In this video, Professor Julienne Stroeve, UCL and Dr Lars Kaleschke, Sea Ice Physics Group, AWI talk about classifying sea ice, and about how the SMOS satellite is used to measure sea ice thickness. Julienne Stroeve also shows us how the scientists make hourly ice watch observations from the Kapitan Dranitsyn.

Sea ice is classified by how long ago it formed. This influences characteristics of the ice such as its salinity (saltiness) and thickness. These different characteristics mean that ice can be classified using remote sensing.

• First-year ice is ice that has been developing for no more than one year
• Multi-year ice is ice that has survived seasonal melts. Multi-year ice is less salty and gets harder each year that it survives, which makes it more difficult for icebreakers to navigate.

Lars explains the difference between First-Year Sea ice and Multi-Year Sea ice – and why it is important for us to be able to measure this.

Multi-year ice and first-year ice scatter microwaves differently, which allows remote sensing to distinguish between the two different types.

In terms of salinity, first-year ice typically contains 6-10% salt. The brine then gradually drains from the upper part of the ice, which leaves multi-year ice containing about 1% salt.

The brine draining leaves bubbles of air in the ice. These bubbles make radar energy bounce back - meaning that SAR is a good tool to measure the extent of first- vs multi-year ice.

Arctic sea ice is getting younger. Multi-year ice cover has fallen from 61% to 34% since 1984. Older is often thicker than first-year ice, which means that the Arctic sea ice is generally getting thinner. We could be heading towards ice-free summers in the Arctic.

It is important to measure sea ice thickness because changes in the volume of sea ice impact the heat and freshwater budgets of the Arctic and it also has wider implications for the global climate.

ESA's Soil Moisture Ocean Salinity (SMOS) Earth Explorer mission is a radio telescope in orbit and it can also be used to measure sea ice thickness.

In addition to the SMOS satellites, sea ice thickness is measured from space using satellites such as ESA’s CryoSat-2 satellite.

CryoSat-2 was launched in 2010 and has given us vital data due to its unparalleled coverage of the Arctic Ocean. This has made it possible to estimate the Northern Hemisphere sea ice thickness and volume.

The instrument that CryoSat uses is SIRAL (SAR Inferometer Radar Altimeter). It emits a radar signal, and the time taken for the signal to return to the satellite after being reflected off the ice allows us to know the height of the sea ice above sea level. The thickness is calculated using this height and the density of the ice. We will go into more detail about SAR in Week 3 of this course.

Roughly speaking, 1/9th of the sea ice is above the waterline and 8/9ths is below. Snow cover may affect this, as a thick layer of snow cover will push the sea ice deeper.

Eero Rinne, Head of the Polar Oceanography and Sea Ice Group at the Finnish Meteorological Institute has provided some Powerpoint presentations to support this topic. They are available under the 'Downloadable Documents' section below.

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Featured Educators:

• Professor Julienne Stroeve
• Dr Lars Kaleschke