Measuring sea ice thickness

Antarctica is surrounded by a zone of sea ice which covers an area nearly three times the size of Australia at its maximum extent in spring. This sea ice zone is important in the global climate system as it helps drive the ocean circulation that redistributes heat from the equator to the poles.

Variations in sea ice extent, concentration and thickness are of interest to climate scientists, as together they provide the total ice volume, which can be monitored for the effects of climate change. Sea ice extent and concentration can be determined daily from satellites. However, the technology to accurately measure sea ice thickness is still developing, requiring scientists to compile information from a wide range of instruments, and rely on numerical models to help them understand how it might be varying.

In July 2006, the Australian Government Antarctic Division co-hosted a three day workshop of international experts on Antarctic sea ice to discuss ways to better measure and monitor sea ice thickness. The workshop brought together 60 participants from 13 countries, including observational scientists, modellers and members of the satellite remote sensing community. The group assessed the current state of knowledge on Antarctic sea ice thickness and examined emerging technologies to measure sea ice thickness from the surface, ships, under-ice vehicles and moorings, airborne sensors and satellite sensors.

In the northern hemisphere, de-classified data from US and British navy submarines has shown conclusively that there has been a significant reduction (10% per decade) in Arctic sea ice thickness over the past several decades. There are no long-term submarine records in the Antarctic. However, in greenhouse scenarios, climate models indicate that Antarctic sea ice thickness decreases more rapidly than sea ice extent (suggesting that thickness may be a better indicator of climate change), and predict that these changes should already have started. In the absence of sufficient data, however, we do not know whether important changes in the thickness of Antarctic sea ice are currently going unnoticed.

International efforts to obtain widespread Antarctic sea ice thickness information are progressing. However the goal of a circumpolar baseline of high resolution Antarctic sea ice thickness data is yet to be achieved. The workshop outlined future collaborative programmes that will aim to achieve this in coming years, in particular during the International Polar Year, during which Australia has a leading programme of sea ice zone research.

ANTHONY WORBY, Antarctic Climate and Ecosystems Cooperative Research Centre and Ice, Ocean, Atmosphere and Climate Programme, AGAD