From BBC by Jonathan AmosThere is going to be a gap of several years in our ability to measure the thickness of ice at the top and bottom of the world, scientists are warning.
The only two satellites dedicated to observing the poles are almost certain to die before replacements are flown.
This could leave us blind to some important changes in the Arctic and the Antarctic as the climate warms.
The researchers have raised their concerns with the European Commission and the European Space Agency.
A letter detailing the problem - and possible solutions - was sent to leading EC and Esa officials this week; and although the US space agency (Nasa) has not formally been addressed, it has been made aware of the correspondence.
At issue is the longevity of the European CryoSat-2 and American IceSat-2 missions.
These spacecraft carry instruments called altimeters that gauge the shape and elevation of ice surfaces.
They've been critical in recording the loss of sea-ice volume and the declining mass of glaciers.
image captionArtwork: CryoSat-2 (top) and IceSat-2 (bottom) will hopefully last until mid-decade
What's unique about the satellites is their orbits around the Earth.
In contrast, most other satellites don't usually go above 83 degrees.
The worry is that CryoSat-2 and IceSat-2 will have been decommissioned long before any follow-ups get launched.
CryoSat-2 is already way beyond its design life. It was put in space in 2010 with the expectation it would work for at least 3.5 years.
IceSat-2 was launched in 2018 with a design life of three years, but with the hope - and expectation - it can operate productively deep into the decade.
"Without successful mitigation, there will be a gap of between two and five years in our polar satellite altimetry capability," the scientists' letter states."
The only satellite replacement currently in prospect is the EC/Esa mission codenamed Cristal.
Industry has started work on the spacecraft but it won't launch until 2027/28, maybe even later because full funding to make this date a reality is not yet in place.
Dr Josef Aschbacher, the director of Earth observation at Esa, said his agency was working as fast as it could to plug the gap.
"This is a concern; we recognise it," he told the BBC. "We've put plans in motion to build Cristal as quick as we can. Despite Covid, despite heavy workloads and video conferences by everyone - we have gone through the evaluation... and Cristal was kicked off in early September."
Just over 10% of the near-600 signatories to the letter are American scientists.
Dr Thomas Zurbuchen, the head of science at Nasa, is not being sent the letter because it is primarily aimed at European funders - and most of the signatories are European.
He said he was hopeful any polar gap could be plugged or minimised.
"I think there are multiple options at this moment in time that we can deploy to that end, in partnership or otherwise," he commented.
One of those solutions in Europe would be to run a version of Nasa's IceBridge project.
This was an airborne platform that the US agency operated in the eight years between the end of the very first IceSat mission in 2010 and the launch of IceSat-2 in 2018.
An aeroplane flew a laser altimeter over the Arctic and the Antarctic to gather some limited data-sets that could eventually be used to tie the two IceSat missions together.
There are many who think a European "CryoBridge" is the most affordable and near-term option to mitigate the empty years between CryoSat-2 and Cristal.
The cost of manufacture of the airborne radar altimeter could be accomplished for perhaps €5m (£4.5m), scientists believe, but its design and fabrication would likely take two years.
The signatories to the letter sent to the EC and Esa include leading scientists using CryoSat and IceSat data, the president of the International Glaciology Society, and lead authors on the United Nations' Intergovernmental Panel on Climate Change, which prepares the authoritative state-of-the-climate reports for world governments.