A 29-year timeseries of polar sea ice thickness and volume that draws on altimetry data from a succession of ESA missions has been presented at the European Geoscience Union general assembly.

The dataset – developed by scientists at the University of Toulouse’s LEGOS laboratory – will be used to uncover long-term trends that are often masked by year-to-year variability.

Sea ice is a key piece of the climate jigsaw and its persistent decline is both a consequence of global warming and a driver of further changes to the environment.

Shifts in the area covered by frozen seawater in the past decades have been well documented, but there is a need to develop improved long-term estimates of ice thickness and volume to build a more complete picture of how sea ice is evolving under Earth’s changing climate.

To address this challenge, scientists created a timeseries of sea ice thickness spanning nearly three decades by combining observations from ESA’s CryoSat mission with data from two ESA Heritage Missions named Envisat and ERS. Part of ESA’s Earth Explorers programme, CryoSat launched in 2010 and will remain in operation until at least 2025. The mission has greatly advanced the study of the cryospheric changes impacting the planet.

CryoSat’s altimeter is able to deliver data at a higher resolution when an higher resolution when compared to previous missions. This allows scientists to discriminate between sea ice floes and ocean leads with a high level of certainty, enabling confident estimates of sea ice thickness.

ESA’s ERS programme – consisting of two satellites named ERS-1 and ERS-2 – was operational between 1991 and 2011, with Envisat launching in 2002 and remaining active for 10 years.

Using CryoSat observations as a benchmark, it is possible to develop high-quality estimates of sea ice thickness from altimetry data gathered by these Heritage Missions.

Scientists have previously produced timeseries containing altimetry data from Envisat and CryoSat but, until now, no continuous dataset that draws on measurements collected by the ERS satellites, CryoSat and Envisat has been developed.

The difficulty in incorporating ERS measurements into wider timeseries stems from a technical issue called the pulse blurring effect.

This phenomenon causes radar pulses emitted from the satellite’s altimeter to spread out and blur as they travel, reducing the accuracy of the distance measurements that are used to estimate sea ice thickness.

To correct for the pulse blurring effect and ensure consistency of the 29-year dataset, scientists developed a network-based method to calibrate Envisat against CryoSat and then ERS against Envisat.

This calibration is trained on discrepancies observed between the altimeter measurements during the mission overlap periods and a set of parameters characterising sea ice state, including several proxies for ice roughness, as well as ice concentration and ice age.

The resulting dataset includes monthly sea ice thickness estimates spanning the 29-year period. For the Antarctic these monthly estimates are available all year round and for the Arctic they are available for only the winter months.

The timeseries shows that Arctic sea ice below the latitude of 81.5 degrees north is melting at a rate of approximately 13.1% per decade. Antarctic sea ice evolution displayed no significant trends along the whole period, but a drop in volume is observed after 2016.

Significant variation between different regions were also observed in both poles.

Marion Bocquet, study lead author and remote sensing doctoral student at LEGOS, says, “This nearly three decades timeseries of sea ice thickness is the most extensive dataset of its kind and the first to incorporate observations from ESA’s ERS programme.

“We expect the dataset to help climate scientists further investigate how sea ice dynamics are changing in response to the evolving climate, and the impacts these changes will have on Earth’s systems.”

The project was completed under the supervision of LEGOS research engineer Sara Fleury.

It was supported by ESA’s Fundamental Data Records for Altimetry (FDR4ALT) project, the French National Centre for Space Studies (CNES) and Collecte Localisation Satellites (CLS).

References:

Bocquet, M., Fleury, S., Rémy, F., Garnier, F., and Moreau, T., 2023. Arctic and Antarctic sea ice thickness and volume changes during the last 29 years from satellites. EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8780, https://doi.org/10.5194/egusphere-egu23-8780

Bocquet, M., Fleury, S., Piras, F., Rinne, E., Sallila, H., Garnier, F., and Rémy, F., 2022. Arctic sea ice radar freeboard retrieval from ERS-2 using altimetry. Toward sea ice thickness observation from 1995 to 2021, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2022-214