An Indian city of 8 million that turned a garbage-clogged lake into a natural biofilter provides a lesson on how to adapt to climate change – Fortune

With surrounding communities burgeoning, Sembakkam Lake functions as a catch basin during seasonal monsoons and a reservoir during the long spells without rain in between. The lake helps prevent both flooding and drought, twin perils that afflict Chennai with increasing frequency and severity because of climate change. Just a few years ago, Sembakkam Lake was so overgrown with invasive weeds and clogged with garbage that it made both of those problems worse.

Sembakkam Lake is among hundreds of local water bodies being restored as part of a comprehensive effort to manage two of Chennai’s biggest challenges: a ballooning population and increased vulnerability to floods, drought, tropical storms, and, on occasion, devastating tsunami.  

At Sembakkam Lake, The Nature Conservancy and local environmental group Care Earth Trust have stepped to the forefront of that effort. The goal of their collaboration is to transform a degraded lake from weed-choked cesspool to a natural biofilter and catch basin.

“It’s basically climate adaptation,” says Nisha Priya Mani, the Nature Conservancy scientist overseeing the restoration. 

Sembakkam Lake is just one battleground in Chennai’s fight against rapid population growth and the intensifying impacts from climate change. The two challenges are linked because often the ravages of climate change inflict the greatest hardship on the poorest and least established residents who make up the biggest source of economic growth. 

The population of cities in developing countries is expected to triple by 2050, compared with 13% growth for cities in developed countries, according to the United Nations. By 2050, 5.6 billion people will live in developing world cities, up from 3.2 billion in 2018. 

“Two-thirds of the world’s population is expected to be urban, many living in unplanned and informal settlements and in smaller urban centers,” the Intergovernmental Panel on Climate Change (IPCC) said in a report issued earlier this year. 

Urban centers like Chennai, spread out over a low, flat plain on India’s southeastern Coromandel Coast, can be powerful drivers of economic development. The city’s population, already India’s sixth-largest, is expected to swell by 6-10% annually for at least the next decade. Chennai boasts a growing technology corridor with a factory producing iPhones run by Apple supplier Foxconn; car and truck makers such as Hyundai, Kia and the Indian manufacturer Mahindra; as well as a plethora of technology centers and back offices of other global companies.

Yet often such booming hubs also bear the brunt of climate change. Low-lying coastal locations like Chennai, which has an average elevation of just 22 feet above sea level, are particularly susceptible to rising oceans and more intense cyclones. Many of the developing world’s largest and fast-growing urban centers are situated on or near the equator where temperatures increasingly strain the limits of human habitability given humidity levels, which are also climbing because warmer air absorbs more water. 

Climate-related disasters threaten economic dynamism in the developing world just as its cities are grappling with an influx of new residents, thus defeating the reason so many flocked to those cities in the first place—to escape grinding poverty. 

Will fast-growing, climate-battered cities of the developing world remain quagmires, multiplying the destructive capacity of climate change? Or can they be transformed into bulwarks of sustainable growth?

Chennai’s historic flood

Chennai’s physical contours—the geographic features that helped make it a megacity even before the British established a trading center here in 1639—can clearly be seen as one descends into Chennai International Airport. Three major rivers—the Adyar, the Cooum and the Kosasthalaiyer—traverse India’s sprawling Eastern Coastal Plains as they flow to the Bay of Bengal from the inland hills. In 1806, the British dug a 500 mile channel—dubbed the Buckingham Canal—that parallels the coast, bisects Chennai, and connects waterways from Kakinada City in the state of Andhra Pradesh to Vilapurram District in the heart of Tamil Nadu, where Chennai is located. The canal was the first of a long series of ambitious interventions in Chennai’s complex hydrology; many of those initiatives changed the city for the worse. More recently, some have changed it for the better.

Over time, as wetlands were filled and rivers rerouted and hemmed in for development, the intricate network of lakes, ponds, streams, and rivers lost its capacity to hold water long enough to recharge underground aquifers before delivering the remaining water smoothly to the sea. It took a historic deluge in 2015 to spur an equally historic response.

In November of that year, record rainfall swamped Chennai’s man-made reservoirs, exceeding what little capacity the concrete and asphalt urban center had to absorb the runoff. Authorities’ decision to open the floodgates to relieve pressure on the reservoirs submerged half the city. In many neighborhoods, the water rose above the ground floors of buildings. By the time flooding subsided days later, more than 400 people had died and some 1.8 million had been forced from their homes.

Chennai authorities had done enough thinking about climate change to understand that disasters like the flood—as well as heat waves, droughts and rising sea levels—would become increasingly common in their city as the planet warms. The flood galvanized an unprecedented effort by city officials to work with environmental and social advocacy groups, businesses and local academic institutions to forge a comprehensive response to the climate challenges ahead, particularly when it came to flooding and drought. 

The trick was to “turn the crisis into an opportunity,” said Wilfred Davidar, a former official with the state of Tamil Nadu, who was involved in the city’s response to the flood and the overhaul of its approach to planning afterwards. 

Chennai’s coordinated response to climate change

Chennai already had launched its own project to try to address water challenges. The city had signed on a year earlier to an international initiative aimed at building resilience in 100 cities around the world, funded by The Rockefeller Foundation. The Atlantic Council, a Washington, D.C.-based think tank, now oversees the program with continued Rockefeller funding. 

The flood super-charged those efforts. The city and Tamil Nadu state government, joined by a range of outside groups, embarked on an unprecedented, coordinated response. The Rockefeller-funded city resilience program, headed by Krishna Mohan Ramachandran, an energetic Chennai native with a background in corporate communications, helped shape a long-term view that infused government actions with the insights and initiatives of the outside groups that had previously been marginalized or uninvolved—environmentalists, scientists from local universities, companies with social responsibility funds to spend, and community advocates for the poor.

A collection of international development organizations brought billions of dollars and organizational expertise, including the World Bank, the Asian Development Bank and international development agencies from the U.S., Germany, and Japan.

Relocating vulnerable populations

Some of the city’s first steps to handle the flooding underscored the drawbacks of established methods for dealing with natural disasters when applied to the chronic challenge of climate change. Authorities leaned into a plan to relocate large but informal communities that had established themselves along the banks of rivers. These enclaves housed tens of thousands of families—some occupying formal, concrete dwellings, others living in more ramshackle shanties. Nearly all were poor. In total, almost 100,000 have been moved about 20 kilometers from the city center to a marshy stretch of field where they now live in more than a dozen high-rise tenements. 

The relocations and the city’s water body restorations have helped prevent a repeat of the massive flooding and acute water shortages for the city center so far. Yet displaced residents and advocacy groups say the relocations haven’t done enough to prepare low-income communities to cope with climate change. In some ways, flood prevention programs have left poor residents worse off.

The informal settlers had adapted to living along the rivers, in many cases building mutually beneficial arrangements with the city’s wealthiest communities adjacent to them. They found work as household help or gardeners. Men drove auto rickshaws, ferrying the wealthier residents who lived nearby. Others survived on odd jobs like sharpening knives from a roaming bicycle set-up or recycling trash. Their children could attend local public schools and there were government health clinics nearby.

When the floodwaters came, usually several times a year, informal residents decamped to the upper floors of nearby community centers or the apartments of extended family. When the waters receded, they would return and clean up.

In their new high-rise tenements in Parumbakkam, the inland area where they have been relocated, there is less work and fewer facilities to provide education and health care.

“Eighty to 90% of the residents are casual laborers. We cannot guarantee social mobility just by providing housing,” says Vanessa Peter, a researcher and advocate for poor communities in the city. “They are being stigmatized and criminalized for the flooding in the city.” 

At a day care center after hours, block leaders, elected community representatives for up to 100 families, do their best to resolve complaints and mobilize the bureaucracy to fix buildings, pick up trash, improve safety and address persistent flooding in the area. The flooding—expected, since the projects were built on marshlands—is an irony many residents are quick to note.

Mohana, a 36-year-old mother of two, tells the group how she got the tenement managers to clean a despoiled water tank. It’s a small win after six years in the building. She relishes it.  

“It took a week to get it to happen,” she reports to the group.

More resilient climate change adaptations

Now pressure is growing to swing the pendulum away from displacing communities along the city’s waterways to development that seeks to make them more resilient to the consequences of climate change. At Sembakkam Lake, for example, The Nature Conservancy is building a facility that uses an aerated lagoon and reed beds to filter waste and sewage before it enters the lake from surrounding communities. It has teamed up with a local branch of the prestigious Indian Institute of Technology and IBM to monitor water quality directly and using satellite photos.

The project has added 100,000 cubic meters of volume to the lake, one-third more than it held before, according to Priya Mani. 

“Cities now have these stacked or reverberating effects, where you’ve got all kinds of things happening at once. That sometimes also creates opportunities for kinds of stacked solutions that solve more than one thing at a time,” says Minal Pathak, a professor at Ahmedabad University who is part of the Intergovernmental Panel on Climate Change assessment of impacts on urban areas. “We can give a whole list of solutions that don’t cost that much money, that can address climate change and that can bring development benefits. And we need those now.”

Other actors are pushing to redevelop communities along the city’s waterways in ways that don’t require massive relocations. The Sponge Collaborative is an architecture and urban planning firm founded by a trio of U.S.-trained Indians to promote nature-based solutions across India. They call themselves a “resiliency planning” firm. In Chennai, where the partnership is based, one of its first projects has been promoting nature-based ideas for restoring the Buckingham Canal that foster healthy population growth while both controlling flooding and retaining water for community use.

One such idea is multi-storied housing with terraced park and recreation land along river banks that accommodates rising water and allows some of it to be absorbed underground before the rest returns to the river and flows out to the nearby sea.

“Climate change poses an opportunity, despite its negative impacts,” says Praveen Raj, a Harvard and California Berkeley trained founder of the Sponge Collaborative. “It allows us to take a new look at things.”