AMNA NAWAZ: Coastal cities in Southeast Asia, including Bangkok, Jakarta and Manila, face a mutually risky future.

They are sinking as sea levels around them are rising.

Fred de Sam Lazaro has our report from Thailand.

FRED DE SAM LAZARO: Just 30 miles from Thailand's capital, Bangkok, sits a temple on a small spit of land.

The only way in is to walk or hitch a ride with a motorcyclist along a narrow concrete footbridge.

Over my shoulder here are about 2.5 square miles of what is today the Gulf of Thailand.

As recently as the mid-1990s, this was the village of Samut Chin.

Community leader Suwan Buaplai points to the lines of what were once power poles disappearing into the distance, marking roads which once connected houses, farms and markets in a thriving fishing village.

SUWAN BUAPLAI, Community Leader (through translator): The soil here erodes by 1.5 to two inches every year.

People have had to move houses six or seven times because the water has kept coming in.

FRED DE SAM LAZARO: 50-year-old Buaplai has been fishing since he was 10.

SUWAN BUAPLAI (through translator): There's been a decline in mangrove forests, which has led to quite a few species of fish and shellfish disappearing almost entirely.

FRED DE SAM LAZARO: Slowly, he says, the buffer zone that shielded the capital from the worst is eroding away.

SUWAN BUAPLAI (through translator): Bangkok has a natural reservoir for water, and if the water comes in, it's very hard for it to get out.

If there's flooding and places like Samut Chin are no longer around, the water could potentially stay in Bangkok for months.

FRED DE SAM LAZARO: Thailand's capital was moved here 240 years ago on the banks of Chao Phraya River, lifeblood to acres of rice paddies, at the time the lifeblood of the economy.

Today, this dense, concrete megacity of some 10 million residents is sinking at a rate of up to two-thirds of an inch every year.

During high tides after some flooding events, the river has risen nearly 10 feet above sea level.

VARAWUT SILPA-ARCHA, Former Thai Former Minister of Natural Resources: Am I worried?

Yes.

FRED DE SAM LAZARO: I met Varawut Silpa-archa, until recently Thailand's minister for natural resources and environment, in his high-rise office.

VARAWUT SILPA-ARCHA: Forty or 50 years from now, we might be sitting at the sea level here on the 20th floor.

FRED DE SAM LAZARO: Many who live at street level already know what that's like.

SURAPOL KEARNAKPU, Bangkok Resident (through translator): It's impossible to live here if you don't have a two-story house.

FRED DE SAM LAZARO: How high the water was.

Surapol Kearnakpu, 65, is a retired soldier and lives on a canal in East Bangkok.

SURAPOL KEARNAKPU (through translator): What we can carry upstairs, we carry, but, sometimes, we just have to let it flood.

So, during the big floods of 2011, we couldn't move appliances like the refrigerators and television, so we just let it flood.

FRED DE SAM LAZARO: The 2011 floods, the worst in half-a-century, inundating the city for almost three months.

More than 800 people died, and it cost the Thai economy 40 billion U.S. dollars in factory shutdowns.

Thailand is a major manufacturing hub serving global supply chains.

The government has built some new dikes and floodgates to hold the water at bay in a future flood, but even Former Minister Varawut is skeptical.

VARAWUT SILPA-ARCHA: I don't think it's adequate enough.

We need to move much quicker.

FRED DE SAM LAZARO: Despite the high stakes, he says, the sense of urgency has waned as memories fade of the 2011 floods.

VARAWUT SILPA-ARCHA: Some of the infrastructure has been developed since 2011.

But, unfortunately, I think not many people is really concerned or realize how the magnitude of the problem is.

FRED DE SAM LAZARO: One with multiple causes.

Rising sea levels are just one factor causing the Thai capital to sink.

Millions of new residents and thousands of new high-rises have drained groundwater levels.

And many canals that drained into the sea have been paved over.

KOTCHAKORN VORAAKHOM, Landscape Architect: We grew, like, rapidly without even thinking about many capacities and many, like - - like sustainable urban planning.

So, we are very addicted to growth.

Like, we still want to grow more.

FRED DE SAM LAZARO: Landscape architect Kotchakorn Voraakhom runs a social enterprise working to increase urban resilience in her hometown.

She says, in recent decades, the city has lost nearly half of its network of 3,000 canals that drain through the Chao Phraya River and into the sea.

KOTCHAKORN VORAAKHOM: Water is life to our culture.

But now, when you develop the city without concerning the benefits of the natural infrastructure, you shift to road, you have to drive more, you have to build more.

FRED DE SAM LAZARO: She's working to build a more climate-friendly city.

Her hallmark design is Centenary Park, opened six years ago, a vast green space in the middle of the city.

Graded to harness gravity, the park collects and holds water in an underground reservoir, reducing the flood risk.

In dry periods, up to a million gallons is available for watering.

Another Kotchakorn project is Chong Nonsi Canal Park above a major canal now reconnected to fresh water to nourish greenery on parkland.

Like many waterways, this one had been disconnected from the canal network, leaving it stagnant and polluted.

KOTCHAKORN VORAAKHOM: This main canal is, like, in the heart of the city and is the true main canal that connects to Chao Phraya.

FRED DE SAM LAZARO: So people moved in here migrating from rural areas to get jobs?

KOTCHAKORN VORAAKHOM: To -- yes.

FRED DE SAM LAZARO: She took us to another canal and a community that was among the hardest-hit in the 2011 flood.

KOTCHAKORN VORAAKHOM: We need these people to serve the cities, but it's so expensive to commute, so they live in informal settlements.

FRED DE SAM LAZARO: Kotchakorn's group is working with community leaders to help rehouse families being displaced as authorities plan to widen canals to move floodwaters through faster.

She's like to see more green space ideas, not more concrete, to mitigate the impact of Bangkok's concrete binge.

KOTCHAKORN VORAAKHOM: It's such a lovely city that we still want to be part of it, even with the flood.

We used to live with the flood.

We are amphibious.

We can live in a wet season and dry season.

It's not about destroying it, but work with the concrete.

FRED DE SAM LAZARO: Are you optimistic that something will happen in time?

VARAWUT SILPA-ARCHA: Oh, I'm... FRED DE SAM LAZARO: Or do you think you're going to lose a lot of this city?

VARAWUT SILPA-ARCHA: I know something will happen.

It's not a matter of if.

It's a matter of when.

And it's not a choice, shall we do it or shall we not.

FRED DE SAM LAZARO: Meantime, not far from the bustling metropolis, in the once-upon-a-time village of Samut Chin, a large statue of Lord Buddha stands on a platform of concrete, facing the sea, hands outstretched as if holding back the tide.

For the "PBS NewsHour," I'm Fred de Sam Lazaro near Bangkok, Thailand.

AMNA NAWAZ: And Fred's reporting is a partnership with the Under-Told Stories Project at the University of St. Thomas in Minnesota.