Sea

The sea is the interconnected system of all oceanic waters on Earth. This massive system includes the Atlantic, Pacific, Indian, Southern, and Arctic Oceans.

While we often use the word "ocean" for the largest parts, the term "sea" can also describe smaller bodies of water. Many seas are marginal seas, which are sections of the ocean partly enclosed by land.

Some are called Mediterranean seas because they are almost entirely surrounded by land. Others are inland seas that are wholly enclosed. An unusual exception is the Sargasso Sea, which has no coastline. It exists within a circular current called the North Atlantic Gyre.

Seawater is defined by its salinity, which is the amount of dissolved salt in the water. The open ocean typically has a salinity of 35 parts per thousand (‰). This means there are 35 grams of solids in every liter of water.

Sodium chloride, or common table salt, makes up about 85 percent of these solids. The water also contains ions of magnesium, calcium, potassium, and mercury. Salinity levels change based on local conditions. High evaporation rates can increase salt levels, as seen in the northern Red Sea. Conversely, heavy precipitation or river runoff can lower salinity. The Baltic Sea is considered brackish because many rivers flow into it.

Temperature and light also change as you move through the water. The amount of solar radiation hitting the surface determines the temperature. In tropical regions, surface layers can exceed 30 °C (86 °F). Near the poles, water can be as cold as −2 °C (28 °F).

Light also behaves differently at various depths. Red light is absorbed in the top few meters. Blue and violet light can penetrate as deep as 1,000 meters. However, photosynthesis cannot occur beyond 200 meters because there is insufficient light. This limit defines where most marine plants can grow.

The ocean is constantly in motion through several different mechanisms. Surface currents are driven by winds blowing over the water. These winds create waves and cause water to circulate through drag. Below the surface, deep-sea currents move in a massive system called the global conveyor belt. This system is driven by changes in temperature and density. Cold, dense water sinks near the poles and travels to every ocean. This movement helps moderate Earth's climate by distributing heat.

Additionally, tides cause sea levels to rise and fall roughly twice a day. Tides are caused by the gravitational effects of the Moon and the Sun.

Scientists use several specialized fields to study these complex systems. Oceanography is the broad study of the sea. Physical oceanography focuses on the sea's motion, forces, and waves. Marine biology, or biological oceanography, examines the organisms living in marine ecosystems. Chemical oceanography studies the behavior of elements like carbon and oxygen.

Marine geology, also called geological oceanography, explores the seabed and tectonic activity. This field provided evidence for continental drift and the structure of the Earth. Finally, marine and maritime geography charts the physical shape of the sea.

The scale of the sea is immense and vital to the planet. Earth's seas contain approximately 1,335,000,000 cubic kilometers of water. This covers about 71 percent of the Earth's surface.

The sea plays a massive role in the carbon cycle. The oceans absorb about 30–40% of the carbon dioxide added to the atmosphere. This absorption can lead to ocean acidification, which lowers the pH of the water. For much of history, the average pH was about 8.2, but it has recently dropped below 8.1. This chemical change is a significant part of modern climate study.

Humans have maintained a deep connection to the sea throughout history. We rely on the sea for food, including fish, shellfish, and seaweed.

Beyond food, the sea is used for trade, travel, and mineral extraction. People also use it for leisure, such as swimming and scuba diving.

However, these human activities can lead to marine pollution. The management of these spaces is governed by the law of the sea and admiralty law. Understanding the sea is essential for protecting both human interests and marine life.