Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast

Abstract

A widely held belief persists that rising land levels since the latter part of the last glaciation will help safeguard much of the Scottish coast from the impact of global sea level rise. Although the landforms of much of Scotland's coast reflect long-term land uplift, recent investigations show that uplift rates are now modest and are less than rising sea levels. When comparisons are made between long-term land-level changes using Glacio-Isostatic Adjustment models, representative of the last few thousand years (Shennan and Horton, 2002; Shennan et al., 2009), and recent land-level changes using Continuous GPS records, representative of the last decade (Bradley et al., 2006), it is apparent that recent rates of uplift are slower than longer-term averages. We show here that when tidal records are considered, they show marked increases over recent decades although the extent to which these are part of a longer-term trend is uncertain. When considered alongside the UKCP09 climate projections, these tidal observations are of value in narrowing or calibrating the wide choice of sea level projections under various climate change scenarios. It appears that Scotland's observed tidal record now lies close to the 95% projection of the UKCP09 High Emission Scenario and isostatic uplift now contributes little towards mitigating the effect of relative sea level rise on the Scottish coast. If the observed recent patterns are maintained, this has significant implications for strategic planning, flood risk management and sustainable development on Scotland's coast, and particularly on low-lying coastal zones around the major cities.

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1Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast
2A.F. Rennie
a
, J.D. Hansom
b,
⁎
3
a
Scottish Natural Heritage, Great Glen House, Leachkin Road, Inverness, IV3 8NW, UK
4
b
Department of Geographical and Earth Science, University of Glasgow, Glasgow, G12 8QQ , UK
5
6
abstractarticle info
7Article history:
8Received 23 March 2010
9Received in revised form 7 September 2010
10 Accepted 16 September 2010
11 Available online xxxx
1213
14
15 Keywords:
16 Sea level trend reversal
17 Scotland
18 Isostatic uplift
19 Continuous Global Positioning System records
20 Tide gauge trends
21 Future sea level trends
22 Coastal planning
23A widely held belief persists that rising land levels since the latter part of the last glaciation will help safeguard
24much of the Scottish coast from the impact of global sea level rise. Although the landforms of much of
25Scotland's coast reflect long-term land uplift, recent investigations show that uplift rates are now modest and
26are less than rising sea levels. When comparisons are made between long-term land-level changes using
27Glacio-Isostatic Adjustment models, representative of the last few thousand years (Shennan and Horton,
282002; Shennan et al., 2009), and recent land-level changes using Continuous GPS records, representative of
29the last decade (Bradley et al., 2006), it is apparent that recent rates of uplift are slower than longer-term
30averages. We show here that when tidal records are considered, they show marked increases over recent
31decades although the extent to which these are part of a longer-term trend is uncertain. When considered
32alongside the UKCP09 climate impact projections, these tidal observations are of value in narrowing or
33calibrating the wide choice of sea level projections under various climate change scenarios. It appears that
34Scotland's observed tidal record now lies close to the 95% projection of the UKCP09 High Emission Scenario
35and isostatic uplift now contributes little towards mitigating the effect of relative sea level rise on the Scottish
36coast. If the observed recent patterns are maintained, this has significant implications for strategic planning,
37flood risk management and sustainable development on Scotland's coast, and particularly on low-lying
38coastal zones around the major cities.
39© 2010 Published by Elsevier B.V.
4041
42
43
44 1. Introduction
45 The impact of changes in relative sea level on coastal erosion and
46 flooding has long been recognised but it is only more recently that an
47 increase in the rate of sea level change and the resultant coastal
48 erosion has become attributed to the forcing effect of global climate
49 (Parry et al., 2007). For example, a recent scoping review of coastal
50 flooding in Scotland confirmed that Mean Sea Level was the key
51 variable when forecasting future flood risk, since no evidence of
52 increased storminess could be found (SNIFFER, 2008). In Great Britain
53 and particularly in Scotland, there persists a widely held belief in parts
54 of Government and the media that the full impact of global sea level
55 rise on the coast is significantly reduced, if not reversed, by the
56 ongoing isostatic emergence of the Scottish land mass (Dawson et al.,
57 2001; Scottish Government, 2009; The Telegraph, 2010). However
58 accurate this may have been for some parts of the Scottish coast over
59 much of the Holocene, it remains that for many other parts of the
60 Scottish coast long-term relative sea level (RSL) rise has been the
61 norm over the Holocene (Shennan et al., 2006, 2009). Recent
62 technological developments in the analysis of present rates of land-
63 level changes via absolute gravity (AG) and Continuous Geographic
64Positioning Systems (CGPS) now allow the longstanding land-level
65changes known from the Holocene geomorphological record to be
66placed into perspective. This allows land-level change to be integrated
67with the eustatic sea level change to better constrain RSL trends over
68the most recent decades.
69This paper summarises the rates associated with the long-term
70processes of land uplift and sea level change and compares them with
71recent observed tide gauge data that document more recent trends of
72RSL in Scotland. The temporal and spatial implications of the resulting
73patterns are significant for coastal managers and planners charged
74with mitigation of, and adaptation to, the impact of present and future
75sea level change on the Scottish coast. The situation on the Scottish
76coast may then serve as a model for the reversals that may face other
77isostatically uplifting coasts that have thus far outpaced sea level rise,
78for example, in parts of North America and Southern Scandinavia.
792. Global sea level changes
80Averaged over the past 100 years or so, widely distributed tide
81gauges show the average rate of global sea level rise to be 1.7 mm/
82year (Cazenave and Nerem, 2004). However, estimates over more
83recent time periods have indicated faster and more variable rates. For
84example, between 1961 and 2001, global sea levels were increasing at
85an average rate of 1.8 mm/year, with an increased rate of 3.1 mm/year
86between 1993 and 2001 (Bindoff et al., 2007). Using satellite altimetry
Geomorphology xxx (2010) xxx–xxx
⁎Corresponding author.
E-mail address: jim.hansom@glasgow.ac.uk (J.D. Hansom).
GEOMOR-03379; No of Pages 10
0169-555X/$ –see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.geomorph.2010.09.015
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

87 data from TOPEX/Jason-1, Beckley et al. (2007) obtained a global rate
88 of 3.36±0.41 mm/year over the 14-year period from 1993 to 2007,
89 which they attributed principally to an increase in ocean volume due
90 to thermal expansion, with some contribution from meltwater from
91 glaciers and ice sheets. Global sea level may now be following a
92 delayed, upward trajectory similar to that of 20th century global
93 temperature trends, with the sea level curve showing a low rate of rise
94 over much of the 20th century being replaced by a steeper rising curve
95 over more recent decades (Beckley et al., 2007).
96 2.1. How level is global sea level?
97 Behind these general trends lies considerable uncertainty about
98 the regional variation in sea level, since strong basin-scale polarities,
99 pronounced inter-decadal variability (Beckley et al., 2007) and forcing
100 by natural climatic signals such as the North Atlantic Oscillation and El
101 Niño all occur. Measured to the centre of the Earth, the surface of the
102 sea varies by over 100 m in altitude, this geoidal variation reflecting
103 gravity changes due to the different density of the rocks underneath
104 the oceans (Gehrels and Long, 2008). Other factors also affect the
105 volume and resultant altitude of global sea level to the extent that sea
106 level is unlikely to be uniformly affected either by thermal expansion
107 of the world's oceans or by glacier meltwater contributions and the
108 gravitational effects that these may produce (Katsman et al., 2008).
109 For example, Mitrovica et al. (2001) and Gehrels and Long (2008)
110 show greater sea level elevations on coasts further away from the
111 main melt source areas (Antarctica, Greenland and small valley
112 glaciers), reflecting the post-melt reduced gravitational pull at these
113 source areas.
114 Such regional variation has cast doubt on the utility of global
115 values, suggesting that regional estimates of eustatic changes are
116 more meaningful (Gehrels and Long, 2008). The difficulty is that
117accurate estimation of regional values is problematic due to the
118number and complexity of local processes involved and the often
119substantial errors that are currently attached to such estimates. In
120spite of this, recent developments in the measurement of vertical land
121movements serve to enhance the accuracy of the estimates of regional
122sea level change.
1232.2. Rates of eustatic sea level rise in Great Britain
124Recent satellite altimetry shows that rates of global sea level rise
125spanning the period 1993–2007 lie in the region of 3.36 mm/year,
126although Teferle et al. (2006) estimate the regional eustatic sea level
127rise for Great Britain waters to lie within the range 0.6–1.9 mm/year
128with a central value of 1.1± 0.7 mm/year. These estimates of modern
129rates compare fairly well with estimates of global average sea level
130change over the 20th century centring on 1.7 mm/year (Church and
131White, 2006) and the 1.4 mm/year estimate for the British Isles of
132Woodworth et al. (2009a) over the same period. However, for these
133regional eustatic rates to be of value to coastal engineers and planners,
134they need to be put into context by adding or subtracting the land-
135level changes, driven in the Scottish context by isostatic change. This
136allows the actual RSL for any particular part of Scotland's coast to be
137more accurately estimated.
1383. Isostasy and vertical land movements in Scotland
1393.1. Scotland's isostatic inheritance
140The long-term sea level position is a function of a combination of
141isostatic or tectonic processes (the local vertical movement of the
142Earth's crust) and eustatic processes (the global volume of the world's
143oceans). In Scotland, isostatic processes are mainly driven by the
Fig. 1. Characteristic sea level curves within Scotland (based on Shennan and Horton (2002) and Rennie (2006)).
2A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

144 effects of glaciation with the development of the Scottish Ice Sheet
145 (fully in place before 32 k calibrated years BP (Sejrup et al., 2009)
146 resulting in isostatic depression of the crust. Proportional to the
147 thickness of the ice, the amount of depression varied radially from the
148 centre of the ice mass towards the periphery where the ice was
149 thinner or absent (Smith, 1997; Dawson et al., 2001). Since
150 deglaciation (fully complete about 11.7 k calibrated years BP (Jacobi
151 et al., 2009), the crust has been in a state of recovery over the
152 Holocene, uplifting rapidly in those places where depression was
153 greatest and subsiding where the ice sheet was thin or absent
154 (Bradley et al., 2009).
155 However, this relatively simple regional isostatic pattern was
156 complicated by a rapid rise in eustatic sea level following the final
157 collapse of the Laurentide ice sheet in North America (Milne et al.,
158 2006). In Scotland, the net effect was to temporarily reverse the falling
159 sea level trend in the isostatically uplifting areas and accentuate the
160 rising sea level trend elsewhere. Before about 7.8 k calibrated years BP
161 (Smith, 2005) most of Scotland was subject to a rising Holocene sea
162level, after which the eustatic rate slowed markedly to leave a pattern
163dominated by isostatic uplift close to the centre and relative sea level
164rise at the periphery. This imparted very different RSL curves to parts
165of the Scottish coast (Shennan and Woodworth, 1992; Shennan et al.,
1662000; Shennan and Horton, 2002:Smith, 2005; Shennan et al., 2006)
167(Fig. 1) and greatly influenced the macro-scale geomorphology of
168Scotland's coastline, allowing the development of landforms of
169emergence close to the uplift centre and landforms of submergence
170towards the western and northern peripheries (Hansom, 2001)
171(Fig. 2a, b). The average relative sea level rise over the late Holocene
172(the last 1000 years) has been recently summarised by Shennan et al.
173(2009) in a map of modelled RSL change in the British Isles that shows
174an elliptical pattern of uplift over central Scotland and RSL rise over
175Shetland, the southern west of England and the southern North Sea
176(Fig. 3a). Such reconstructions are useful since they portray a long
177term average RSL that is independent of any late 20th century
178acceleration in RSL rise and thus they act as a baseline against which
179such accelerations might be assessed.
Fig. 2. The west coast of Islay (a) close to the Scottish uplift centre has been characterised in the past by falling RSL and emerged shorelines, whereas the west coast of North Uist
(b) distant from the uplift centre has been characterised by rising RSL and a drowned landscape (Photo: P MacDonald/SNH).
3A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

180 3.2. Aconstricting zero isobase?
181 Glacio-Isostatic Adjustment (GIA) diminishes slowly following
182 glaciation with the shape of the land adjustment curve flattening with
183 time (Andrews, 1970), contraction of the zone of uplift eventually being
184 reflected in migration of the GIA zero-isobase toward the uplift centre.
185 Previously emergent coasts may then become submergent (with
186 attendant flooding and erosional impacts), a process that may be
187 already occurring in parts of Arctic Canada (Lageunesse and Hanson,
188 2008Q3 ). In Scotland, the area enclosed within the GIA zero-isobase
189 boundary generally follows an elliptical path to encircle most of central
190 Scotland'sinterior(Dawson et al., 2001; Smith et al., 2006; Shennan et
191 al., 2009). In theory any change in position of this boundary should be
192 identifiable from the time-series RSL signal (i.e. combined isostatic and
193 eustatic) captured by tide gauge records for the Scottish coast or from
194 accurately establishing the current rate of regional vertical land
195 movements together with the regional expression of global sea level
196 trends. Such an exercise, albeit using the crude measure of annual sea
197 level maxima rather than Mean Sea Level, was undertaken within the
198 Firth of Forth by Pethick (1999) based on data from Graff (1981). This
199 highlighted a fallingtidal trend up to themid-1970 s followed by a rising
200 trend that occurs first in the outer coast sites. Tidal data for the outer
201 Firth of Forth at Leith confirms a rising RSL trend over recent decades
202 (see Table 2 below). Reassessment of Scottish Postglacial sea level data
203 to take account of the long-term exponential decay in glacio-isostatic
204 uplift, indicates that the minimum rates of uplift are between 0.2 and
205 1.0± 0.1 mm/
14
C year near the centre and 0.2 0±0.1 mm/
14
Cyear
206 near the margin (Firth and Stewart, 2000).
207 3.3. Recent isostatic changes
208 Since the isostatic component diminishes over the Holocene, then
209 the spatial expression of the current rate of isostatic adjustment
210becomes a factor in the RSL equation. Although Vertical Land
211Movement (VLM) velocities over millennia have been established
212for some time using Glacio-Isostatic Adjustment models (GIA models)
213(Fig. 3a), recent developments with Absolute Gravity (AG) and
214Continuous GPS (CGPS) data have revolutionised these studies,
215allowing vertical land velocity estimates for the last decade or so to
216be established (Bradley et al., 2009). Although the number of CGPS
217stations and AG stations in Scotland is as yet limited, they provide a
218valuable database of recent movements. CGPS investigations have
219been undertaken for 11 Permanent Sea Level Monitoring Stations in
220the UK (Bingley et al., 2007). All of these stations show a significant
221movement north and east of about 15 mm/year reflecting plate
222movements, but several widely distributed Scottish stations also
223identify negative vertical changes, including Aberdeen (−0.16 mm/
224year± 0.54), Inverness (−0.07 mm/year ±0.74) and Lerwick
225(−0.30 mm/year±0.70) (Bingley et al., 2007)(Table 1).
226Comparisons between the Late-Holocene GIA record and the
227combined models which include AG and CGPS measurements over the
228last decade (Bradley et al., 2009) show lower recent uplift rates than
229the long-term averages and it is these lower estimates that have been
230adopted into the UK Climate Impacts Programme (UKCP09) (Fig. 3b).
2314. The observed record
2324.1. Recent changes in RSL –observed tide gauge data
233Records derived from tide gauges sited in coastal ports reflect the
234combined effect of eustatic and isostatic changes, and so they should
235reflect the sum of these processes (i.e. an unmodified RSL signal).
236However, tide gauge records may also be influenced by a range of
237oceanographic, neotectonic and meteorological conditions as well as
238by harbour and reclamation works that alter the local basin geometry
239and bathymetry. Firth and Stewart (2000) in re-evaluating Postglacial
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Fig. 3. a Average UK RSL rise (mm/year) over the Late Holocene (1000 BP to A.D. 1950) showing positive values in Scotland's interior (isostatic emergence) and negative values in
Scotland's peripheries & England (from Shennan et al., 2009). b Present rates of Vertical Land Movement (mm/year) for the UK used in UKCP09. The model uses both Late Holocene
VLM and modern AG corrected CGPS estimates (2000–2003) modified from UKCP09 (2009) and Bradley et al. (2009) and shows present day estimates to be lower than the longer-
term estimates. Note scale difference between panels a and b.
4A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

240 neotectonic activity, seismic activity and shoreline sequences sug-
241 gested that whilst the mid- to Late Holocene was less seismically
242 active than the early Holocene, a full understanding of the influence of
243 neotectonics on shorelines (and thus RSL) remains unclear and its
244 influence problematic. Similarly, whilst it is possible to filter out
245 episodic meteorological effects from the data, the influence of harbour
246 works on tide levels is also problematic, especially if these have
247 occurred over several decades. Accepting the above caveats, exami-
248 nation of the tidal records for Scottish ports reveals a pattern of RSL
249 rise over extensive areas of the Scottish coast including those areas
250 where RSL had been falling, as suggested for tidal maxima in the Firth
251 of Forth by Pethick (1999). For example, there appears to be a global
252 acceleration in sea level rise around the beginning of the 20th century
253 which also appears in proxy records (Gehrels, 2010). At the regional
254 scale tidal records for Aberdeen from the 1860s indicate a point of
255 inflection just before 1900, where the mean sea level trend changed
256 from a falling to a rising long-term trend. Woodworth et al. (2009a)
257 show the average rate of mean sea level rise since records began at
258 Aberdeen to be +0.87 mm/year. Recent work by the Scotland and
259 Northern Ireland Forum for Environmental Research (SNIFFER, 2008)
260 confirms that more recent observed increases in RSL tidal records at
261 Aberdeen, Millport and Stornoway can be attributed to a systematic
262 increase in Mean Sea Level rather than changes in the magnitude of
263 tidal surges. Elsewhere on the Scottish coast, tide gauge data between
264 1957 and 2006/7 mainly show RSL rises of 1.5–4.6 mm/year, but with
265 Lerwick showing a RSL fall of 0.68 mm/year over this time span
266 (Table 2). Shetland has been characterised by relative sea level rise
267 over the Holocene that differs from the pattern shown by mainland
268 Scotland (Shennan et al., 2009). Nevertheless, the cause of the
269 apparent fall in RSL at Lerwick between 1957 and 2006/7 remains
270 unclear although it is possible that the tidal record may have been
271 compromised by reclamation and development of the harbour over
272 this period (Herald, 2008).
273 Many of the newer tide gauges have records dating from only 1992
274 but using the minimum selection criteria of 15 complete years of
275record (Woodworth et al., 1999), some of these gauges can be used
276now to identify trends. There are clear dangers in extrapolating long-
277term trends from short-term data since sea level changes are subject
278to significant inter-annual, decadal and inter-decadal variability
279(Woodworth et al., 2009b), and it remains that GIA land movements
280will compensate for some of sea level rise caused by other forcings.
281Nevertheless, it is clear that the highest rates in Table 2 relate to
282recently established gauges (e.g. 1992–2007, such as Islay and
283Kinlochbervie), suggesting that the 1957–2007 (i.e. longer) time-
284averaged rates may mask more recent changes in RSL rates in much in
285the same way as averages spanning the Late Holocene mask recent
286increases in relative sea level. However, it is not yet known whether
287the recent short-term rates in Table 2 are part of a wider and
288systematic increase in sea level or part of a short-term fluctuation in
289the trend. In any case, a short-term increase in the rate of rise will
290produce enhanced coastal risk even where the land is uplifting. The
291observed 1992–2007 rates for all of the tide gauges in Table 2 lie in the
292range 2.6–6.2 mm/year, most of these rates being markedly higher
293than the long-term rates. The Lerwick gauge shows a rising RSL of
2943.17 mm/year over the 15 years of short-term data up to 2007, this
295trend now in line with other Scottish gauges. One additional influence
296on these rates is the effect of variations in tidal magnitude. The most
297influential of these is the 18.61 year-period M2 nodal modulation
298(Pugh, 2004), which can affect tidal range by up to 4% (i.e. 12 cm
299within a nominal 3 m tidal range). If this modulation is compared
300alongside the tidal trends since 1992, it becomes apparent that the
301observed increases do not match the M2 18.61 year cycle (Fig. 4). This
302refocuses attention towards possible accelerations in eustatic con-
303tributions at a time when isostatic contributions are less than before.
304Accepting the warning that extrapolating a long-term sea level trend
305from such short-term data may be problematic (Woodworth et al.,
3062009b), it remains possible that the increase in the rate of RSL since
3071992 may signal the start of a phase of more rapid sea level rise that
308future projections need to address. In other words, there is a very
309distinct possibility that the upward trend in the RSL curve may now
310apply to the whole of the UK and that any compensation from land
311uplift in central Scotland may now be a relatively minor component.
3125. Discussion
3135.1. Observed RSL rise
314If we assume that the expected range of RSL rise is the crude
315balance of isostatic contributions (a maximum uplift rate of
316+1.17 mm/year at Edinburgh and a minimum rate of −2.36 mm/
317year at Aberdeen) (Table 1) and the regional estimate of eustatic
318change for UK waters (a minimum of 0.6 mm/year rise and a
Table 1t1:1
Summary table of observed eustatic sea level rise and isostatic rates.
t1:2
t1:3Process Rate (mm/yr) + SE Source
t1:4Present Eustatic global +3.36 ±0.41 Beckley et al. (2007)
t1:5Present Eustatic local +1.10 ±0.7 Teferle et al. (2006)
t1:6Isostatic Aberdeen −0.16±0.54 Bingley et al. (2007)
t1:7Isostatic Edinburgh +1.17 ±0.53 Bingley et al. (2007)
t1:8Isostatic Glasgow +1.01 ± 0.55 Bingley et al. (2007)
t1:9Isostatic Inverness −0.07 ± 0.74 Bingley et al. (2007)
t1:10 Isostatic Lerwick −0.30±0.70 Bingley et al. (2007)
t1:11 Isostatic Mallaig +0.94 ±1.10 Bingley et al. (2007)
Table 2t2:1
Relative sea level trends as measured from tide gauges at Scottish ports and N.E. England. Over the long term, heavily fragmented records (e.g. Leith) and poorer quality outliers (e.g.
Rosyth, Woodward et al., 1999
Q1 ) are excluded. Over the short term, only gauges with data from 1992 to 2007 are included but some gauges have missing data.
t2:2
t2:3Tide gauge Rate (mm/yr)+SE Years Rate (mm/yr)+ SE Years Data source
t2:4Lerwick −0.68 ± 0.34 1957–2005 +3.18 ±3.54 1992–2007 Woodworth et al., 2009a & POL, 2009
t2:5Wick +1.55 ± 0.43 1965–2006 + 5.54 ± 3.23 1992–2007 Woodworth et al., 2009a & POL, 2009
t2:6Inverness +2.66 ± 4.54 1992–2007 POL (2009)
t2:7Aberdeen composite + 0.87 ± 0.1 1901–2006 Woodworth et al. (2009a)
t2:8Aberdeen 1 +6.03 ±2.15 1992–2007 POL (2009)
t2:9Leith 2 +4.04 ±4.18 1992–2007 POL (2009)
t2:10 Dunbar +0.47 ± 0.31 1914–1950 Woodworth et al. (2009a)
t2:11 Portpatrick +1.95 ± 0.44 1968–2004 + 4.80 ± 3.87 1992–2007 POL (2009)
t2:12 Millport +1.20 ± 0.53 1969–2006 Woodworth et al. (2009a)
t2:13 Islay +6.23 ± 3.24 1992–2007 POL (2009)
t2:14 Fort William +2.98 ±4.90 1992–2007 SEPA (Corpach)
t2:15 Stornoway + 2.20 ± 0.90 1977–2006 + 5.70 ± 3.08 1992–2007 POL (2009)
t2:16 Ullapool +2.12 ± 1.15 1983–2006 Woodworth et al. (2009a)
t2:17 Kinlochbervie +3.57 ± 4.04 1992–2007 POL (2009)
t2:18 North Shields +1.92 ±0.12 1901–2006 + 6.04 ± 2.40 1992–2007 Woodworth et al., 2009a & POL, POL, 2009
Q2
5A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

Kinlochbervie MSL Changes
y = 3. 5662x + 7024. 6
R2 = 0.4893
7000
7025
7050
7075
7100
7125
7150
1992 1994 1996 1998 2000 2002 2004 2006
Yea r Year Yea r
Yea rYear
Year
Level (mm )
-20
-10
0
10
20
30
40
50
60
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Wick MSL Changes
y = 5. 5433x + 6931. 2
R2 = 0.5738
6900
6925
6950
6975
7000
7025
7050
1992 1994 1996 1998 2000 2002 2004 2006
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Lerwick MSL Changes
y = 3. 1796x + 6996. 4
R2 = 0.4519
6950
6970
6990
7010
7030
7050
7070
7090
1992 1994 1996 1998 2000 2002 2004
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Stornoway MSL Changes
RSL: y = 3.1796x + 6996.4
R2 = 0.4519
6950
6975
7000
7025
7050
7075
7100
1992 1994 1996 1998 2000 2002 2004 2006
Year
Year
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Inverness MSL Changes
y = 2. 6628x + 359. 94
R2 = 0.1535
330
355
380
405
430
455
480
1992 1994 1996 1998 2000 2002 2004 2006
Level (mm)
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulation
Linear (MSL)
Aberdeen MSL Changes
y = 6. 0278x + 7000
R2 = 0.77
6990
7015
7040
7065
7090
7115
7140
1992 1994 1996 1998 2000 2002 2004 2006
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Islay MSL Changes y = 6. 2279x + 6956. 8
R2 = 0.5666
6940
6960
6980
7000
7020
7040
7060
7080
7100
1992 1994 1996 1998 2000 2002 2004 2006
Level (mm)
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Portpatrick MSL Changes y = 3. 6764x + 7013. 3
R2 = 0.2927
6970
6995
7020
7045
7070
7095
7120
1992 1994 1996 1998 2000 2002 2004
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Leith MSL Changes RSL: y = 3.1796x + 6996.4
R2 = 0.4519
7000
7025
7050
7075
7100
7125
7150
1992 1994 1996 1998 2000 2002 2004 2006
Yea r
Level (mm )
-40
-20
0
20
40
60
80
100
Tidal modulation contribution (mm)
MSL
M2 Tidal modulati on
Linear (MSL)
Fig. 4. Mean sea level changes at various Scottish ports, plotted to represent their approximate spatial locations, showing that the M2 tidal modulation (right-hand axis) has little impact on the rising trends shown.
6A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
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Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

319 maximum of 1.9 mm/year rise) (Teferle et al., 2006), then the recent
320 observed relative sea level rise rates for Scottish ports should broadly
321 lie between the most benign rate of + 0.57 mm/year (i.e. +1.17 –
322 0.6 mm/year) and the least benign rate of 4.26 mm/year (i.e. 2.36
323 +1.9 mm/year). It is clear from Table 2 that the real RSL rise as
324 measured by observed tide gauge data exceeds these estimates, the
325 mismatch most likely being a function of errors associated with the
326 estimates of regional eustatic rates. Indeed, if the recent satellite
327 estimate of global sea level rise from 1993 to 2007 of 3.36 ±0.41 mm/
328 year (Beckley et al., 2007) is briefly considered instead of the regional
329 eustatic estimates for UK waters, then the RSL rise rates for Scottish
330 ports should broadly lie within the range (3.2–4.53 mm/year rise).
331 Although this represents a closer match to the 1992–2007 observed
332 rates of 2.6–6.2 mm/year, particularly at the lower rates, the upper
333 rates remain underestimates.
334 Accepting the Shennan et al. (2009) observation that Lerwick is
335 one of 3 relative foci of relative sea level rise in Great Britain, then
336 plotting the observed tide gauge data for the remaining ports in its
337 spatial context reveals clear patterns. The data are spatially patchy but
338 the longer-term record spanning various dates since 1901 shows the
339 RSL distribution to broadly follow an ellipsoidal shape, with Scotland's
340 interior experiencing an RSL rise of less than 1 mm/year, increasing
341 towards the periphery to around 2 mm/year (Fig. 5a). However, if
342 only the more recent tidal trends between 1992 and 2007 are used,
343 the data show a similar but more accentuated ellipsoid whose pattern
344 is, unsurprisingly, inversely affected by the isobases that depict
345 Holocene isostatic uplift (Fig. 5b). This shows a rising RSL at Fort
346 William of about 3 mm/year (reflecting the offsetting contribution of
347 uplift close to the uplift centre), increasing to 4–6 mm/year for much
348 of mainland Scotland and the coastal periphery (reflecting a reduced
349 relative uplift contribution further away from the uplift centre). It
350should be re-emphasised that these maps are based on a much sparser
351distribution of sites and much shorter runs of data than are desirable,
352especially over the short term. Nevertheless, they are of interest since
353they show recent increases in RSL rates over the short term that echo
354the recent short-term increases in a range of co-related indicators of
355climate change, such as the upward curve of increased global
356temperature and the increase in sea surface temperature and
357hurricane power (Archer, 2007), as well as the satellite data record
358of global sea level change (Beckley et al., 2007).
3595.2. Informing predictions of future RSL rise
360Identifying the direction, rate and spatial variation of future RSL
361change is clearly important to the development of coastal strategies
362targeted at avoidance, adaptation and mitigation of negative impacts
363on society and its coastal infrastructure. The latest UK Climate
364Projections (Lowe et al., 2009 Q4; UKCP09) seek to outline the ways in
365which key climate change variables might be expected to change over
366the coming century and impact on the UK. The UKCP09 models use
367individual projections for 25 km× 25 km cells, and, although not all
368parts of the coast are covered by these grid cells, those that are
369incorporate estimates of RSL rise. The UKCP09 projections also reflect
370the uncertainties associated with the various greenhouse gas
371emissions scenarios by presenting a central estimate (50%) alongside
3725% and 95% values which reflect the lower and higher estimates for
373each of the Low, Medium and High Emissions Scenarios. The UKCP09
374‘user interface’is designed to allow coastal managers to retrieve RSL
375projections for areas of particular interest. For example, based on a
376High Emissions Scenario, Fig. 6a shows the central (or 50% cumulative
377distribution function) estimate of relative sea level rise in metres
378above the 1990 levels for key Scottish ports as extracted from UKCP09,
- 0.68
2.2
2.2
2.2
2.1
2.1
2.1 1.55
0.87
0.47
1.2
1.95 1.92
3.183.18
3.18
3.18
5.545.54
5.54
5.54
3.573.57
3.57
3.57
5.75.7
5.7
5.7
2.66
2.982.98
2.98
2.98
6.036.03
6.03
6.03
4.044.04
4.04
4.04
6.056.05
6.05
6.05
4.84.8
4.8
4.8
6.236.23
6.23
6.23
1
1
1
2
2
2
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
6
6
6
5
5
5
4
4
4
1
1
1
50 km 50 km
ab
Fig. 5. Scottish tide gauge trends plotted spatially to compare (a) longer-term average rates between 1957 and 2007 and (b) more recent rates between 1992 and 2007.
7A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

379 with Fig. 6c showing the higher (or 95%) estimate (i.e. based on a High
380 Emissions scenario, by year 2010 it is highly unlikely that sea level will
381 exceed ca. 0.1 m above 1990 levels). Fig. 6banddshowthe
382 corresponding rates expressed in mm/year. Using the central 50%
383 projection from Fig. 6c, UKCP09 predicts RSL rises of between ca. 2 and
384 4 mm/year at 2000 and ca. 2.2 and 4.2 mm/year at 2010. For the
385 higher (95%) projection the rates are ca. 3.8 and 5.8 mm/year at 2000
386 and 4.5 and 6.2 mm/year at 2010 (Fig. 6d). When the observed RSL
387 rates between 1992 and 2007 from Table 2 are plotted onto Fig. 6b and
388 d, it becomes clear that the present (short term) rates of RSL rise
389 presently experienced at Scottish ports lie above the central estimate
390 for the High Emissions Scenario and more closely align with the
391 higher (or 95%) projection for the High Emissions Scenario. When
392 compared with the recent observed record of RSL rise at ca. 2006/7,
393 the UKCP09 95% projection of a High Emission Scenario results in an
394 over-estimation of 2.8 mm/year for Lerwick and an under-estimation
395 of 1.7 mm/year for Islay. Irrespective of the detail, it is clear that over
396 the past 15 years, the present observed rates of RSL rise in Scotland lie
397 at the upper end of the High Emissions Scenario produced by UKCP09.
398 6. Discussion
399 Whilst it is acknowledged that the RSL maps based on recent tide
400 gauge data are drawn from a sparser distribution of sites and shorter
401 runs of data than is desirable, they do highlight short-term trends that
402 are of interest. They show recent increases in regional RSL rates over
403 the short term that echo climate change indicators on a global scale,
404 such as global sea level change (Beckley et al., 2007), global
405temperature (Hansen et al., in press), sea surface temperature and
406hurricane power (Archer, 2007). If these are indicators of the long-
407term trend rather than a short-term fluctuation then comparison with
408the recent geological record serves to highlight the significance of
409such rates of RSL rise. For example, in the Orkney Isles the fastest rate
410of relative sea level rise since deglaciation was ~ 6 mm/year, achieved
411between 8 and 6 k calibrated years BP (Shennan and Horton, 2002).
412This resulted in large changes in coastal configuration with substantial
413submergence, loss of land and the creation of the archipelago that we
414see today (Rennie, 2006). Over the same time period, the coastal
415outline of other areas in Scotland, such as the Firth of Forth and the
416Dornoch and Moray Firths, were effectively reconfigured by excep-
417tionally rapid coastal change driven by a swiftly rising RSL that
418inundated to substantial distances inland (Hansom, 2001; Smith et al.,
4192010). The period since then (at least for the past 4 k years) has been
420characterised by much slower rates of RSL change in Scotland and
421thus may be a poor analogy for predicting future response, even for
422the areas that lie out with the zero isobase of uplift such as the
423Western and Northern Isles of Scotland.
424Since the present rates of RSL for Islay, Aberdeen, Wick and
425Stornoway lie in the range 5.5–6.2 mm/year, these locations and their
426adjacent coastlines are now experiencing rates of RSL rise that have
427not been matched for the last 7 k years or so. Such modern RSL rise
428rates also exceed the 3–4 mm/year thresholds identified as points
429beyond which widespread reorganisation of coastal landforms begins
430to be forced (Carter et al., 1989; Pethick, 1999; Orford and Pethick,
4312006). For example, over the last 4 k years in Ireland a slowly rising
432RSL of less than 1 mm/year led to the development of barrier systems
Comparison of RSLR (m) of Scottish Ports
[50% value, Hi Emiss ions & b ase d on 1990 leve ls]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Yea r
Yea r
Yea r
Yea r
Relative Sea Level Rise ( m)
Lerwick
Wick
Inve rne ss
Aberdeen
Dundee
Rosyth
Dunbar
Portpatrick
Millport
Glasgow (ers kine)
Islay
Mallaig
Ullapool
Stornoway
Kinlochbervie
Comparison of RSLR (m) of Scottish Ports
[95% value, Hi Emiss ions & bas ed on 1990 le vels]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Relative Sea Level Rise (m )
Lerwick
Wick
Inve rne ss
Aberdeen
Dundee
Rosyth
Dunbar
Portpatrick
Millport
Glasgow (ers kine)
Islay
Mallaig
Ullapool
Stornoway
Kinlochbervie
Comparison of RSLR (mm/yr) of Scottish Ports
[50% value, Hi Emiss ions & bas ed on 1990 le vels]
0
2
4
6
8
10
12
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Relative Sea Level Rise ( mm/yr)
Lerwick
Wick
Inverness
Aberdeen
Dundee
Rosyth
Dunbar
Portpatrick
Millport
Glasgow (ers kine)
Islay
Mallaig
Ullapool
Stornoway
Kinlochbervie
Com parison of RSLR (mm /yr) of Scott ish Ports
[95% value, Hi Emiss ions & b ase d on 1990 leve ls]
0
2
4
6
8
10
12
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Relative Sea Level Rise ( mm/yr)
Lerwick
Wick
Inverness
Aberdeen
Dundee
Rosyth
Dunbar
Portpatrick
Millport
Glasgow (ers kine)
Islay
Mallaig
Ullapool
Stornoway
Kinlochbervie
ab
cd
Fig. 6. Projected RSL rise above 1990 levels for various Scottish Ports based on UKCP09 High Emissions Scenarios. Panel a shows the Central (50%) estimate of RSL rise in metres and
panel b the rate of relative sea level rise in mm/year. Panel c shows the Upper (95%) estimates of RSL in metres and panel d the rate of relative sea level rise in mm/year. The vertical
bars overprinted onto panels b and d depict the range in current rate of RSL rise as shown by the short term tidal records plotted in Fig. 5b.
8A.F. Rennie, J.D. Hansom / Geomorphology xxx (2010) xxx–xxx
Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015

433 or closed cell sediment systems and limited rates of morphological
434 change (Carter et al., 1989). On the other hand in Nova Scotia, over the
435 same time period, a rapidly rising RSL of up to 5 times the Irish rate
436 forced rapid coastal changes that were strongly associated with
437 sediment supply factors and these shorelines continue to display
438 many rapid coastal changes over periods as short as 10–20 years
439 (Carter et al., 1989). The short term RSL rates now being experienced
440 in Scotland are comparable with those experienced in Nova Scotia and
441 it is likely that substantial coastal modification may already be under
442 way. Indeed Angus et al. (in press) report on a site in the Dornoch
443 Firth that has been prograding for the last 7 k years but is now
444 undergoing pronounced erosion along its frontal edge and is subject
445 to invasive salt marsh colonisation of low lying mature sand dune
446 environments in its interior. Such RSL related concerns about rapid
447 changes to coastal landforms and habitats are compounded by current
448 deficits in the supply of coastal sediment needed to maintain beaches,
449 dunes and salt marshes as protective morphologies, deficits that will
450 be exacerbated by enhanced RSL rise in the future (Orford and
451 Pethick, 2006).
452 From a strategic planning viewpoint, it is clear that the projections
453 based on Low or Medium greenhouse gas Emissions Scenarios as a
454 planning guide are underestimates and have already been overtaken
455 by current rates of RSL rise. Indeed, the rates of RSL indicated by tide
456 gauge trends in Scotland show that present rates match those
457 predicted at the 95% upper limit of a High Emissions Scenario and a
458 strong argument exists that a High Emissions Scenario should now be
459 used as the minimum baseline for coastal planning. This argument
460 may be supported by recent upward revisions of the amount of global
461 sea level rise (+1.4 m by 2100) resulting from enhanced glacier
462 melting in high latitudes (Turner et al., 2009).
463 Since an isostatic slowdown is entirely consistent with geomor-
464 phological theory (Andrews, 1970), it should come as no surprise that
465 the mitigation it provides from the direct flooding and erosional
466 impact of rising RSL might be first diminished and then overtaken as
467 the rate of RSL rise increases. This presents a considerable challenge
468 for the policy makers of the Scottish Government and its statutory
469 advisory agencies as well as for coastal planners and managers that
470 more routinely deal with planning applications for new coastal
471 development in low lying and often vulnerable coastal land. All Local
472 Authorities may now need to consider what the UKCP09 projections,
473 along with the analysis presented here, mean for their responsibilities.
474 Operationally, Local Authorities'Structural Plans should be informed
475 by the UKCP09 projections as well as being supplemented by analysis
476 of regional tidal records. It follows that coping strategies need to be
477 urgently developed in order to allow plans to be sustainable in the
478 future. The key contribution of geomorphology to the planning
479 process lies in a unique ability to establish the impact of RSL rise both
480 on coastal landform stability and position and on the additional
481 forcing of already depleted coastal sediment supplies. In terms of the
482 spatial and temporal changes expected, a geomorphological perspec-
483 tive also embraces the intergenerational aspects of sustainable
484 management by recognising the need for accommodation space to
485 be made available, on a rolling basis, for several iterations of coastal
486 position beyond the present plus one.
487 7. Conclusions
488 Comparison of the rate of long-term land and sea level change with
489 the recent trends of RSL derived from tide gauges suggests that
490 isostatic uplift no longer continues to significantly reduce the adverse
491 effects of sea level rise on the Scottish coast. With observed RSL rise
492 rates of between 2.6 and 6.2 mm/year, the mitigation provided by
493 land uplift now appears to be limited. Although it is openly
494 acknowledged that employing only 15 years of records to establish
495 meaningful sea level trends carries risk, it is also clear that the recent
496 rates of RSL rise from tide gauges are unquestionably higher than the
497longer-term averages for the 20th century and are of differing
498magnitudes depending on location. However, only time will tell if
499these increases are the start of a longer trend or part of a temporary
500short-lived acceleration. From a planning perspective therein lies the
501conundrum since any delay in actioning national and local mitigation
502and adaptation measures may serve to increase the impacts of faster
503RSL rise on the coast in the future. Again from a planning viewpoint, it
504follows that the single estimates provided by the IPCC are of little real
505value to planners in search of regional sea level trends and future
506predictions (Gehrels, 2010). If the trend demonstrated by the Scottish
507tide gauges since 1992 continues then strategic planning in Scotland
508needs to address the consequences of more rapidly rising regional
509RSLs and recognise that the coastal changes expected over the next
510several decades are likely to differ markedly from those in the past.
511Many coasts will change much more rapidly and with a greater
512magnitude than previously, indeed coastal trends over the late
513Holocene may be a poor model from which to project the trends of
514the future coast. For low gradient coastal zones, both rocky and
515depositional, the future coastline may relocate far inland and, as a
516result, planning time frames need to be replaced by an inter-
517generational time-frame that provides the accommodation space to
518allow the coast to adjust and migrate in an iterative fashion. A
519geomorphological perspective thus has the potential to add real
520substance to future planning strategies especially since the future soft
521and low gradient coastline is unlikely to be located within the
522recognised limits of the present coastline.
5238. Q5Uncited reference
524Bingley et al., 2001
525Acknowledgements
526The authors would like to thank Andy Plater and two anonymous
527referees for their helpful and constructive comments on an earlier
528draft of this paper.
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Please cite this article as: Rennie, A.F., Hansom, J.D., Sea level trend reversal: Land uplift outpaced by sea level rise on Scotland's coast,
Geomorphology (2010), doi:10.1016/j.geomorph.2010.09.015