The region is home to a fifth of the world's population and is already vulnerable to climate extremes: seasonal floods, cyclones and droughts that ravage vast swathes of agricultural land and displace hundreds of thousands of people every year.
The costs of countering climate change in South Asia will also increase over time and will be prohibitively high in the long term, the ADB's "Assessing the Costs of Climate Change and Adaptation in South Asia" report said.
"Without global deviation from a fossil-fuel-intensive path, South Asia could lose an equivalent of 1.8 percent of annual GDP by 2050, which will progressively increase to 8.8 percent by 2100 on the average under the business-as-usual scenario," it said.
The Maldives will also be hardest hit in the next few decades, with a loss of 2.3 percent of GDP. Bangladesh, Bhutan, India, Nepal and Sri Lanka will lose 2 percent, 1.4 percent, 1.8 percent, 2.2 percent, and 1.2 percent, respectively, by 2050.
Those countries, excluding Sri Lanka, will see more frequent severe weather, damaging property, infrastructure, agriculture and human health, the ADB said. Between 1990 and 2008, more than 750 million people in South Asia were affected by at least one natural disaster, resulting in almost 230,000 deaths, it said.
The cost of shielding the region against climate change could be lowered if the world's governments significantly cut greenhouse gas emissions and, if the rise in global temperatures was kept below 2.5 degrees Celsius, that cost could be nearly halved to about $40.6 billion, or 0.48 percent of GDP, it said.
South Asia also needs to introduce flood- and saline-resistant crop varieties, better coastal zone management, improved disease surveillance, protection of groundwater and greater use of recycled water.
India, one of the world's largest agrarian economies, is badly at risk, the report said, and may see GDP losses of up to 8.7 percent by 2100.
"Agriculture provides employment and livelihood opportunities to most of India's rural population and changes in temperature and rainfall, and an increase in floods and droughts linked to climate change, would have a devastating impact on people's food security, incomes, and lives," ADB Vice-President Bindu Lohani said in a statement.