Bangladesh is a land of water. Its silty rivers rush down from the Himalayas, spill into a filigreed maze of ponds, wetlands and tributaries before emptying into the blustery, black Bay of Bengal.

Now, its most profound threat is water, in its many terrible incarnations: drought, deluge, cyclones, saltwater. All are aggravated to varying degrees by Climate change, and all are forcing millions of people to do whatever they can to keep their heads above it.

This matters to the rest of the world, because what the 170 million people of this crowded, low-lying delta nation face today is what many of us will face tomorrow.

The people of Bangladesh are rushing to harvest rice as soon as they get word of heavy rains upstream. They’re building floating beds of water hyacinths to grow vegetables beyond the reach of floodwaters. Where Shrimp farms have turned the soil too salty to cultivate crops, they’re growing okra and tomatoes not in soil, but in compost, stuffed into plastic boxes that had once carried shrimp. Where the land itself is washing away, people have to move to other villages and towns. And where they’re running out of even drinking water, they’re learning to drink every drop of Rain.

Saber Hossain Chowdhury, a governing party lawmaker and the prime minister’s climate envoy, compared his country’s efforts to plugging a leaky barrel. “It’s like when you have a drum that’s got seven leaks, and you’ve got two hands,” he said. “What do you do? It’s not an easy thing.”

Bangladesh has succeeded in saving lives during cyclones and floods. But there’s a host of other challenges to address, all at once: finding new sources of drinking water for millions along the coast, extending crop insurance, preparing cities for the inevitable influx of migrants from the countryside, even cultivating good relations with neighboring countries to share weather data.

All this, with little help from the rich countries of the world. There’s mounting frustration in places like Bangladesh that wealthy nations have not shored up the funds that developing countries need to adapt to the hazards they already face. It’s a theme of the Paris climate finance summit this week.

Among the 64 districts of Bangladesh, half are considered to be vulnerable to climate change.

An Early Warning System

In the middle of April, Rakibul Alam, an agricultural extension worker in the lowlands of the north, received a warning from his boss in the nearest city, Sunamganj, who had himself been warned by his superiors in the capital, Dhaka.

Mr. Alam was told there could be heavy rains in northeastern India in a couple of weeks, which could send floodwaters rushing across the border to drown the fields in his area just as the rice was ripening.

He knew he had to persuade local farmers to get as much of their rice off the fields as quickly as possible. And that meant helping them to overcome a psychological hurdle. Even in an area prone to flash floods, farmers want to squeeze as much rice as they can from their tiny plots of land. “They want to wait till the grain gets ripened 100 percent to get the best yield,” Mr. Alam said.

This year, he knew, waiting could be catastrophic.

Mr. Alam turned to his local networks. Calls and texts went out to farmers association leaders. Volunteers went from village to village with bullhorns. Imams used their mosque loudspeakers. The message was straightforward: Flash floods could be on the way, harvest what rice is almost ready, don’t wait.

To Mr. Alam’s relief, farmers took the warning to heart. They worked nonstop, even through the Eid al-Fitr holidays. By April 25, almost all the fields were cleared.

Luckily, this time, the rains weren’t heavy, and there were no flash floods — but the harvest was protected.

It was a dry run, so to speak, for what could be a more frequent occurrence as climate change intensifies the rains and increases the risk of flash floods in these lowlands. It was also an extension of the early warning system that’s been used to get people out of harm’s way when a cyclone approaches the coast. This time, it was used to save a harvest.

The government, for its part, has an ambitious national adaptation plan with expensive projects, like dredging rivers of silt and building embankments to hold back the sea.

But much of that is yet to be realized, and critics say big infrastructure jobs are rife with potential for mismanagement and graft. “Climate vulnerability is there,” said Zakir Hossain Khan, who analyzes climate finance for a local nonprofit group called Change Initiative. “Also, corruption vulnerability.”

What do you do when the rivers swell and drown your crops?

If you’re Shakti Kirtanya, you grow your crops on top of the water. If the water rises, they rise, too. They float and bob. “If you see the harvest, it will fill your heart with joy,” he said.

Mr. Kirtanya learned this farming technique from his father, who learned it from his. It’s been practiced for 200 years in his low-lying district, Gopalganj, where land is usually inundated for half the year.

Now, because climate change is spreading the risk of flooding to many other areas, the floating gardens of Gopalganj are spreading. Over the past five years, the government has supported floating gardens in 24 of the country’s 64 districts.

Mr. Kirtanya uses what he has. He cuts the stems of water hyacinths in the lake near his house, lets the pile stew in the sun, and shapes it into long, wide seedbeds on top of the water. He sows watermelon and amaranth in summer, cabbage and cauliflower in winter. The garden is a source of income and, for his family, a source of fresh produce grown without chemicals.

“Whether the rains are late or early, it doesn’t affect it,” Mr. Kirtanya said. “It doesn’t get hurt in the heat either.”

There is one looming threat. Seawater is coming farther inland. Partly it’s because of sea level rise, elevating the tides. Partly it’s because rivers have been dammed upstream, and not enough freshwater is flowing down. Partly it’s because too much groundwater is pulled up.

Mr. Kirtanya saw a glimpse of a salty future last year. Leaves turned red. Plants became frail.

That salty future is already present in the 3,860-square-mile mangrove forest, the Sundarbans, on the edge of the Bay of Bengal.

The forest is the country’s main defense against storm surges. The roots of the sundari, the mangrove species for which the forest is named, stick out of the mud like fingers of the dead. Tigers leave their prints in the ground.

Today, the almost unthinkable is happening. The water is becoming too salty for the sundari. They are dying. Other mangrove species are taking over. The landscape is changing. Likely forever.

“I don’t think the sundari will come back unless the salinity decreases,” said Nazrul Islam, the son of a forest officer who grew up in the area and now runs river tours in the forest. “And I don’t see the possibility of salinity decreasing.”

Capturing the Rain

Sheela Biswas faces the crisis of salinity every single day. Salt has intruded into canals and ponds that her village relies on for drinking and washing. An estimated 30 million people who live along the coast face the problem of saltwater intrusion to varying degrees. The area where Ms. Biswas lives is among the worst hit.

It wasn’t like this when she came as a bride 30 years ago. Then, most people ate rice that they grew on their land. They drank water they collected in their pond.

Then came “white gold,” shrimp. Shrimp farms spread. People let in saltwater through a canal from the river, so saltwater spread, too. Ms. Biswas’s pond turned too salty to drink.

First, she hired a cart to buy water. Then she turned to a neighbor who built an underground tank to collect rainwater. She invented her own rainwater harvesting system with what she had at home, jiggering plastic pipes to channel rainwater from her tin roof through a fishing net and into earthen jars. She still had to bathe in her salty pond, which brought on a skin rash, a common complaint in the area. Doctors say rates of hypertension are high, too; they suspect their patients unintentionally ingest too much salt.

The latest solution to Ms. Biswas’s problem came in the form of a hot pink 2,000-liter plastic water tank, the equivalent of about 530 gallons, with a filter on top. It sits in her courtyard collecting the monsoon rains, one of nearly 4,000 such tanks distributed over the past three years by a development organization, BRAC, that assists the poor.

Shrimp is no longer white gold. Intensive shrimp production has brought new risks, including diseases that cut into profits. A few of her neighbors have begun closing their shrimp ponds, filling them with sand and waiting for the rains to flush out the salt below.

That’s rare. Most people here have very little land, and they can’t afford to leave it idle so it can recover. They are stuck. “They can’t rely on shrimp, and they can’t change,” Ms. Biswas said.

Even if they could, sea level rise, combined with the subsidence of the land for other reasons, now threatens to aggravate the menace of salt in the water. If the land is sinking, even a little bit of sea level rise is very dangerous. Embankments sometimes collapse in tidal surges, which are growing stronger.

Like Ms. Biswas, the people of the southwestern coast have tried all kinds of things to hustle for drinking water.

A few entrepreneurs are selling water that they desalinate using small reverse osmosis systems in their homes, but that ends up dumping salty slime into nearby ponds. Some people are moving to the busy port town of Mongla, but there, too, freshwater is scarce.

Farther south, where the soil is too salty to plant crops, women have started growing vegetables in pots filled with compost and manure. Or they’ve turned empty rice sacks into planters, even plastic boxes that once took shrimp to market.

Their slapdash efforts to secure the most basic human needs, food and water, are a glimpse into the epic struggle of hundreds of millions of people who are trying to cope with climate risks every day.

Money for adaptation, $29 billion to all developing nations in 2020, is a small fraction of what is needed: at least $160 billion a year, according to United Nations estimates. This explains the fury of developing countries’ leaders in international climate politics.

Unless global emissions are reduced quickly and dramatically, Bangladesh can do little to stay above the surface, said Mr. Chowdhury, the lawmaker. “Whatever we do is not going to be enough,” he said.

Julfikar Ali Manik contributed reporting from Bangladesh.