Misleading claims about sea level rise


The IPCC claims a faster rate in sea level rise in the period 1993-2003 (3.1 mm/year) compared with 1961-2003 (1.8 mm/yr), see WG1 SPM p 5,7, table SPM1.  To make this claim, the IPCC have employed two of their familiar misleading tricks simultaneously - (a) compare a short period with a longer period, (b) change the measurement technique.

Prior to 1993 IPCC uses the tide gauge record of sea level, which records measurements at several shorelines; in 1993 this was changed to satellite altimetry, which measures the entire ocean. The change in method coincides with an apparent acceleration of sea level rise over previous periods, which IPCC attributes to AGW, throwing out the tide gauge record, which shows significant fluctuations but no such acceleration.

To compare one set of results using one method over one time period (prior to 1993) with another set of results using a different method over another time period (after 1993) and then using this cobbled-together record to claim an accelerating trend between the two time periods is bad science, at best, especially if the record for the latter time period which uses the same method for both periods shows no acceleration is ignored.

These false claims are repeated in the main body of AR4 WG1, in section 5.5. On page 409 it is stated that "global sea level gradually rose in the 20th century and is currently rising at an increased rate" and "This decade-long satellite altimetry data set shows that since 1993, sea level has been rising at a rate of around 3 mm yr–1, significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation...", with no supporting evidence. This last statement is contradicted by the papers by Holgate and Woodworth and by Douglas below.

Later on in chapter 5 (p 413), the authors acknowledge that there has been no acceleration, and illustrate their astonishing bias: "Interannual or longer variability is a major reason why no long-term acceleration of sea level has been identified using 20th-century data alone (Woodworth, 1990; Douglas, 1992). Another possibility is that the sparse tide gauge network may have been inadequate to detect it if present (Gregory et al., 2001)." The IPCC authors are in denial of the facts - they 'know' that sea level rise must be accelerating, and if the data doesn't show it, then there must be something wrong with the data.  On the same page is the following figure, that shows just how misleading the IPCC SPM claim is. Note the large fluctuations, and the tiny section of green line from the satellite data.




Research papers on this subject include:

J. Church and N. J. White,  A 20th century acceleration in global sea level rise, Geophys. Res. Lett., 33, L01602 (2006). They find a minute acceleration in sea level rise (0.013 mm/yr/yr).

B. C. Douglas, Global sea level acceleration, J. Geophys. Res., 97, 12,699–12,706 (1992). He finds no acceleration (despite his title!) and in fact finds a slight deceleration of -0.011 mm/yr/yr. "Thus there is no evidence for an apparent acceleration in the past 100+ years".

S. J. Holgate and P. L. Woodworth, Evidence for enhanced coastal sea level rise during the 1990s, Geophys. Res. Lett. 31, L07305 (2004).

Abstract:

Sea level rise over the last 55 years is estimated to have been 1.7 ± 0.2 mm yr−1, based upon 177 tide gauges divided into 13 regions with near global coverage and using a Glacial Isostatic Adjustment (GIA) model to correct for land movements. We present evidence from altimeter data that the rate of sea level rise around the global coastline was significantly in excess of the global average over the period 1993–2002. We also show that the globally-averaged rate of coastal sea level rise for the decade centered on 1955 was significantly larger than any other decade during the past 55 years. In some models of sea level rise, enhanced coastal rise is a pre-cursor of global average rise. It remains to be seen whether the models are correct and whether global-average rates in the future reflect the high rates of coastal rise observed during the 1990s.

 

More recent research (not available for AR4) confirms the lack of acceleration, and in fact finds a slight deceleration: 

S. J. Holgate, On the decadal rates of sea level change during the twentieth century, Geophys. Res. Lett. 34, L01602 (2007).

Abstract:

Nine long and nearly continuous sea level records were chosen from around the world to explore rates of change in sea level for 1904–2003. These records were found to capture the variability found in a larger number of stations over the last half century studied previously. Extending the sea level record back over the entire century suggests that the high variability in the rates of sea level change observed over the past 20 years were not particularly unusual. The rate of sea level change was found to be larger in the early part of last century (2.03 ± 0.35 mm/yr 1904–1953), in comparison with the latter part (1.45 ± 0.34 mm/yr 1954–2003). The highest decadal rate of rise occurred in the decade centred on 1980 (5.31 mm/yr) with the lowest rate of rise occurring in the decade centred on 1964 (−1.49 mm/yr). Over the entire century the mean rate of change was 1.74 ± 0.16 mm/yr.

Holgate is clearly a proper scientist. He uses the same method throughout (tide gauges) and compares a fifty year trend with another fifty year trend. 


A more recent paper confirms that there has been no acceleration:

P. L. Woodworth et al, Evidence for the accelerations of sea level on multi-decade and century timescales, Int J Climatol, 29(6), 777-789 (2009).

Abstract:

A modification in the rate of change of sea level (i.e. an acceleration or nonlinear trend) is an important climate-related signal, which requires confirmation and explanation. In this study, the evidence for accelerations in regional and global average sea level on timescales of several decades and longer is reviewed by inter-comparison of the recent findings of different researchers and by inspection of original tide gauge records. Most sea-level data originate from Europe and North America, and both the sets display evidence for a positive acceleration, or inflexion, around 1920-1930 and a negative one around 1960. These inflexions are the main contributors to reported accelerations since the late 19th century, and to decelerations during the mid- to late 20th century. However, these characteristic features are not always found in records from other parts of the world. Although some aspects of the sea-level time series are consistent with changes in rates of globally averaged temperature changes, volcanic eruptions and natural climate variability, modelling undertaken so far has been unable to describe these features adequately. This emphasizes the need for a major enhancement of the sea-level data set, especially for those parts of the world without long tide gauge records, in order to obtain greater insight into the spatial dependence of accelerations. A number of complementary methods must be employed, of which salt marsh techniques offer the possibility of obtaining time series similar to those that would have been obtained from coastal tide gauges.


Update 2011: Two more papers showing no acceleration, in fact a deceleration:

J. R. Houston and R. G. Dean, Sea-Level Acceleration Based on U.S. Tide Gauges and Extensions of Previous Global-Gauge Analyses  Journal of Coastal Resarch, 27, 409 – 417 (2011).

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.

P. J. Watson, Is There Evidence Yet of Acceleration in Mean Sea Level Rise around Mainland Australia?  Journal of Coastal Research, 27, 368 – 377 (2011). 

Abstract:  ...The analysis reveals a consistent trend of weak deceleration at each of these gauge sites throughout Australasia over the period from 1940 to 2000...