Sea

• 1. Introduction

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  The sea holds about 97.2 percent of all the water humans know of, and it covers roughly 71 percent of the surface of the Earth. Earth is the only planet known to carry seas of liquid water on its surface. Beneath that vast skin of salt water, ice crystals knit themselves into pancakes, whales thirty metres long swim above abyssal trenches, and cold currents creep along a thousand-year loop that touches every ocean. Mars has ice caps but no liquid sea of its own. This is a body of water that humans have crossed since they first lashed together a reed boat, yet much of it still lies in permanent darkness, unmapped and unmet. How does a single substance freeze, glow, surge, and breathe all at once? Why does the Moon, twenty-seven million times less massive than the Sun, pull harder on the tides? And what lives in water so deep that no plant can grow there? The answers begin with what the sea is actually made of.

• 2. Salt And Substance

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  Sodium chloride, the same compound as table salt, is the most abundant solid dissolved in seawater. Chloride and sodium together make up about 85 percent of the dissolved solids, with chloride alone at 19.3 parts per thousand and sodium at 10.8. The open ocean carries roughly 35 grams of solids per litre, a salinity expressed as 35 parts per thousand. Magnesium, calcium, potassium, sulphate, carbonate, and bromide round out the mix, alongside trace amounts of elements down to mercury. Salinity is not uniform, even if the recipe stays steady. The Mediterranean Sea sits a little higher at 38 parts per thousand, and the northern Red Sea can climb to 41 because so much water evaporates there. The Baltic Sea runs the other way, fed by many rivers, brackish rather than fully marine. The Dead Sea, a landlocked hypersaline lake, holds 300 grams of dissolved solids per litre, a figure that dwarfs the open ocean. Evaporation and brine rejection during ice formation push salinity up, while rain, melting sea ice, and river runoff dilute it. Despite all this variation, the relative proportions of the dissolved salts stay remarkably stable across the world's oceans. One consequence is unavoidable: seawater is too salty for a person to drink, because human kidneys cannot make urine as salty as the sea. That chemistry shapes not just thirst but the very water that freezes at the poles.

• 3. Ice, Heat, And Light

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  Seawater at a salinity of 35 parts per thousand freezes at about -1.8 degrees Celsius, lower than fresh water. When the surface chills enough, ice crystals form and break into small pieces that gather into a thick suspension called frazil. In calm water this sets into a thin sheet known as nilas, thickening from below. In rougher seas the crystals clump into flat discs called pancakes, which slide together to build floes. Fresh nilas can hold a salinity of 12 to 15 parts per thousand, but a year-old sheet of sea ice drops to 4 to 6 as trapped salt drains away. Temperature governs more than freezing. In the tropics, with the sun overhead, surface layers can climb past 30 degrees Celsius, while polar surface water in balance with sea ice hovers near -2. Deep seawater everywhere on the globe stays between -2 and 5 degrees Celsius, a cold reservoir circulating beneath the warmth. Light fades fast as it sinks. Red light is absorbed within the top few metres, yellow and green reach deeper, and blue and violet may travel as far as 1,000 metres down. Below about 200 metres there is too little light for photosynthesis, so no plant can grow. That dark boundary divides a sunlit garden from a vast lightless interior.

• 4. Waves And Rogue Walls

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  Most ocean waves stand less than 3 metres high, yet rogue waves have been documented above 25 metres. Waves form perpendicular to the wind, beginning as ripples from a gentle breeze and building into ridges as stronger air pushes against the water. Their size depends on the fetch, the distance the wind has blown, along with its strength and how long it lasts. In the Southern Hemisphere's Roaring Forties, where the wind blows almost without pause, long organised masses called swell roll across open water. A wave carries energy, not a horizontal river of water. The crest is its high point, the trough its low point, and the wavelength the gap between crests. When waves from different directions meet, interference can break the surface into irregular seas, and constructive interference can stack into a single unexpected rogue wave far higher than its neighbours. As a wave nears land it changes character. Bending around headlands is refraction, wrapping around rocks is diffraction. When the wave's deepest oscillations brush the seabed, it slows, the crests crowd together, and its height rises in a process called shoaling. Once the ratio of height to depth crosses a limit, the wave breaks, toppling into foaming water that rushes up the beach before gravity drags it back. A different and far rarer wall of water comes not from wind but from the floor of the sea itself.

• 5. Tsunamis And The Restless Crust

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  An underwater earthquake, a landslide, a meteorite impact, or a volcanic eruption can launch a tsunami by lifting or dropping the sea surface across a wide area. In the deep open ocean a tsunami may stretch 80 to 300 miles between crests, race at over 600 miles per hour, and stand less than three feet tall, so it often passes unnoticed. As it reaches shallow water it slows, its wavelength shortens, and its amplitude swells enormously. Either the trough or the crest can arrive first. When the trough comes first, the sea draws back and exposes the shore, a warning that the crest is coming to flood inland. A single geological event often spawns several tsunamis arriving between eight minutes and two hours apart, and the first is not always the largest. These shocks rise from the movements of tectonic plates. The Earth's rigid outer shell, the lithosphere, is fractured into plates that float on the hotter mantle below. At mid-ocean ridges magma pushes up through the seabed and drives plates apart, while elsewhere one oceanic plate slides beneath another in subduction, carving deep trenches. The deepest is the Mariana Trench, running about 2,500 kilometres across the seabed near the Mariana Islands in the West Pacific. Its lowest point lies 10.994 kilometres beneath the surface, nearly seven miles down. That same grinding of plates raises underwater mountains and the chains of volcanic islands that ring the trenches.

• 6. Currents And The Conveyor Belt

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  Five main gyres turn in the world's oceans, two in the Pacific, two in the Atlantic, and one in the Indian Ocean, with a single gyre circling Antarctica. These great circular currents form because wind drags surface water along, and other water flows in to fill the gap. They flow clockwise in the Northern Hemisphere and anticlockwise in the Southern, steered for millennia by the land's shape, the wind, and the Coriolis effect. Warm water carried away from the equator cools, moderating the climate by warming higher latitudes. Beneath the surface runs a slower system. The thermohaline circulation, the global conveyor belt, is driven by differences in water density set by salinity and temperature. Near Greenland, cold salty water sinks and flows south through the Atlantic. Reaching Antarctica it gathers more sinking water, turns east, then splits into streams that climb north into the Indian and Pacific Oceans, warming and rising before looping back. The full circuit takes a thousand years. Smaller, shorter currents work the coastlines. A longshore current runs parallel to shore when waves strike at an angle, shifting sand and pebbles, building spits, and silting channels. A rip current funnels piled-up water back out to sea through a gap in a sandbar, reaching speeds that can carry off an unwary swimmer. Where wind pushes surface water away from land, cold nutrient-rich water wells up, feeding blooms of phytoplankton that lift the whole productivity of the sea. That upwelled abundance feeds a food web reaching from the smallest drifting larvae to the largest whales.

• 7. A World Of Living Water

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  Half the world's oxygen is estimated to come from phytoplankton, the drifting plants of the sea, and about 45 percent of marine primary production is contributed by diatoms alone. Marine life ranges from whales 30 metres long down to microscopic plankton, fungi, and bacteria, spread across habitats from sunlit reefs to lightless trenches. Coral reefs, called the rainforests of the sea, cover less than 0.1 percent of the ocean surface yet shelter 25 percent of all marine species, among them Australia's Great Barrier Reef. Most marine life clusters in coastal habitats, even though the continental shelf is only 7 percent of the total ocean area. Where light fails, life finds other fuel. Around deep-sea hydrothermal vents, communities are built on sulphide-oxidising chemoautotrophic bacteria rather than sunlight, feeding bivalves, crabs, worms, and fish found nowhere else. A dead whale drifting to the bottom feeds its own assembly of organisms relying on sulphur-reducing bacteria. Life itself may have begun in the sea. The Miller-Urey experiments suggested a dilute chemical soup in open water, while other proposals point to volcanic hot springs, fine clay sediments, or deep-sea black smoker vents, each sheltered from the ultraviolet radiation the early atmosphere did not block. Humans have been reaching across that living water for thousands of years.

• 8. Humans Across The Sea

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  By about 3000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia, and their Lapita descendants navigated from the Bismarck Archipelago to Fiji, Tonga, and Samoa, later finding Hawaii, Rapa Nui, and New Zealand on outrigger canoes. In the early fifteenth century the Chinese Ming Dynasty sent a fleet of 317 ships and 37,000 men under Zheng He across the Indian and Pacific Oceans. Bartolomeu Dias rounded the Cape of Good Hope in 1487, Vasco da Gama reached India by that route in 1498, and Christopher Columbus sailed from Cadiz in 1492 only to make landfall in the Caribbean. In 1519 Ferdinand Magellan led the Spanish Magellan-Elcano expedition that became the first to sail around the world. Knowing position at sea took new instruments. Latitude could be read with an astrolabe, Jacob's staff, or sextant, but longitude waited on an accurate clock, and in 1759 the clockmaker John Harrison built one that James Cook later carried. The science of the sea grew from these voyages. The Challenger expedition of 1872 to 1876 effectively created oceanography, logging about 4,700 new marine species and 492 deep-sea soundings across a 68,890-nautical-mile journey. Edward Forbes had argued in 1854 that no life could exist below about 600 metres, a claim disproved in 1868 when W. B. Carpenter and C. Wyville Thomson dredged living creatures from deep water. Humans also fought, traded, and fed on these waters. Battles have been waged at sea for more than 3,000 years, from the Hittite king Suppiluliuma II burning an Alashiya fleet around 1210 BC to the carrier clashes of the Pacific War. Today seaborne trade carries more than 4 trillion US dollars of goods a year, and in 2011 the world produced an estimated 154 million tonnes of fish. The same sea that may have birthed life now carries the conveyor belt of human commerce, one container ship at a time.