sea levels
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Throughout 2023, the area of ocean around Antarctica covered by sea ice was so far below the norm that scientists have struggled to communicate their shock.
This month, as the sea ice shrinks to its smallest point of the year, it is once again tracking well below its previous levels.
Research released in September 2023 shows that ocean warming was a key contributor to the dramatic change in sea ice.
The question is where the heat comes from.
A new satellite launched recently may provide the key to understanding how the ocean transports heat to Antarctica’s margins where it has a devastating impact on sea ice and ice shelves.
Sea ice insulates the ocean, reflects heat, drives currents, supports ecosystems and protects ice shelves.
Parts of the ocean 100–200m below the surface began to warm in 2015, and those same regions lost substantial sea ice in 2016. Since then, the warm subsurface ocean seems to have maintained the low sea-ice coverage.
The record-breaking low sea ice of 2023 may be the new abnormal, the beginning of the inevitable decline in Antarctic sea ice, long projected by climate models.
For millions of years, the icy continent has been ring-fenced by the Antarctic Circumpolar Current, separating the warm northern waters from the cold polar ocean.
Flowing clockwise around Antarctica and driven by westerly winds, the current is the world’s strongest, with a flow 100 times stronger than all rivers combined.
The Antarctic Circumpolar Current flows around Antarctica, keeping warm water out — but eddies can let heat through.
The current ‘feels’ the seafloor and the mountains in its path. Where it encounters barriers like ridges or seamounts, ‘wiggles’ are created in the water flow that form eddies.
Ocean eddies are the weather systems of the seas, and they play a key role in transporting heat through the circumpolar current to the ocean around Antarctica. But they’re small and hard for satellites to see.
Broad-scale ocean mapping identifies at least five major ‘heat flux gates’ or eddy hotspots in the circumpolar current.
One is south of Australia, about halfway between Tasmania and Antarctica.
To understand the ocean dynamics happening now and how these may change in the future, we need much higher-resolution data to see smaller-scale features like the eddy hotspots.
Enter the Surface Water and Ocean Topography (Swot) satellite. Jointly developed by Nasa and French space agency Centre National d’Études Spatiales (CNES), the Swot satellite measures differences in the height of the ocean within a few centimetres from an orbit of more than 890km above the surface.
The advanced radar altimeters on the two-tonne satellite detect surface water features with 10 times better resolution than previous technologies.
Oceanographers say it’s like a short-sighted person looking at a tree in the distance, and then putting on glasses to reveal all the leaves.
As Swot passes over the Southern Ocean, the high-resolution topography it records of the shape of the ocean surface shows the fine streams of current to capture the eddy hotspots spinning off the Antarctic Circumpolar Current.
This means scientists can monitor these smaller-scale circulation features thought to be responsible for transporting most of the heat and carbon from the upper ocean to deeper layers – a critical buffer against global warming.
For the first time we can see them on the surface in detail – but we still need to work out what’s happening beneath the waves.
In November 2023, scientists were able to validate the Swot satellite data from an eddy hotspot in the Southern Ocean in an ambitious voyage on CSIRO research vessel investigator.
The five-week FOCUS voyage travelled 850 nautical miles south of Hobart to the Macquarie meander, one of the five eddy hotspots.
A meander may sound gentle and slow, but in fact it’s where the world’s strongest current races through a series of hairpin bends, steered by mountains on the seafloor.
As the satellite passed overhead, the team led by CSIRO and the Australian Antarctic Program Partnership deployed a variety of high-tech observational equipment.
Researchers and crew anchored a tall mooring 3.6km high at the centre of the survey area, carrying over 54 instruments on a cable stretching from the seafloor to near the surface.
They also released free-floating autonomous instruments like floats, drifters and gliders into the eddies, while more than a hundred CTDs – conductivity, temperature, and depth sensors – plumbed the depths and a Triaxus was towed behind the ship through the satellite’s path.
Researchers use a variety of instruments to understand the ocean. Some float along the surface, some dive deep in the water, and some follow directed paths using motors.
The wealth of information gathered by all these instruments ‘ground-truths’ and validates the satellite data from the surface.
The Antarctic is rapidly changing, and with further disruptions to the sea-ice cycle on the cards, there’s a race to understand why.
Strong winds over the Southern Ocean have been increasing for decades and are likely to continue. It’s expected this will send more heat southward through leaky meanders, accelerating ice shelf melting in Antarctica and sea level rise.
Ultimately, this research aims to turn daily maps of ocean sea surface height from satellites into daily maps of the movement of heat in the Southern Ocean toward Antarctica.
This is vital information in a climate crisis. It will help governments plan how to respond to ocean warming and rising sea levels and how quickly action is needed.
At the same time, as the transition to a net-zero world gathers momentum and carbon levels in the atmosphere start to level out, we need to be able to track the response of the Southern Ocean and the global climate system.
Ariaan Purich is a lecturer in the School of Earth, Atmosphere and Environment at Monash University, and a Chief Investigator with the Australian Research Council Special Research Initiative Securing Antarctica’s Environmental Future. She has worked previously with CSIRO.
Edward Doddridge is a physical oceanographer working in the Australian Antarctic Program Partnership (AAPP) based at the Institute for Marine and Antarctic Studies (IMAS) in Hobart, Tasmania.
Benoit Legresy works with CSIRO as climate scientist leading the Oceans group and is a co-leader of the oceanography project with the Australian Antarctic Program Partnership.
Edward Doddridge (University of Tasmania) and Ariaan Purich (Monash University) both receive funding from the Australian Research Council.
The research of the FOCUS voyage is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility which is supported by the Australian Government’s National Collaborative Research Infrastructure Strategy (NCRIS).
The Australian Antarctic Program Partnership is funded by the Australian Government Department of Climate Change, Energy, the Environment and Water through the Antarctic Science Collaboration Initiative.
Authors:
Ariaan Purich is a lecturer in the School of Earth, Atmosphere and Environment at Monash University, and a Chief Investigator with the Australian Research Council Special Research Initiative Securing Antarctica’s Environmental Future. She has worked previously with CSIRO.
Edward Doddridge is a physical oceanographer working in the Australian Antarctic Program Partnership (AAPP) based at the Institute for Marine and Antarctic Studies (IMAS) in Hobart, Tasmania.
Benoit Legresy works with CSIRO as climate scientist leading the Oceans group and is a co-leader of the oceanography project with the Australian Antarctic Program Partnership.
Edward Doddridge (University of Tasmania) and Ariaan Purich (Monash University) both receive funding from the Australian Research Council.
The research of the FOCUS voyage is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility which is supported by the Australian Government’s National Collaborative Research Infrastructure Strategy (NCRIS).
The Australian Antarctic Program Partnership is funded by the Australian Government Department of Climate Change, Energy, the Environment and Water through the Antarctic Science Collaboration Initiative.
Authors:
YanaBu, Shutterstock
Katrin Meissner, UNSW Sydney; Deepashree Dutta, University of Cambridge, and Martin Jucker, UNSW Sydney
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Our planet has warmed by about 1.2°C since 1850. But this warming is not uniform. Warming at the poles, especially the Arctic, has been three to four times faster than the rest of the globe. It’s a phenomenon known as “polar amplification”.
Climate models simulate this effect, but when tested against the past 40 years of warming, these models fall short. The situation is even worse when it comes to modelling past climates with very high levels of greenhouse gases.
This is a problem because these are the same models used to project into the future and forecast how the climate will change. They are likely to underestimate what will happen later this century, including risks such as ice sheet melting or permafrost thawing.
In our new research published today in Nature Geoscience we used a high-resolution model of the atmosphere that includes the stratosphere. We found a special type of cloud appears over polar regions when greenhouse gas concentrations are very high. The role of this type of cloud has been overlooked so far. This is one of the reasons why our models are too cold at the poles.
Polar Stratospheric Clouds over Norway (Night Lights Films – Adrien Mauduit)
Back to the future
Looking into past climates can give us glimpses of possible futures for a range of extreme conditions. For us, this means we can use Earth’s history to find out how well our climate models perform. We can test our models by simulating episodes in the past when Earth was much warmer. The advantage of this is that we have temperature reconstructions for these episodes to evaluate the models, as opposed to the future, for which measurements are not available.
If we go back 50 million years or so, our planet was very hot. Carbon dioxide (CO₂) concentrations ranged between 900 and 1,900 parts per million (ppm), compared with 415 ppm today. Methane (CH₄) concentrations were likely also much higher.
Canada’s arctic archipelago was covered in lush rainforests inhabited by alligators, turtles, lizards and mammals.
For these plants and animals to survive, conditions must have been warm and ice-free year-round. Indeed, surface ocean temperatures exceeded 20°C near the north pole (at about 87°N) and 25°C in the Southern Ocean (at about 67°S).
This period called the early Eocene is a perfect test bed for our models, because it was globally very warm, and the poles were even warmer, meaning it was a climate with extreme polar amplification. In addition, the Eocene is recent enough for temperature reconstructions to be available.
But as it turns out, the models fail again. They are much too cold at high latitudes. What are our models missing?
Polar stratospheric clouds
In 1992 American paleoclimatologist Lisa Sloan suggested polar stratospheric clouds might have caused extreme warming at high latitudes in the past.
These clouds are a rare and beautiful sight today. They are also called nacreous or mother-of-pearl clouds for their vivid and sometimes luminous colours.
They form at very high altitudes (in the stratosphere) and at very low temperatures (over the poles). In the present day climate, they appear mainly over Antarctica, but have also been observed during winter months over Scotland, Scandinavia and Alaska, at times when the stratosphere was particularly cold.
Just like greenhouse gases, they absorb infrared radiation emitted by the Earth’s surface and re-emit a portion of this energy back to the surface. This suggests polar stratospheric clouds could be one of the missing puzzle pieces.
They warm the surface. And their effect could be significant, especially in winter, when the sun does not rise. But they are difficult to simulate in a climate model, so most models ignore them. This omission could explain why climate models miss some of the polar warming, because they miss a process that warms the poles.
Three decades after Sloan’s paper, a few atmosphere models are finally complex enough to allow us to test her hypothesis. In our research we use one of them and find that under certain conditions, the additional warming due to these polar stratospheric clouds exceeds 7°C during the winter months. This significantly reduces the gap between climate models and temperature evidence from the early Eocene. Sloan was right.
Implications for future projections
Our research explains why climate models don’t work so well for past climates when greenhouse gas levels were much higher than they are today. But what about the future? Should we be concerned?
There is some good news. While polar stratospheric clouds do warm the poles, they won’t be as common in the future as they were in the distant past, even if both CO₂ and CH₄ reach very high levels.
This is due to another difference between the Eocene and today: the position of continents and mountains, which were different back then and which also influence the formation of polar stratospheric clouds. So even if we hit early Eocene levels of CH₄ and CO₂ in the future, we would expect less polar stratospheric cloud to be formed. This suggests the standard climate models are better at predicting the future than the past.
It’s therefore unlikely the Arctic and Antarctica will be covered by these beautiful clouds anytime soon. But our research shows evidence from past climates can reveal processes that only become important when greenhouse gas concentrations are high. Some of these processes are not included in our models because models are tested against present day observations and other processes simply seemed more important to include. Looking into the past is a way of broadening our horizon and learning for the future.
Authors: Kerrylee Rogers, Associate Professor, University of Wollongong; Jeffrey Kelleway, Postdoctoral Research Fellow in Environmental Sciences, Macquarie University, and Neil Saintilan, Head, Department of Environmental Science, Macquarie University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Coastal wetlands don’t cover much global area but they punch well above their carbon weight by sequestering the most atmospheric carbon dioxide of all natural ecosystems.
Termed “blue carbon ecosystems” by virtue of their connection to the sea, the salty, oxygen-depleted soils in which wetlands grow are ideal for burying and storing organic carbon.
In our research, published in Nature, we found that carbon storage by coastal wetlands is linked to sea-level rise. Our findings suggest as sea levels rise, these wetlands can help mitigate climate change.
Sea-level rise benefits coastal wetlands
We looked at how changing sea levels over the past few millennia has affected coastal wetlands (mostly mangroves and saltmarshes). We found they adapt to rising sea levels by increasing the height of their soil layers, capturing mineral sediment and accumulating dense root material. Much of this is carbon-rich material, which means rising sea levels prompt the wetlands to store even more carbon.
We investigated how saltmarshes have responded to variations in “relative sea level” over the past few millennia. (Relative sea level is the position of the water’s edge in relation to the land rather than the total volume of water within the ocean, which is called the eustatic sea level.)
What does past sea-level rise tell us?
Global variation in the rate of sea-level rise over the past 6,000 years is largely related to the proximity of coastlines to ice sheets that extended over high northern latitudes during the last glacial period, some 26,000 years ago.
As ice sheets melted, northern continents slowly adjusted elevation in relation to the ocean due to flexure of the Earth’s mantle.
For much of North America and Europe, this has resulted in a gradual rise in relative sea level over the past few thousand years. By contrast, the southern continents of Australia, South America and Africa were less affected by glacial ice sheets, and sea-level history on these coastlines more closely reflects ocean surface “eustatic” trends, which stabilised over this period.
Our analysis of carbon stored in more than 300 saltmarshes across six continents showed that coastlines subject to consistent relative sea-level rise over the past 6,000 years had, on average, two to four times more carbon in the upper 20cm of sediment, and five to nine times more carbon in the lower 50-100cm of sediment, compared with saltmarshes on coastlines where sea level was more stable over the same period.
In other words, on coastlines where sea level is rising, organic carbon is more efficiently buried as the wetland grows and carbon is stored safely below the surface.
Give wetlands more space
We propose that the difference in saltmarsh carbon storage in wetlands of the southern hemisphere and the North Atlantic is related to “accommodation space”: the space available for a wetland to store mineral and organic sediments.
Coastal wetlands live within the upper portion of the intertidal zone, roughly between mean sea level and the upper limit of high tide.
These tidal boundaries define where coastal wetlands can store mineral and organic material. As mineral and organic material accumulates within this zone it creates layers, raising the ground of the wetlands.
New accommodation space for storage of carbon is therefore created when the sea is rising, as has happened on many shorelines of the North Atlantic Ocean over the past 6,000 years.
To confirm this theory we analysed changes in carbon storage within a unique wetland that has experienced rapid relative sea-level rise over the past 30 years.
When underground mine supports were removed from a coal mine under Lake Macquarie in southeastern Australia in the 1980s, the shoreline subsided a metre in a matter of months, causing a relative rise in sea level.
Following this the rate of mineral accumulation doubled, and the rate of organic accumulation increased fourfold, with much of the organic material being carbon. The result suggests that sea-level rise over the coming decades might transform our relatively low-carbon southern hemisphere marshes into carbon sequestration hot-spots.
How to help coastal wetlands
The coastlines of Africa, Australia, China and South America, where stable sea levels over the past few millennia have constrained accommodation space, contain about half of the world’s saltmarshes.
A doubling of carbon sequestration in these wetlands, we’ve estimated, could remove an extra 5 million tonnes of CO₂ from the atmosphere per year. However, this potential benefit is compromised by the ongoing clearance and reclamation of these wetlands.
Preserving coastal wetlands is critical. Some coastal areas around the world have been cut off from tides to lessen floods, but restoring this connection will promote coastal wetlands – which also reduce the effects of floods – and carbon capture, as well as increase biodiversity and fisheries production.
In some cases, planning for future wetland expansion will mean restricting coastal developments, however these decisions will provide returns in terms of avoided nuisance flooding as the sea rises.
Finally, the increased carbon storage will help mitigate climate change. Wetlands store flood water, buffer the coast from storms, cycle nutrients through the ecosystem and provided vital sea and land habitat. They are precious, and worth protecting.
The authors would like to acknowledge the contribution of their colleagues, Janine Adams, Lisa Schile-Beers and Colin Woodroffe.
Authors: Kaitlin Naughten, Ocean-Ice Modeller, British Antarctic Survey; Jan De Rydt, Associate Professor of Polar Glaciology and Oceanography, Northumbria University, Newcastle, and Paul Holland, Ocean and Ice Scientist, British Antarctic Survey
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The rate at which the warming Southern Ocean melts the West Antarctic ice sheet will speed up rapidly over the course of this century, regardless of how much emissions fall in coming decades, our new research suggests. This ocean-driven melting is expected to increase sea-level rise, with consequences for coastal communities around the world.
The Antarctic ice sheet, the world’s largest volume of land-based ice, is a system of interconnected glaciers comprised of snowfall that remains year-round. Coastal ice shelves are the floating edges of this ice sheet which stabilise the glaciers behind them. The ocean melts these ice shelves from below, and if melting increases and an ice shelf thins, the speed at which these glaciers discharge fresh water into the ocean increases too and sea levels rise.
In West Antarctica, particularly the Amundsen Sea, this process has been underway for decades. Ice shelves are thinning, glaciers are flowing faster towards the ocean and the ice sheet is shrinking. While ocean temperature measurements in this region are limited, modelling suggests it may have warmed as a result of climate change.
We chose to model the Amundsen Sea because it is the most vulnerable sector of the ice sheet. We used a regional ocean model to find out how ice-shelf melting will change here between now and 2100. How much melting can be prevented by reducing carbon emissions and slowing the rate of climate change – and how much is now unavoidable, no matter what we do?
Rapid change is locked in
We used the UK’s national supercomputer ARCHER2 to run many different simulations of the 21st century, totalling over 4,000 years of ocean warming and ice-shelf melting in the Amundsen Sea.
We considered different trajectories for fossil fuel burning, from the best-case scenario where global warming is limited to 1.5°C in line with the Paris Agreement, to the worst, in which coal, oil and gas use is uncontrolled. We also considered the influence of natural variations in the climate, such as the timing of events such as El Niño.
The results are worrying. In all simulations there is a rapid increase over the course of this century in the rate of ocean warming and ice-shelf melting. Even the best-case scenario in which warming halts at 1.5°C, something that is considered ambitious by many experts, entails a threefold increase in the historical rate of warming and melting.
What’s more, there is little to no difference between the scenarios up to 2045. Ocean warming and ice-shelf melting in the 1.5°C scenario is statistically the same as in a mid-range scenario, which is closer to what existing pledges to reduce fossil fuel use over the coming decades would produce.
The worst-case scenario shows more melting than the others, but only from around mid-century onwards, and many experts think this amount of future fossil fuel burning is unrealistic anyway.
The results imply that we are now committed to rapid ocean warming in the Amundsen Sea until at least 2100, regardless of international policies on fossil fuels.
The increases in warming and melting are the result of ocean currents strengthening and driving more warm water from the deep ocean towards the shallower ice shelves along the coast. Other studies have suggested this process is behind the ice shelf thinning measured by satellites.
How much will the sea level rise?
Melting ice shelves are a major cause of sea-level rise, but not the whole story. We can’t put a number on how much sea levels will rise without also simulating the flow of Antarctic glaciers and the rate of snow accumulating on the ice sheet, which our model didn’t include.
But we have every reason to believe that increased ice-shelf melting in this region will cause the rate at which sea levels are rising to speed up.
The West Antarctic ice sheet is already contributing substantially to global sea-level rise and is losing about 80 billion tonnes of ice a year. It contains enough ice to cause up to 5 metres of sea-level rise, but we don’t know how much of it will melt, and how quickly. Our colleagues around the world are working hard to answer this question.
Courage and hope
There are some consequences of climate change that can no longer be avoided, no matter how much fossil fuel use falls. Substantial melting of West Antarctica up to 2100 may now be one of them.
How do you tell a bad news story? The conventional wisdom is that you’re supposed to give people hope: to say that there’s a disaster behind one door, but we can avoid it if only we choose a different one. What do you do when your science tells you that all doors lead to the same disaster?
Kate Marvel, an atmospheric scientist, said that when it comes to climate change, “we need courage, not hope … Courage is the resolve to do well without the assurance of a happy ending”. In this case, courage means shifting our attention to the longer term.
The future will not end in 2100, even if most people reading this will no longer be around. Our simulations of the 1.5°C scenario show ice-shelf melting starting to plateau by the end of the century, suggesting that further changes in the 22nd century and beyond may still be preventable. Reducing sea-level rise after 2100, or even slowing it down, could save many coastal cities.
Courage means accepting the need to adapt, protecting coastal communities where it’s possible to do so, and rebuilding or abandoning them where it’s not. By predicting future sea-level rise in advance, we’ll have time to plan for it – rather than wait until the ocean is on our doorstep.
Kaitlin Naughten, Ocean-Ice Modeller, British Antarctic Survey; Jan De Rydt, Associate Professor of Polar Glaciology and Oceanography, Northumbria University, Newcastle, and Paul Holland, Ocean and Ice Scientist, British Antarctic Survey
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The situation in Antarctica, both what’s currently being observed and the latest research, and the consequences are succinctly explained by Prof. Nerilie Abrahm from the Australian National University. Further consequences are discussed in the second half of the video.
From Carbon Brief:
‘UNCHARTED TERRITORY’: Since mid-March, global average sea surface temperatures have been higher than at any time since records began in 1982, fuelling concerns that Earth is entering “uncharted territory” because of climate change, the Guardian reported. Global sea surface temperatures reached 21.1C at the start of April, topping the previous high of 21C in March 2016, according to data from the US National Oceanic and Atmospheric Administration.
The Guardian added that these temperatures typically dip towards the end of April after reaching their annual peak in March or early April, but that temperatures have remained high this year for the first time since records began. The record ocean heat has “scientists scratching their heads”, Prof Mike Meredith, a researcher from the British Antarctic Survey, said to the Guardian. He told the publication: “The fact that it is warming as much as it has been is a real surprise and very concerning. It could be a short-lived extreme high, or it could be the start of something much more serious.”
EL NIÑO INCOMING: The Guardian noted that the world is currently on the cusp of an El Niño, a periodic natural phenomenon in the tropical Pacific Ocean that affects many regions, causing a warming impact globally. “But the El Niño system is yet to develop, so this oscillation cannot explain the recent rapid heating,” the publication said. However, Axios reported that some climate scientists do think the pulse in ocean heat could be related to El Niño – specifically, the transition to El Niño from La Niña, the climate pattern that acts as El Niño’s opposite. Axios said: “When a La Niña event gives way to an El Niño, as is happening now, large amounts of ocean heat that had been lurking beneath the ocean surface is drawn upwards, according to Michael Mann, a climate scientist at the University of Pennsylvania. The result, Mann told Axios via email, is ‘a sizable increase’ in tropical Pacific and global ocean surface temperatures during the transition.”
STEP CHANGE: The ocean surface temperature spike is likely to also reflect the fact that, since the last major El Niño in 2016, global average surface temperatures on land and sea have increased because of climate change, scientists told Axios. It reported: “This means the 2023 El Niño is elevating global average temperatures from a higher starting point, making it easier to set records. This is like a basketball player playing on a court with a steadily higher floor, making it easier to dunk the basketball.” BBC News also reported on how the spike in ocean temperatures could represent the impact of El Niño combining with human-caused climate change. It reported: “Some research has shown that the world is warming in jumps, where little changes over a period of years and then there are sudden leaps upwards, like steps on a stairs, closely linked to the development of El Niño.”
Several scientists contacted by BBC News were “reluctant to go on the record about the implications” of such step changes, according to the article. It added: “One spoke of being ‘extremely worried and completely stressed’.”
We can’t just sit around and wait to see what will happen next. We need positive action.
I’ve read a lot of Climate Adaptation Plans and Strategies over the past the last few years, but He Toka Tū Moana Mō Maketū (Maketū Climate Change Adaptation Plan) is hand-down the best. It’s clearly laid out, outlines the community’s priorities, and can readily serve as a template to help every community around Aoteara develop their own Climate Adaptation Plans. Most important of all:
It’s iwi led, community driven, it’s a plan that’s been decided by those who live here. – Elva Conroy, Kaitohotohu / Facilitator (Video; to listen Watch on Youtube)
Winner of the 2023 Supreme Planning Awards, the Maketū Climate Change Adaptation Plan was developed by Ngāti Pikiao Environmental Society, Te Rūnanga o Ngāti Whakaue ki Maketū , Whakaue Marae Trustees, and Conroy and Donald Consultants.
In the words of the Maketu Iwi Collective, ‘we will be resilient like the anchor stone Takaparore – strong and steadfast against the elements and tides of change and uncertainty. Regardless of what happens as a result of a changing environment, we will remain standing’. – New Zealand Planning Institute, April 2023.
By: Edward Doddridge, Research Associate in Physical Oceanography, University of Tasmania
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The rhythmic expansion and contraction of Antarctic sea ice is like a heartbeat.
But lately, there’s been a skip in the beat. During each of the last two summers, the ice around Antarctica has retreated farther than ever before.
And just as a change in our heartbeat affects our whole body, a change to sea ice around Antarctica affects the whole world.
Today, researchers at the Australian Antarctic Program Partnership (AAPP) and the Australian Centre for Excellence in Antarctic Science (ACEAS) have joined forces to release a science briefing for policy makers, On Thin Ice.
Together we call for rapid cuts to greenhouse gas emissions, to slow the rate of global heating. We also need to step up research in the field, to get a grip on sea-ice science before it’s too late.
The shrinking white cap on our blue planet
One of the largest seasonal cycles on Earth happens in the ocean around Antarctica. During autumn and winter the surface of the ocean freezes as sea ice advances northwards, and then in the spring the ice melts as the sunlight returns.
We’ve been able to measure sea ice from satellites since the late 1970s. In that time we’ve seen a regular cycle of freezing and melting. At the winter maximum, sea ice covers an area more than twice the size of Australia (roughly 20 million square kilometres), and during summer it retreats to cover less than a fifth of that area (about 3 million square km).
In 2022 the summer minimum was less than 2 million square km for the first time since satellite records began. This summer, the minimum was even lower – just 1.7 million square km.
The annual freeze pumps cold salty water down into the deep ocean abyss. The water then flows northwards. About 40% of the global ocean can be traced back to the Antarctic coastline.
By exchanging water between the surface ocean and the abyss, sea ice formation helps to sequester heat and carbon dioxide in the deep ocean. It also helps to bring long-lost nutrients back up to the surface, supporting ocean life around the world.
Not only does sea ice play a crucial role in pumping seawater across the planet, it insulates the ocean underneath. During the long days of the Antarctic summer, sunlight usually hits the bright white surface of the sea ice and is reflected back into space.
This year, there is less sea ice than normal and so the ocean, which is dark by comparison, is absorbing much more solar energy than normal. This will accelerate ocean warming and will likely impede the wintertime growth of sea ice.
Headed for stormy seas
The Southern Ocean is a stormy place; the epithets “Roaring Forties” and “Furious Fifties” are well deserved. When there is less ice, the coastline is more exposed to storms. Waves pound on coastlines and ice shelves that are normally sheltered behind a broad expanse of sea ice. This battering can lead to the collapse of ice shelves and an increase in the rate of sea level rise as ice sheets slide off the land into the ocean more rapidly.
Sea ice supports many levels of the food web. When sea ice melts it releases iron, which promotes phytoplankton growth. In the spring we see phytoplankton blooms that follow the retreating sea ice edge. If less ice forms, there will be less iron released in the spring, and less phytoplankton growth.
Krill, the small crustaceans that provide food to whales, seals, and penguins, need sea ice. Many larger species such as penguins and seals rely on sea ice to breed. The impact of changes to the sea ice on these larger animals varies dramatically between species, but they are all intimately tied to the rhythm of ice formation and melt. Changes to the sea-ice heartbeat will disrupt the finely balanced ecosystems of the Southern Ocean.
A diagnosis for policy makers
Long term measurements show the subsurface Southern Ocean is getting warmer. This warming is caused by our greenhouse gas emissions. We don’t yet know if this ocean warming directly caused the record lows seen in recent summers, but it is a likely culprit.
As scientists in Australia and around the world work to understand these recent events, new evidence will come to light for a clearer understanding of what is causing the sea ice around Antarctica to melt.
Antarctic sea ice is highly variable, but there has been less ice than normal for almost all of the last seven years. This chart of monthly sea ice extent anomaly shows the difference between the long-term average sea ice and the observed sea ice in each month. By removing the annual cycle due to sea ice formation and melt, we can see the longer term variability underneath, and the extreme low sea ice events in recent years. Dr Phil Reid, BoM, Author provided
If you noticed a change in your heartbeat, you’d likely see a doctor. Just as doctors run tests and gather information, climate scientists undertake fieldwork, gather observations, and run simulations to better understand the health of our planet.
This crucial work requires specialised icebreakers with sophisticated observational equipment, powerful computers, and high-tech satellites. International cooperation, data sharing, and government support are the only ways to provide the resources required.
After noticing the first signs of heart trouble, a doctor might recommend more exercise or switching to a low-fat diet. Maintaining the health of our planet requires the same sort of intervention – we must rapidly cut our consumption of fossil fuels and improve our scientific capabilities.
By: Christine Batchelor, Lecturer in Physical Geography, Newcastle University and Frazer Christie, Postdoctoral Research Associate, University of Cambridge
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The Antarctic Ice Sheet, which covers an area greater than the US and Mexico combined, holds enough water to raise global sea level by more than 57 metres if melted completely. This would flood hundreds of cities worldwide. And evidence suggests it is melting fast. Satellite observations have revealed that grounded ice (ice that is in contact with the bed beneath it) in coastal areas of West Antarctica has been lost at a rate of up to 30 metres per day in recent years.
But the satellite record of ice sheet change is relatively short as there are only 50 years’ worth of observations. This limits our understanding of how ice sheets have evolved over longer periods of time, including the maximum speed at which they can retreat and the parts that are most vulnerable to melting.
So, we set out to investigate how ice sheets responded during a previous period of climatic warming – the last “deglaciation”. This climate shift occurred between roughly 20,000 and 11,000 years ago and spanned Earth’s transition from a glacial period, when ice sheets covered large parts of Europe and North America, to the period in which we currently live (called the Holocene interglacial period).
During the last deglaciation, rates of temperature and sea-level rise were broadly comparable to today. So, studying the changes to ice sheets in this period has allowed us to estimate how Earth’s two remaining ice sheets (Greenland and Antarctica) might respond to an even warmer climate in the future.
Our recently published results show that ice sheets are capable of retreating in bursts of up to 600 metres per day. This is much faster than has been observed so far from space.
Pulses of rapid retreat
Our research used high-resolution maps of the Norwegian seafloor to identify small landforms called “corrugation ridges”. These 1–2 metre high ridges were produced when a former ice sheet retreated during the last deglaciation.
Tides lifted the ice sheet up and down. At low tide, the ice sheet rested on the seafloor, which pushed the sediment at the edge of the ice sheet upwards into ridges. Given that there are two low tides each day off Norway, two separate ridges were produced daily. Measuring the space between these ridges enabled us to calculate the pace of the ice sheet’s retreat.
During the last deglaciation, the Scandinavian Ice Sheet that we studied underwent pulses of extremely rapid retreat – at rates between 50 and 600 metres per day. These rates are up to 20 times faster than the highest rate of ice sheet retreat that has so far been measured in Antarctica from satellites.
The highest rates of ice sheet retreat occurred across the flattest areas of the ice sheet’s bed. In flat-bedded areas, only a relatively small amount of melting, of around half a metre per day, is required to instigate a pulse of rapid retreat. Ice sheets in these regions are very lightly attached to their beds and therefore require only minimal amounts of melting to become fully buoyant, which can result in almost instantaneous retreat.
However, rapid “buoyancy-driven” retreat such as this is probably only sustained over short periods of time – from days to months – before a change in the ice sheet bed or ice surface slope farther inland puts the brakes on retreat. This demonstrates how nonlinear, or “pulsed”, the nature of ice sheet retreat was in the past.
This will likely also be the case in the future.
A warning from the past
Our findings reveal how quickly ice sheets are capable of retreating during periods of climate warming. We suggest that pulses of very rapid retreat, from tens to hundreds of metres per day, could take place across flat-bedded parts of the Antarctic Ice Sheet even under current rates of melting.
This has implications for the vast and potentially unstable Thwaites Glacier of West Antarctica. Since scientists began observing ice sheet changes via satellites, Thwaites Glacier has experienced considerable retreat and is now only 4km away from a flat area of its bed. Thwaites Glacier could therefore suffer pulses of rapid retreat in the near future.
Ice losses resulting from retreat across this flat region could accelerate the rate at which ice in the rest of the Thwaites drainage basin collapses into the ocean. The Thwaites drainage basin contains enough ice to raise global sea levels by approximately 65cm.
Our results shed new light on how ice sheets interact with their beds over different timescales. High rates of retreat can occur over decades to centuries where the bed of an ice sheet deepens inland. But we found that ice sheets on flat regions are most vulnerable to extremely rapid retreat over much shorter timescales.
Together with data about the shape of ice sheet beds, incorporating this short-term mechanism of retreat into computer simulations will be critical for accurately predicting rates of ice sheet change and sea-level rise in the future.