GEOFF BENNETT: Across the U.S.

and around the world, coastal cities are grappling with rising seas and worsening floods.

Now a look at a place facing those threats on an even larger scale.

Indonesia's capital, Jakarta, now the world's largest city and home to 42 million people, is sinking rapidly, as climate change and overdevelopment collide.

For our climate series tipping point, Fred de Sam Lazaro reports now from North Jakarta.

FRED DE SAM LAZARO: In Jakarta's north, only a seawall stands between this megacity and the Java Sea, holding back the tide, but not always, especially between October and April, as the rainy season and rising seas collide.

Entire neighborhoods are submerged, motorbikes stall in water, and children find new joy in the streets filled with contaminated floodwaters.

For millions like 35-year-old Ratini, who, like many Indonesians, goes by just one name, this flooding is now routine.

RATINI, Jakarta, Indonesia, Resident (through translator): The water can reach thigh high.

Cars and motorcycles can't enter the area.

FRED DE SAM LAZARO: She spent her entire life here and cares for a family of seven.

RATINI (through translator): It's difficult for everyone, because, when there is a flood, it's hard to make a living for me and the children.

FRED DE SAM LAZARO: When we visited, one son was sick, a reminder of the flood season's impact.

RATINI (through translator): We often get sick during floods.

Everything becomes chaotic.

Sometimes, electricity goes out as well.

FRED DE SAM LAZARO: Her husband, like many here, supports the family by collecting scrap, earning roughly $6 a day.

During floods, that slashed in half.

About 40 percent of Jakarta is below sea level, and it's estimated nearly half of the city could be inundated and uninhabitable by 2050.

Water is everywhere in Indonesia, an archipelago of 17,000 islands.

Farmers here depend on the rains to flood rice paddies that help feed the world's fourth most populous country.

But in the densely populated capital, water is no longer a lifeline; it's a threat.

M. ABDUL BAITS, Indonesian Forum for the Environment (through translator): It's reached the second floor.

FRED DE SAM LAZARO: M. Abdul Baits, with one of Indonesia's most prominent environmental groups, showed us the impact years of flooding has had in Ratini's neighborhood.

M. ABDUL BAITS (through translator): This is proof that Jakarta has sunk below sea level here.

This used to be a two-story house.

FRED DE SAM LAZARO: Nearby, this submerged mosque has become a symbol of what's been lost in the world's largest Muslim-majority country, its dome disappearing over recent years.

The capital city has been sinking as much as six inches per year, and, at the same time, sea levels have risen about four inches in the last three decades.

That's led to more severe flooding, in 2020 reaching record levels that killed more than 60 and forced tens of thousands to evacuate.

Decades of growth has depleted groundwater, slowly hollowing out the land and causing it to sink.

M. ABDUL BAITS (through translator): During the 1970s and '80s, there was massive development, especially in industrial and commercial areas.

That level of construction required huge amounts of water.

FRED DE SAM LAZARO: And the glittering high-rises that now stretch out for miles add to the burden.

M. ABDUL BAITS (through translator): Most structures are made of concrete.

The weight of those buildings adds significant pressure to the land.

FRED DE SAM LAZARO: This construction covered wetlands and mangrove swamps.

M. ABDUL BAITS (through translator): It is no longer ecologically functional.

Jakarta is like a sponge.

If the sponge is sealed, it can no longer absorb water, and eventually it collapses.

FRED DE SAM LAZARO: By law, a third of Jakarta is supposed to be green, open space.

In reality, Baits says, it's less than 5 percent, leaving rainwater nowhere to go.

Trash and debris often block existing drainage points.

M. ABDUL BAITS (through translator): Flooding in Jakarta has a huge impact on people, especially those living in slums and densely populated areas.

This is deeply unfair.

They're not the ones causing the flooding, yet they suffer the most.

KARITEM, Jakarta, Indonesia, Resident (through translator): Tidal flooding is the worst.

Luckily, this house is on stilts now.

We couldn't cook at all.

We even had to boil water using candles.

FRED DE SAM LAZARO: Karitem (ph), Ratini's mother, lost her nearby home to flooding and moved in with her daughter.

They feel safe for now a level above the street.

RATINI (through translator): After the embankment was built, we don't get flooded as much.

The water is blocked now, so it feels much safer.

FRED DE SAM LAZARO: The government began building coastal defenses and pumps in the early 2000s.

Today, roughly nine miles of seawall protect parts of the shoreline.

Last year, the government announced plans for a massive 435-mile seawall project.

And, in 2019, Indonesia also announced plans to move its capital, now on the island of Java, to Borneo, a transition scheduled for 2028.

Pitched as a green, futuristic city by former President Joko Widodo, it was meant to ease Jakarta's burdens.

JOKO WIDODO, Former Indonesian President (through translator): This is a massive project with a timeline of 15 to 20 years.

This isn't a one-to two-year project.

FRED DE SAM LAZARO: But progress has been slow.

Only parts are built, mainly government offices and housing, with few residents so far.

And environmental and indigenous groups warn, the project could endanger one of the world's largest surviving tropical rain forests.

M. ABDUL BAITS (through translator): Relocating the capital does not address the root environmental and social problems.

Those issues remain unresolved.

FRED DE SAM LAZARO: Baits wants to see existing laws about green space enforced, and local communities consulted to find the solutions, instead of what he calls top-down big infrastructure projects.

M. ABDUL BAITS (through translator): This is not a long-term solution.

Seawater gradually erodes concrete, and, over time, it will weaken and fail.

In fact, it could become a time bomb.

People may feel safe because the wall is there, but it does not eliminate the risk.

If a larger disaster happens, the impact could be far worse.

FRED DE SAM LAZARO: In a city that has adapted to the sea since its very beginning 500 years ago, Baits says that skill will be put to its severest test in generations to come.

For the "PBS News Hour," I'm Fred de Sam Lazaro.