EXPERT REACTION: Antarctic ice loss

These papers present authoritative reviews on the state of the Antarctic ice sheet. Their publication is very timely: the UK Met Office analysis shows that six of the hottest years globally have all occurred in the last six years.

In the Australian context, 2019 broke the temperature record by a wide margin, and seven of our hottest years have occurred in the last eight years. These records are being smashed as the global atmosphere is presently showing extraordinary levels of greenhouse-producing carbon dioxide of 415 parts per million.

Antarctic ice core records show that the atmosphere has not exhibited concentrations anywhere near this level for the last million years at least. Most of this human-activity rise has come in the last 70 years, and the rate of increase is accelerating.

In this warming world, the papers make use of the latest data and satellite information to show that the Antarctic continent is losing ice, particularly in West Antarctica, with important implications for sea level rise.

The papers point to the need for us to understand the complex changes and interactions between the Southern Ocean, the atmosphere, sea ice and ice cap to prepare for future changes. The reviews present timely reminders of climate system tipping points and threshold behaviour which can lead to rapid changes in this high latitude environment.

New research published in the journal Science today provides further evidence for chronic and accelerated ice loss in Antarctica.

Even small rates of sea level rise can have a big impact on coastal flooding. As a rule of thumb, every 10cm of sea level rise increases the frequency of a given coastal flood by a factor of three.

This means that a 0.6m rise in sea level - predicted for Australia in the next few decades - will turn a flood event that is expected to occur on average only once every century to one that occurs every 5 to 6 years.

The problem for coastal managers today is that the future rate of sea level rise remains highly uncertain. Current coastal planning assumes a 1m rise in sea level by the end of century relative to present-day, but a rise of 2-3m is not implausible because of large unknowns around ice sheet instabilities in Antarctica.

The most critical of coastal infrastructure – such as airports – should now be considering these more extreme scenarios given the amount the economy depends on these systems.

The review and summary of sea ice loss in Antarctica is of immense importance for the scientific community. There is and has been high uncertainty in regards to projections of sea ice cover for the coming decades. Providing a detailed understanding of the extent of sea ice is relevant not only for sea level predictions but for the survival of a number of iconic species including whales.

We are currently working on the impacts of climate change on humpback whales in an interdisciplinary team of scientists from Australia, South Africa and South America for the Southern Hemisphere.

The past five years have shown drastic sea ice loss in some areas of Antarctica. We are yet to work out the consequences for whales feeding in Antarctica and relying on predictable sea ice dynamics, however, we have documented shifts in migration, calving and feeding behaviour.

If fast sea ice loss and fast melting rates in spring in Antarctica are to continue there will be dramatic changes in the Antarctic ecosystem leading to shifts in plankton and krill blooms. As krill is one of the main food sources for baleen whales in the Southern Hemisphere this has consequences for a range of species from blue whales to humpback whales.

We need more reliable prediction models for Antarctica to foresee if recent trends of fast sea ice loss will also extend to other regions in Antarctica and how sea ice dynamic will vary over time.

Several groups of international of scientists have reported in the prestigious journal Science that as global warming increases along with consequent increasing ocean temperatures, Antarctic ice is melting more rapidly with a resulting increase in the rate of sea level rise.

The Antarctic ice sheets make up the greatest store of fresh water in the world. The East Antarctic ice sheet is 4600 metres thick in places and is 800,000 years old. If this, together with the smaller West Antarctic ice sheet, melted completely sea level would rise by 60 metres. Fortunately, this is very unlikely. 

Nevertheless, since 1992 the melting of Antarctic ice has contributed 8 millimetres to overall sea level rise, which does not seem much, but if global temperatures rise by another 2°C melting will increase and a sea level rise of a metre or more is likely. 

This is of great concern to communities living in low-lying coastal regions, particularly to those of the low-lying Pacific islands in our vicinity, and also should be to those of the large coastal areas of China and the United States; the two major greenhouse gas emitters. 

All of this emphasises yet again the importance of rapidly decreasing global greenhouse gas emissions such that we keep well below the 1.5°C maximum temperature rise target of the Paris Accord.

That one of the most prestigious scientific journals globally – urgently publishing the greatest medical and scientific breakthroughs – would devote an issue to Antarctica says that humanity needs to understand Antarctica quickly. In the last decades, pioneering scientists have taken us from total ignorance to understanding a lot, but we’re still swimming in a sea of unknowns.

If anything the system is more complex than the papers present – yes, we need to understand the ice, ocean, atmosphere, solid Earth and how they interact, but the ecosystems also come into the deep inter-connectedness of Antarctica. We need to understand them all, and how they interact, if we’re going to better-project how Antarctica will change in the future. It’s devilishly complicated and it’s the most remote and hostile place on Earth. 

Australia needs to step up in this space - we claim 42 per cent of Antarctica and there are key regions that are both hugely vulnerable but also rarely visited by anyone. All of Australia’s claim is in East Antarctica and the work summarises that it likely collapsed in past warm periods – warm periods we are heading toward with just 1.5 or 2 degrees of warming.

Fortunately, Australia’s new icebreaker is coming online next year. We’ve also developed new underwater vehicles to go tens of kilometres under the vast ice shelves that hold back the ice sheet. It’s big, expensive science, that involves humans going where they haven’t gone before. But the expense and effort of doing the work is tiny compared to the $200 billion of Australian infrastructure vulnerable to just 1m of sea level rise, not to mention the impact on our regional neighbours.

Australia has developed a world-class Antarctic research workforce and we will soon have the icebreaker, so everything is coming together – now we need government urgency and financial support to enable the researchers to quickly shift us from deep uncertainty to giving clear guidance on the consequences of our choices in mitigating climate change.

The series of three papers published in Science on 20 March together overview the development of the Antarctic ice sheet (over the last 50+ million years) its structure, dynamic processes and vulnerabilities, and its interaction with the Southern Ocean.

Three different and largely independent satellite-based methods have been available for the last ~30 years to estimate Antarctic mass changes. There is broad agreement between these that Antarctica is losing ice into the ocean, and contributing to sea level rise.

This is additional to the sea level rise from the Greenland ice sheet, shrinking glaciers and thermal expansion of a warming ocean. The changes to Antarctica are due to warm ocean water reaching and eroding the floating ice shelves fringing the ice sheet. Shrinking ice shelves remove buttressing from the grounded ice which then flows more rapidly into the ocean.

The largest of these changes are in the Amundsen Bay region, West Antarctica, but similar changes are also occurring to the Totten Glacier near Australia’s Casey station. Present and future changes are also comprehensively reviewed in the recent IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC).

The SROCC projection of the Antarctic contribution to global mean sea level rise by 2100 is from 3 to 28 centimetres for a business-as-usual emission scenario. But with continuing warming, sea level rise will continue well beyond 2100, with Antarctica capable of adding several metres to sea level over a number of centuries.

The Antarctic sea-ice extent can reach 20 million km2, an area about twice the size of Europe. In addition to the process of sea-ice formation (salt extrusion) that drive the ‘haline’ part of ‘thermohaline’ ocean current circulation (the global ‘conveyer belt’), sea-ice is the ‘home’ of sea ice algae. Sea ice algae use sunlight as energy to grow. In so doing they capture carbon dioxide from the atmosphere and convert it into biological matter (themselves).

This process is called ‘primary production’ and represents the beginning of the ocean food web. All larger organisms (krill, fish, sea mammals, sea birds) rely (in/directly) on sea ice algae and other light-harvesting microbes (phototrophs) as food.

As the global sea ice extent diminishes, less ‘houses’ are available for the sea ice algae, so they cannot grow and therefore every other part of the ocean food web is affected. In a simple sense ocean life increasingly starves and this causes shifts in food web structures, e.g. less krill and more salps.

Understanding the impacts of microbes of climate change and climate change on microbes is critical for achieving an environmentally sustainable future – ignoring them risks the fate of humanity.