The Center for Operational Oceanographic Products and Services has been measuring sea level for over 150 years, with tide stations of the National Water Level Observation Network operating on all U.S. coasts. Changes in Mean Sea Level (MSL), either a sea level rise or sea level fall, have been computed at 128 long-term water level stations using a minimum span of 30 years of observations at each location. These measurements have been averaged by month to remove the effect of higher frequency phenomena (e.g. storm surge) in order to compute an accurate linear sea level trend. The trend analysis has also been extended to 240 global tide stations using data from the Permanent Service for Mean Sea Level (PSMSL).
The mean sea level (MSL) trends measured by tide gauges that are presented on this web site are local relative MSL trends as opposed to the global sea level trend. Tide gauge measurements are made with restpect to a local fixed reference level on land; therefore, if there is some long-term vertical land motion occurring at that location, the relative MSL trend measured there is a combination of the global sea level rate and the local vertical land motion. The global sea level trend has been recorded by satellite altimeters since 1992 and the latest calculation of the trend can be obtained from NOAA's Laboratory for Satellite Altimetry, along with maps of the regional variation in the trend. The University of Colorado's Sea Level Research Group compares global sea level rates calculated by different research organizations and provides detailed explanations about the issues involved.
March, 2012 -- Between January 2003 and September 2008, the Sea Levels Online website has displayed sea level trends for CO-OPS stations calculated with data up to the year 1999. Since September 2008, recalculated trends with data up to the year 2006 have been displayed. CO-OPS has now adopted a new policy of recalculating sea level trends every year since 2006 for the stations still in operation and making updated trends available on Sea Levels Online. This does not mean that any use of the older trends is now inaccurate or obsolete. Since the original trends are based on long data sets (of at least 30 years), the use of a few more years of data only makes the calculated trends slightly more precise. In fact, if in recent years there was a high or low sea level anomaly occurring in an area, the most recent trends may move slightly away from the true long-term trend. Comparisons of small year-to-year changes in trend should not be taken as evidence of acceleration or decceleration in the rate of sea level change.
January, 2012 -- For most stations, MSL trends were calculated with all the recorded water level data. At a few stations, with an obvious offset in the series caused by an earthquake, the series were broken into a pre-earthquake and post-earthquake trend. This was done for the effect of the March 1957 earthquake on Adak Island (9451380) and Unalaska (9452620) and the March 1964 earthquake on Seward (9455090) and Kodiak Island (9457292). For these stations, the post-earthquake trends have been listed on this site. For Guam (1630000), the trend before the August 1993 earthquake has been listed until now because of the short duration of the post-earthquake record. Although Guam has a large interannual variability, making it difficult to get a precise trend with a short period of data, the post-earthquake trend will now be listed because it is comparable to the rapid sea level rise measured by satellite altimetry in the western Pacific Ocean since 1992.
In other cases, a change in trend may not be evident until there is sufficient data recorded to see that the series is no longer linear. This now seems to be the case for Yakutat (9453220), Cordova (9454050), and Valdez (9454240). It is not certain when there was a change of trend, but in order to best estimate the present rate of sea level change, trends were re-calculated before and after the February 1979 earthquake located in between these three stations. It is not known whether the change in trend was caused by changing motions of the tectonic plates in the region due to the earthquake or by an increase in the rate of melting of the large Alaskan glaciers in the vicinity and the subsequent elastic rebound of the earth's crust.