During the Antarctic summer, thousands of mesmerising blue lakes form around the edges of the continent’s ice sheet, as warmer temperatures cause snow and ice to melt and collect into depressions on the surface. Colleagues of mine at Durham University have recently used satellites to record more than 65,000 of these lakes.
Though seasonal meltwater lakes have formed on the continent for decades, lakes had not been recorded before in such great numbers across coastal areas of East Antarctica. This means parts of the world’s largest ice sheet may be more vulnerable to a warming climate than previously thought.
Much of Antarctica is surrounded by floating platforms of ice, often as tall as a skyscraper. These are “ice shelves”. And when some of these ice shelves have collapsed in the past, satellites have recorded networks of lakes growing and then abruptly disappearing shortly beforehand. For instance, several hundred lakes disappeared in the weeks before the the catastrophic disintegration of the Larsen B Ice Shelf – when 3,250 km² of ice broke up in just two months in 2002.
The collapse may have depended on water from these lakes filling crevasses and then acting like a wedge as the weight of the water expanded the crevasses, triggering a network of fractures. The weight of lakes can also cause the ice shelf surface to flex, leading to further fracturing, which is thought to have helped the shelf become unstable and collapse.
Ice shelves act as door stops, supporting the huge mass of ice further inland. Their removal means the glaciers feeding the ice shelf are no longer held back and flow faster into the ocean, contributing to sea-level rise.
Scientists already knew that lakes form on the Antarctic ice sheet. But the latest study, published in Scientific Reports, shows that many more lakes are forming than previously thought, including in new parts of the ice sheet and much further inland and at higher elevations.
Since the cold and remoteness makes it logistically challenging to measure and monitor Antarctica’s lakes in the field, we largely know all this thanks to satellite imagery. In this case, one of the satellites used was the European Space Agency’s Sentinel-2 which provides global coverage of the Earth’s surface every five days and can detect features as small as ten metres.
My colleagues analysed satellite images of the East Antarctic Ice Sheet taken in January 2017. In total, the images covered 5,000,000 km² (that’s more than 20 times the area of the United Kingdom).
Because water reflects certain wavelengths very strongly compared to ice, lakes can be detected in these images by classifying pixels in the image as “water” or “non-water”. From these images we can pinpoint when lakes form, their growth and drainage, and how their extent and depth change over time. The largest lake detected so far was nearly 30 km long and estimated to hold enough water to fill 40,000 Olympic-sized swimming pools.
In a warming world, scientists are particularly interested in these lakes because they may contribute to destabilising the ice shelves and ice sheet in future.
Like a sponge, the more that ice shelves become saturated with meltwater, the less they are able to absorb, meaning more water pools on their surfaces as lakes. More surface lakes mean a greater likelihood that water will drain out, fill crevasses and potentially trigger flexing and fracturing. If this were to occur, other ice shelves around Antarctica may start to disintegrate like Larsen B. Glaciers with floating ice tongues protruding into the ocean may also be vulnerable.
Meanwhile in Greenland, scientists have observed entire lakes draining away within a matter of days, as meltwater plunges through vertical shafts in the ice sheet known as “moulins”. A warm, wet base lubricated by meltwater allows the ice to slide quicker and flow faster into the ocean.
Could something similar be happening in Antarctica? Lakes disappearing in satellite imagery suggests they could be draining in this way, but scientists have yet to observe this directly. If we are to understand how much ice the continent could lose, and how much it could contribute to global sea-level rise, we must understand how these surface meltwater lakes behave. Though captivating, they are potentially a warning sign of future instability in Antarctica.
We know that our planet has experienced warmer periods in the past, during the Pliocene geological epoch around three million years ago.
Our research, published today, shows that up to one third of Antarctica’s ice sheet melted during this period, causing sea levels to rise by as much as 20 metres above present levels in coming centuries.
We were able to measure past changes in sea level by drilling cores at a site in New Zealand, known as the Whanganui Basin, which contains shallow marine sediments of arguably the highest resolution in the world.
Using a new method we developed to predict the water level from the size of sand particle moved by waves, we constructed a record of global sea-level change with significantly more precision than previously possible.
The Pliocene was the last time atmospheric carbon dioxide concentrations were above 400 parts per million and Earth’s temperature was 2°C warmer than pre-industrial times. We show that warming of more than 2°C could set off widespread melting in Antarctica once again and our planet could be hurtling back to the future, towards a climate that existed three million years ago.
Last week we saw unprecedented global protests under the banner of Greta Thunberg’s #FridaysForFuture climate strikes, as the urgency of keeping global warming below the Paris Agreement target of 2°C hit home. Thunberg captured collective frustration when she chastised the United Nations for not acting earlier on the scientific evidence. Her plea resonated as she reminded us that:
With today’s emissions levels, that remaining CO₂ budget [1.5°C] will be entirely gone in less than eight and a half years.
At the current rate of global emissions we may be back in the Pliocene by 2030 and we will have exceeded the 2°C Paris target. One of the most critical questions facing humanity is how much and how fast global sea levels will rise.
According to the recent special report on the world’s oceans and cryosphere by the Intergovernmental Panel on Climate Change (IPCC), glaciers and polar ice sheets continue to lose mass at an accelerating rate, but the contribution of polar ice sheets, in particular the Antarctic ice sheet, to future sea level rise remains difficult to constrain.
If we continue to follow our current emissions trajectory, the median (66% probability) global sea level reached by the end of the century will be 1.2 metres higher than now, with two metres a plausible upper limit (5% probability). But of course climate change doesn’t magically stop after the year 2100.
To better predict what we are committing the world’s future coastlines to we need to understand polar ice sheet sensitivity. If we want to know how much the oceans will rise at 400ppm CO₂, the Pliocene epoch is a good comparison.
Back in 2015, we drilled cores of sediment deposited during the Pliocene, preserved beneath the rugged hill country at the Whanganui Basin. One of us (Timothy Naish) has worked in this area for almost 30 years and identified more than 50 fluctuations in global sea level during the last 3.5 million years of Earth’s history. Global sea levels had gone up and down in response to natural climate cycles, known as Milankovitch cycles, which are caused by long-term changes in Earths solar orbit every 20,000, 40,000 and 100,000 years. These changes in turn cause polar ice sheets to grow or melt.
While sea levels were thought to have fluctuated by several tens of metres, up until now efforts to reconstruct the precise amplitude had been thwarted by difficulties due to Earth deformation processes and the incomplete nature of many of the cycles.
Our research used a well-established theoretical relationship between the size of the particles transported by waves on the continental shelf and the depth to the seabed. We then applied this method to 800 metres of drill core and outcrop, representing continuous sediment sequences that span a time period from 2.5 to 3.3 million years ago.
We show that during the Pliocene, global sea levels regularly fluctuated between five to 25 metres. We accounted for local tectonic land movements and regional sea-level changes caused by gravitational and crustal changes to determine the sea-level estimates, known as the PlioSeaNZ sea-level record. This provides an approximation of changes in global mean sea level.
Our study also shows that most of the sea-level rise during the Pliocene came from Antarctica’s ice sheets. During the warm Pliocene, the geography of Earth’s continents and oceans and the size of polar ice sheets were similar to today, with only a small ice sheet on Greenland during the warmest period. The melting of the Greenland ice sheet would have contributed at most five metres to the maximum 25 metres of global sea-level rise recorded at Whanganui Basin.
Of critical concern is that over 90% of the heat from global warming to date has gone into the ocean. Much of it has gone into the Southern Ocean, which bathes the margins of Antarctica’s ice sheet.
Already, we are observing warm circumpolar deep water upwelling and entering ice shelf cavities in several sites around Antarctica today. Along the Amundsen Sea coast of West Antarctica, where the ocean has been heating the most, the ice sheet is thinning and retreating the fastest. One third of Antarctica’s ice sheet — the equivalent to up to 20 metres of sea-level rise — is grounded below sea level and vulnerable to widespread collapse from ocean heating.
Our study has important implications for the stability and sensitivity of the Antarctic ice sheet and its potential to contribute to future sea levels. It supports the concept that a tipping point in the Antarctic ice sheet may be crossed if global temperatures are allowed to rise by more than 2℃. This could result in large parts of the ice sheet being committed to melt-down over the coming centuries, reshaping shorelines around the world.
A record start to summer ice melt in Greenland this year has drawn attention to the northern ice sheet. We will have to wait to see if 2019 continues to break ice-melt records, but in the rapidly warming Arctic the long-term trends of ice loss are clear.
But what about at the other icy end of the planet?
Antarctica is an icy giant compared to its northern counterpart. The water frozen in the Greenland ice sheet is equivalent to around 7 metres of potential sea level rise. In the Antarctic ice sheet there are around 58 metres of sea-level rise currently locked away.
Like Greenland, the Antarctic ice sheet is losing ice and contributing to unabated global sea level rise. But there are worrying signs Antarctica is changing faster than expected and in places previously thought to be protected from rapid change.
On the Antarctic Peninsula – the most northerly part of the Antarctic continent – air temperatures over the past century have risen faster than any other place in the Southern Hemisphere. Summer melting already happens on the Antarctic Peninsula between 25 and 80 days each year. The number of melt days will rise by at least 50% when global warming hits the soon-to-be-reached 1.5℃ limit set out in the Paris Agreement, with some predictions pointing to as much as a 150% increase in melt days.
But the main threat to the Antarctic ice sheet doesn’t come from above. What threatens to truly transform this vast icy continent lies beneath, where warming ocean waters (and the vast heat carrying capacity of seawater) have the potential to melt ice at an unprecedented rate.
Almost all (around 93%) of the extra heat human activities have caused to accumulate on Earth since the Industrial Revolution lies within the ocean. And a large majority of this has been taken into the depths of the Southern Ocean. It is thought that this effect could delay the start of significant warming over much of Antarctica for a century or more.
However, the Antarctic ice sheet has a weak underbelly. In some places the ice sheet sits on ground that is below sea level. This puts the ice sheet in direct contact with warm ocean waters that are very effective at melting ice and destabilising the ice sheet.
Scientists have long been worried about the potential weakness of ice in West Antarctica because of its deep interface with the ocean. This concern was flagged in the first report of the Intergovernmental Panel on Climate Change (IPCC) way back in 1990, although it was also thought that substantial ice loss from Antarctica wouldn’t be seen this century. Since 1992 satellites have been monitoring the status of the Antarctic ice sheet and we now know that not only is ice loss already underway, it is also vanishing at an accelerating rate.
The latest estimates indicate that 25% of the West Antarctic ice sheet is now unstable, and that Antarctic ice loss has increased five-fold over the past 25 years. These are remarkable numbers, bearing in mind that more than 4 metres of global sea-level rise are locked up in the West Antarctic alone.
Thwaites Glacier in West Antarctica is currently the focus of a major US-UK research program as there is still a lot we don’t understand about how quickly ice will be lost here in the future. For example, gradual lifting of the bedrock as it responds to the lighter weight of ice (known as rebounding) could reduce contact between the ice sheet and warm ocean water and help to stabilise runaway ice loss.
On the other hand, melt water from the ice sheets is changing the structure and circulation of the Southern Ocean in a way that could bring even warmer water into contact with the base of the ice sheet, further amplifying ice loss.
There are other parts of the Antarctic ice sheet that haven’t had this same intensive research, but which appear to now be stirring. The Totten Glacier, close to Australia’s Casey station, is one area unexpectedly losing ice. There is a very pressing need to understand the vulnerabilities here and in other remote parts of the East Antarctic coast.
Sea ice forms and floats on the surface of the polar oceans. The decline of Arctic sea ice over the past 40 years is one of the most visible climate change impacts on Earth. But recent years have shown us that the behaviour of Antarctic sea ice is stranger and potentially more volatile.
The extent of sea ice around Antarctica has been gradually increasing for decades. This is contrary to expectations from climate simulations, and has been attributed to changes in the ocean structure and changing winds circling the Antarctic continent.
But in 2015, the amount of sea ice around Antarctica began to drop precipitously. In just 3 years Antarctica lost the same amount of sea ice the Arctic lost in 30.
So far in 2019, sea ice around Antarctica is tracking near or below the lowest levels on record from 40 years of satellite monitoring. In the long-term this trend is expected to continue, but such a dramatic drop over only a few years was not anticipated.
There is still a lot to learn about how quickly Antarctica will respond to climate change. But there are very clear signs that the icy giant is awakening and – via global sea level rise – coming to pay us all a visit.
Nerilie Abram, ARC Future Fellow, Research School of Earth Sciences; Chief Investigator for the ARC Centre of Excellence for Climate Extremes, Australian National University; Matthew England, Australian Research Council Laureate Fellow; Deputy Director of the Climate Change Research Centre (CCRC); Chief Investigator in the ARC Centre of Excellence in Climate System Science, UNSW, and Matt King, Professor, Surveying & Spatial Sciences, School of Technology, Environments and Design, University of Tasmania
The ocean that surrounds Antarctica plays a crucial role in regulating the mass balance of the continent’s ice cover. We now know that the thinning of ice that affects nearly a quarter of the West Antarctic Ice Sheet is clearly linked to the ocean.
The connection between the Southern Ocean and Antarctica’s ice sheet lies in ice shelves – massive slabs of glacial ice, many hundreds of metres thick, that float on the ocean. Ice shelves grind against coastlines and islands and buttress the outflow of grounded ice. When the ocean erodes ice shelves from below, this buttressing action is reduced.
While some ice shelves are thinning rapidly, others remain stable, and the key to understanding these differences lies within the hidden oceans beneath ice shelves. Our recently published research explores the ocean processes that drive melting of the world’s largest ice shelf. It shows that a frequently overlooked process is driving rapid melting of a key part of the shelf.
Rapid ice loss from Antarctica is frequently linked to Circumpolar Deep Water (CDW). This relatively warm (+1C) and salty water mass, which is found at depths below 300 metres around Antarctica, can drive rapid melting. For example, in the south-east Pacific, along West Antarctica’s Amundsen Sea coast, CDW crosses the continental shelf in deep channels and enters ice shelf cavities, driving rapid melting and thinning.
Interestingly, not all ice shelves are melting quickly. The largest ice shelves, including the vast Ross and Filchner-Ronne ice shelves, appear close to equilibrium. They are largely isolated from CDW by the cold waters that surround them.
The contrasting effects of CDW and cold shelf waters, combined with their distribution, explain much of the variability in the melting we observe around Antarctica today. But despite ongoing efforts to probe the ice shelf cavities, these hidden seas remain among the least explored parts of Earth’s oceans.
It is within this context that our research explores a new and hard-won dataset of oceanographic observations and melt rates from the world’s largest ice shelf.
In 2011, we used a 260 metre deep borehole that had been melted through the north-western corner of the Ross Ice Shelf, seven kilometres from the open ocean, to deploy instruments that monitor ocean conditions and melt rates beneath the ice. The instruments remained in place for four years.
The observations showed that far from being a quiet back water, conditions beneath the ice shelf are constantly changing. Water temperature, salinity and currents follow a strong seasonal cycle, which suggests that warm surface water from north of the ice front is drawn southward into the cavity during summer.
Melt rates at the mooring site average 1.8 metres per year. While this rate is much lower than ice shelves impacted by warm CDW, it is ten times higher than the average rate for the Ross Ice Shelf. Strong seasonal variability in the melt rate suggests that this melting hotspot is linked to the summer inflow.
To assess the scale of this effect, we used a high-precision radar to map basal melt rates across a region of about 8,000 square kilometres around the mooring site. Careful observations at around 80 sites allowed us to measure the vertical movement of the ice base and internal layers within the ice shelf over a one-year interval. We could then determine how much of the thinning was caused by basal melting.
Melting was fastest near the ice front where we observed short-term melt rates of up to 15 centimetres per day – several orders of magnitude higher than the ice shelf average rate. Melt rates reduced with distance from the ice front, but rapid melting extended far beyond the mooring site. Melting from the survey region accounted for some 20% of the total from the entire ice shelf.
Why is this region of the shelf melting so much more quickly than elsewhere? As is so often the case in the ocean, it appears that winds play a key role.
During winter and spring, strong katabatic winds sweep across the western Ross Ice Shelf and drive sea ice from the coast. This leads to the formation of an area that is free of sea ice, a polynya, where the ocean is exposed to the atmosphere. During winter, this area of open ocean cools rapidly and sea ice grows. But during spring and summer, the dark ocean surface absorbs heat from the sun and warms, forming a warm surface pool with enough heat to drive the observed melting.
Although the melt rates we observe are far lower than those seen on ice shelves influenced by CDW, the observations suggest that for the Ross Ice Shelf, surface heat is important.
Given this heat is closely linked to surface climate, it is likely that the predicted reductions in sea ice within the coming century will increase basal melt rates. While the rapid melting we observed is currently balanced by ice inflow, glacier models show that this is a structurally critical region where the ice shelf is pinned against Ross Island. Any increase in melt rates could reduce buttressing from Ross Island, increasing the discharge of land-based ice, and ultimately add to sea levels.
While there is still much to learn about these processes, and further surprises are certain, one thing is clear. The ocean plays a key role in the dynamics of Antarctica’s ice sheet and to understand the stability of the ice sheet we must look to the ocean.
Craig Stewart, Marine Physicist, National Institute of Water and Atmospheric Research
Greenland has been in the news a bit lately. From Huskies seemingly walking on water, to temperatures soaring to 20℃ above average for the time of year, to predictions of the vast ice sheet being lost entirely, what is going on?
At its most simple: ice melts when it gets too warm.
Of course, some ice melts every time summer rolls around, but the amount of Arctic ice that melts each summer is growing, and we’re waiting to see whether this turns out to be a record-breaking year for Greenland ice melt.
No part of the planet is free from the impacts of human-caused climate change. But Greenland, and the Arctic more generally, is experiencing the impacts particularly severely. Temperatures in the planet’s extreme north are rising twice as fast as the global average.
Greenland is warming so rapidly because of what climate scientists refer to as a “positive feedback”. Despite the name, these are not good. A better term might be “climate change amplifier”.
The Arctic has many “positive feedbacks” or “amplifiers” that worsen the effects of climate change here. For example, as snow and ice begin to melt, the surface darkens, allowing it to absorb more heat and thus melt even more.
This effect is most dramatic when snow and ice are lost completely, as in the case of the dramatic loss of the sea ice covering the Arctic ocean. Arctic sea ice loss is one of the major factors that explains why the Arctic is warming so much faster than the rest of the planet.
Another worrisome characteristic of climate change in the Arctic is the potential for ice melt to accelerate. The temperature threshold at which ice begins to melt means that once the climate has warmed enough to start melting ice, any further warming will rapidly cause an even larger amount of melting to occur. That is the reality beginning to play out in Greenland.
Last month, ice melt across the surface of Greenland made headlines. Surface melting spiked rapidly and was unusually strong for June. Melting was most intense around the edges of the Greenland ice sheet, and about 40% of the entire ice sheet surface was affected to some extent.
Greenland ice melt is typically very irregular during each summer, spiking as weather systems bring warm air masses over the ice sheet. Given this variability, it is not yet clear whether 2019 is going to be an unusually bad year for melting over Greenland – and whether it will rival the worst year on record, 2012, when the entire surface of the ice sheet experienced melting.
But what is very clear from observations since the 1970s (and completely consistent with simple physics) is that as the Arctic climate warms, the Greenland summer melt season is starting earlier, lasting longer, and becoming more intense.
Samples of older ice from inside Greenland’s ice sheet paint an even clearer picture of the changes that climate warming is causing. The amount of summer melting first began to increase in the mid-1800s, not long after human-driven climate warming began. Summer melt over the past two decades has reached levels roughly 50% higher than before the Industrial Revolution, and the speed of ice loss from the Greenland sheet has increased nearly sixfold since the 1980s.
An ice sheet has existed on Greenland for millions of years. But the geological timescales of ice sheet growth and renewal are vastly outpaced by the human-caused changes we see today.
A study published in June this year, at the same time surface melting of the ice sheet was spiking, predicts that if human greenhouse emissions continue unabated, by the end of this century ice loss from the Greenland ice sheet could see the ocean rise by up to 33cm.
If all of the Greenland ice sheet were to melt, global sea level would rise by more than 7 metres. According to the same study, that could potentially happen within 1,000 years.
The evidence is abundantly clear: the rising temperature of the planet is causing more Arctic ice to melt during the northern summer. We cannot avoid further ice loss in coming decades, and people and ecosystems will have to adapt to this.
But there is still a window of opportunity to avoid the worst impacts of future climate change in the longer term. The evidence tells us that the only way to prevent the destruction of the Greenland ice sheet, and multi-metre rises in global sea level, is to make rapid, deep cuts to greenhouse gas emissions. That is a choice we still have a chance to make.
Twenty-six of the forty-six fish species known to live in the Murray-Darling basin are listed as rare or threatened. Recent fish kills in the iconic river system are a grim reminder of how quickly things can take a turn for the worst.
A sudden drop in population size can push a species towards extinction, but there may be hope for resurrection. Frozen zoos store genetic material from endangered species and are preparing to make new individuals if an extinction occurs.
Cryopreservation: the field of possibilities
Unfortunately, poor response to freezing has hindered the introduction of fish into frozen zoos in the past. Now new techniques may provide them safe passage.
A frozen zoo, also known as a biobank or cryobank, stores cryopreserved or “frozen” cells from endangered species. The primary purpose of a frozen zoo is to provide a backup of endangered life on Earth allowing us to restore extinct species.
Reproductive cells, such as sperm, oocytes (eggs) and embryos, are cooled to -196ºC, at which point all cellular function is paused. When a sample is needed, the cells are warmed and used in breeding programs to produce new individuals, or to study their DNA to determine genetic relationships with other species.
There are several cryobanking facilities in Australia, including the Australian Frozen Zoo (where I work), the CryoDiversity Bank and the Ian Potter Australian Wildlife Biobank, as well as private collections. These cryobanks safeguard some of Australia’s most unique wildlife including the greater bilby, the golden bandicoot, and the yellow-footed rock wallaby as well as other exotic species such as the black rhino and orangutans.
Internationally, frozen zoos are working together to build a “Noah’s Ark” of frozen tissue. The Frozen Ark project, established in 2004 at the University of Nottingham, now consists of over 5,000 species housed in 22 facilities across the globe.
As more and more species move into frozen zoos, fish are at risk of being left out. Despite years of research, no long-term survival has been reported in fish eggs or embryos after cryopreservation. However, precursors of sperm and eggs known as gonial cells found in the developing embryo or the ovary or testis of adult fish have been preserved successfully in several species including brown trout, rainbow trout, tench and goby.
By freezing these precursory cells, we now have a viable method of storing fish genetics but, unlike eggs and sperm, the cells are not mature and cannot be used to produce offspring in this form.
To transform the cells into sperm and eggs, they are transplanted into a surrogate fish. Donor cells are injected into the surrogate where they follow instructions from surrounding cells which tell them where to go and when and how to make sperm or eggs.
Once the surrogate is sexually mature they can mate and produce offspring that are direct decedents of the endangered species the donor cells were originally collected from. In a way, we are hijacking the reproductive biology of the surrogate species. By selecting surrogates that are prolific breeders we can essentially “mass produce” sperm and eggs from an endangered species, potentially producing more offspring than it would have been able to within its own lifetime.
The combination of cryopreservation and surrogacy in conservation is promising but has only successfully been used in one endangered species so far, the Manchurian trout.
The “store now, save later” strategy of frozen zoos sounds simple but alas it is not. The methods needed to reproduce many species from frozen tissue are still being developed and may take years to perfect. The cost of maintaining frozen collections and developing methods of resurrection could divert funding from preventative conservation efforts.
Even if de-extinction is possible, there could be problems. The Australian landscape is evolving – temperatures fluctuate, habitats change, new predators and diseases are being introduced. Extinction is a consequence of failing to adapt to these changes. Reintroducing a species into the same hostile environment that lead to its demise may be a fool’s errand. How can we ensure reintroduced animals will thrive in an environment they may no longer be suited for?
Reducing human impact on the natural environment and actively protecting threatened species will be far easier than trying to resurrect them once they are gone. In the case of the Murray Darling Basin, reversing the damage done and developing policies that ensure its long-term protection will take time that endangered species may not have.
Frozen zoos are an insurance policy, and we don’t want to have to use them. But if we fail in our fight against extinction, we will be glad we made the investment in frozen zoos when we had the chance.