This is the “uncertainty margin,” or the range from the mean (average) within which there is a high probability that the true number resides. For example, when we say that global mean sea level for a particular month is 50mm +/- 4 mm, the true value for that month has a high probability of being between 46 and 54 mm. The uncertainty margin exists both because of sea level variability (sea level above the first point in the time series) in the ocean and the accuracy with which we measure the individual values.
Sea level rise is caused primarily by two factors related to global warming: the added water from melting ice sheets and glaciers and the expansion of seawater as it warms. The first graph tracks the change in sea level since 1993 as observed by satellites.
The second graph, derived from coastal tide gauge data, shows how much sea level changed from about 1870 to 2000.
Data shown are latest available, given time needed to allow for processing.