, as well as the possibility of more glacial lake outburst floods — the sudden release of an enormous quantity of water when an ice block holding back water melts. In 2013, heavy rain followed by a glacial lake outburst caused devastating floods and landslides in the Himalayan state of Uttarakhand. The floods killed upwards of 4,000 people, destroyed the Hindu pilgrimage town of Kedarnath, and damaged roads, bridges, and dams, causing economic losses of $3.8 billion.

Flooding in the city of Jamkhandi in southwestern India in August. STR/AFP/GETTY IMAGES

Meanwhile, on India’s coastal plains — where rivers and sea meet large urban populations — another kind of flood risk could increase. In a study published last year, Mishra and his colleagues found that short bursts of heavy rainfall, lasting only hours, are likely to increase by 20 percent if the global mean temperature rises above 1.5 degrees C. Such “sub-daily” extremes have the most impact on urban flooding, Mishra says, noting that storm water systems need to be redesigned for the new extremes.

Unplanned urbanization is already increasing flooding. India added 90 million people to its urban population between 2001 and 2011, and will add 416 million more by 2050, according to the UN. Rain runoff automatically goes up when permeable soil is replaced by impervious surfaces, such as roads and parking lots. In addition, sprawling Indian cities have been building on wetlands and expanding into floodplains. And urbanization itself may be affecting rainfall patterns. Studies from the Indian Institute of Technology Bombay have found that urban heat islands could be intensifying thunderstorms over Mumbai.

“At different levels,” says Koll, “we are seeing a human imprint on rainfall.”

A small concrete shed is tucked away on the banks of the Ulhas River at Badlapur, a suburban town on the outskirts of the coastal metropolis of Mumbai, population 21 million. On one wall is painted a list of the high flood line in meters since 1991. The river crossed the danger mark 12 times in this period, the highest in 2005 when record-breaking rainfall, coupled with high tides, submerged the metropolitan region. The second-worst flood came this July, when the river rose with heavy monsoon rainfall, affecting more than 50,000 people. The deluge swamped cars, shops, and ground-floor apartments in the new buildings that had sprung up on land close to the river.

The absence of flood zoning has enabled risky development in Mumbai and other Indian cities.

The absence of flood zoning has enabled that risky development in Mumbai and other Indian cities. New residents in Badlapur, many of them first-time homebuyers, say they are shaken. Revati Rahate, who had moved into her new house only in May, had woken up in the middle of the night to feed her 10-day-old baby when she discovered water rushing into her ground-floor apartment. The Rahates fled to higher floors, and when the waters receded after two days, moved to her sister’s home for 10 days. Their building lost water and power, their home was full of mud, and with continued heavy rains, Revati says she lives in fear of another flood.

Badlapur was just one area in the western state of Maharashtra to be hit by floods during this summer’s monsoon. In July and August, floods affected almost 300,000 people and damaged nearly 1 million acres of farmland. Another round of heavy rain submerged parts of Mumbai earlier this month, requiring emergency forces to rescue stranded commuters. In response to recent flooding events, the Maharashtra government has established a task force on climate change. The Indian government is planning a new law to better manage major river basins.

This family had to evacuate its ground-floor home in Badlapur in late July when floodwaters reached the ceiling. VAISHNAVI CHANDRASHEKHAR / YALE E360

What can India do to reduce its flood risk? More, and better, science would be a start. Gaps include modelling small-scale cloud dynamics and understanding the exact role of the Bay of Bengal in the monsoon, says Koll. More research is also needed on river hydrology, says IIT Gandhinagar’s Jain. Flood models don’t incorporate sediment dynamics, for instance, which play a role in Himalyan river flooding. High sediment loads can reduce the capacity of a channel, leading to overtopping of the banks.

Beyond forecasting, flood management policies need an overhaul, experts say. Historically, in India as elsewhere, flood management has been about creating embankments and dams to control floods. But a 2017 audit in 17 Indian states found delays in completion of river management projects and deficiencies in flood protection structures.

Meanwhile, new ecosystem-based flood management approaches being promoted by international agencies have yet to catch on in India. That includes planning cities with ponds and permeable surfaces, restoring wetlands and forests, and regulating development in floodplains and hills, said Raghu Murtugudde, professor of earth sciences at the University of Maryland. Flood zone mapping and regulation of the sort that would have helped the townships along the Ulhas River was proposed decades ago, but has not been implemented.

“In the long run, the solution is better risk-informed land-use planning: Where do you build and how do you build,” says UNEP’s Thummarukudy, who consulted with the Kerala government after the floods last year. “Disaster risk reduction and environment protection go hand in hand.”

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