Sea level is the average height of the sea surface. Scientists have calculated a mean sea level based on observations around the world. The mean average sea level takes into account all stages of ocean tides over a 19-year period. Numerous irregularities and slopes on the sea surface make it difficult to calculate an accurate measure of sea level.

What is the difference between an ocean and a sea?

There is no neatly defined distinction between ocean and sea. One definition says the ocean is a great body of interconnecting saltwater that covers 71 percent of Earth’s surface. There are five major oceans—the Arctic, Atlantic, Indian, Pacific, and Southern—but some sources do not include the Arctic Ocean, calling it a marginal sea. The terms “ocean” and “sea” are often used interchangeably but a sea is generally considered to be smaller than an ocean. The name is often given to saltwater areas on the margins of an ocean, such as the Mediterranean Sea.

How much salt is in brackish water?

Brackish water has a saline (salt) content between that of freshwater and sea water. It is neither fresh nor salty, but somewhere in between. Brackish waters are usually regarded as those containing 0.5 to 30 parts per thousand salt, while the average saltiness of seawater is 35 parts per thousand.