October 28, 2021
The sea is a large salt water surrounded wholly or partly by land. In a broader sense, "sea" is a system of interconnected saltwater oceanic waters on Earth that are considered as one global ocean or as several major oceans. The ocean influences Earth's climate and has an important role in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been explored and navigated since prehistoric times, the modern scientific study of the sea, namely oceanography, only started with the HMS Challenger expedition from Great Britain in the 1870s. The oceans are generally divided into five major oceans which include the four oceans recognized by the International Hydrographic Organization (Atlantic, Pacific, Indian and Arctic Oceans) and the Southern Ocean. Due to continental drift, the Northern Hemisphere currently has a larger land-to-sea ratio. balanced (approximately 2:3) than the Southern Hemisphere, which is almost entirely ocean (1:4,7). Salinity levels in the high seas are generally around 3.5%, but variations can be found in waters that are more landlocked, near the mouths of large rivers, or at great depths. About 85% of the dissolved substance in the open ocean is sodium chloride. Differences in salinity and temperature between ocean areas give rise to thermohaline currents. The influence of waves, generated by wind and by ocean tides, creates surface currents. The direction of current flow is regulated by land at the surface and under the sea and by the Coriolis effect due to the rotation of the Earth. Changes in sea level in the past left the continental shelf, which is a shallow area in the sea that is close to land. This nutrient-rich area is inhabited by life that is a source of food for humans such as fish, mammals, crustaceans, molluscs, and seaweed, both caught from the wild and raised in ponds. The most diverse marine biodiversity is found in tropical coral reef areas. In the past, whaling on the high seas was common, but declining whale numbers have prompted conservation efforts from various countries that have resulted in a moratorium on commercial whaling. Life in the sea can also be found at depths far from the reach of sunlight. Deep sea ecosystems are supported by the availability of nutrients from hydrothermal vents. Life on Earth probably started there and water microbes are generally thought to be the trigger for the ancient increase in oxygen in Earth's atmosphere. Both plants and animals first evolved in the sea. The sea is also an important element for human trade, transportation, and industrial activities as well as a source of power generation. These things make the sea taken into account in the strategy of war. On the other hand, the sea can also be a source of disaster threats such as tsunamis and tropical cyclones. These influences make the sea an important aspect of human culture. From the various sea gods that can be found in various cultures, the epic poems by the Ancient Greek writer Homer, or the burial of humans at sea to the changes brought about by the Columbus Exchange, hyperrealist marine art, and sea-inspired music such as "The Sea and Sinbad Ship" by Nikolai Rimsky-Korsakov. The sea is also a place for human leisure activities such as swimming, diving, surfing, and sailing. However, population growth, industrialization, and intensive agriculture are now causing marine pollution. The increasing amount of carbon dioxide in the atmosphere lowers the pH value of the oceans through the process of ocean acidification. Overfishing is also a problem for the jointly owned sea.