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Jonathan Amos @jcdamos.bsky.social · 12/04/2026

I'm reasonably convinced now that Iceberg A23a has completely collapsed. The USNIC is no longer tracking a berg of that name. But A23k (also not listed) does however lumber on, albeit in a disheveled state. This view is from Thursday (9 April) NOAA-21 VIIRS. usicecenter.gov/Products/Ant...

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Jonathan Amos @jcdamos.bsky.social · 03/04/2026

End days for Iceberg A23. The two largest remaining fragments - A and K - had major blowouts on or just after the 28th of March. The largest chunks are ~50 sq km. A23 calved from the Filchner-Ronne Ice Shelf in 1986. Quite the journey it's had over four decades.

2 April. A23k after fragmentation event (Nasa)

2 April. A23a after fragmentation event (Nasa)

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Jonathan Amos @jcdamos.bsky.social · 26/03/2026

The last two big fragments of the once mighty A23 iceberg. The US National Ice Center calls the bottom-left object A23k and the top-right, A23a. Its latest report (20/3/26) said both were about 140 sq km. Latitude is roughly 50 deg S.

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Jonathan Amos @jcdamos.bsky.social · 17/02/2026

Iceberg A23a today (17 Feb). A totally clear sky. Utterly stunning. Still refusing to die. Still 500 sq km in area. Image from the Modis instrument on the venerable Aqua satellite, another oldie that's still plugging away.

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Jonathan Amos @jcdamos.bsky.social · 13/01/2026

One big paddling pool becomes three. Today's view of iceberg A23a (left) and a view from the end of December (right). Notice how the raised rim effect, holding in surface meltwater, has re-established on all three main segments of the broken berg. A23a is turning to slush puppie.

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Jonathan Amos @jcdamos.bsky.social · 12/01/2026

💥 Boom! And it's just happened. The huge volumes of meltwater that had collected on the surface of iceberg A23a have triggered a massive and catastrophic hydrofracture event. Much of the berg has turned to mush. Three major segments remain, the largest about 500 sq km.

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Jonathan Amos @jcdamos.bsky.social · 29/12/2025

Want to see an iceberg turn to mush? What remains of iceberg A23a (<1,000 sq km) is about to experience death by hydrofracture. Look at all the meltwater sitting atop the berg. The whole thing could just fall apart any day now.

NOAA 20 / VIIRS - Iceberg A23a viewed on 24 December 2025

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Jonathan Amos @jcdamos.bsky.social · 03/11/2025

"Such records are vital for distinguishing between rapid grounding-line retreat, as is reported to have occurred at Hektoria Glacier, versus the retreat of a floating ice margin through “conventional” iceberg calving processes," says polar SAR specialist Frazer Christie at Airbus Defence and Space.

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Jonathan Amos @jcdamos.bsky.social · 03/11/2025

But the paper has caused a stir because there is wide disagreement in the glaciology community about precisely where Hektoria had been fully grounded on bedrock due to a lack of high-accuracy satellite records.

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Jonathan Amos @jcdamos.bsky.social · 03/11/2025

Swansea Uni co-author @adrianluckman.bsky.social put this Sentinel-1 movie together. "Although the paleo record indicates some very rapid retreats in the past, the pace of retreat of Hektoria Glacier and its neighbours is unprecedented in the observational record," Adrian says.

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Jonathan Amos @jcdamos.bsky.social · 03/11/2025

Hektoria lost ~25km in 15 months, with 8.2km fragmenting in just the Nov/Dec of 2022. Naomi Ochwat et al point to a particular vulnerability: an "ice plain" - a big section of glacier ice sitting on flat seabed rock that suddenly goes afloat, spitting off icebergs like crazy.

Satellite images of the terminus of Hektoria glacier taken on 26 October 2022 (left) and 23 February 2023 (Copernicus/ESA/A.Luckman/Swansea University)

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Jonathan Amos @jcdamos.bsky.social · 04/10/2025

Ooops! It's happened again. The remnants of iceberg A23a have been caught on another Taylor Column, this time above the NorthWest Georgia Rise to the north of South Georgia BOT. The fragments have spun around and around for more than a month now.

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Jonathan Amos @jcdamos.bsky.social · 30/08/2025

Really clear view today of Iceberg A23a's big fragmentation event. Going back through the data, I think it happened on the 25th/26th. Image from NASA/Terra/Modis. Each new fragment is probably big enough to get a US National Ice Center designation. Likely A23g, A23h and A23i.

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Jonathan Amos @jcdamos.bsky.social · 29/08/2025

🚨🚨🚨Boom! Iceberg A23a looks to have undergone a major fragmentation event in the past 24 hours. I count at least three "daughter" bergs (g, h and i ?). Further satellite imagery will confirm. The last intact measurement I made was ~2,250 sq km. @bas.ac.uk

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Jonathan Amos @jcdamos.bsky.social · 15/07/2025

Iceberg A23a update. It's lost another couple of large chunks in the past few days. Are we getting close to a major fragmentation event? Likely so.

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Jonathan Amos @jcdamos.bsky.social · 07/07/2025

In case you were wondering... Iceberg A23a seen cruising around South Georgia today by Meteosat 12. The old berg still has an area of 2,846 sq km.

Iceberg A23a on 7/7/25 Eumetsat/Meteosat 12

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Jonathan Amos @jcdamos.bsky.social · 15/06/2025

Iceberg D15a, grounded in the Amery Sea, is now officially "the world's biggest iceberg".

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Jonathan Amos @jcdamos.bsky.social · 15/06/2025

Sentinel-1 has finally got a clear view of iceberg A23a (13/6/25). The berg is much diminished as it skirts around South Georgia. And the US National Ice Center has updated its stats: A23a is now only the world's second biggest berg at 829.79 sqNM, or 2846,11 sqKM.

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Jonathan Amos @jcdamos.bsky.social · 04/06/2025

You can see the original grounded position in this BBC map. The berg is following the expected track, riding the prevailing current and hugging the continental shelf. It could always ground again. But if not, it should sweep around SG and go north.

Grounded position of iceberg A23a in early March off the coast of South Georgia

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Jonathan Amos @jcdamos.bsky.social · 04/06/2025

The movement of iceberg A23a over the past seven days. It's moved about 80km to the southeast of the position where it grounded off the coast of South Georgia in early March. Zoomed in animation from Meteosat-12.

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Jonathan Amos @jcdamos.bsky.social · 01/06/2025

And I'm pretty sure now that A23a is no longer the "World's Biggest Iceberg". D15a is larger in my view. But only the US National Ice Center can make that determination and its listing doesn't yet reflect this.

Nasa Worldview measurement of iceberg A23a

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Jonathan Amos @jcdamos.bsky.social · 01/06/2025

Somebody's got itchy feet! Iceberg A23a has moved in the past couple of days. It grounded close to South Georgia back at the beginning of March. But the erosion and melting since then has clearly allowed it to break free. Does it now reground or sweep off around the island?

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Jonathan Amos @jcdamos.bsky.social · 30/05/2025

With days shortening at South Georgia (it's about 8 hours from sunrise to sunset), optical satellite coverage in the region is getting more restricted. Not so for Meteosat, which images the Europe/Africa hemisphere regardless of season. Hence we get a few views of iceberg A23a. Still grounded.

Meteosat view of South Georgia and iceberg A23a. 30/5/25

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Jonathan Amos @jcdamos.bsky.social · 06/05/2025

There was a spectacular view of iceberg A23a on 3 May. Clear skies over the berg and South Georgia. By my measurement, A23a is no longer the world's biggest iceberg. At ~2850 sq km, it's now bested by D15a - but only just. The US National Ice Center will be the final judge on the matter.

Iceberg A23a on 3 May. Copernicus / Sentinel-3

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Jonathan Amos @jcdamos.bsky.social · 24/04/2025

Mickey's recent GRL paper on trends/extremes in big Antarctic bergs is here: doi.org/10.1029/2024... And you can read the Kirkham paper in Nature Comms here: doi.org/10.1038/s414... And yours truly with the lead story on New Scientist online here: newscientist.com/article/2477... 😎

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Jonathan Amos @jcdamos.bsky.social · 24/04/2025

This underscores thinking about Antarctica. Large calvings like A23a are not necessarily a sign of instability. “Rather, ice shelves disintegrate via death by a thousand cuts. We should be concerned when we stop seeing the large calving events,” Mickey MacKie tells me.

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Jonathan Amos @jcdamos.bsky.social · 24/04/2025

In Antarctica, megabergs calve from ice shelves. The recognition that tabular bergs once existed in the North Sea tells us the seaward margins of the retreating British and Irish Ice Sheet at the end of the last glacial maximum also had ice shelves.

British and Irish Ice Sheet 20k years ago - BRITICE Project

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Jonathan Amos @jcdamos.bsky.social · 24/04/2025

"We can estimate from the extent of the scours and what is known about ancient sea levels that these bergs were probably five to a few tens of kilometres wide and perhaps a couple of hundred metres thick." ie Think blocks of ice the size of Cambridge or Bristol 😲

Iceberg and scourmarks graphic - Kirkham et al

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Jonathan Amos @jcdamos.bsky.social · 24/04/2025

It's crazy to think the UK once had its own colossal icebergs. @jdkirkham.bsky.social et al have found the preserved scour marks the megabergs made when their undersides ploughed through the muddy bottom of the North Sea some 20k-18k years ago. 🧵

Megaberg scour marks in North Sea - Kirkham et al

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Jonathan Amos @jcdamos.bsky.social · 22/04/2025

US National Ice Center formally catalogues A23c, the big off-cut from grounded behemoth A23a. Daughter berg is ~130km² - or 2x Manhattan Island. A23a (~3,100km²) only needs to lose another ~30km² to hand "World's Biggest Iceberg" title to D15a (~3,070km²) in Amery Sea.

A23a and A23c pictured off South Georgia on 22/4/25 Nasa Worldview Modis

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Jonathan Amos @jcdamos.bsky.social · 16/04/2025

A big chunk has broken away from iceberg A23a, currently grounded at South Georgia. I calculate ~110 sq km. Looks to have fractured sometime on 11/12 April. It will get a US National Ice Center designation. It should be A23c, unless I'm mistaken.

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Jonathan Amos @jcdamos.bsky.social · 02/04/2025

You don't need to be a seismologist to look at this figure and understand the huge forces that ripped through Myanmar on March 28. The M7.7 quake ruptured fully 500km of the Sagaing Fault, moving its western flank north, with >3m of slip at the surface. Data from EU Sentinel satellites.

NASA/JPL/Caltech/ARIA/Copernicus Product. POC: R Zinke/G Bato/C Speed/E Fielding.

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Jonathan Amos @jcdamos.bsky.social · 29/03/2025

A new lander was required when ESA pulled out of its Russian joint venture bc of the Ukraine war. The Brits will assemble the main structure, along with the thermal and propulsion systems. The engines themselves will be Skycrane heritage, from NASA. Avionics from TAS Italy.

ExoMars/Rosalind Franklin lander on descent - Airbus

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Jonathan Amos @jcdamos.bsky.social · 29/03/2025

Mars update: We've had the formal announcement of work that has been ongoing since late 2022 to develop a new landing mechanism for ESA's Rosalind Franklin rover. Covered by a £150m contract for Airbus UK that also includes modifications to the rover itself.

ExoMars/Rosalind Franklin on Mars' surface - Airbus

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Jonathan Amos @jcdamos.bsky.social · 28/03/2025

The Rainfall Rescue project was a stunning achievement. Announced at the beginning of lockdown, it caught a wave. The glamorous Lady Bayning, who fastidiously collected rain gauge data from 1835 to 1887, would have been thrilled.

Norfolk's Lady Bayning, who collected rainfall data through her life, even taking her gauge to London for the social season. National Portrait Gallery - https://www.bbc.co.uk/news/science-environment-60860397

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Jonathan Amos @jcdamos.bsky.social · 27/03/2025

15 years ago I was in the Airbus UK cleanroom and captured this picture. What was it? It was the electrical and service module for what would eventually become ESA's Gaia satellite - the discovery machine that mapped some two billion stars and other objects on the night sky. Adieu 👋

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Jonathan Amos @jcdamos.bsky.social · 24/03/2025

Iceberg A23a pictured from the ISS. The space station is almost as far south as it can reach here (51.5° S, 49.0° W - altitude of 422km). Picture taken on 14 March, so the berg had been grounded at South Georgia for a fortnight by this time.

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Jonathan Amos @jcdamos.bsky.social · 18/03/2025

Stupendous views yesterday of our friend A23a. The berg is listed with an official (usicecenter.gov) area of 1,010.13 sq nautical miles. It only needs to lose 118 sqNM to hand the title of the "World's biggest Iceberg" to D15a, which remains grounded off Princess Elizabeth Land.

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Jonathan Amos @jcdamos.bsky.social · 15/03/2025

Great report from Sky’s Tom Clark, who flew over iceberg A23a, now grounded at South Georgia. Clear imagery of the “footloose” terracing that’s driven by wave and warm surface water erosion. The terraces are ultimately ripped away by buoyant forces.

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Jonathan Amos @jcdamos.bsky.social · 14/03/2025

Read the Bedmap3 paper from Pritchard et al in Scientific Data here: nature.com/articles/s41... and it's possible to get a physical copy of the map here: bas.ac.uk/data/our-dat...

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Jonathan Amos @jcdamos.bsky.social · 14/03/2025

Bedmap3 is not just a nice-to-have; it's fundamental to understanding ice dynamics, for modellers to try to gauge where and how fast ice might slide off the continent in an ever warming world.

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Jonathan Amos @jcdamos.bsky.social · 14/03/2025

Explorers have deployed planes, satellites, ships, tractors, even dog-drawn sleds to get this data, using a variety of techniques including ice-penetrating radar, laser altimetry, seismics, gravimetry, and sonar. Essentially, every tool in the box. It's a stunning achievement.

So-cats in Est Antarctica 1960/61 (US Navy)

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Jonathan Amos @jcdamos.bsky.social · 14/03/2025

We know this now thanks to the publication this week of Bedmap3, an international effort led from the British Antarctic Survey, to describe the landscape *under* the White Continent's 27 million cu km of ice. The sumptuous new map incorporates >60 years of survey data.

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Jonathan Amos @jcdamos.bsky.social · 14/03/2025

Ever wondered where the thickest ice is in Antarctica? It's here - in un-named canyon at 76.052°S, 118.378°E in Wilkes Land. A whopping 4,757 metres thick, or more than 15 times the height of the Shard, the UK's tallest skyscraper.

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Jonathan Amos @jcdamos.bsky.social · 03/03/2025

No doubt about it. Iceberg A23a is now grounded on the edge of South Georgia's continental shelf. It became wedged in the sediments on Saturday and hasn't moved since. Will it free itself? Perhaps this is where we see the megaberg break apart?

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Jonathan Amos @jcdamos.bsky.social · 03/03/2025

🚨 Unless my eyes deceive me, iceberg A23a has indeed grounded on the continental shelf at South Georgia. No movement between Saturday and Sunday. @bas.ac.uk

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Jonathan Amos @jcdamos.bsky.social · 27/02/2025

NASA/USGS Landsat-9 took this very pretty picture yesterday of Iceberg A23a on approach to South Georgia. The 3,200 sq km berg looks like it's going to try to squeeze past the northwestern tip of the British Overseas Territory, but it risks grounding in the shallows.

NASA/USGS Landsat-9 image of Iceberg A23a near South Georgia 26/2/25

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