Relative sea level changes around Scotlands coastline Alastair Dawson Society of Cartographers University of Aberdeen 2008

Relative sea level change around Scotlands 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 accelerated relative sea level rise). Note disappearance of worlds last great ice sheets took place by ca. 7,000 years ago. Scotland ice sheet melt 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 PHASE with timing of melting of ice sheets worldwide …………….. ….caused complex change in relative sea level around Scotlands coast. The legacy of these changes is presently being registered in Scotlands tide gauges!

In areas where ice was thickest, glacio-isostatic rebound exceeded the glacio-eustatic increase in ocean volume….. …..leading to raised shorelines (e.g western Jura).

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

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

Landscapes of submergence

Differences in regional vertical movements mean that relative sea level rise countered by crustal uplift in some areas and exaggerated by relative subsidence in other areas

Present crustal rebound rates derive from continuing glacio-isostatic rebound that started ca. 15,000 years ago. Pattern of rebound reflects dimensions of former ice sheet.

Satellite data show unequivocal evidence for recent sea level rise – BUT these are global average values Source – University of Colorado

Tide Gauge Trends for Scotland - relative sea level rise during recent decades typically between +1 mm/yr and +2.5 mm/yr +2.5 mm/yr maximum no change Source – Woodworth pers.comm mm/yr

blue line = smoothing filter Aberdeen tide gauge points to average rise of 100 mm over last ca. 120 years

Most recent tide gauge estimates - Scotland Lerwick – mm/yr +/ mm Wick mm/yr +/ mm Aberdeen – mm/yr +/ mm Rosyth – mm/yr +/ mm Dunbar – mm/yr +/ mm Portpatrick – mm/yr +/ mm Millport – mm/yr +/ mm Ullapool – mm/yr +/ mm Stornoway – mm/yr +/ 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 sea level 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 Gaugerise of mm/yr Crustal subsidencefall of - 1 mm/yr Rise due to glacial eustasy and thermal expansionrise of 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.