One of the regions hardest hit by the impacts of climate change - floods, typhoons, droughts and saltwater intrusion

BANGKOK, Feb 23 (Thomson Reuters Foundation) - Falling crop yields, damage to infrastructure and heat-related illnesses brought on by climate change could cost the four Southeast Asian countries of the lower Mekong River basin $34 billion per year, researchers say.

Southeast Asia is one of the regions hardest hit by the impacts of climate change, such as floods, typhoons, droughts and saltwater intrusion - when seawater flows up rivers, threatening agriculture and infrastructure.

A report released on Monday found that climate change could cost Cambodia, Laos, Thailand and Vietnam $16 billion per year in lost worker productivity, crop production and natural resource assets, plus $18 billion in infrastructure damage because of flooding, storms and extreme heat.

The report - by the Washington-based World Resources Institute think tank for the USAID Mekong Adaptation and Resilience to Climate Change - drew findings from a 2013 USAID analysis that forecasts higher temperatures, more rainfall and sea level rise for the region by 2050.

Worker productivity is projected to suffer heavily, costing $8 billion per year in lost work days due to illnesses such as heat rash, fatigue and stroke, particularly among farmers and construction workers.

"There are tens of millions of open-air workers likely to experience greater levels of heat stress and heat-related illnesses when temperatures start rising above 40 degrees Celsius," report author John Talberth said in a statement.

"So much of the Lower Mekong Basin's economy is based on outdoor labor, worker productivity should be front and center for any adaptation plans, and fast-tracked."

The report suggests preventive actions such as changes to working hours and redirecting spending on urban growth towards greener cities "to make life more hospitable as temperatures rise".

The cost of falling crop yields due to storms, rising sea levels, flooding and higher temperatures was projected at $2.5 billion, and $430 million for hydroelectric power production.

Most Mekong River tributaries have dams in place or planned, with 71 projects expected to be operational by 2030, the report said, identifying 11 hydropower facilities in locations of projected increases in temperature and potential drought.

An aim of the analysis is to help governments make plans and investments to avoid exposure to climate risks.

"If we address the problem early enough - and it is still early enough - we can make investments that are likely to pay off many times over," Talberth said.

(Reporting by Alisa Tang, Editing by Ros Russell.)

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