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Scientists using satellites to study Antarctica and global sea level rise were recently surprised to discover that the surface of a vast, remote glacier 1,000 metres thick was rising and falling like clockwork, twice a day.

They concluded in 2024 that the ebb and flow of tides were heaving the Thwaites glacier, which rests on the bed of the Amundsen Sea off West Antarctica, up and down from below by tens of centimetres. The tides were pushing warmer, salty seawater far beneath the underbelly of the glacier and causing "vigorous melting".

The findings add to evidence that climate change could weaken the Thwaites and accelerate global sea level rise from the glacier, which covers an area the size of Florida or Great Britain and is sometimes dubbed the "Doomsday Glacier" because of its vulnerability to thaw. Other glaciers flowing into the oceans from Greenland to Antarctica may also be at risk from water seeping underneath.

"It’s mind-boggling to think that a little change in water pressure from the sea is lifting up a kilometre of ice, lifting up the whole glacier," says Eric Rignot, a professor of Earth System Science at the University of California–Irvine, and lead author of the Thwaites study. Rignot, who also works for the United States' space agency NASA's Jet Propulsion Laboratory, called the zone where ice rests on the seafloor "the Achilles heel of the glaciers" in a warming world.

A special focus is currently being placed on glacier monitoring due to climate change, but scientists have always been interested in them. Among the illustrious examples is the creation in 1894 of the International Glacier Commission to monitor glaciers and study how ice ages occur. Today, scientists continue to carry out in situ monitoring, visiting the ice to measure surface temperatures or inserting long metal stakes into glaciers and returning months later to see whether ice has thinned or thickened. 

But the dangers and costs of visiting heavily crevassed glaciers are spurring ever more use of technology. Scientists are enlisting technologies including satellites, robot submarines, hot water drills, ice cores, and aerial photography by drones and planes.

These techniques are effective, but they are also expensive. The United States and the United Kingdom invested US$50 million from 2018-25 to study the Thwaites, whose total collapse would add 65 centimetres to sea levels. The remoteness of Antarctica and bone-chilling temperatures makes such projects hugely challenging. 

Impressive technological equipment is deployed to examine this ice giant. Scientists camped on the glacier and worked from ships, deployed robot submarines, sonars, radars, and even tagged seals with high-tech sensors. They used a hot water drill to open a hole 600 metres through the ice, and lowered a torpedo-shaped robot known as Icefin to study seawater beneath the ice shelf, the tongue of ice floating at the end of the glacier.

In Norway, drones and laser scanners are used to monitor small glaciers. "In the north of the country, there were record glacier ice losses this year. It was a bit shocking to see how negative the numbers were," explains Liss Marie Andreassen, research professor at the Norwegian Water Resources and Energy Directorate. 

Scientists are racing against time. Global warming has already led to the disappearance of thousands of glaciers worldwide. The Copernicus Climate Service, a European initiative providing information about the past, present and future climate, reports that in 2023 – the warmest year on record – glaciers collectively lost 600 billion tonnes of water, equivalent to about 1.7 mm of sea level rise. In some parts of the world, glacier monitoring is going out of business as whole countries are losing glaciers.

In Slovenia, a camera that monitors ice below the summit of Triglav, the nation's highest peak at 2,864 metres, shows mere remnants. Temperatures there are now sometimes above freezing for six months a year, against just four months in the 1950s.

"There are just two small patches of ice – the leftovers of the former real glacier... they don't have crevasses and don't really move any more," laments Miha Pav?ek of the Anton Melik Geographical Institute in Slovenia.