Late Holocene vertical land motion and relative sea‐level changes: lessons from the British Isles

Abstract

Vertical land motion caused by continuing glacial isostatic adjustment is one of several important components of sea-level change and is not limited just to previously glaciated regions. A national-scale analysis for the British Isles shows an ellipse of present-day relative uplift (relative sea-level fall), $1.2 mm a À1 , broadly centred on the deglaciated mountains of Scotland. The pattern of three foci of relative subsidence, $1 mm a À1 , results from the additional interactions of the deglacial meltwater load on the Atlantic basin and the continental shelf, and the signal due to far-field ice sheets. At a local scale, sediment compaction can more than double the rate of relative land subsidence. Relative land-level change (the negative of relative sea-level change) is not the same as vertical land motion. There is a spatial pattern in the difference between relative land-level change and vertical land motion, with differences at present of approximately À0.1 to À0.3 mm a À1 around the British Isles and þ2.5 to À1.5 mm a À1 globally. For the wider scientific and user community, whether or not the differences are considered significant will depend upon the location, time frame and spatial scale of the study that uses such information. Context and aimsIs sea level rising? A reasonable question, but the answer depends on what is meant or perceived about the meaning of the term sea level. You must frame the relevant location, time frame and spatial scale. This may seem obvious to the scientific community familiar with the complexities and details of decades of research but it may not be to those outside of that relatively small community. Controversies regarding data quality, management, analysis and display in the run up to the 2009 UN Climate Change Summit in Copenhagen and the ensuing media coverage marked a tipping point in the relationship between science and the public (Berkhout, 2010). Debates on past and future climate and environmental changes occur in many different forums, not only through academic conferences and through scientific journals. Online sources, ranging from web pages of the television and newspaper industry to discussion-boards and blogs, influence public opinion and ultimately filter through to political decisions. It is a key challenge to promote the findings of academic research to all of these communities. While the scientific community can debate issues and separate different contributions to sea-level change (Gehrels, 2010b), other stakeholders, practitioners and the wider public audience seek, appreciate and use information presented in a single, userfriendly format. One of the best formats is a map (Defra, 2006;Lowe et al., 2009;UKCIP, 2005) but confusion within the scientific literature will fuel scepticism and must be avoided. Studies of Late Holocene relative sea-level change provide critical information on the spatial variation of current rates and future scenarios of sea-level change. But we cannot expect the wider public audience to appreciate what may appear as a subtle diff...

Observed mean sea level changes around the North Sea coastline from 1800 to present

“…Trends are generally lower for the northern British, Danish and Norwegian coastlines which are subject to land uplift (e.g. Hansen et al, 2011;Shennan et al, 2012). Higher trends are derived for the subsiding southern English, Dutch and German coastlines (e.g.…”

“…Shennan and Woodworth, 1992;Shennan and Horton, 2002;Gehrels and Woodworth, 2012) or models capable of simulating glacial isostatic adjustment, the rebound effect resulting from the last de-glaciation (e.g. Peltier, 2004;Bradley et al, 2009;Hansen et al, 2011;Shennan et al, 2012). GIA is the only vertical land movement process that can be modelled on a global basis and for which we can get VLM predictions with uncertainties similar to those of RMSL rates from tide gauges.…”

“…2). The results from GIA modelling for the UK sites were taken from Shennan et al (2012), as these are considered the current best estimates for this area. Their GIA model incorporates geological information.…”

“…Their GIA model incorporates geological information. However, Shennan et al (2012) are not located at the actual tide gauge sites. This raises the question whether the nearby measured vertical motions represent the exact motion at the tide gauge (Wöppelmann and Marcos, 2012).…”

Late Holocene sea- and land-level change on the U.S. southeastern Atlantic coast

“…Shennan et al, 2012). During the late Holocene (last 45 ~2000-3000 years), RSL change along the passive U.S. Atlantic margin was dominated by 46 spatially-variable land subsidence and geoid fall.…”