Sea Level Rise Threatens Some Areas More than Others

Flooding in Bangladesh and India that accompanies the seasonal monsoon could get even worse due to changes in ocean levels, as some areas experience higher sea levels than others, a new study suggests.

Sea levels in the Indian Ocean are rising in some areas and falling in others, which is at least partly a result of climate change influencing how the wind circulates air above the surface of the ocean, said study team member Weiqing Han, a climatologist at the University of Colorado in Boulder.

If human-caused warming continues to alter these complex circulation patterns , the study's models predict more rain in the eastern tropical regions of the Indian Ocean and drought in the western equatorial Indian Ocean region.

Modeling sea level changes in the Indian Ocean is complex, however, and natural changes not caused by man-made warming could also be at play, Han told OurAmazingPlanet.

The study also shows that the melting poles aren't the only concern when it comes to rising sea levels.

"If you mention sea level rise to some people, they think about the ice sheets melting and dumping water into the ocean," said climatologist Glenn Milne of the University of Ottawa in Canada, who was not involved in the research. "But this study shows it's more complicated than that."

Sea level shift

The key player in the Indian Ocean's sea level change is the Indo-Pacific warm pool, which is an enormous, bathtub-shaped area spanning a swath of the tropical oceans from the east coast of Africa to the International Date Line in the Pacific. The warm pool has heated by about 1 degree Fahrenheit (0.6 degrees Celsius) in the past 50 years, primarily because of human-generated emissions of greenhouses gases, suggest the study's authors.

Two wind patterns in the Indian Ocean, known as the Hadley circulation and the Walker circulation, interact with the Indo-Pacific warm pool to drive sea level changes. These wind patterns could become supercharged due to human-caused climate change.

"Our results from this study imply that if future anthropogenic warming effects in the Indo-Pacific warm pool dominate natural variability, mid-ocean islands such as the Mascarenhas Archipelago, coasts of Indonesia, Sumatra, and the north Indian Ocean may experience significantly more sea level rise than the global average," Han said.

Uneven rise

Sea levels are rising, on average, 5 inches (13 centimeters) per century along the north Indian Ocean. But in reality, sea levels do not rise the same amount everywhere, Han said.

Using sophisticated ocean and climate models, past data and observations, the researchers found that while sea levels are rising in a number of areas in the Indian Ocean, sea levels are falling in other areas.

Sea level rise is particularly high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java. Sea levels in parts of this region could rise as high as 7 inches (18 cm) per century.

The Seychelles Islands and the island of Zanzibar off Tanzania's coast show the largest sea level drop. Coastal towns in this region could see sea levels retreat by up to 6 inches (15 cm) per century, the study suggests.

Understanding these sea level patterns and how they will play out in different spots along the Indian Ocean is important for risk assessment, Han said.

Towns where the sea level is creeping further inland may flood easier during the summer monsoons (seasonal reversals in the wind) that bring torrential rains from the southwest.

"It is important for us to understand the regional changes of the sea level, which will have effects on coastal and island regions," said study co-author Aixue Hu of the U.S. National Center for Atmospheric Research in Boulder, Colo.

The study is detailed in the July 11 edition of the journal Nature Geoscience.

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