Bridging the gap in polar altimetry

With the accelerated melting of the ice sheets and the sea ice cover, Earth’s Polar Regions are major witnesses to global warming. Arctic Amplification already modifies lifestyles, economies, ecologies, industries, and transportation across the region. But as a result of teleconnections with the climate system, the Polar Regions also impact on a global scale, affecting sea level rise, ocean circulation and weather patterns, which, in turn, disrupt the natural environment and society. Because of their scale and inaccessibility, observation of the Polar Regions requires a collection of space-based techniques.  Satellite altimetry provides a unique capability to monitor changes in the thickness of land ice and sea ice, and in the Polar Oceans. This information is essential for charting the response of the Polar Regions to climate change, and for predicting their future interactions with, and impacts on, the global climate system.

Although at least 7 satellite altimeters are in orbit today, only two reach polar latitudes: CryoSat-2 and ICESat-2.  CryoSat-2 was launched in 2010 and although it is still operational, it is projected to reach end of life between 2024 and 2026 due to known fuel leakage and battery degradation. ICESat-2 was launched in 2018 with a design-life of 3 years. Other satellite altimeters in lower inclination orbits, including Sentinel-3, survey only minor fractions of the Arctic sea ice pack and the polar ice sheets, and are therefore unable to provide observations of their overall imbalance. The European Commission has initiated the CRISTAL polar altimeter as a high priority candidate mission in partnership with the European Space Agency, in view of their Arctic policy, and based on user requirements. The earliest launch date for CRISTAL is in the final quarter of 2027.

Without successful mitigation, there will be a gap of between 2 and 5 years in our polar satellite altimetry capability. This gap will introduce a decisive break in the long-term records of ice sheet and sea ice thickness change and polar oceanography and this, in turn, will degrade our capacity to assess and improve climate model projections. These capabilities are of major societal importance. In order to ensure the continuity of polar altimetry, there is an urgent need to consider mitigation measures. This paper aims to stimulate a community discussion and position on possible solutions, including extending the lifetime of CryoSat-2 or ICESat-2, manoeuvring an alternative satellite into a high-inclination orbit, accelerating the launch of CRISTAL, and initiating a systematic airborne measurement programme as a bridging capability.