To most people sea level is the point at which the surface of the land and sea meet. Officially known as the sea level datum plane, it is a reference point used in measuring land elevation and water depths. It refers to the vertical distance from the surface of the ocean to some fixed point on land, or a reference point defined by people. Sea level became a standardized measure in 1929. Mean sea level is the average of the changes in the level of the ocean over time, and
it is to this measure that we refer when we use the term sea level.
Constant motion of water in the oceans causes sea levels to vary. Sea level in Maine is about 10 in (25 cm) higher than it is in Florida. Pacific coasts sea level is approximately 20 in (50 cm) higher than the Atlantic.
Rotation of Earth causes all fluids to be deflected when they are in motion. This deflection (or curvature of path) is known as the Coriolis effect. Ocean water and atmospheric winds are both influenced in the same way by the Coriolis effect. It creates a clockwise deflection in the northern hemisphere and a counterclockwise deflection in the southern.
Mean sea level can also be influenced by air pressure. If the air pressure is high in one area of the ocean and low in another, water will flow to the low pressure area. Higher pressure exerts more force against the water, causing the surface level to be lower than it is under low pressure. That is why a storm surge (sea level rise) occurs when a hurricane reaches land. Air pressure is unusually low in the eye of a hurricane, and so water is forced towards the eye, creating coastal flooding.
Increases in temperature can cause sea level to rise. Warmer air will increase the water temperature, which causes water molecules to expand and increase the volume of the water. The increase in volume causes the water level to become higher.
Mean sea level has risen about 4 in (10 cm) during the last hundred years. Several studies indicate this is due to an average increase of 1.8°F (1°C) in world-wide surface temperatures. Some scientists believe rising sea levels will create environmental, social, and economic problems, including the submerging of coastal lands, higher water tables, salt water invasion of fresh water supplies, and increased rates of coastal erosion.
Sea level can be raised or lowered by tectonic processes, which are movements of Earth's crustal plates. Major changes in sea level can occur over geologic time due to land movements, ice loading from glaciers, or increase and decrease in the volume of water trapped in ice caps.
About 30,000 years ago, sea level was nearly the same as it is today. During the ice age 15,000 years ago, it dropped and has been rising ever since.