Giadecontaminated Tide Gauge Estimates Of The Rate Of Global Sea Level Rise

For the purpose of producing the best tide gauge-based estimate of the global rate of relative sea level rise which is not contaminated by the influence of the glacial isostatic adjustment process, I will focus first upon the results obtained by applying the highest resolution version of the ICE-4G (VM2) model for which results have been computed. These results are presented in Table 4.1 in which analyses are shown for 27 tide gauges, each of which is characterized by more than 70 years of observations. As discussed in detail by Douglas (1991), and as demonstrated by the earlier results of Peltier and Tushingham (1989) shown previously in Fig. 4.1, it is extremely important for the stability of the global estimate that one employ only very long records in such analyses since the interannual variability of relative sea level is intense and must be effectively averaged out if an accurate estimate is to be obtained. In Table 4.1 the individual columns give the latitude and longitude locations of each of the gauges, the number of years in the record and the estimate by Douglas ("Doug.") (Chapter 3) for the rate of relative sea level rise determined on the basis of a best fitting linear model to the raw annually averaged tide gauge data. Five columns of additional information labeled with the numbers -0.5, 0.5,1.5, 2.5, 3.5 list the difference between the observed rates of RSL rise of Douglas and the GIA predictions for the ICE-4G (VM2) model of the rate of RSL rise that is predicted to occur at each of the tide gauge sites and at each of the times before present (measured in 1000 years) represented by the numbers —0.5 to +3.5, if the only process acting in the Earth System were the global process of glacial isostatic adjustment. The column labeled LSQ presents the difference between the Douglas calculation and the rate of relative sea level rise associated with the GIA process at each gauge determined by a linear least-squares fit to the theoretical predictions over the age range from 3.5 kyr before present to 0.5 kyr in the future. By including the LSQ result in our analysis, we will be able to estimate the magnitude of the error that would be committed by using the 14C-dated geological data in the way employed by Gornitz (1995) in her analysis of U.S. east coast data. She obtained an estimate of the regional rate of RSL rise based upon GIA decontaminated tide gauge data from this coast of 1.5 mm/yr, a number that is significantly lower than that later obtained by Peltier (1996b), whose analysis of the data from the same region gave an average value near 1.9 mm/yr. At the bottom of the table is listed the average rate of RSL rise determined by simply averaging the observations of Douglas together with a standard deviation of the individual observations from this average value. For each of the

Table 4.1

Rates of Sea Level Rise, Adjusted from Tide Gauge Data

Diffs [Douglas-Peltier] (mm/yr)

Table 4.1

Rates of Sea Level Rise, Adjusted from Tide Gauge Data

Diffs [Douglas-Peltier] (mm/yr)

Station

LAT

LON

YRS

Doug, (mm/yr)

LSQ

-0.50

0.50

1.50

2.50

3.50

Lagos

37.10

-8.40

83

1.40

1.66

1.73

1.67

1.65

1.66

1.68

San Diego (Quara)

32.40

-117.00

74

1.90

1.78

1.98

1.85

1.76

1.71

1.68

Pensacola

30.20

—87.00

73

2.10

1.60

1.84

1.76

1.59

1.48

1.37

Fernandina

30.40

-81.30

96

2.00

1.38

1.60

1.59

1.38

1.25

1.07

Boston

42.20

-71.00

76

2.70

1.91

2.37

2.22

1.95

1.69

1.27

Halifax

44.40

-63.40

76

3.40

1.86

2.58

2.29

1.97

1.53

0.96

Aberdeen I & II

57.15

-2.08

97

0.70

1.20

1.18

1.12

1.19

1.24

1.33

Newlyn

50.10

-5.55

82

1.70

1.15

1.51

1.33

1.18

0.99

0.81

Brest

48.38

-4.50

91

1.30

0.73

1.10

0.92

0.76

0.57

0.37

Cascais

38.68

-9.42

88

1.60

1.84

1.91

1.84

1.82

1.84

1.86

Marseille

43.30

5.35

96

1.20

1.26

1.36

1.30

1.24

1.23

1.23

Genoa

44.40

8.90

92

1.20

1.30

1.38

1.32

1.29

1.28

1.30

Trieste

45.65

13.75

92

1.10

1.17

1.27

1.23

1.15

1.12

1.12

Auckland

-36.87

174.80

85

1.30

1.58

1.84

1.65

1.53

1.53

1.54

Dunedin

-45.88

170.50

89

1.40

1.79

1.93

2.00

1.68

1.67

1.68

Lyttelton

-43.60

172.72

85

2.30

2.68

2.88

2.70

2.65

2.65

2.68

Wellington

-41.28

174.78

87

1.70

2.10

2.33

2.16

2.06

2.04

2.06

Honolulu

21.32

-157.87

92

1.50

1.99

1.97

2.18

1.79

1.91

2.12

San Francisco

37.80

-122.47

80

1.80

1.52

1.50

1.63

1.52

1.44

1.35

Balboa

8.97

-79.57

72

1.50

1.74

1.80

1.74

1.68

1.73

1.83

Buenos Aires

-34.60

-58.37

75

1.10

1.59

2.17

1.98

1.35

1.36

1.41

Key West

24.55

-81.80

84

2.20

1.68

1.91

1.87

1.69

1.55

1.38

Charleston I

32.78

-79.93

75

3.30

2.47

2.86

2.74

2.51

2.33

1.85

Baltimore

39.27

-76.58

94

3.10

1.79

2.31

2.18

1.87

1.51

1.00

Atlantic City

39.35

-74.42

85

3.10

1.41

1.89

1.83

1.52

1.09

0.52

New York

40.70

-74.02

97

3.00

1.77

2.33

2.16

1.84

1.50

0.98

Portland

43.67

-70.25

85

1.90

1.83

2.07

2.02

1.85

1.70

1.46

Average

1.91

1.66

1.91

1.83

1.65

1.54

1.40

Standard deviation

0.75

0.40

0.47

0.45

0.41

0.41

0.50

five individual times for which the theoretical rates have been computed are listed the average differences between the Douglas observations and the GIA predictions together with a standard deviation. Also shown is the average difference between the observations and the LSQ estimate deduced by least-squares best fit of a straight line to the theoretical predictions over the age range from -3.5 kyr BP to +0.5 kyr in the future.

Careful inspection of the data presented in Table 4.1 indicates that there is no significant difference between the magnitude of the estimate of the globally averaged rate obtained by direct averaging of Douglas observations (this gives 1.91 mm/yr) and the result 1.87 mm/yr determined by averaging the GIA-corrected observations using the GIA prediction for the present day (by averaging the results for +0.5 and -0.5 kyr BP). There is, however, an extremely significant impact achieved by properly correcting the raw tide gauge data for the influence of GIA. Note that the standard deviation of the raw observations of Douglas from the average value is 0.75 mm/yr, whereas the standard deviation of the GIA-corrected rates from their average value is reduced to 0.45 mm/yr. It is therefore abundantly clear that when the tide gauge data are corrected for the GIA influence, the result is an estimate for the global rate of RSL rise that is much less variable as a function of position on Earth's surface. Also of considerable interest in this table is the difference between the globally averaged rates of relative sea level rise (a) determined by the GIA-corrected tide gauge data that are obtained when the GIA rates are accurate estimates for the present epoch and (b) those obtained when the GIA rate is taken to be represented by an average over the most recent 4 kyr of geological history. In the former case the globally averaged rate is 1.87 mm/yr, whereas in the latter case it is 1.66 mm/yr. The impact of employing the latter methodology of Gornitz (1995) is therefore severe, even when viewed from a global perspective. It inevitably leads to an underestimate of the average rate of RSL rise that could be associated with ongoing climate change in the Earth system. However, when viewed locally, as at the locations along the eastern seaboard of the continental United States, the impact of employing methodology (b) is further increased. On consideration of the results listed in Table 4.1 for the Key West, Charleston I, Baltimore, Atlantic City, and New York locations, application of method (b) for making the GIA correction, based upon the direct use of 14C-controlled geological data averaged over the past 4 kyr to determine a rate, will reduce the strength of the inferred climate related contribution by amounts of 0.2, 0.32, 0.45, 0.44, 0.45 mm/yr for these five locations. The average value of the underestimate of the climate-related rate of RSL rise that is made by employing the Gornitz procedure at these five locations is therefore 0.37 mm/yr, which is essentially identical to the difference between the 1.5 mm/yr estimate of Gornitz (1995) and the 1.9 mm/yr estimate of Peltier (1996b) for the U.S. east coast region when the geological data are employed directly to decontaminate the observations. One must accurately estimate the GIA rate appropriate to the present day in order to accurately effect a decontamination of the tide gauge observations.

Several further consequential issues must be addressed before being satisfied that we have determined the best possible estimate of the global rate of RSL rise that could be associated with modern climate change. The first of these concerns the issue of the geographical representativeness of the raw average data presented in Table 4.1. There are clearly several regions of the Earth's surface that are oversampled by the analysis procedure employed in constructing Table 4.1. Specifically the east coast of North America is highly oversampled, there being six sites from the northern part of this coast (Baltimore, Atlantic City, New York, Boston, Portland, Halifax) and four from the southern part (Pensacola, Key West, Fernandina, Charleston I). Therefore

10 of the 27 gauges employed to produce a "global" estimate of the rate of RSL rise are actually located along one coast of the North Atlantic Ocean. To correct for this over sampling effect we choose to cluster the tide gauge sites as in Table 4.2 in order first to produce an average for each of the individual clusters and then to produce our estimate of the global rate of RSL rise by analyzing the average over these clusters. Inspection of the partition documented in Table 4.2 reveals the tide gauges have been amalgamated into

11 clusters, one of which consists of the data from Aberdeen I and II, both in Scotland and thus from a region that was heavily glaciated at Last Glacial Maximum. We will eliminate these data from further analysis for this reason. At the bottom of Table 4.2 we list the new globally averaged rate of RSL rise that is obtained by averaging over the individual cluster averages of the raw rates, which gives 1.71 mm/yr, and by averaging over the GIA-corrected cluster averages, which gives a value of 1.84 mm/yr. Of equal importance as the fact that the average rate is now somewhat increased when the GIA correction is made is the fact that the standard deviation of the individual cluster averages is reduced from 0.55 to 0.35 mm/yr. If we recall that the equivalent standard deviations for the unclustered data were 0.75 and 0.45 mm/yr, it will be clear that recognition of the oversampling problem has enabled us to further refine our estimate of the global rate of RSL rise that could be related to ongoing climate change in the Earth system.

We still need to understand the meaning of our best estimate, 1.84 mm/yr, of this average. What relative weight should be attached to the individual cluster averages in Table 4.2, considering the different fractional areas of Earth's surface that the individual clusters should represent? Because it would be arbitrary to assign a degree of importance to each of the clusters, I will leave this question as a caveat on the results that have been presented. It is worth noting, however, that the Northwestern European cluster represented by the tide gauges at Newlyn and Brest delivers results that are somewhat lower than those of most of the other clusters when the GIA correction is applied. This need not be taken to imply a problem with the analysis since there is no reason to believe that the influence of modern climate change

Table 4.2

Clustered Rates of Sea Level Rise, Adjusted from Tide Gauge Data

Table 4.2

Clustered Rates of Sea Level Rise, Adjusted from Tide Gauge Data

Station

LAT

LON

YRS

Doug, (mm/yr)

Diffs [Douglas-Peltier] (mm/yr) (Time = 0.00)

Aberdeen I & II

57.15

-2.08

97

0.70

1.15

1 location

Av

rate of

RSL

rise, std

dev

= 0.70,

0.00; Av diff,

std dev

= 1.15, 0.00

Newlyn

55.10

-5.55

82

1.70

1.42

Brest

48.38

-4.50

91

1.30

1.01

2 locations

Av

rate of

RSL

rise, std

dev

= 1.50,

0.28; Av diff,

std dev

= 1.21, 0.29

Cascais

38.68

-9.42

88

1.60

1.88

Lagos

37.10

-8.40

83

1.40

1.70

2 locations

Av

rate of

RSL

rise, std

dev

= 1.50,

0.14; Av diff,

std dev

= 1.79, 0.12

Marseille

43.30

5.35

96

1.20

1.33

Genova

44.40

8.90

92

1.20

1.35

Trieste

45.65

13.75

92

1.20

1.25

3 locations

Av

rate of

RSL

rise, std

dev

= 1.17,

0.06; Av diff,

std dev

= 1.31, 0.05

Auckland

-36.87

174.80

85

1.30

1.74

Dunedin

-45.88

170.50

89

1.40

1.97

Lyttelton

-43.60

172.72

85

2.30

2.79

Wellington

-41.28

174.78

87

1.70

2.24

4 locations

Av

rate of

RSL

rise, std

dev

= 1.67,

0.45; Av diff,

std dev

= 2.19, 0.45

Honolulu

21.32

-157.87

92

1.50

2.07

1 location

Av

rate of

RSL

rise, std

dev

= 1.50,

0.00; Av diff,

std dev

= 2.07, 0.00

San Francisco

37.80

-122.47

80

1.80

1.56

San Diego (Quara)

32.40

-117.10

74

1.90

1.91

2 locations

Av

rate of

RSL

rise, std

dev

= 1.85,

0.07; Av diff,

std dev

= 1.74, 0.24

Balboa

8.97

-79.57

72

1.50

1.77

1 location

Av

rate of

RSL

rise, std

dev

= 1.50,

0.00; Av diff,

std dev

= 1.77, 0.00

Buenos Aires

-34.60

-58.37

75

1.10

2.08

1 location

Av

rate of

RSL

rise, std

dev

= 1.10,

0.00; Av diff,

std dev

= 2.08, 0.00

Pensacola

30.20

-87.10

73

2.10

1.80

Key West

24.55

-81.80

84

2.20

1.89

Fernandina

30.40

-81.30

96

2.00

1.59

Charleston I

32.78

-79.93

75

3.30

2.80

4 locations

Av

rate of

RSL

rise, std

dev

= 2.40,

0.61; Av diff,

std dev

= 2.02, 0.53

Baltimore

39.27

-76.58

94

3.10

2.25

Atlantic City

39.35

-74.42

85

3.10

1.86

New York

40.70

-74.02

97

3.00

2.24

Boston

42.20

-71.00

76

2.70

2.29

Portland

43.67

-70.25

85

1.90

2.04

Halifax

44.40

-63.40

76

3.40

2.43

6 locations

Av

rate of

RSL

rise, std

dev

= 2.87,

0.52; Av diff,

std dev

= 2.19, 0.20

Average of group values RSL rate from tide gauge records, std dev = 1.71, 0.55 RSL rate corrected for GIA, std dev = 1.84, 0.35

should cause sea level to rise at the same rate everywhere, especially if the global rise of level is caused to significant degree by the thermal expansion of the oceans as appears to be the case.

It will be noted on the basis of the data presented in both Tables 4.1 and 4.2 that the sign of the rate of RSL rise that is predicted to be occurring at all locations in the far field of the Pleistocene ice sheets is negative at all of the tide gauge locations from which long time series of data are available. Clearly when the individual tide gauge rates of RSL rise from this region are corrected by removing the GIA contamination, the result is an increase in the rate inferred to be due to ongoing climate change. As it happens the existence of this far field signature of the GIA process may be invoked to place strong constraints on the possible sources of the global rate of RSL rise that is presently occurring in the Earth system. The penultimate section of this chapter will address this interesting but insufficiently appreciated line of argument.

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