Oceans are large bodies of saltwater that surround the continents and occupy the basins between them. Ocean basins are the part of the seafloor that lies beyond the margins of the continents, generally in water deeper than 600 feet (183 meters).
Background and Scientific Foundations
As Earth formed in a cloud of gas and dust more than 4.5 billion years ago, a huge amount of lighter elements, including hydrogen (H) and oxygen (O), became trapped inside the planet as the gases condensed and formed molten rock. Materials of different densities separated out. In the young planet's molten interior, heavy elements sank and light elements rose.
Gases rose through thousands of miles of molten and melting rock. They erupted on the surface through volcanoes and fissures. Within the planet and above the surface, oxygen combined with hydrogen to form water (H20). Enormous amounts of water—enough to fill oceans if it were liquid—covered the globe as an incredibly dense atmosphere of water vapor. Near the top of the atmosphere, where heat could be lost to outer space, water vapor condensed to liquid and fell back into the water vapor layer below, cooling the layer. This atmospheric cooling process continued until the first raindrops fell to Earth's surface. Over thousands of years, and perhaps at different times during the life of Earth, the surface depressions filled to create the oceans.
It may be that most of the water on Earth today has been cycling between the oceans, the land, and Earth's atmosphere for more than four billion years. Small amounts of "new" water continue to escape the planet's interior from volcanoes today. The ocean is the largest part of the hydrologic cycle. Changes to it can alter eustatic sea level.
Water absorbs a large amount of heat energy before its temperature changes. In other words, water must lose a large amount of heat energy before it cools noticeably. Water, more than air or earth, tends to remain at the temperature at which it is already. Water is not given to sudden, wild extremes of temperature. It has a strong moderating effect on climates. Where there is water, there are more moderate temperatures. A maritime (oceanlike) climate tends to be moister and subject to less change in temperature than regions far from the ocean such as the interior of the United States.
Nearly all coasts experience this maritime effect. It is especially apparent along coasts where there are large ocean currents. The moderating effects of the Gulf Stream are a good example. Caribbean sunshine warms the waters of the Gulf Stream in the tropics. This warm water flows up the east coast of the United States and Canada. It then crosses the Atlantic to the coast of Western Europe. This warm current is why England's climate is so much warmer than areas at about the same latitude in North America. When the Gulf Stream moves southward, Western Europe experiences extreme cold. The last such event, during the fourteenth through nineteenth century, is known as the "Little Ice Age."
Issues and Developments
In June 2012, Australia started the world's largest network of marine reserves (protected conservation areas). When complete, these will include the Great Barrier Reef and the Coral Sea. Australia plans to increase its number of marine reserves from 27 to 60 covering 1.2 million square miles (3.1 million km2) of ocean.
In September 2016, the International Union for Conservation of Nature (IUCN) released a report asserting that the world's oceans are near their limit in absorbing the excess heat produced thus far by global warming. Dying corals and rapid depletion of fish stocks are warning signals of more looming extremes in weather events, including droughts and hurricanes. The report surveys data showing that the world's oceans have acted as a buffer, absorbing 93 percent of the carbon dioxide released by human activities. Oceans are predicted to rise and grow warmer by 1.8 to 7.2 degrees Fahrenheit (1 to 4 degrees Celsius) by the year 2100.
Increasing levels of atmospheric carbon dioxide also are increasing oceanic acidification. Reports in both 2013 and 2019 report by the scientists from the Arctic Monitoring and Assessment Program (AMAP) found that the greatest changes occurred in the Arctic where cooler waters absorb CO2 faster and sea-ice loss has exposed more sea-surface area to airborne emissions.
In 2019, a United Nations Environment Programme (UNEP) report showed that solid waste and trash pollution continue to threaten the world's oceans. Those reports are similar to those by the U.S. National Oceanic and Atmospheric Administration (NOAA) that estimate that hundreds of thousands of marine mammals may die each year from trash-related ocean pollution. Discoveries of floating trash and small particles of plastic in both the Atlantic and Pacific Oceans increased prior estimates of the extent of floating surface pollution in the world's oceans.
In addition to dead zones created by fertilizer and other chemical runoff, scientists are concerned that the quantity of plastic in the world's oceans is growing. Some estimates say it has increased more than one hundred-fold over the last 40 years. Scientists and environmentalists say that use of non-biodegradable materials and overuse of disposable products that end up in waterways are prime causes of the floating trash. Plastics in both marine and terrestrial environments can pose special life-threatening dangers to wildlife populations. Plastic components can strangle animals and birds, obstruct digestive and excretory systems, and/or release toxic poisons that result in illness or death. In 2020, the Ocean Conservancy noted that each year 8 million metric tons of plastic trash is added to the estimated 150 million tons already in our marine environments.