Ask a Scholar: Ocean Level

Mark R. Abbott

Dear Ask a Scholar,

How do we know that the level of the oceans is rising?

- Jeanne Moersch

Answered by Mark R. Abbott, dean and professor in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University. Dr. Abbott's research focuses on the interaction of biological and physical processes in the upper ocean and relies on both remote sensing and field observations. Dr. Abbott is a pioneer in the use of satellite ocean color data to study coupled physical/biological processes. He has also advised the Office of Naval Research and the National Science Foundation on ocean information infrastructure.

Sea level changes on a wide range of time scales, ranging from hours for tides caused by the Sun and the Moon to millions of years for tectonic processes that raise and lower continents. 

Sea level rise is not uniform globally. Some areas of the coast are subsiding because of the pumping of groundwater or oil, while other areas are subject to sporadic inundation from tropical cyclones.  

Measuring sea level rise is thus complex because of these local to global-scale processes and of the short to long time scale variability. Thus a single tide gauge in a single location may reveal a lowering of the local sea level while another location might show a local rise. Moreover, the precision tide gauge record is relatively sparse and only extends back a few decades. It is thus difficult to derive a global perspective of sea level rise from tide gauges for much longer than the last 50 years.  Since 1993, high precision satellite altimetry has provided a much more complete view of sea level variability, and when combined with networks of in situ measurements, it has been possible to develop a high quality record of global sea level rise. However, uncertainties remain, such as the larger scale tectonic adjustment by land masses as ice from the last Ice Age melts, changes in ocean density and circulation which can affect local sea level, and the exchanges of water between land and ocean. 

Long time series of global sea level rise generally have accounted for the glacial isostatic effect (sometimes called the post-glacial rebound which describes the rise of land masses that were depressed by the enormous weight of the ice sheets during the last Ice Age.)  Direct measurements from satellite altimetry and in situ networks show that global sea level is rising at about 2 mm/year, which is faster than the rate from the previous 2000 years which has been derived from a variety of proxy indicators of sea level. This increase is thought to be the result of the continuing release of water to the ocean from the melting of land ice (e.g., glaciers and ice sheets) and the thermal expansion of the ocean.   

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Image: DSC_0539.jpg by Kyle Mortara // CC BY

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