James R. Houston

Sea-Level Acceleration Based on U.S. Tide Gauges and Extensions of Previous Global-Gauge Analyses

Abstract Without sea-level acceleration, the 20th-century sea-level trend of 1.7 mm/y would produce a rise of only approximately 0.15 m from 2010 to 2100; therefore, sea-level acceleration is a critical component of projected sea-level rise. To determine this acceleration, we analyze monthly-averaged records for 57 U.S. tide gauges in the Permanent Service for Mean Sea Level (PSMSL) data base that have lengths of 60–156 years. Least-squares quadratic analysis of each of the 57 records are performed to quantify accelerations, and 25 gauge records having data spanning from 1930 to 2010 are analyzed. In both cases we obtain small average sea-level decelerations. To compare these results with worldwide data, we extend the analysis of Douglas (1992) by an additional 25 years and analyze revised data of Church and White (2006) from 1930 to 2007 and also obtain small sea-level decelerations similar to those we obtain from U.S. gauge records.

Comparisons at Tide-Gauge Locations of Glacial Isostatic Adjustment Predictions with Global Positioning System Measurements

Abstract Houston, J.R. and Dean, R.G., 2012. Comparisons at tide-gauge locations of glacial isostatic adjustment predictions with global positioning system measurements. Glacial isostatic adjustment (GIA) is routinely used to adjust sea-level trends determined from tide-gauge data to improve estimates of worldwide sea-level rise. This adjustment may be appropriate for formerly glaciated high-latitude (referred to as FGHL) areas where vertical land motions due to GIA are large compared with motions produced by other phenomena. However, since GIA is only one component of vertical motion, does adjusting for it outside FGHL areas improve sea-level rise estimates or bias them? We compare global positioning system (GPS) gauge measurements with the vertical land-motion component of GIA predictions at 147 worldwide locations that are near tide gauges and outside FGHL areas and find remarkably little correlation. We analyze the data in several ways to determine the source of the lack of correlation. We also find that the average vertical motion for the 147 locations measured by GPS is subsidence, whereas the average GIA prediction is zero.

Effects of Sea-Level Decadal Variability on Acceleration and Trend Difference

ABSTRACT Houston, J.R., and Dean, R.G. 2013. Effects of sea-level decadal variability on acceleration and trend difference. Previous research has shown that sea-level acceleration determined from individual tide gauge records has remarkably large scatter as record lengths decrease due to decadal variations in sea level. We extend previous data sets to the present time and find even greater acceleration scatter. Using analytic solutions, sinusoidal oscillations with amplitudes and periods of typical decadal variations are shown to basically account for the relationship between record length and both acceleration and trend difference. Data show that decadal variations will obscure estimates of underlying accelerations if record lengths of individual gauges are not greater than at least 75 years. Although worldwide data are less affected by decadal variations than individual gauge data, decadal variations still significantly affect estimates of underlying accelerations, in particular for record lengths less than about 60 years. We give two examples of recent studies that use record lengths of about 30 to 60 years to determine acceleration or related trend difference. Previous authors dismissed the importance of decadal variations on their results and, as a result, reached invalid conclusions.

Accounting for the Nodal Tide to Improve Estimates of Sea Level Acceleration

Abstract The 18.6-year nodal tide is a component of all tide gauge records. It can affect estimates of sea level acceleration, in particular for tide gauge records with lengths of less than 60 years. We provide an analytic solution that shows the effect of the nodal tide on estimates of sea level trend and acceleration. By adding a term to the least squares formulation used to estimate sea level trend and acceleration, we can account for the nodal tide and eliminate its effect on the estimate. Using representative world-wide tide gauge records, we demonstrate that accounting for the nodal tide can improve estimates, particularly of acceleration.

Discussion of: Baart, F.; van Koningsveld, M., and Stive, M., 2012. Trends in Sea-Level Trend Analysis. Journal of Coastal Research, 28(2), 311–315

Baart, van Koningsveld, and M. Stive (2012) are commended for a stimulating discussion of sea-level trend analysis. We were surprised by the reaction to Houston and Dean (2011) because we reached conclusions in agreement with several earlier studies and made no attempt to project sea-level rise into the 21st century. The issue is not whether data show a small acceleration (as found by Church and White, 2011) or deceleration (as we and others found) of sea level in the 20th century. In either case, the values are so close to zero that the trend is essentially linear. Woodworth et al (2009) note, ‘‘However, little evidence has been found in individual tide gauge records for an ongoing positive acceleration of the sort suggested for the 20th century itself by climate models.’’ This mirrors the conclusion in the seminal article by Douglas (1992) that said, ‘‘There is no evidence for an apparent acceleration in the past 100+ years that is significant either statistically, or in comparison to values associated with global warming.’’ Houston and Dean (2011) highlight the lack of understanding of 20th century sea-level rise and the challenge this offers to projecting into the 21st century.