The Rising Indian Ocean
Changing sea levels have happened before and will happen again in a dynamic world. Newly detected rising sea levels in parts of the Indian Ocean, including the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java, appear to be at least partly a result of human induced increases of atmospheric greenhouse gases, says a study led by the University of Colorado at Boulder. The study, which combined sea surface measurements going back to the 1960s and satellite observations, will threaten inhabitants of some coastal areas and islands.
Sea level has changed often over geologic time.
During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 400 feet lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice. The majority of this melted by about 10,000 years ago causing sea levels to rise.
Hundreds of similar glacial cycles have occurred throughout the Earth's history. Geologists who study the positions of coastal sediment deposits through time have noted dozens of shifts of shorelines associated with a later recovery. This results in sedimentary cycles which in some cases can be correlated around the world with great confidence.
The key player in the rising Indian Ocean process is the Indo-Pacific warm pool, an enormous, bathtub shaped area of the tropical oceans stretching from the east coast of Africa west to the International Date Line in the Pacific. The warm pool has heated by about 1 degree Fahrenheit, or 0.5 degrees Celsius, in the past 50 years.
"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," said Han of Colorado University at Boulder's atmospheric and oceanic sciences department.
While a number of areas in the Indian Ocean region are showing sea level rise, the study also indicated the Seychelles Islands and Zanzibar off Tanzania's coastline show the largest sea level drop. Global sea level patterns are not geographically uniform, and sea rise in some areas correlate with sea level fall in other areas.
The Indian Ocean is the world's third largest ocean and makes up about 20 percent of the water on Earth's surface. The ocean is bounded on the west by East Africa, on the north by India, on the east by Indochina and Australia, and on the south by the Southern Ocean off the coast of Antarctica.
The international research team used several different sophisticated ocean and climate models for the study, including the Parallel Ocean Program -- the ocean component of the Community Climate System Model. In addition, the team used a wind driven, linear ocean model for the study.
"Our new results show that human caused changes of atmospheric and oceanic circulation over the Indian Ocean region -- which have not been studied previously -- are the major cause for the regional variability of sea level change," wrote the authors in Nature Geoscience.
Han said that based on all season data records, there is no significant sea level rise around the Maldives. But when the team looked at winter season data only, the Maldives show significant sea level rise. The smallest Asian country, the Maldives is made up of more than 1,000 islands -- about 200 of which are inhabited by about 300,000 people -- and are on average only about five feet above sea level with a maximum of eight feet.
The effects on the Maldives are here now. In 1987, unusually high tides swept over the country and inundated the capital city, Male. The Asian tsunami of December 26, 2004 killed 82 people, displaced an additional 12,000 and caused extensive damage to the country's important tourism industry. As a result, the Maldives' GDP contracted by 3.6 percent in 2005.
The complex circulation patterns in the Indian Ocean may also affect precipitation by forcing even more atmospheric air down to the surface in Indian Ocean subtropical regions than normal, Han speculated. "This may favor a weakening of atmospheric convection in the subtropics, which may increase rainfall in the eastern tropical regions of the Indian Ocean and increase drought in the western equatorial Indian Ocean region, including east Africa," Han said.
The new study indicates that in order to document sea level change on a global scale, researchers also need to know the specifics of regional sea level changes that will be important for coastal and island regions, said Hu who is one of the authors. Along the coasts of the northern Indian Ocean, seas have risen by an average of about 0.5 inches per decade.
For further information: http://www.colorado.edu/news/r/4db6c47a28ba81f1853061bfd4260128.html