̽��ֱ�� of Cambridge - Sea Level

Antarctic ice shelves hold twice as much meltwater as previously thought

sc604 — Thu, 27 Jun 2024 08:56:09 +0000

Slush – water-soaked snow – makes up more than half of all meltwater on the Antarctic ice shelves during the height of summer, yet is poorly accounted for in regional climate models.

Ice shelves fracture under weight of meltwater lakes

sc604 — Fri, 03 May 2024 14:31:26 +0000

When air temperatures in Antarctica rise and glacier ice melts, water can pool on the surface of floating ice shelves, weighing them down and causing the ice to bend. Now, for the first time in the field, researchers have shown that ice shelves don’t just buckle under the weight of meltwater lakes — they fracture.

As the climate warms and melt rates in Antarctica increase, this fracturing could cause vulnerable ice shelves to collapse, allowing inland glacier ice to spill into the ocean and contribute to sea level rise.

Ice shelves are important for the Antarctic Ice Sheet’s overall health as they act to buttress or hold back the glacier ice on land. Scientists have predicted and modelled that surface meltwater loading could cause ice shelves to fracture, but no one had observed the process in the field, until now.

̽��ֱ��new study, published in the Journal of Glaciology, may help explain how the Larsen B Ice Shelf abruptly collapsed in 2002. In the months before its catastrophic breakup, thousands of meltwater lakes littered the ice shelf’s surface, which then drained over just a few weeks.

To investigate the impacts of surface meltwater on ice shelf stability, a research team led by the ̽��ֱ�� of Colorado Boulder, and including researchers from the ̽��ֱ�� of Cambridge, travelled to the George VI Ice Shelf on the Antarctic Peninsula in November 2019.

First, the team identified a depression or ‘doline’ in the ice surface that had formed by a previous lake drainage event where they thought meltwater was likely to pool again on the ice. Then, they ventured out on snowmobiles, pulling all their science equipment and safety gear behind on sleds.

Around the doline, the team installed high-precision GPS stations to measure small changes in elevation at the ice’s surface, water-pressure sensors to measure lake depth, and a timelapse camera system to capture images of the ice surface and meltwater lakes every 30 minutes.

In 2020, the COVID-19 pandemic brought their fieldwork to a screeching halt. When the team finally made it back to their field site in November 2021, only two GPS sensors and one timelapse camera remained; two other GPS and all water pressure sensors had been flooded and buried in solid ice. Fortunately, the surviving instruments captured the vertical and horizontal movement of the ice’s surface and images of the meltwater lake that formed and drained during the record-high 2019/2020 melt season.

GPS data indicated that the ice in the centre of the lake basin flexed downward about a foot in response to the increased weight from meltwater. That finding builds upon previous work that produced the first direct field measurements of ice shelf buckling caused by meltwater ponding and drainage.

̽��ֱ��team also found that the horizontal distance between the edge and centre of the meltwater lake basin increased by over a foot. This was most likely due to the formation and/or widening of circular fractures around the meltwater lake, which the timelapse imagery captured. Their results provide the first field-based evidence of ice shelf fracturing in response to a surface meltwater lake weighing down the ice.

“This is an exciting discovery,” said lead author Alison Banwell, from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the ̽��ֱ�� of Colorado Boulder. “We believe these types of circular fractures were key in the chain reaction style lake drainage process that helped to break up the Larsen B Ice Shelf.”

“While these measurements were made over a small area, they demonstrate that bending and breaking of floating ice due to surface water may be more widespread than previously thought,” said co-author Dr Rebecca Dell from Cambridge’s Scott Polar Research Institute. “As melting increases in response to predicted warming, ice shelves may become more prone to break up and collapse than they are currently.”

“This has implications for sea level as the buttressing of inland ice is reduced or removed, allowing the glaciers and ice streams to flow more rapidly into the ocean,” said co-author Professor Ian Willis, also from SPRI.

̽��ֱ��work supports modelling results that show the immense weight of thousands of meltwater lakes and subsequent draining caused the Larsen B Ice Shelf to bend and break, contributing to its collapse.

“These observations are important because they can be used to improve models to better predict which Antarctic ice shelves are more vulnerable and most susceptible to collapse in the future,” Banwell said.

̽��ֱ��research was funded by the U.S. National Science Foundation (NSF) and the Natural Environment Research Council (NERC), part of UK Research and Innovation (UKRI). ̽��ֱ��team also included researchers from the ̽��ֱ�� of Oxford and the ̽��ֱ�� of Chicago. Rebecca Dell is a Fellow of Trinity Hall, Cambridge.

Reference:
Alison F Banwell et al. ‘Observed meltwater-induced flexure and fracture at a doline on George VI Ice Shelf, Antarctica.’ Journal of Glaciology (2024). DOI: 10.1017/jog.2024.31

Adapted from a CIRES press release.

Heavy pooling meltwater can fracture ice, potentially leading to ice shelf collapse

Ian Willis

Ali Banwell and Laura Stevens installing the time-lapse camera used in this study on the George VI Ice Shelf in Antarctica.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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̽��ֱ��Fens of eastern England once held vast woodlands

sc604 — Fri, 24 Nov 2023 05:23:59 +0000

̽��ֱ��Fens of eastern England, a low-lying, extremely flat landscape dominated by agricultural fields, was once a vast woodland filled with huge yew trees, according to new research.

Accelerating melt rate makes Greenland Ice Sheet world’s largest ‘dam’

sc604 — Mon, 21 Feb 2022 19:47:29 +0000

̽��ֱ��world’s second-largest ice sheet is melting from the bottom up – and generating huge amounts of heat from hydropower.

Arctic Ocean started getting warmer decades earlier than we thought

sc604 — Wed, 24 Nov 2021 18:56:45 +0000

̽��ֱ��Arctic Ocean has been getting warmer since the beginning of the 20th century – decades earlier than records suggest – due to warmer water flowing into the delicate polar ecosystem from the Atlantic Ocean.

Fibre-optics used to take the temperature of Greenland Ice Sheet

sc604 — Fri, 14 May 2021 17:09:27 +0000

Scientists have used fibre-optic sensing to obtain the most detailed measurements of ice properties ever taken on the Greenland Ice Sheet. Their findings will be used to make more accurate models of the future movement of the world’s second-largest ice sheet, as the effects of climate change continue to accelerate.

Lakes on Greenland Ice Sheet can drain huge amounts of water, even in winter

sc604 — Wed, 31 Mar 2021 23:19:20 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, used radar data from a European Space Agency satellite to show that even when the heat from the Sun is absent, these lakes can discharge large amounts of water to the base of the ice sheet. These ‘drainage events’ are thought to play a significant role in accelerating the movement of the ice by lubricating it from below.

Previous studies of draining lakes have all been carried out during the summer months, through a combination of direct field observations and optical satellite data, which requires daylight.

̽��ֱ��approach developed by the Cambridge researchers uses the radar ‘backscatter’ – the reflection of waves back to the satellite from where they were emitted – to detect changes in the lakes during the winter months, when Greenland is in near-total darkness.

̽��ֱ��results, reported in the journal ̽��ֱ��Cryosphere, imply that the ‘plumbing’ system beneath the Greenland Ice Sheet doesn’t just slowly leak water from the previous summer, but even in the depths of the Arctic winter, it can be ‘recharged’, as large amounts of surface lake water cascade to the base of the ice sheet.

Many previous studies have shown that the Greenland Ice Sheet is losing mass, and the rate of loss is accelerating, due to melting and runoff.

“One of the unknowns in terms of predicting the future of the ice sheet is how fast the glaciers move – whether they will speed up and if so, by how much,” said co-author Dr Ian Willis from Cambridge’s Scott Polar Research Institute (SPRI). “ ̽��ֱ��key control on how fast the glaciers move is the amount of meltwater getting to the bottom of the ice sheet, which is where our work comes in.”

Lakes form on the surface of the Greenland ice sheet each summer as the weather warms. They exist for weeks or months but can drain in a matter of hours due to hydrofracturing, transferring millions of cubic metres of water and heat to the base of the ice sheet. ̽��ֱ��affected areas include sensitive regions of the ice sheet interior where the impact on ice flow is potentially large.

“It’s always been thought that these lakes drained only in the summer, simply because it’s warmer and the sun causes the ice to melt,” said co-author Corinne Benedek, also from SPRI. “In the winter, it’s dark and the surfaces freeze. We thought that the filling of the lakes is what caused their eventual drainage, but it turns out that isn’t always the case.”

Benedek, who is currently a PhD candidate at SPRI, first became interested in what happens to surface lakes in the winter while she was a Master’s student studying satellite thermal data.

“ ̽��ֱ��thermal data showed me that liquid water can survive in the lakes throughout the winter,” she said. “Previous studies using airborne radar had also identified lakes buried a few metres beneath the surface of the ice sheet in the summer. Both of these things got me thinking about ways to observe lakes all year long. ̽��ֱ��optical satellite imagery we normally use to observe the lakes isn’t available in winter, or even when it’s cloudy.”

Benedek and Willis developed a method using data from the Sentinel-1 satellite, which uses a type of radar called synthetic aperture radar (SAR). SAR functions at a wavelength that makes it possible to see through clouds and in the dark. Ice and water read differently using SAR, and so they developed an algorithm that tracks when sudden changes in SAR backscatter occur.

Over three winters, they identified six lakes that appeared to drain over the winter months. These lakes were buried lakes or surface lakes that were frozen over. ̽��ֱ��algorithm was able to identify where the backscatter characteristics of the lake changed markedly between one image and the next one recorded 12 days later.

̽��ֱ��SAR data was backed up with additional optical data from the previous autumn and subsequent spring, which confirmed that lakes areas shrank considerably for the six drained lakes. For three of the lakes, the optical data, as well as data from other satellites, was used to show the snow- and ice-covered lakes collapsed, dropping by several metres, again confirming the water had drained.

“ ̽��ֱ��first lake I found was surprising,” said Benedek. “It took me a while to be sure that what I thought I was seeing was really what I was seeing. We used surface elevation data from before and after the events to confirm what we were thinking. We know now that drainage of lakes during the winter is something that can happen, but we don’t yet know how often it happens.”

“Glaciers slow down in the winter, but they’re still moving,” said Willis. “It must be this movement that causes fractures to develop in certain places allowing some lakes to drain. We don’t yet know how widespread this winter lake drainage phenomenon is, but it could have important implications for the Greenland Ice Sheet, as well as elsewhere in the Arctic and Antarctic.”

Reference:
Corinne L. Benedek and Ian C. Willis. ‘Winter drainage of surface lakes on the Greenland Ice Sheet from Sentinel-1 SAR imagery.’ ̽��ֱ��Cryosphere (2021). DOI: 10.5194/tc-15-1-2021

Using satellite data to ‘see in the dark’, researchers have shown for the first time that lakes on the Greenland Ice Sheet drain during winter, a finding with implications for the speed at which the world’s second-largest ice sheet flows to the ocean.

We don’t yet know how widespread this winter lake drainage phenomenon is, but it could have important implications for the Greenland Ice Sheet, as well as elsewhere in the Arctic and Antarctic

Ian Willis

Lake on the surface of the Greenland Ice Sheet

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Drone images show Greenland Ice Sheet becoming more unstable as it fractures

sc604 — Tue, 03 Dec 2019 09:49:11 +0000

̽��ֱ��world’s second-largest ice sheet, and the single largest contributor to global sea level rise, is potentially becoming unstable because of fractures developing in response to faster ice flow and more meltwater forming on its surface.