Can the SIN’XS project finally resolve polar sea ice to give us estimates we all agree on?

Researchers are invited to delve into a treasure trove of satellite data, including those of the European Space Agency’s CryoSat mission, to agree a reconciled estimate of sea ice thickness.
 

Sea ice is changing rapidly

In the Arctic, multi-year ice was once the norm, forming a thick crust over much of the Arctic Ocean throughout winter and persisting through the warmer summer months.

Now that is no longer the case. Much of the more persistent multi-year ice is now seasonal. It’s thinner, covers less area, and might disappear entirely in the summer months if global heating continues unabated.

The Antarctic was, until recently, showing signs of being resilient to the effects of climate change, with sea ice extent increasing in some areas. That seems to have changed in the past few years, during which sea ice in the Southern Ocean has declined to unprecedented lows.

The reason we know all of this is because, in addition to the work of climate scientists on the ice and in the air above the polar regions, we have satellites like ESA’s ice mission, CryoSat.

CryoSat has now been in orbit nearly 15 years, using radar altimetry to measure ice sheet elevation and sea ice freeboard up to 88 degrees north and south, giving us one of the longest unbroken single mission records of global ice in existence.
 

Many satellites but no consensus

Alongside CryoSat, NASA’s ICESat-2, which carries a laser altimeter, also studies polar ice up to 88 degrees north and south.

The two satellites are currently working in tandem in a project known as Cryo2ice. Since 2020, their observations have been brought closer together over the polar regions, crossing over on 6 February 2025, to provide collocated measurements over the same ice at the same time.

Other satellites also provide sea ice data. ESA’s Soil Moisture and Ocean Salinity (SMOS) mission shares a widely used sea ice product with CryoSat. The Copernicus Sentinel-3 satellites can also measure sea ice thickness, as can the CNES/ISRO satellite SARAL/AltiKa. The suite of available data is completed by reanalyses and numerical models.

Instruments of different types, such as CryoSat’s radar and ICESat-2’s lidar, or even the different types of radar on board CryoSat and SARAL/AlitKa, measure freeboard –the height of sea ice compared to the sea surface- slightly differently. Some penetrate through the snow layer more than others, which is a major cause of uncertainty in measurements.

There are many different products and algorithms being used, each with different assumptions, to put our wealth of satellite altimetry data into sea ice models. Although the different products tend to agree that sea ice is changing, there is little consensus about precisely how much it is changing.

But accurate and consistent estimates are very important for a range of fields, from climate modelling to marine operations.
 

Defining a reconciled estimate of sea ice thickness

That’s why the Sea ice-thickness product intercomparison exercise (SIN’XS) was born.

Led by NOVELTIS, along with AWI, LEGOS and UCL, and funded by ESA, its objective is to identify the differences between a wide range of satellite and model sea-ice and snow thickness products by carrying out in-depth intercomparisons, and to propose a reconciled estimate of sea-ice thickness.

The SIN’XS team is calling for researchers to make use of the treasure trove of data, which can be analysed directly on the SIN’XS platform and downloaded to perform dedicated analysis. There is also a forum to discuss the results and ask questions about the data.

The first stage of the exercise was to collect as many sources of satellite ice thickness data as possible, which concluded in June 2024. Around 40 datasets have been collected to date from satellite observations, in-situ measurements, and models, uniting contributions from the entire sea ice community.

The data encompasses both Northern and Southern Hemisphere measurements, as specified by SIN’XS guidelines, and spans multiple variables, with sea ice thickness (SIT) and snow thickness (SNT) as the primary measurements of interest. Additional variables such as radar freeboard were also collected and, thanks to the unique conversion library available directly in the online tool, these will be combined to infer SIT measurements.

Now, the work of comparing the data begins

Protocols and metrics have been agreed upon to identify the best way to compare the data, which are being used to investigate the agreement between products and their readiness for applications and operations.

Reference data will be an integral part of the assessment and of broad relevance for the community. The project team is also evaluating the sensitivity of sea ice thickness products from altimetry data to varying auxiliary parameters like snow thickness or freeboard.

Work is also underway to derive a best-guess reconciled estimate of sea-ice thickness. “A reconciled estimate of sea ice thickness and an improved understanding of the uncertainties will be extremely valuable for the modelling community, where sea ice thickness is a central input quantity for assimilation,” says Valentin Ludwig of AWI. “It is also a valuable piece of information for navigation in polar waters.”

An improved understanding of the extent to which products agree with each other and their sensitivity towards input parameters will be important for future missions like the Copernicus Polar Ice and Snow Topography Altimeter, CRISTAL, and Copernicus Imaging Microwave Radiometer, CIMR.

The first results will be presented during a SIN’XS workshop in March, the date of which will be announced soon.