Relative sea level changes around Scotland’s coastline

Alastair DawsonSociety of CartographersUniversity of Aberdeen2008

Relative sea level change around Scotland’s coastline

Underlying long-term patterns of change

• role of glacial eustasy

• role of glacio- isostasy

Present and future patterns of change

Typical trend of relative sea level from low latitudes(MWP episodes denote periods of acceleratedrelative sea level rise). Note disappearance of world’slast great ice sheets took place by ca. 7,000 years ago.

Scotlandice sheetmelt history

Ice age glacio-eustatic lowering of ca. 120 m of average sea level

Ice age glacio-isostatic depression of lithosphere (crust)

Long-term vertical deformation (rebound and subsidence) across Scotland

BUT – precise knowledge of maximum ice cover uncertain

Uncertainty – therefore ice cover and sea level change data used to test models of crustal response

courtesy K Lambeck

Knowledge of ice cover, thickness and timing needs to be precise!

Timing of melting of last Scottish ice sheet OUT OF PHASEwith timing of melting of ice sheets worldwide ……………..….caused complex change in relative sea level aroundScotland’s coast.

The legacy of these changes is presently being registered in Scotland’s tide gauges!

In areas where ice was thickest, glacio-isostaticrebound exceeded theglacio-eustatic increasein ocean volume…..…..leading to raisedshorelines (e.g westernJura).

Raasay intertidal peat deposits (=lower relative sea level)with raised shoreline on top (=higher and later relative sealevel).

The carselands – where residual glacio-isostatic reboundhas exceeded rate of glacio-eustatic rise

Landscapes of submergence

Differences in regional vertical movements meanthat relative sea level rise countered by crustal uplift in some areas andexaggerated by relative subsidence inother areas

Present crustalrebound rates derivefrom continuingglacio-isostatic reboundthat started ca. 15,000years ago.

Pattern of rebound reflectsdimensions of formerice sheet.

Satellite data show unequivocal evidence for recentsea level rise – BUT these are global average values

Source – University of Colorado

Tide Gauge Trends for Scotland - relative sea level riseduring recent decades typically between +1 mm/yr and +2.5 mm/yr

+2.5 mm/yr maximum

no change

Source – Woodworth pers.comm.

+2.25 mm/yr

blue line = smoothing filter

Aberdeen tide gauge points to average rise of 100 mmover last ca. 120 years

Most recent tide gauge estimates - Scotland

Lerwick 38 1957 – 2005 -0.68 mm/yr +/- 0.34 mm Wick 34 1965 - 2006 +1.55mm/yr +/- 0.43 mm Aberdeen 105 1901 – 2006 +0.87 mm/yr +/- 0.10 mm Rosyth 27 1964 – 1993 +1.99 mm/yr +/- 0.92 mm Dunbar 37 1914 – 1950 +0.47 mm/yr +/- 0.31 mm Portpatrick 31 1968 – 2004 +1.95 mm/yr +/- 0.44 mm Millport 20 1969 – 2006 +1.20 mm/yr +/- 0.53 mm Ullapool 17 1983 – 2006 +2.12 mm/yr +/- 1.15 mm Stornoway 18 1977 – 2006 +2.22 mm/yr +/- 0.90 mm

Woodworth pers. comm.

What is driving the RSL rise – melting ice or thermal expansion ?

Quantities of relative sea level rise (plus lowering) since AD 1995 due to effects of thermal expansion (or thermal shrinkage?)

Melting ice (glacio-eustatic) component of sealevel change:

•Glaciers and ice caps +0.93 mm/yr

•Greenland ice sheet +0.36 mm/yr

•West Antarctic ice sheet +0.30 mm/yr

•East Antarctic ice sheet -0.19 mm/yr

•Total contribution +1.40 mm/yr

•(Ramillien et al. 2006; Dyurgerov and Meier 2004;•Shepherd and Wingham 2007)

Solving the Conundrum:

Stornoway

Tide Gauge rise of +2.25 mm/yr

Crustal subsidence fall of - 1 mm/yr

Rise due to glacial eustasy andthermal expansion rise of +1.25 mm/yr

Ice melt contribution (latest data) +1.40 mm/yr

Implies thermal shrinkage -0.15 mm/yr

BUT thermal expansion component is the key driver of sea level rise into the future.