Measuring Antarctic sea ice thickness from space

This project offers an exciting opportunity to work at the forefront of polar science by creating new measurements of Antarctic sea ice thickness using satellite measurements.

Sea ice is a key component of Earth’s climate system, regulating the exchanges of heat and freshwater in the polar regions, and influencing global ocean circulation. Typically covering an area larger than South America in austral winter and shrinking dramatically in summer, Southern Ocean sea ice is one of the most dynamic surfaces on Earth (Shepherd et al., 2018). Highly sensitive to changes in both the atmosphere and ocean, it is also one of our planet’s main indicators of climate change.

Sea ice concentration during sea ice maximum of 2016 (September) and following minimum (February 2017). February 2017 was the lowest Antarctic sea ice extent since records began in 1980. Image courtesy of NASA Earth Observatory.

The launch of the European Space Agency (ESA) CryoSat-2 satellite in 2010 began a new era of high resolution, Synthetic Aperture Radar (SAR) altimetry, a method for measuring surface elevation via transmission and receipt of a radar pulse. CryoSat-2 has transformed our ability to monitor sea ice thickness in the Arctic, and to track its response to changes in climate [Tilling et al., 2015, 2016]. Despite these advances, we still know little about Antarctic sea ice because heavy snowfall in the southern hemisphere complicates our ability to measure its thickness from space.  But in 2018, NASA launched ICESat-2, a laser altimeter which provides an exciting opportunity to measure Antarctic sea ice thickness from space. Sea ice thicknesses is calculated from measurements of its freeboard – the portion protruding above the ocean surface – and although the CryoSat-2 radar signal passes through the snow layer, the ICESat-2 laser does not, and so the measurements can be combined to determine the snow and ice thickness at the same time.

Sea ice thickness is calculated from measurement of the ice freeboard – the portion protruding above the ocean – and an estimate of the snow depth. Because the CryoSat-2 radar and ICESat-2 laser scatter from the bottom and top of the snow layer, they can be combined to remove the need for a snow thickness estimate, which has proved challenging in Antarctica.

The aim of this PhD is therefore to produce the first continental-scale measurements of Antarctic sea ice thickness, and to explore this dataset to provide the first understanding of spatial and temporal trends which may impact on the region’s climate, natural habitat, and maritime activity. While sea ice coverage of the Arctic Ocean has been on a downward trend for decades, Antarctic sea ice extent was, until recently, growing. The last few years have seen dramatic losses in sea ice coverage of the Southern Ocean, shrinking to a 40-year minimum in 2017 and 2018. The reasons for the observed trends in extent remain unclear, with understanding currently hampered by a lack of Antarctic sea ice thickness and volume measurements. This is therefore a critical time to develop these datasets and help illuminate the drivers of recent change.

Antarctic sea ice thickness will be determined from a combination of CryoSat-2 and ICESat-2 satellite altimetry, a model of dynamic snow loading, and a physical model of the sea ice buoyancy and radar scattering informed by ship-based measurements. There may also be opportunities to participate in polar field campaigns, which play an essential role in calibrating and validating satellite observations.

Sampling Antarctic sea ice thickness from RRS Ernest Shackleton (Courtesy Rachel Tilling)

As a PhD candidate, you will have the opportunity to (1) work with state-of-the-art satellite data, (2) develop expertise in the most recent instrument and processing techniques, and (3) address globally-significant questions relating to climate science.

This PhD is based within the Centre for Polar Observation and Modelling (CPOM), in partnership with the British Antarctic Survey (BAS) and the European Space Agency (ESA). CPOM provides the UK with core strategic expertise in the exploitation of satellite measurements to study the Earth’s cryosphere, using state-of-the-art Earth observation techniques and numerical models to study and predict changes in the polar ice sheets and sea ice cover. CPOM also develops and maintains near real time measurements of Arctic sea ice thickness in partnership with ESA (http://www.cpom.ucl.ac.uk/csopr/seaice.html).

The project will be supervised by Professor Andy Shepherd and Dr Isobel Lawrence at Leeds, with input from ESA and Dr Paul Holland at BAS. Andy is the Principal Scientific Advisor for ESA’s CryoSat-2 mission and an expert in polar climate processes, Isobel is a specialist in radar altimetry, and Paul is an expert in Antarctic sea ice and its recent changes.

The project would suit a numerate candidate with a degree in a discipline such as Physics, Mathematics, Earth Sciences, Computer Sciences or Physical Geography.

Objectives

This PhD project will establish the first continental-scale measurements of, and spatial and temporal trends in, Antarctic sea ice thickness using high resolution SAR and laser altimetry data.  The precise objectives will depend upon the successful candidate’s specific expertise and interests, but may include:

  • Developing novel approaches to Antarctic sea ice retrievals from SAR and laser altimetry.
  • Improving the certainty of altimetry estimates of Antarctic sea ice through advances in processing techniques.

Potential for high impact outcome

  • Establishing and monitoring changes to Antarctica’s sea ice is of global significance because these trends affect not only the Antarctic region itself but also the global climate system.
  • The ability to map and predict sea ice trends is, for example, highly valuable for tourist and research expeditions in the area whilst helping to understand changes to marine life beneath the ice, and global ocean circulation.
  • The project also provides the opportunity to be at the forefront of advances in the altimetry of Antarctic sea ice, an emerging area of polar science.
  • We therefore anticipate that the work will lead to several publications, with at least one submission to a high impact journal.

Training

The PhD student will work under the supervision of Prof. Andrew Shepherd and Dr Isobel Lawrence within the ICAS Polar Earth Observation group. The group, which currently includes 3 PhD students and 2 postdoctoral researchers, offers a supportive and collaborative environment for training as a polar Earth Observation scientist. The project will provide specialist training in geodetic Earth Observation techniques, algorithm development and data processing. The student will also benefit from being a member of CPOM, a world-renowned polar research institute, with ample opportunity to participate in workshops, meetings and conferences. There will also be access to a broad spectrum of training provided by the faculty, including computer programming, degree management and communication.

References

Near-real-time Arctic sea ice thickness and volume from CryoSat-2.  Tilling, R.L., Ridout, A., Shepherd, A. (2016), Cryosphere doi:10.5194/tc-10-2003-2016

Increased Arctic sea ice volume after anomalously low melting in 2013.  Tilling, R.L., Ridout, A., Shepherd, A., Wingham, D.J. (2015), Nature Geoscience doi:10.1038/ngeo2489

Holland, P. R. , Bruneau, N. , Enright, C. , Losch, M. , Kurtz, N. T. and Kwok, R. (2014): Modeled Trends in Antarctic Sea Ice Thickness , Journal of Climate, 27 (10), pp. 3784-3801 . doi: 10.1175/JCLI-D-13-00301.1

Shepherd, A., Fricker, H.A. & Farrell, S.L. Trends and connections across the Antarctic cryosphere. Nature 558, 223–232 (2018). https://doi.org/10.1038/s41586-018-0171-6