Speakers

Ian Shennan (Durham University, Durham)

Roland Gehrels (University of York, York)

Why is sea level study important?

Sea level varies globally, responding to: the melting of ice sheets; isostatic rebound (as crust adjusts following ice sheet decay); tectonics; and thermal expansion.  Understanding how sea level has responded to, or driven, past environmental changes is of interest to Quaternary and present day climate scientists.  Understanding the interplay between eustatic and isostatic sea level change is vitally important, and can help us to better understand the interplay of ice sheets and sea level change.  This in turn develops our understanding of other parts of the Earth system, and the role sea level has in them.  Finally, it is only with an understanding of the past changes in sea level that we can place current changes and future predictions into a longer term context. Given this understanding of long term trends, this helps us to better understand the processes causing changes, and whether the current changes are unusual.

James Scourse, Sarah Bradley, Ian Shennan, and Roland Gehrels discussing sea level change

James Scourse, Sarah Bradley, Ian Shennan, and Roland Gehrels discussing sea level change

 

What have been the major developments in sea level science?

After the discovery and development of of radiocarbon dating methods, sea level scientists realised they could use it to understand the height of sea level in the past.  This is because this technique allowed corals and other beach deposits above sea level to be dated.  Deposits at varying heights above and below present day sea level are used as ‘sea level index points’ – which tell us about the past water level. This allows a sea level curve to be drawn for a particular location, essentially marking out the change in sea level through time.  During the 1960s and 1970s scientists worked to collect sea level index points from as many locations as possible in order to understand the spatial variability in sea level change.

Isostatic sea level change (sea level change caused by the depression of the Earth’s crust, often by ice sheets) was originally recognised during the 1800s by Thomas Jamieson. This was followed by work from Nathanial Shaler in the 1870s.  By the early 1900s it had been realised that the amount of glacioisostatic adjustment experienced at a given place depends on the position of that location relative to the former ice mass.  This allowed scientist to develop conceptual models of isostacy, and the priciples of forebulges (where land bulges up vertically in front of large ice sheets), and how this can affect sea level.  By the 1960s, scientists were able to use the amount of isostatic rebound, based on multiple sea level index points, to create isobase maps.  This, as Sisson’s demonstrated, could be used to reconstruct ice sheet contours across Britain.

Over the past few years, sea level modellers and geophysicists have been able to fingerprint sea level rise in order to understand when contributions from ice sheet melting occur.  Sea level will actually fall close to a melting ice sheet due to isostatic rebound and a reduction in the gravitational attraction of the ice sheet and the ocean.  This can be used to pinpoint the cause of sea level rise (e.g. the source of the freshwater from melting ice), and provide independent estimates rates of ice sheet melting.

  

What are the major challenges?

Many of the recent developments, and likely future changes, will not be completely new ideas but advances on existing knowledge made possible by developments in technology, such as coring, dating methods, and computing power.  The likely questions, or challenges to be faced are:

  • the development of palaeotidal modelling, and its full inclusion in models.
  • full integration of sea level data with current climate and ocean models to improve predictions.
  • encouragement of policy makers to use palaeo-sea level data to inform future sea level predictions.