University of Tasmania, Australia

Sea level change is an important climate indicator – modern satellite techniques now provide near global coverage, yet the trajectory of sea level change prior to the satellite era is complicated by a geographically sparse and incomplete network of land based instrumentation. This is particularly the case in and around the Southern Ocean. Therefore an open question is how have sea levels varied in this region over the previous century?

Macquarie Island (54° 30' S, 158° 57' E) is the lone observing station of its type in the Southern Ocean, providing one of very few sea level records south of 50 degrees latitude. As part of a five-year ARC Discovery Project “Environmental Geodesy: Variations of Sea Level and Water Storage in the Australian Region” this project revisits rare historical sea level observations collected during Sir Douglas Mawson’s 1911–1914 Australasian Antarctic Expedition (AAE), together with modern sea level data, space geodetic estimates of crustal displacement and modelling of earthquake deformation to provide a valuable insight into the evolution of sea level and land level change over the twentieth century at this location. Related work now continues using data from Commonwealth Bay, Antarctica – a second treasure trove of invaluable sea level data from Mawson’s pioneering voyage of the AAE.

By pairing and analysing historical data and modern day observations of sea level and land level change, we have found sea level has risen relative to the land at Macquarie Island at close to 5 mm/yr. Contrary to longer term geological evidence, the isthmus / Garden Cove region has been shown to be subsiding, yielding a rate of absolute sea level rise of +2.0 ± 0.8 mm/year over this period.

An interesting dimension of this project is the integration of sea level observations, space geodetic observations of ground motion (from the Global Positioning System) and earthquake modelling to better understand both the response of the ocean and the land over the past 100 years. The deformation of the crust caused by tectonic forcing and resultant earthquakes is vitally important when assessing sea level change in areas such as this close to plate margins. Analysis of the great earthquake that occurred north of the Island on 23 December 2004, provided some fascinating insights. We found that the Australian plate is less rigid than previously thought (with earthquake related deformation observed throughout southeast Australia), and were able to infer that the land subsidence at Macquarie Island is most probably occurring as a response to a thrust earthquake that occurred just to the south of the Island in 1924.

This study provides one of few estimates of observed sea level change in this region of the Southern Ocean, highlighting the value of historical data and continued geodetic and oceanographic observing programs in remote areas.

Caption: The measurement of sea level at a place like Macquarie Island is an achievement in itself (Image of RSV Aurora Australis beyond large surf at Macquarie Island by Angela Bender). Image on the right shows the tide gauge operated on the Island between 1912/1913.

Watson, C.S., Burgette, R., Tregoning, P., White, N., Hunter, J., Coleman, R., Handsworth, R., and Brolsma, H., (2010), Twentieth century constraints on sea level change and earthquake deformation at Macquarie Island, Geophysical Journal International. 182(2), doi: 10.1111/j.1365-246X.2010.04640.x. pp781-796.

McClusky, S., Tregoning, P, Watson, C.S., Burgette, R. and Lejeune, S. (2011) Great Earthquakes and the Stability of the Australian Plate: Implications for Terrestrial Reference Frame Definition (Invited). Session JG04S1, IUGG General Assembly, June 28 – July 7, 2011. Melbourne, Australia.

 Outreach:

UTAS (2010) Sea-level change at Macquarie Island, 100 years after Mawson. Research to Reality, Edition 6, 2010. (http://www.utas.edu.au/__data/assets/pdf_file/0018/124146/2010-edition-6.pdf).

Project Leaders (External)

Dr Christopher Watson – cwatson@utas.edu.au

Project investigator

Space geodesist, Surveying and Spatial Science Group

School of Geography and Environmental Studies