One major reason for sea level change is simply the amount of water available to the global ocean. Glaciers hold large amounts of fresh water that can be released when global temperatures rise and melt some of the ice. As discussed in The Enhanced Greenhouse Effect, scientists have found that global temperatures have recently increased primarily due to the addition of greenhouse gases in our atmosphere. Although the added heat increases sea level because there is a greater volume of water in the oceans, it also has the effect of thermally expanding ocean water. In fact, sea level rise due to thermal expansion contributes more to sea level rise than does simply adding more water to the system.

One important consideration in sea level fluctuation is the location of the ice. The only ice that will cause a sea level increase (if melted) is the ice on land. Sea ice displaces a proportionate amount of water equal to the mass of the ice. Therefore, when the ice melts sea level is unaffected. (This effect is illustrated better by putting ice cubes in a glass of water. When the ice melts, the liquid level does not change.)