LONG XUYEN, Vietnam (Reuters) - About a year ago some farmers from Binh Thanh commune in Vietnam's southern rice growing heartland suspected the worst -- that their irrigation water had become too salty.
They telephoned Vo Thanh, the head of An Giang province's hydro-meteorology center, and he came to take water samples from the commune, which is about 20 km (12 miles) from the sea.
The farmers' hunch turned out to be right. The brackish water would damage their crops, so Thanh advised officials to tell farmers to stop pumping it into their rice fields immediately. Not everyone took heed.
"Those who didn't suffered losses," Thanh said. "Some 4,000 hectares (9,880 acres) of rice were damaged."
Cropland salination represents just one of the many increasingly acute environmental challenges in Vietnam, exacerbated by climate change, that are testing the government's ability to coordinate countermeasures.
While negotiators in Cancun work to lay the foundation for a deal to try to turn down the global thermostat, or at least slow its rise, Vietnam is in the early stages of cobbling together plans to adapt to changes already starting to take place.
Study after study flag Vietnam as one of the most vulnerable countries on earth to the effects of climate change, such as a sea level rise and volatile weather.
The Mekong Delta is particularly at risk. Nearly half of the country's rice is grown in the Delta, including almost all that Vietnam sends abroad to make it the world's second-biggest rice exporter after Thailand. A fifth of Vietnam's 86 million people live there, and it is one of earth's most biodiverse regions.
The government said in a report last year a third of the Mekong Delta could be submerged if the sea rose by 1 meter (3 ft). Other parts of the beach-lined country will be swamped, volatile weather patterns will hurt flood- and drought-prone areas and warmer temperatures will trim rice yields.
A study by the International Food Policy Research Institute this year estimated that a sea level rise of 17 cm (6.7 in) accompanied by other changes in climate could slash rice yields country-wide by as much as 18.4 percent by 2030.
Thanh had seen salty irrigation water before, but never so far inland from the sea. What was troubling about Binh Thanh's case, though, was not the salt. It was that the problem was caused by an increasingly complex network of dykes and sluice gates built precisely to prevent salination, he said.
"The other gates were closed to keep fresh water in, so the salty water flowed there," he said.
It is an example of the type of problem experts say Vietnam will face more often as hard choices are made to adapt.
"Things are happening already, it's not in the future, and it's going to get worse," said Koos Neefjes, the United Nations policy advisor for climate change in Vietnam.
"It is easy to say what needs to be there by the year 2100, but it is very difficult to say what is tomorrow's priority."
Consultants and non-governmental organizations give the government high marks for its relatively early recognition of the risks and the need to adapt.
This year, the ruling Communist Party included the need to face the effects of climate change in public drafts of policy documents prepared for a five-yearly Party Congress planned for January, underscoring its commitment.
The government approved a National Target Program to deal with climate change two years ago and is collecting submissions of provincial plans to incorporate into a national plan.
But there is a wide divergence in how local governments understand the problem and approach it.
"I think that they've done a lot over the past two years. But I think that an urgent situation needs them to act faster in the future," said Nguyen Thi Yen, climate change coordinator for the non-governmental organization CARE International in Vietnam.
"We feel like there's a need for support for the communities, for the local levels, on how to adapt to climate change, how to understand the situation in the local context, how to mainstream climate change into the local planning. It's very urgent."
In some of the mountainous northern provinces where she recently conducted surveys, the level of understanding and action seemed very limited, she said.
In the Mekong Delta and some other coastal areas, by contrast, local governments appear to have a better understanding.
Adapting will also require the government to think comprehensively, encompassing social, economic, land use and other policies -- something experts say will be a challenge for a polity still emerging from an era of stovepiped central planning.
"The knee-jerk response is engineering," said Jeremy Carew-Reid with the Hanoi-based International Center for Environmental Management.
Many localities, for instance, think dykes are the answer to projections of increased flooding. As an example he notes the government approved a $650 million plan to encompass Ho Chi Minh City, Vietnam's commercial hub, in a system of dykes.
He modeled the impact for a widely read study and showed that while the plan might stop flooding it would create a whole new set of problems for the city of more than 7 million people.
"Since then it's been on hold as more and more people have been criticizing it," he said.
For places like Binh Thanh commune, environmental challenges will only increase. But the action-reaction cycle of change and responses will play itself out, as it has in the flood-prone region for centuries.
Le Van Banh, a rice exert at the Cuu Long Delta Rice Research Institute, says the salty water situation will worsen -- but researchers are creating new strains of rice that can withstand ever saltier water.
Standing by his muddy fields that yield three rice crops a year, farmer Nguyen Van Banh poked holes in a paddy dyke with a staff and planted beans with his wife.
Asked if he was worried about climate change, his answer was telling: "We don't have time to worry about that stuff."
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