Observing the Disintegration of the A68A Iceberg from Space

Abstract

<p>Icebergs impact the physical and biological properties of the ocean along their drift trajectory by releasing cold fresh meltwater and nutrients. This facilitates sea ice formation, fosters biological production and influences the local ocean circulation. The intensity of the impact depends on the amount of meltwater. A68 was the sixth largest iceberg ever recorded in satellite observations, and hence had a significant potential to impact its environment. It calved from the Larsen-C Ice Shelf in July 2017, drifted through the Weddell and Scotia Sea and approached South Georgia at the end of 2020. Finally, it disintegrated near South Georgia in early 2021. Although this is a common trajectory for Antarctic icebergs, the sheer size of A68A elevates its potential to impact ecosystems around South Georgia through release of fresh water and nutrients, through blockage and through collision with the benthic habitat.</p><p>In this study we combine satellite imagery data from Sentinel 1, Sentinel 3 and MODIS and satellite altimetry from CryoSat-2 and ICESat-2 to chart changes in the A68A iceberg’s area, freeboard, thickness, volume and mass over its lifetime to assess its disintegration and melt rate in different environments. We find that A68A thinned from 235 ± 9 to 168 ± 10 m, on average, and lost 802 ± 35 Gt of ice in 3.5 years. While the majority of this loss is due to fragmentation into smaller icebergs, which do not melt instantly, 254 ± 17 billion tons are released through melting at the iceberg’s base - a lower bound estimate for the fresh water input into the ocean. Basal melting peaked at 7.2 ± 2.3 m/month in the Northern Scotia Sea. In the vicinity of South Georgia we estimate that 152 ± 61 Gt of freshwater were released over 96 days, potentially altering the local ocean properties, plankton occurrence and conditions for predators. The iceberg may also have scoured the sea floor briefly. Our detailed maps of the A68A iceberg thickness change will be useful to investigate the impact on the Larsen-C Ice Shelf, and for more detailed studies on the effects of meltwater and nutrients released off South Georgia. Our results could also help to model the disintegration of other large tabular icebergs that take a similar path and to include their impact in ocean models.</p>

Impact of an acceleration of ice sheet melting on monsoon systems

“…The potential large number of retroactive events in snow-covered regions resulting from rising temperatures (Fettweis et al, 2013) suggests that tipping points can be very rapid and sudden. Recent trends are worrying: the recent melting of the A68 iceberg was estimated to have released about 152 Gt of fresh water and nutrients near South Georgia (Braakmann-Folgmann et al, 2022). In addition, nearly twice as much lightning was detected north of 80 • N in 2021 than in the previous 9 years combined, denoting an increase in liquid precipitation at high latitudes (Network, 2022).…”

Ocean currents break up a tabular iceberg

“…Iceberg A68 (surface area 5800 km 2 ) calved from the Larsen C Ice Shelf (Antarctic Peninsula) between the 10 and 12 July 2017 and, a few days later, split into two bergs, the 5710 km 2 A68a and 90 km 2 A68b ( 16 , 17 ). By December 2020, iceberg A68a had reduced in area to 3900 km 2 while drifting north to near South Georgia Island.…”

“…1 ), whereby large (>5 km 2 ) child bergs break off from a larger parent berg ( 18 ). The first rift calving event occurred on December 17 near a shallow topographic feature and was likely triggered when A68a briefly contacted the seafloor ( 16 ). However, the cause of the second rift calving event, where the “finger” that comprised the southern half of A68a broke off around December 21, was unclear.…”