Many scientists are interested in the long-term mean sea level change, especially in connection with global climate change. By taking such long-term measurements, these scientists are hoping to confirm the predictions of several climate models, including the idea that global warming is a result of the “greenhouse” gases from either human or natural sources.

Mt. Everest (right peak, with Mt. Nuptse at left) in Nepal, rising to a height of 29,022 feet, is the tallest mountain on Earth when measuring height compared to sea level.

There are two major ways to determine such sea level variations. The first estimates sea level changes using tide gauge measurements, mathematically averaging the numbers. Graphs of the most recent estimates using this method show a 0.669 to 0.960 inch (1.7 to 2.44 millimeter) rise in sea level per year. The second method uses global positioning system (GPS) devices and satellite altimeter measurements, both of which accurately pinpoint global ocean heights quickly and more efficiently. For example, from 1994 to 2004, scientists mathematically constructed graphs from satellite altimeter measurements, showing that the global mean sea levels have risen anywhere between 1.10 and 1.18 inches (2.8 and 3.0 millimeters).

No matter what the method, scientists do know the global mean sea levels are slowly rising. Many believe that about one quarter of the rise is caused by thermal expansion as the oceans warm, and another one quarter by small glaciers melting around the world. Some rise may also be caused by such human activities as burning trees, pumping ground water, and draining wetlands. Currently, scientists are not quite certain about the true rate of sea level rise, mainly because of the intensity of working on the data: Ocean-tide gauge records must be averaged, over many decades, and corrected for variable ocean dynamics and distortions of Earth’s crust.