Off the coast of Vietnam, the country’s economic planners have set out on what could be their biggest-ever engineering project: A network of walls designed to hold back rising sea levels that are swamping fertile rice-growing regions.

At first glance, it is the kind of multidecade effort that the world’s richer countries might throw their weight behind. In Paris in December, industrialized nations committed to providing $100 billion a year to help poorer ones cope with the threat posed by climate change. Vietnam’s hulking, concrete sea walls would seem to fit the bill.

Not everyone is waiting to see how the dikes work out, though.

On a chilly January day recently, Do Van Duy slugged back another shot of rice liquor. It had been a good year for raising fish in the Red River delta of northern Vietnam. He and other villagers in Nam Dien had gathered to toast their success as the Lunar New Year approached—and question whether climate change is such a bad thing after all.

“We live better now,” said Mr. Duy, 31 years old, who now farms grouper, shrimp and crab in the brackish waters of the delta after giving up rice a few years ago. “If you can make the switch there’s a lot more money to be made.”

Nearly three-quarters of households in Nam Dien have abandoned rice farming, said Bui Van Cuong, a fisheries official with the People’s Commune in Nam Dien, as salt water flows farther into the delta’s farmland. “The changes are very apparent over the past 10 years,” Mr. Cuong said.

The shift is focusing attention on a difficult question: Is it better to invest resources in fighting the effects of climate change, or in helping people adapt?

The idea behind the Paris pact is that richer countries—the ones mostly to blame for pollution that scientists say is contributing to rising temperatures and higher sea levels—should pay to help poorer nations such as Vietnam, Bangladesh and the Philippines adjust. Some of the money is intended to help developing countries acquire cleaner power and limit further rises in the planet’s temperature.

What isn’t so straightforward is how developing nations should protect themselves or change practices when faced with rising sea levels and other risks associated with climate change.

Consider Vietnam’s Mekong and Red River deltas. Like the Ganges and Nile deltas, they are already feeling the effects of environmental changes. In the lower reaches of the Mekong, salt water is penetrating as far as 60 kilometers (37 miles) inland during the dry season, according to government meteorologists, killing crops and forcing farmers to leave or find a new way of making a living. Some 30 million people live in the Mekong delta area, with another 18 million in the Red River delta. Many have been growing rice for generations.

Vietnam’s initial response to the threat was to build a wall.

In Nam Dien, construction was completed a few months ago on a 20-kilometer (12.4-mile) dike designed to slow the flow of salt water into the area and prevent erosion. It is part of the broader system of dikes and sea walls with which the government aims to defend a centuries-old network of earthen levees. The project will add up to hundreds of miles of walls and take 40 years to build.

That is a long time, though. Some experts say Vietnam’s communist leaders should sharpen their focus on helping farmers such as Mr. Duy adapt to the changing environment.

“Their competitive advantage is changing,” said Le Anh Tuan, a director at the Institute for Climate Change Studies at Can Tho University. “The delta might not always be the best place to grow rice, but people can raise shrimp instead.”

There are signs that the government is now listening. Mr. Cuong, the fisheries director, is among a network of officials helping rice farmers switch to raising fish, rather than fighting the environmental changes.

It isn’t easy. He says farmers have to learn a whole new way of working, from filtering water to investing in healthy fingerlings. “The biggest problem is land,” says Mr. Cuong, 38.

Farmers typically need to borrow from relatives or local banks to buy large plots of land to accommodate a commercially viable fish pond. On average, a pond is around 2,000 square meters (21,500 square feet) in the Red River delta, and sells for around $90,000—a significant sum in Vietnam, where incomes average about $5,600 a year.

If run well, though, fish farms can be three to five times as productive as rice farming, he says. About 70% of their output is exported to China, with the rest going to the nearby cities of Hanoi and Haiphong. Living standards have improved as a result, with Mr. Duy and his neighbors earning up to $20,000 a year. Brand-new motor scooters are parked outside many of the houses in Nam Dien, and the occasional SUV bumps down country roads that wind past Buddhist pagodas and French-built Catholic churches.

The bigger problem for Vietnam might be how to help those who can’t or are unwilling to scrape together enough money to invest in a new way of life.

“The rich are getting richer and the poor and getting poorer, just like everywhere else,” Mr. Cuong said.

By James Hookway - The Wall Street Journal - February 25, 2016