Sea levels are rising unevenly in the Indian Ocean, placing millions at risk along low-lying coastlines in Bangladesh, Indonesia and Sri Lanka, scientists say in a study.
Researchers from the University of Colorado and the National Center for Atmospheric Research say the rising sea levels are caused in part by climate change and are triggered by warming seas and changes to atmospheric circulation patterns.
In his acceptance speech for the Nobel Peace Prize last year, US President Barack Obama warned that if the world does nothing to confront climate change, "we will face more drought, famine and mass displacement that will fuel more conflict for decades".
The authors of the latest study say higher seas could exacerbate monsoon flooding, placing crops, homes and livelihoods at greater risk. They argue a better understanding of the changes are needed to improve risk assessment planning for the future.
Sea levels in general are rising globally by about 3 mm (0.1181 inch) a year. Scientists blame rising temperatures caused by the growing amounts of greenhouse gases, such as carbon dioxide from burning fossil fuels, that trap heat in the atmosphere.
Oceans are absorbing a large part of this extra heat, causing them to expand and sea levels to rise. Warmer temperatures are also causing glaciers and parts of the ice blanketing Greenland and West Antarctica to melt.
The team of researchers in their study used long-term tide gauge data, satellite observations and computer climate models to build a picture of sea level rises in the Indian Ocean since the 1960s.
They found that sea-level rise is particularly high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java and that these areas could suffer rises greater than the global average.
But they also found that sea levels are falling in other areas. The study indicated that the Seychelles Islands and Zanzibar off Tanzania's coast show the largest sea-level drop.
"Global sea level patterns are not geographically uniform," said co-author Gerald Meehl of NCAR in Boulder, Colorado.
The study is published in the latest issue of the journal Nature Geoscience.
A key player in the process is the Indo-Pacific warm pool, a large oval-shaped area spanning the tropical oceans from the east coast of Africa to the International Date Line in the Pacific.
The pool has warmed by about 0.5 degrees Celsius (1 degree Fahrenheit) over the past 50 years, primarily because of mankind's greenhouse gas emissions. The warmer water has strengthened two major atmospheric circulation patterns that have a major impact on sea levels.
"Our new results show that human-caused atmosphericoceanic circulation changes over the Indian Ocean, which have not been studied previously,contribute to the regional variability of sea-level change," the researchers say in the study.
The two main wind patterns in the region are the Hadley and Walker circulations.
In the Hadley circulation, air currents rise above strongly heated tropical waters near the equator and flow poleward at upper levels, then sink to the ocean in the subtropics and cause surface air to flow back toward the equator.
The Walker circulation causes air to rise and flow westward at upper levels, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool.
Strengthening of these two patterns could have far-reaching impacts on AsianAustralian monsoons, Indonesian floods and drought in Africa, the study says.