Indian Ocean
The Indian Ocean covers 70560000 square kilometres, roughly 20% of the water on Earth's surface, yet it does not stretch from pole to pole like its larger siblings. It is enclosed by Asia to the north, Africa to the west, and Australia to the east, an ocean hemmed in by land on three sides. Geologists call it the youngest of the oceans. It is also the warmest, and its waters carry a memory that reaches back tens of thousands of years.
More than two billion people live in countries that border it. Their crops, their faiths, their fortunes, and even their disasters have moved across this water for thousands of years. Why does an ocean only 20 million years removed from a vanished sea behave so differently from the Atlantic or the Pacific? How did calm waters and a seasonal wind turn this basin into what some historians call the first global economy? And what happens to a sea already running a fever as the planet grows warmer? The answers begin with the wind.
Asia blocks the heat from escaping the Indian Ocean, trapping warmth and preventing the ventilation of its thermocline. This makes the basin the core of a large-scale Tropical Warm Pool, and that warm pool, interacting with the atmosphere, shapes climate both regionally and globally. The Indian Ocean is the warmest ocean in the world.
The monsoon driven by Asia is the strongest on Earth. Strong north-east winds blow from October until April, while south and west winds prevail from May until October. The word itself comes from mawsim, the Arabic word for season, used by sailors long before a navigator named Hippalus was credited with discovering the winds in the 1st century. When the seasons turn, the ocean obeys, even reversing the direction of the Somali Current.
Some 80% of India's total annual rainfall arrives in a single summer surge. The dependence runs so deep that many civilisations perished when the monsoon failed. That fragility is ancient. Records show a strong, wet phase between 33,500 and 32,500 years before present, a weak dry phase, and a very weak phase between 17,000 and 15,000 years before present, tracking dramatic global swings like the Younger Dryas.
The warming now underway is rapid and continuous. Between 1901 and 2012 the ocean rose about 1.2 degrees Celsius, with the sharpest heating in the northwestern Indian Ocean and the Arabian Sea. Climate models predict accelerated warming of 1.7 to 3.8 degrees Celsius per century from 2020 to 2100. By the end of the century, projections push the tropical Indian Ocean toward a near-permanent heatwave, with marine heatwaves rising from 20 days a year to between 220 and 250 days a year.
The Tethys Ocean fragmented only 20 million years ago, splitting into the Mediterranean Sea and the Indian Ocean. That makes this the youngest of the major oceans, and its floor still bears the marks of recent birth: narrow continental shelves that make up just 15% of the basin, and an average depth of 3,741 metres dropping to a maximum of 7,290.
The opening began far earlier, around 156 million years ago, when Africa separated from East Gondwana. The Indian Subcontinent began pulling away from Australia and Antarctica between 135 and 125 million years ago, and as the Tethys closed north of India, the Indian Ocean opened behind it. The basin is still moving. Since 20 million years ago the East African Rift System has been dividing the African plate into the Nubian and Somalia plates.
Hotspots have written their tracks across the sea floor. The Reunion hotspot, active 70 to 40 million years ago, links Reunion and the Mascarene Plateau to the Chagos-Laccadive Ridge and the Deccan Traps of north-western India. The Kerguelen hotspot, active 100 to 35 million years ago, connects the Kerguelen Islands to the Ninety East Ridge and the Rajmahal Traps of north-eastern India.
Unlike the trench-ringed Pacific, the Indian Ocean has only two trenches: the 6000 kilometre Java Trench and the 900 kilometre Makran Trench south of Iran and Pakistan. Spreading ridges meet at the Rodrigues Triple Point, where three plates pull apart at once. One of those ridges spreads so slowly that its floor never goes bare, an oddity that would later reveal life no one expected to find there.
The western Indian Ocean hosts one of the largest concentrations of phytoplankton blooms in summer, fed by monsoon winds that drive upwelling and lift nutrients into the sunlit zone. Those blooms form the base of a food web that feeds the larger fish, and the ocean accounts for the second-largest share of the most economically valuable tuna catch. Fishing fleets from Russia, Japan, South Korea, and Taiwan work these waters for shrimp and tuna alongside the bordering countries.
Mangroves cover 80984 square kilometres here, almost half of the world's mangrove habitat, and 42500 of those square kilometres lie in Indonesia alone. Coral reefs cover roughly 200000 square kilometres. These coastal systems are among the most productive on Earth, with coastal areas yielding 20 tonnes of fish per square kilometre, even as rising sea surface temperatures spread coral bleaching.
The basin shelters endangered and vulnerable creatures: the blue whale, the dugong, the Irrawaddy dolphin of Southeast Asia, and sea turtles including the green, leatherback, and olive ridley. Their numbers tell mixed stories, some increasing, many decreasing. Meanwhile the marine plankton has declined by up to 20% over the past six decades, and tuna catch rates have fallen between 50 and 90% over the past half-century.
In 2016, six new animal species were found at hydrothermal vents on the Southwest Indian Ridge, among them a crab nicknamed the Hoff and a giant peltospirid snail. The most famous survivor was found far earlier. The West Indian Ocean coelacanth, a lobe-finned fish thought extinct for 66 million years, turned up off South Africa in the 1930s, and a second species was discovered off Sulawesi in the late 1990s.
Nine of Earth's 36 biodiversity hotspots, fully a quarter of them, sit on the margins of the Indian Ocean. Madagascar and the western islands alone hold 13,000 species of plants, 11,600 of them found nowhere else, a concentration that has made the region a textbook case for the study of evolution.
The chameleons tell a strange story. Rather than spreading from a continent to an island, they first diversified on Madagascar and then colonised Africa, a reverse colonisation. Dung beetles, day geckos, and lemurs all became classic examples of adaptive radiation on these islands.
In the Mare aux Songes swamp on Mauritius, scientists have found bones packed 250 to a square metre, including the remains of the Dodo bird and the Cylindraspis giant tortoise. The same remains point to an aridification of the southwest Indian Ocean that began around 4,000 years ago.
On the African mainland, the white rhinoceros of the Maputaland-Pondoland-Albany hotspot recovered from fewer than 20 individuals in 1895 to more than 17,000 by 2013. Other hotspots fared worse. In Somalia, the centre of a 1500000 square kilometre hotspot, mismanagement and overgrazing have left only about 5% of the original habitat, home to the rare Somali cyclamen, the only cyclamen outside the Mediterranean.
Food globalisation began on the Indian Ocean littoral around 4,000 years ago. Five African crops, including sorghum and pearl millet, somehow reached Gujarat in India during the Late Harappan period between 2000 and 1700 BCE. Gujarati merchants became the first explorers of the ocean, trading African ivory and tortoise shells. Around 2000 BCE black pepper and sesame appeared in Egypt, and the banana reached Africa around 3000 years ago.
The calmer waters opened this basin to trade earlier than the Atlantic or Pacific. The powerful monsoons let ships sail west early in the season, wait a few months, then return eastwards, a rhythm that carried ancient Indonesian peoples all the way to Madagascar around 1 CE. Sherds of Ubaid pottery found at Dilmun, present-day Bahrain, mark an exchange network reaching back to Mesopotamia, where Sumerians traded grain and bitumen for copper, tin, dates, and pearls.
Greek geographers added the ocean to their maps after Scylax of Caryanda journeyed to India for the Persian king Darius in the 6th century BCE. Alexander the Great ordered a circumnavigation of the Arabian Peninsula in 323 BCE. The Ptolemies of Egypt explored the African coast, though their main interest was military: they hunted for war elephants.
Historians sometimes call this the first global economy, a web linking Asia, China, India, and Mesopotamia through the monsoon. It developed independently from the trade of the Mediterranean and Atlantic and stayed largely separate from them until European colonial dominance in the 19th century. The guide known as the Periplus of the Erythraean Sea, written in the first century CE, shows that Roman and Greek sailors had already mastered the monsoon winds.
Arabic missionaries and merchants began spreading Islam along the western shores from the 8th century, if not earlier. A Swahili stone mosque dating to the 8th to 15th centuries has been found at Shanga in Kenya, and trade across the ocean gradually carried Arabic script and rice into Eastern Africa. From 1405 to 1433, the Ming admiral Zheng He is said to have led great fleets on treasure voyages that reached the coastal countries of East Africa.
Vasco da Gama rounded the Cape of Good Hope in 1497 and became the first European to sail to India. Along the African coast he met the Swahili, who already traded with India and China. The Portuguese kidnapped most of their pilots in coastal raids, though a sailor from Gujarat was given as a gift by a ruler in Malindi, Kenya, and helped them reach India. After 1500 the Portuguese attacked and colonised cities along the African coast.
The slave trade ran through these waters for a thousand years. Muslim merchants traded an estimated 1,000 African slaves a year between 800 and 1700, a figure that grew to roughly 4,000 in the 18th century. The Dutch East India Company drove the volume sharply higher, and some 4,000 African slaves were used to build the Colombo fortress in Dutch Ceylon. The island of Zanzibar became the center of the trade in the 19th century, with as many as 50,000 slaves passing through its port each year.
The Ottoman Empire pushed into the ocean in 1517 with Sultan Selim I's conquest of Egypt. The region was new to them, yet Muslim geographers had mapped it centuries earlier, and the scholar Ibn Battuta had visited most of the known world in the 14th century. The Arab navigator Ahmad ibn Majid, a contemporary of da Gama, had already compiled his own guide to navigating these seas.
More than 80 percent of the world's seaborne oil trade passes through the Indian Ocean and its chokepoints: 40 percent through the Strait of Hormuz, 35 percent through the Strait of Malacca, and 8 percent through the Bab el-Mandab Strait. An estimated 40% of the world's offshore oil production comes from this ocean, drawn from fields off Saudi Arabia, Iran, India, and Western Australia. The maritime Silk Road threads through it, from Chinese container ports through Malacca and past Colombo, Male, and Mombasa to Djibouti and the Suez Canal.
The Suez Canal opened in 1869 as the Industrial Revolution reshaped global shipping, shifting trade toward East Asia and Australia. The canal also carried life it never intended to. The goldband goatfish replaced the red mullet in the Mediterranean, and since the 1980s huge swarms of jellyfish have clogged power and desalination plants along the Levantian coast.
Disaster has struck this ocean hard. On the 26th of December 2004, fourteen countries were hit by tsunamis from the 2004 Indian Ocean earthquake. The waves crossed the ocean at speeds above 500 kilometres per hour, rose as high as 20 metres, and killed an estimated 236,000 people. A decade later, on the 8th of March 2014, Malaysia Airlines Flight 370, a Boeing 777 carrying 239 occupants, vanished and is thought to have crashed into the southern Indian Ocean about 2500 kilometres from southwest Western Australia. Despite an extensive search, the wreckage has never been found.
The basin still holds open questions of sovereignty and isolation. The Sentinelese people of North Sentinel Island have been called the most isolated people in the world. And in February 2019, the International Court of Justice in The Hague issued an advisory opinion that the United Kingdom must transfer the disputed Chagos Archipelago to Mauritius, a quarrel over a few specks of land in the youngest of the oceans.
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Common questions
How big is the Indian Ocean and where is it located?
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering 70560000 square kilometres or about 20% of Earth's water surface. It is bounded by Asia to the north, Africa to the west, Australia to the east, and the Southern Ocean or Antarctica to the south.
Why is the Indian Ocean the warmest ocean in the world?
The Indian Ocean is the warmest ocean because Asia blocks heat from escaping and prevents the ventilation of its thermocline, forming the core of a large-scale Tropical Warm Pool. It warmed about 1.2 degrees Celsius between 1901 and 2012, with models predicting 1.7 to 3.8 degrees Celsius of further warming per century from 2020 to 2100.
How old is the Indian Ocean geologically?
The Indian Ocean is the youngest of the major oceans. It was directly preceded by the Tethys Ocean, which fragmented into the Mediterranean Sea and the Indian Ocean only 20 million years ago, and its opening began around 156 million years ago when Africa separated from East Gondwana.
What is the Indian Ocean monsoon and why does it matter?
The Indian Ocean monsoon is the strongest on Earth, driven by Asia, with north-east winds from October to April and south and west winds from May to October. Some 80% of India's annual rainfall arrives during summer, and many civilisations perished in the past when the monsoon failed.
Why is the Indian Ocean important for global trade?
The Indian Ocean carries more than 80 percent of the world's seaborne oil trade through chokepoints including the Strait of Hormuz at 40 percent, the Strait of Malacca at 35 percent, and the Bab el-Mandab Strait at 8 percent. An estimated 40% of the world's offshore oil production comes from the Indian Ocean.
What major disasters have happened in the Indian Ocean?
On the 26th of December 2004, tsunamis from the Indian Ocean earthquake hit fourteen countries, reaching up to 20 metres in height and causing an estimated 236,000 deaths. On the 8th of March 2014, Malaysia Airlines Flight 370, carrying 239 occupants, disappeared and is thought to have crashed into the southern Indian Ocean, with the wreckage never found.