2. ESTIMATES OF EUSTATIC SEA LEVEL RISE 



2.1 INTRODUCTION 



Eustatic sea level rise is the global average sea level rise primarily 

 due to: 1) additional water mass in the oceans through release of water 

 contained in polar ice caps and alpine glaciers, and 2) steric expansion of 

 water presently in the oceans due to increased temperature, thereby 

 increasing the volume of an existing water mass. Sea level change data 

 from 20,000 years before present (BP) to 1,000 years BP have been obtained 

 from radiometric dating of plants and animals that lived only in intertidal 

 or shallow marine waters . Data from the last 100 or so years are based on 

 measurements from long-term tide gages. Both of these sources include not 

 only the "signal" of eustatic sea level change, but the "noise" or 

 contamination by local vertical movement of the land where the measurements 

 are made. Additionally, local and temporal oceanographic and 

 meteorological factors may contribute to anomalously high or low water 

 levels for periods of many years. The degree of contamination in any one 

 tide gage record may be severe with the annual contamination exceeding up 

 to 40 years of eustatic trend. Much of the contamination is spatially and 

 temporally coherent over fairly long distance and time scales and the 

 physics of this contamination is poorly understood. If the available tide 

 gage data provided a representative distribution over the world's oceans, 

 the noise could be eliminated by simply averaging over these gages. 

 However, the available tide gage data are heavily concentrated in the 

 northern hemisphere and along continental margins . 



Tide gages measure the local relative sea level which is important and 

 is the water level relevant to that area. However, an understanding of 

 recent eustatic sea level rise is critical, because models developed for 

 predicting future sea level rise are calibrated based on estimates of 

 recent rise. Most of these estimates suggest a rate of 10-15 cm/century 

 (1 to 1.5 mm/yr) with some investigators inferring an increase in the rate 

 of rise over the past 40 or so years. Most of the studies leading to the 

 above estimates have been based on gages located in reasonably stable low- 

 to mid-latitude areas. Clearly the most significant neotectonic 



