Robot submarine looks under Antarctic to find out why sea ice is getting thicker

by Edward Hanna Edward Hanna is Professor of Climate Change at University of Sheffield 25.11.2014
For several years now climatologists have puzzled over an apparent conundrum: why is Antarctic sea ice continuing to expand, albeit at the relatively slow rate of about one to two percent per decade, while Arctic sea ice has been declining rapidly (by some 13% per decade in late summer)?

Just a few weeks ago the Antarctic saw a third consecutive record year of sea ice coverage. The two previous records were set in 2012 and 2013.

To help get to the bottom of this mystery, one team of scientists have enlisted an underwater robot to help measure the thickness of the ice. Their vehicle, known as SeaBED, has an upwards looking sonar which maps the underside of ice floes and provides novel, highly-detailed three-dimensional maps of Antarctic sea ice. The researchers present their findings in the journal Nature Geoscience.

Measuring a total of ten ice floes covering more than 500,000 square metres, they found mean ice thicknesses of 1.4 to 5.5 metres. In some places the ice was up to 16 metres thick. This is much thicker than has been gauged by previous more limited field-based (mainly ship-based) measurements, possibly because ships tend to avoid the areas of thicker sea ice, so there may very well be a sampling selection bias.

Satellites would ideally be able to assess ice thickness over a much wider area. However, although they have had some success in the Arctic, at the other end of the world satellites are severely hampered by our poor knowledge of how much snow there is on top of any given area of Antarctic sea ice.

The cartographic submarine at work. WHOI

The researchers report that the ice they measured was in its first year of growth. This is important because it is the multi-year sea ice (ice that survives more than one summer melt season) which is more susceptible to thickness growth through deformation and ridging.

Previous estimates of mean thickness for first-year Antarctic sea-ice – which date back to at least 1986 – suggest it is no more than around a metre thick on average. It has long been recognised however that much thicker multi-year ice floes exist – especially near the coast and the Antarctic Peninsula, where sea-ice ridges can be the size of a house.

Although the new study is important, especially from an innovation/technological point of view, I would like to see this kind of analysis repeated across a range of different areas, and if possible seasons and years. At the end of winter around 20 million square kilometres of sea around the Antarctic is covered by ice – an area larger than Russia. The surveyed zone is tiny in comparison.

Good news

If the results are confirmed by future work, they suggest Antarctic sea-ice may be more resilient towards climate warming than has previously been appreciated.

Also, changes in the sea ice will in turn affect land-based glacial ice and free-floating ice-shelves if the sea-ice suddenly gets removed (or is thicker than realised). This is especially important in regions next to the strongly warming Antarctic Peninsula where some ice shelves have dramatically broken away into the ocean.

However, it will be some time yet before we know answers to the crucial question of what has caused recent sea-ice growth in Antarctica: is it changes in ocean currents, maybe related to an increase in fresher, colder sub-surface meltwaters running off from the great continental ice sheets?

Nevertheless thicker Antarctic sea-ice cover has profound implications. Ice thickness closely controls the exchange of energy between the ocean beneath and the air above – without ice cover, too much heat will leave the oceans and join the atmosphere.

Although ice is a very effective insulator, as soon as it reaches a few tens of centimetres thickness, equally important are holes in the ice cover. These holes, knows as leads and polynyas (leads are long rectilinear channels in the ice, while “polynya” comes from the Russian from “open” and is a larger, lake-like opening) act as natural vents or chimneys, releasing hundreds of watts of heat per square metre into the overlying atmosphere.

Polynyas near Pine Island Glacier, Antarctica. NASA, CC BY

Any changes in the distribution of ice thickness can dramatically affect the points where these features form and their persistence. Thus it is fair to say that Antarctic sea ice thickness plays a pivotal role in what we call sea ice-climate feedbacks, where levels of ice cover are strongly linked to ongoing global climate change and vice versa.

It is therefore crucial for scientists modelling sea ice behaviour to have a good knowledge of thickness distribution to feed their models. This study represents a significant step forward.

The Conversation

Edward Hanna does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

This article was originally published on The Conversation. Read the original article.

About Edward Hanna

Edward Hanna received a BSc in Planetary Science (First Class Honours) from University College London in 1995 and completed a PhD in Satellite Remote Sensing of Antarctic Sea Ice and Climatic Couplings at the University of Bristol in 1998. This was followed by a period of postdoctoral research in the Department of Meteorology, University of Reading between 1998 and 2000 before being appointed as lecturer in Meteorology at the Institute of Marine Studies, University of Plymouth in November 2000, and promotion to a senior lectureship in September 2002.

In September 2003 Edward was appointed as Lecturer in Climate Change at the Department of Geography, University of Sheffield, promoted to Senior Lecturer in 2006, Reader in 2010 and was awarded a personal chair in 2013.

Edward is a Fellow of both the Royal Meteorological and Royal Astronomical Societies, member of the International Glaciological Society, member of the American Geophysical Union, has served as a Council member of the British Astronomical Association, is an Editor of The Cryosphere (European Geosciences Union), and has previously served on the Editorial Board of Weather (RMetS).He has published 80 research papers in international peer-reviewed journals, which have attracted over 1,500 citations according to ISI Web of Science (November 2013).

Research Interests 

Polar ice and climate change; Meteorology/climate of Greenland, Iceland and other high-latitude regions; Mass balance of Greenland and Antarctic Ice Sheets; Sea ice and satellite remote sensing of sea ice; Solar forcing of climate; Meteorology during solar eclipses.

More generally (last but not least!) climatology, applied/historical meteorology including instruments and observations, astronomy and planetary science. Public understanding of science, especially meteorology and astronomy; related media work.

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