We interrupt coverage of the current disaster to inform you that a greater but slower-moving calamity has not taken a holiday. A major glacier in East Antarctica has retreated 5.4 kilometers (3.3 miles) in 22 years, satellite observations have revealed.
Despite a handful of exceptions driven by local conditions, the vast majority of the planet's rivers of ice are in retreat. However, a finding from East Antarctica suggests more than half the world's ice is locked up in this one region – enough to raise sea levels by 1.5 meters (5 feet). Several factors also make it one of the most resistant parts of the planet to global heating-induced melting, so if glaciers are retreating there, nowhere is safe.
A paper in Geophysical Research Letters reports that the Denman Glacier has lost 5 kilometers (3 miles) of length between 1996 and 2018. Since 1979, 268 billion tons of the glacier's ice has melted, a volume sufficient to cover all of England more than 2 meters (7 feet) deep.
The Denman holds around 3 percent of East Antarctica's ice – on its own it could add 1.5 meters (5 feet) to global sea level. The expression “glacial pace” may need reworking.
“East Antarctica has long been thought to be less threatened, but as glaciers such as Denman have come under closer scrutiny by the cryosphere science community, we are now beginning to see evidence of potential marine ice sheet instability in this region,” said Professor Eric Rignot of the University of California, Irvine, in a statement. “The ice in West Antarctica has been melting faster in recent years, but the sheer size of Denman Glacier means that its potential impact on long-term sea level rise is just as significant.”
The retreat hasn't been noticed before because to most observers the boundary between the glacier itself and the Shackleton Ice Shelf is invisible. However, the Denman extends as much as 3.5 kilometers (2.2 miles) down into the deepest continental canyon on Earth, where it touches bedrock. As the planet heats up, it is this submarine component that is melting, leaving only a much thinner tongue of sea-ice extending into the ocean. Rignot and colleagues used the Italian Space Agency's COSMO-SkyMed satellites' radar capacity to study the depth beneath the glacier and compare its grounding line with older observations.
The retreat would have been even more extensive if a subglacial ridge was not holding the eastern flank in place. Unfortunately, the western flank not only lacks any similar stabilizing force at the glacier's current front but the underwater trough only gets deeper behind it, allowing the glacier to keep on retreating if warming continues.
A minor comfort is that smaller nearby glaciers that Rignot studied appear more stable.