Satellites track vanishing Antarctic ice

Satellites track vanishing Antarctic ice

Monitoring Antarctica from space has revealed how its ice is being lost to the oceans, providing crucial insight into the continent’s response to a warming climate.

Scientists from CPOM the University of California San Diego and University of Maryland reviewed decades of satellite measurements to reveal how and why Antarctica’s glaciers, ice shelves, and sea ice are changing.

Their report, published in Nature’s special issue on Antarctica, explains how ice shelf thinning and collapse have triggered an increase in the continent’s sea level contribution. It also explains that although the total area of sea ice surrounding Antarctica has shown little overall change during the satellite era, there are signs of a longer-term decline when mid-twentieth century ship-based observations are considered.

Lead author and CPOM Director Professor Andrew Shepherd said: “Antarctica is way too big to survey from the ground, and we can only truly understand the trends in its ice cover by looking at the continent from space.”

In West Antarctica, ice shelves are being eaten away by warm ocean water, and those in the Amundsen and Bellingshausen seas are up to 18 per cent thinner than in the early 1990s. At the Antarctic Peninsula, where air temperatures have risen sharply, ice shelves have collapsed as their surfaces have melted. Altogether, 34,000 km2 of ice shelf area has been lost since the 1950s

“Although breakup of the ice shelves does not contribute directly to sea-level rise since ice shelves, like sea ice, are already floating, we now know that these breakups have implications for the inland ice: without the ice shelf to act as a natural buffer, glaciers can flow faster downstream and out to sea,” said Professor Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography at UC San Diego.

More than 150 studies have tried to determine how much ice the continent is losing. The biggest changes have occurred in places where ice shelves – the continents protective barrier – have either thinned or collapsed.

In the Amundsen Sea, for example, ice shelf thinning of up to 6 metres per year has triggered a 1.5 km per year acceleration of the Pine Island and Thwaites glaciers.  These glaciers have the potential to raise sea levels by more than a metre, and are now widely considered to be unstable.

Satellite observations have meanwhile provided an increasingly detailed picture of the sea ice cover, allowing us to map the extent, age, motion and thickness of the ice.

The combined effects of climate variability, atmosphere and ocean circulation, and even ice shelf melting have driven regional changes, including reductions in sea ice in the Amundsen and Bellingshausen seas.

Dr. Sinead Farrell, from the Earth System Science Interdisciplinary Centre at the University of Maryland said: “The waxing and waning of the sea ice controls how much sunlight is reflected back to space, cooling the planet. Regional sea ice loss impacts the temperature and circulation of the ocean, as well as marine productivity.”

New and improved missions, such as Sentinel-3, the recently launched Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), and the eagerly awaited ICESat-2, will continue to give us insights into the disappearing ice in even greater detail.

Further information:

The paper trends and connections across the Antarctic cryosphere is published in Nature special issue 14 June 2018.

Antarctica ramps up sea level rise

Ice losses from Antarctica have increased global sea levels by 7.6 mm since 1992, with two fifths of this rise (3.0 mm) coming in the last five years alone.

The findings are from a major climate assessment known as the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE), and are published today in Nature. It is the most complete picture of Antarctic ice sheet change to date. 84 scientists from 44 international organisations combined 24 satellite surveys to produce the assessment.

Sea level contribution due to the Antarctic ice sheet between 1992 and 2017. Credit: IMBIE/Planetary Visions

The assessment, led by CPOM Director Professor Andrew Shepherd at the University of Leeds and Dr Erik Ivins at NASA’s Jet Propulsion Laboratory in California, was supported by the European Space Agency (ESA) and the US National Aeronautics and Space Administration (NASA).

Their findings show that, prior to 2012, Antarctica lost ice at a steady rate of 76 billion tonnes per year – a 0.2 mm per year contribution to sea level rise. However, since then there has been a sharp, threefold increase. Between 2012 and 2017 the continent lost 219 billion tonnes of ice per year – a 0.6 mm per year sea level contribution.

Animation illustrating changes in thickness and sea level contribution due to Antarctica between 1992 and 2017” (1 Minute 22 seconds). Credit: planetaryvisions.com

Antarctica stores enough frozen water to raise global sea level by 58 metres, and knowing how much ice it is losing is key to understanding the impacts of climate change today and in the future.

Professor Shepherd said: “We have long suspected that changes in Earth’s climate will affect the polar ice sheets. Thanks to the satellites our space agencies have launched, we can now track their ice losses and global sea level contribution with confidence.

“According to our analysis, there has been a steep increase in ice losses from Antarctica during the past decade, and the continent is causing sea levels to rise faster today than at any time in the past 25 years. This has to be a concern for the governments we trust to protect our coastal cities and communities.”

Dr Ivins said: “The added duration of the observing period, the larger pool of participants, various refinements in our observing capability and an improved ability to assess both inherent and interpretive uncertainties, each contribute to making this the most robust study of ice mass balance of Antarctica to date.”

Summer clouds swirl in around the Staccato Peaks of Alexander Island, Antarctic Peninsula. High snowfall and strong weather gradients in this mountainous area make assessment of glacier mass balance particularly challenging. Credit: Hamish Pritchard, BAS

The threefold increase in ice loss from the continent as a whole is a combination of glacier speedup in West Antarctica and at the Antarctic Peninsula, and reduced growth of the ice sheet in East Antarctica.

West Antarctica experienced the largest change, with ice losses rising from 53 billion tonnes per year in the 1990s to 159 billion tonnes per year since 2012.  Most of this came from the huge Pine Island and Thwaites Glaciers, which are retreating rapidly due to ocean melting.

At the northern tip of the continent, ice shelf collapse at the Antarctic Peninsula has driven a 25 billion tonne per year increase in ice loss since the early 2000s.

The East Antarctic ice sheet has remained close to a state of balance over the past 25 years, gaining just 5 billion tonnes of ice per year on average.

Josef Aschbacher, ESA’s Director of Earth Observation Programmes, said: “CryoSat and Sentinel-1 are clearly making an essential contribution to understanding how ice sheets are responding to climate change and affecting sea level, which is a major concern.

“While these impressive results demonstrate our commitment to climate research through efforts such as our Climate Change Initiative and scientific data exploitation activities, they also show what can be achieved by working with our NASA colleagues. Looking to the future, however, it is important that we have satellites to continue measuring Earth’s ice to maintain the ice-sheet climate data record.”

Further information:

The paper Mass balance of the Antarctic ice sheet from 1992 to 2017 by The IMBIE Team is published in Nature on 14 June, doi: 10.1038/s41586-018-0179-y