Climate change threatens Antarctic meteorites

Using artificial intelligence, satellite observations, and climate model projections, a team of researchers from Switzerland and Belgium calculate that for every tenth of a degree of increase in global air temperature, an average of nearly 9,000 meteorites disappear from the surface of the ice sheet. This loss has major implications, as meteorites are unique samples of extraterrestrial bodies that provide insights into the origin of life on Earth and the formation of the Moon.

Disappearing at an alarming rate

By 2050, about a quarter of the estimated of 300,000 -- 800,000 meteorites in Antarctica will be lost due to glacial melt.

By end of the century, researchers anticipate that number could rise approaching a loss of meteorites closer to three-quarters of the meteorites on the continent under a high-warming scenario.

Published in the journal Nature Climate Change, Harry Zekollari co-led the study while working under Professor Daniel Farinotti in the Laboratory of Hydraulics, Hydrology and Glaciology at the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich.

Zekollari and co-lead Veronica Tollenaar, Université Libre de Bruxelles, reveal in the study that ongoing warming results in the loss of about 5,000 meteorites a year, outpacing the collection efforts of Antarctic meteorites by a factor five.

Meteorites -- time capsules of the universe

Zekollari, now an Associate Professor of Glaciology at Vrije Universiteit Brussel, calls for a major international effort to preserve the scientific value of meteorites, "We need to accelerate and intensify efforts to recover Antarctic meteorites. The loss of Antarctic meteorites is much like the loss of data that scientists glean from ice cores collected from vanishing glaciers -- once they disappear, so do some of the secrets of the universe."

Meteorites are fragments from space that provide unique information about our solar system.

Antarctica is the most prolific place to find meteorites, and to date, about 60 percent of all meteorites ever found on Earth have been collected from the surface of the Antarctic ice sheet.

The flow of the ice sheet concentrates meteorites in so-called "meteorite stranding zones," where their dark crust allows them to be easily detected.

In addition to intensifying recovery operations, there is potential to increase the efficiency of meteorite recovery missions in the short term.

This potential relies mainly on data-driven analysis to identify unexplored meteorite stranding zones and mapping areas exposing blue ice where meteorites are often found.

Extraterrestrial heritage slipping away

Due to their dark colour, meteorites preferentially heat up with respect to the surrounding ice.

As this heat transfers from the meteorites to the ice, it can warm up the ice, and eventually cause the ice to locally melt, leading to a sinking of meteorites underneath the surface of the ice sheet.

Once the meteorites enter the ice sheet, even at shallow depths, they cannot be detected anymore, and they are thus lost for science.

As atmospheric temperatures increase, so does the surface temperature of the ice, intensifying the loss.

"Even when temperatures of the ice are well below zero, the dark meteorites warm-up so much in the sun that they can melt the ice directly beneath the meteorite. Through this process, the warm meteorite creates a local depression in the ice and over time fully disappears under the surface," says Tollenaar.

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80 mph speed record for glacier fracture helps reveal the physics of ice sheet collapse

There's enough water frozen in Greenland and Antarctic glaciers that if they melted, global seas would rise by many feet. What will happen to these glaciers over the coming decades is the biggest unknown in the future of rising seas, partly because glacier fracture physics is not yet fully understood.

A critical question is how warmer oceans might cause glaciers to break apart more quickly. University of Washington researchers have demonstrated the fastest-known large-scale breakage along an Antarctic ice shelf. The study, recently published in AGU Advances, shows that a 6.5-mile (10.5 kilometer) crack formed in 2012 on Pine Island Glacier -- a retreating ice shelf that holds back the larger West Antarctic ice sheet -- in about 5 and a half minutes. That means the rift opened at about 115 feet (35 meters) per second, or about 80 miles per hour.

"This is to our knowledge the fastest rift-opening event that's ever been observed," said lead author Stephanie Olinger, who did the work as part of her doctoral research at the UW and Harvard University, and is now a postdoctoral researcher at Stanford University. "This shows that under certain circumstances, an ice shelf can shatter. It tells us we need to look out for this type of behavior in the future, and it informs how we might go about describing these fractures in large-scale ice sheet models."

A rift is a crack that passes all the way through the roughly 1,000 feet (300 meters) of floating ice for a typical Antarctic ice shelf. These cracks are the precursor to ice shelf calving, in which large chunks of ice break off a glacier and fall into the sea. Such events happen often at Pine Island Glacier -- the iceberg observed in the study has long since separated from the continent.

"Ice shelves exert a really important stabilizing influence on the rest of the Antarctic ice sheet. If an ice shelf breaks up, the glacier ice behind really speeds up," Olinger said. "This rifting process is essentially how Antarctic ice shelves calve large icebergs."

In other parts of Antarctica, rifts often develop over months or years. But it can happen more quickly in a fast-evolving landscape like Pine Island Glacier, where researchers believe the West Antarctic Ice Sheet has already passed a tipping point on its collapse into the ocean.

Satellite images provide ongoing observations. But orbiting satellites pass by each point on Earth only every three days. What happens during those three days is harder to pin down, especially in the dangerous landscape of a fragile Antarctic ice shelf.

For the new study, the researchers combined tools to understand the rift's formation. They used seismic data recorded by instruments placed on the ice shelf by other researchers in 2012 with radar observations from satellites.

Glacier ice acts like a solid on short timescales, but it's more like a viscous liquid on long timescales.

"Is rift formation more like glass breaking or like Silly Putty being pulled apart? That was the question," Olinger said. "Our calculations for this event show that it's a lot more like glass breaking."

If the ice were a simple brittle material, it should have shattered even faster, Olinger said. Further investigation pointed to the role of seawater. Seawater in the rifts holds the space open against the inward forces of the glacier. And since seawater has viscosity, surface tension and mass, it can't just instantly fill the void. Instead, the pace at which seawater fills the opening crack helps slow the rift's spread.

"Before we can improve the performance of large-scale ice sheet models and projections of future sea-level rise, we have to have a good, physics-based understanding of the many different processes that influence ice shelf stability," Olinger said.

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Greenland's ice sheet is melting -- and being replaced by vegetation

An estimated 11,000 sq miles or 28,707 sq kilometres of Greenland's ice sheet and glaciers have melted over the last three decades, according to a major analysis of historic satellite records.

The total area of ice loss is equivalent to the size of Albania, and represents about 1.6 % of Greenland's total ice and glacier cover.

Where there was once ice and snow, there is now barren rock, wetlands and areas of shrub.

A team of scientists from the University of Leeds, who have tracked the changes across Greenland from the 1980s through to the 2010s, say warmer air temperatures are causing the ice to retreat, which in turn is having an impact on the temperature of the land surface, greenhouse gas emissions and the stability of the landscape.

Permafrost -- a permanently frozen layer below the Earth's surface -- is being "degraded" by the warming and in some areas, the scientists warn that it could have an impact on the infrastructure, buildings and communities that exist above it.

Their findings -- "Land cover changes across Greenland dominated by a doubling of vegetation in three decades"- are reported today (13/02) in the journal Scientific Reports.

Impact of global warming

Greenland is part of the Arctic region. It is the world's biggest island, around 836,330 sq miles in size (2.1 million sq km). Most of the land is covered by ice and glaciers and it is home to almost 57,000 people.

Since the 1970s, the region has been warming at double the global mean rate. On Greenland, average annual air temperatures between 2007 and 2012 were 3 degrees C warmer, compared with the 1979 to 2000 average.

And the researchers warn that more extreme temperatures are likely in the future.

Jonathan Carrivick, an Earth scientist based in the Faculty of Environment at Leeds and one of the authors of the study, said: "Warmer temperatures are linked to the land cover changes that we are seeing on Greenland.

"By analysing high resolution satellite images, we have been able to produce a detailed record of the land cover changes that are taking place."

Ice disappears to be replaced by bare rock and shrubs

Ice loss was concentrated around the edges of present-day glaciers but also in the north and south- west of Greenland. There were also high-levels of ice loss in localised areas in the west, mid-north-west and south-east.

Over the three decades, the amount of land with vegetation growing on it increased by 33,774 sq miles (87,475 sq km), more than doubling over the study period.

A pronounced increase in vegetation was seen across the south-west, east and north-east. The greatest increase in dense wetland vegetation occurred in the vicinity of Kangerlussuaq in the south-west and in isolated areas in the north-east.

Analysis by the researchers revealed that vegetation had increased along a latitudinal gradient between 63 degrees North and 69 degrees North and declined north of this.

Jonathan Carrivick said: "We have seen signs that the loss of ice is triggering other reactions which will result in further loss of ice and further 'greening' of Greenland, where shrinking ice exposes bare rock that is then colonised by tundra and eventually shrub.

"At the same time, water released from the melting ice is moving sediment and silt, and that eventually forms wetlands and fenlands."

Loss of ice triggers further warming

The loss of ice affects land surface temperatures because of albedo, which is the measure of how reflective a surface is.

Snow and ice are good reflectors of the sun's energy hitting the Earth's surface and this helps to keep the Earth cooler. As the ice retreats, it exposes bedrock which absorbs more solar energy, raising the temperature of the land surface.

Similarly, as ice melts it increases the quantity of water in lakes. Water absorbs more solar energy than snow and this also increases the temperature of the land surface.

Greenhouse gas emitter

The analysis shows a near quadrupling of wetlands across Greenland, particularly in the east and north-east. The wetlands are a source of methane emissions.

Writing in the paper, the researchers noted: "Expansion of vegetation and especially in wetland areas indicates but also exacerbates permafrost thaw, active layer thickening and thus emissions of greenhouse gasespreviously stored in these Arctic soils."

The researchers also developed a model to predict those areas on Greenland that are likely to see "marked and accelerated" change in the future.

Dr Michael Grimes, the lead author of the report who conducted the research as part of their PhD, added: "The expansion of vegetation, occurring in tandem with the retreat of glaciers and the ice sheet, is significantly altering the flow of sediments and nutrients into coastal waters.

"These changes are critical, particularly for the indigenous populations whose traditional subsistence hunting practices rely on the stability of these delicate ecosystems.

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Ice cores provide first documentation of rapid Antarctic ice loss in the past

Researchers from the University of Cambridge and the British Antarctic Survey have uncovered the first direct evidence that the West Antarctic Ice Sheet shrunk suddenly and dramatically at the end of the Last Ice Age, around eight thousand years ago.

The evidence, contained within an ice core, shows that in one location the ice sheet thinned by 450 metres -- that's more than the height of the Empire State Building -- in just under 200 years.

This is the first evidence anywhere in Antarctica for such a fast loss of ice. Scientists are worried that today's rising temperatures might destabilize parts of the West Antarctic Ice Sheet in the future, potentially passing a tipping point and inducing a runaway collapse. The new study, published in Nature Geoscience, sheds light on how quickly Antarctic ice could melt if temperatures continue to soar.

"We now have direct evidence that this ice sheet suffered rapid ice loss in the past," said Professor Eric Wolff, senior author of the new study from Cambridge's Department of Earth Sciences. "This scenario isn't something that exists only in our model predictions and it could happen again if parts of this ice sheet becomes unstable."

The Antarctic ice sheets, from west to east, contain enough freshwater to raise global sea levels by around 57 metres. The West Antarctic Ice Sheet is considered particularly vulnerable because much of it sits on bedrock that lies below sea level.

Model predictions suggest that a large part of the West Antarctic Ice Sheet could disappear in the next few centuries, causing sea levels to rise. Exactly when and how quickly the ice could be lost is, however, uncertain.

One way to train ice sheet models to make better predictions is to feed them with data on ice loss from periods of warming in Earth's history. At the peak of Last Ice Age 20,000 years ago, Antarctic ice covered a larger area than today. As our planet thawed and temperatures slowly climbed, the West Antarctic Ice Sheet contracted to more or less its current extent.

"We wanted to know what happened to the West Antarctic Ice Sheet at the end of the Last Ice Age, when temperatures on Earth were rising, albeit at a slower rate than current anthropogenic warming," said Dr Isobel Rowell, study co-author from the British Antarctic Survey. "Using ice cores we can go back to that time and estimate the ice sheet's thickness and extent."

Ice cores are made up of layers of ice that formed as snow fell and was then buried and compacted into ice crystals over thousands of years. Trapped within each ice layer are bubbles of ancient air and contaminants that mixed with each year's snowfall -- providing clues as to the changing climate and ice extent.

The researchers drilled a 651-metre-long ice core from Skytrain Ice Rise in 2019. This mound of ice sits at the edge of the ice sheet, near the point where grounded ice flows into the floating Ronne Ice Shelf.

After transporting the ice cores back to Cambridge at -20oC, the researchers analysed them to reconstruct the ice thickness. First, they measured stable water isotopes, which indicate the temperature at the time the snow fell. Temperature decreases at higher altitudes (think of cold mountain air), so they were able to equate warmer temperatures with lower-lying, thinner ice.

They also measured the pressure of air bubbles trapped in the ice. Like temperature, air pressure also varies systematically with elevation. Lower-lying, thinner ice contains higher pressure air bubbles.

These measurements told them that ice thinned rapidly 8,000 years ago. "Once the ice thinned, it shrunk really fast," said Wolff, "this was clearly a tipping point -- a runaway process."

They think this thinning was probably triggered by warm water getting underneath the edge of the West Antarctic Ice Sheet, which normally sits on bedrock. This likely untethered a section of the ice from bedrock, allowing it to float suddenly and forming what is now the Ronne Ice Shelf. This then allowed neighbouring Skytrain Ice Rise, no longer restrained by grounded ice, to thin rapidly.

The researchers also found that the sodium content of the ice (originating from salt in sea spray) increased about 300 years after the ice thinned. This told them that, after the ice thinned, the ice shelf shrunk back so that the sea was hundreds of kilometres nearer to their site.

"We already knew from models that the ice thinned at around this time, but the date of this was uncertain," said Rowell. Ice sheet models placed the retreat anywhere between 12,000 and 5,000 years ago and couldn't say how quickly it happened. "We now have a very precisely dated observation of that retreat which can be built into improved models," said Rowell.

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Predicting saltwater intrusion into groundwater using Plymouth, Mass. as test case

As the world warms and ice sheets melt, the ocean continually rises. The greater Boston area can expect to see between one and six feet of sea level rise by 2100, according to recent estimates. To find out what this rise might mean for freshwater supplies, a team of hydrogeologists from the University of Massachusetts Amherst, led by David Boutt, professor of Earth, geographic and climate sciences, partnered with the Southeastern Massachusetts Pine Barrens Alliance (SEMPBA) and 13 other grassroots environmental organizations to develop an innovative new model that can not only predict saltwater intrusion over the next 75 years, but also pinpoint the main sources of salt contamination today -- road salt and human development. The team released the results of their study in the recent report, Saltwater Intrusion Vulnerability Assessment in Plymouth, MA.

"For many years now, I've been working with citizen stakeholders in the southeastern corner of Massachusetts," says Boutt, "and in 2021, the Pine Barrens Alliance, an environmental group interested in preserving the area's unique environmental character, approached me with an idea for a project to help assess how communities along the coast could best prepare for climate change."

Boutt and his colleagues, including recent UMass graduate and research assistant Alexander Kirshen, undergraduates Rachel King and Carly Lombardo, graduate student Daniel Corkran and postdoctoral researcher Brendan Moran, jumped at the opportunity to apply their academic research to an urgent, real-world problem close to home.

Plymouth sits on top of a freshwater aquifer -- the town's sole source of water. Because Plymouth extends to the ocean's edge, it is extremely susceptible to rising sea levels. For their study, Boutt, Kirshen and colleagues peeked underground to see what was happening.

Groundwater, flowing beneath the surface of the land, and the ocean's water, which, likewise, flows subterraneanly, push against each other and reach an equilibrium state. A well sunk on the freshwater side will flow with sweet water, but one that drills down into the brackish meeting point between fresh and salt will come up briny. As the oceans rise, that sub-surface saltwater pushes farther inland, and wells that have delivered pure water for generations can suddenly turn salty.

While the theory might seem intuitive enough, actually mapping, to say nothing of predicting, the flows and interactions of both fresh and salt water is an enormously complex task.

To start, the team built a salinity database that gathered all the available data from groundwater wells and surface water, such as ponds and streams, in the Plymouth area and measured them for salinity. This gave them a baseline understanding of the current locations and likely sources of elevated water salinity.

Next, Boutt and Kirshen adopted an existing U.S. Geological Survey hydrogeological model, which only focused on the onshore half of the hydrogeology equation, by extending its reach five kilometers offshore. The model includes ponds, streams, terrestrial recharge -- or the rate and amount of precipitation that seeps down into the aquifer -- as well the various wells that draw from the aquifer and the wastewater that is returned to the aquifer via re-infiltration or septic systems.

Finally, they conducted a series of model runs that took into consideration various scenarios in terms of future precipitation, sea-level rise, groundwater usage and changes in water returned to the aquifer.

"We found that, under the high sea-level rise scenario, areas of the aquifer will increase in salinity by up to 17,000 milligrams per liter by 2100," says Kirshen, "and the mixing zone between the ocean and freshwater will migrate inland by up to 200 meters." While a few ponds might see significant rise in water elevation, by up to 1.8 meters, most ponds would not see their salinity increase from this source of salinization.

The team also learned that water returned to the aquifer by septic systems plays a major role in helping to limit saltwater intrusion. "About 66% of the water that gets pumped out of the aquifer ends up returning to it," says Kirshen.

Perhaps the biggest surprise is that the highest levels of salinity today aren't near the coast, but inland, and especially around the roads. "This surprised me," says Boutt, "and it looks like road salt is one of the main sources of elevated salinity today."

"In partnering with UMass Amherst, we were always thinking beyond the municipal boundaries of Plymouth," says SEMPBA Vice President Frank Mand. "We share an aquifer and a geological foundation with over 30 communities in our ecoregion. So, though the news for Plymouth is good, more importantly we now have a scientific foundation -- and new methods for evaluating susceptibility to saltwater intrusion -- that are transferrable to those other communities and will help inform Plymouth's and other communities' planning for years to come."

"We were not looking to science to help us recover from our mistakes," Mand adds. "We were seeking to avoid problems in the future. That, in and of itself, was a worthy goal."

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Stability inspection for West Antarctica shows: marine ice sheet is not destabilized yet, but possibly on a path to tipping

Antarctica's vast ice masses seem far away, yet they store enough water to raise global sea levels by several meters. A team of experts from European research institutes has now provided the first systematic stability inspection of the ice sheet's current state. Their diagnosis: While they found no indication of irreversible, self-reinforcing retreat of the ice sheet in West Antarctica yet, global warming to date could already be enough to trigger the slow but certain loss of ice over the next hundreds to thousands of years.

"With more and more ice being lost in Antarctica over the last years, concerns have been raised whether a tipping point has already been crossed and an irreversible, long-term collapse of the West Antarctic Ice Sheet has already been initiated," explains Ronja Reese from the Potsdam Institute for Climate Impact Research (PIK) and the Northumbria University, Newcastle. "The results of our studies deliver two messages: First, while a number of glaciers in Antarctica are retreating at the moment, we find no indication of irreversible, self-reinforcing retreat yet, which is reassuring. However, our calculations also clearly indicate that an onset of an irreversible retreat of the ice sheet in West Antarctica is possible if the current state of the climate is sustained."

The main driver of ice loss in West Antarctica is relatively warm ocean water that amplifies melting underneath the ice shelves, which are the floating extensions of the grounded ice sheet. Melting of these ice shelves can enhance ice loss as it speeds up the grounded sections of the ice sheet. That is why the Antarctic margin with its grounding lines -- the zone where the grounded and the floating ice are connected -- is a key indicator of ice sheet health. An accelerated retreat of the grounding lines could indicate a forthcoming collapse of large marine regions of West Antarctica's ice sheet -- those parts of the ice sheet that are grounded below sea level.

Evolving over 10,000 years, triggered today: irreversible ice-loss and sea-level rise

Using state-of-the-art ice sheet models, the researchers not only conducted a thorough inspection of signs of irreversible retreat of marine sectors of the Antarctic ice sheet at present, they also ran simulations to investigate how the ice sheet would evolve over the next 10,000 years if current conditions remained unchanged. These hypothetical experiments indicate that even with no additional warming beyond what we have already experienced today, an irreversible collapse of some marine regions of West Antarctica's ice sheet is possible. Because the ice reacts to changes in temperature very slowly, the authors find that collapse occurs in their simulations at the earliest in 300 to 500 years from now, under current climate forcing. A full collapse would take centuries to millennia.

"The thing with sea-level rise from Antarctica is not that changes would happen overnight as an immediate threat to coastal communities. The process of melting would happen over hundreds or thousands of years. However, the cause could be human actions today, as they have the power to trigger and commit a future of 10,000 years to several meters of global sea-level rise. And stronger warming in the future would even speed up this process," Julius Garbe from PIK stresses.

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Thinning ice sheets may drive sharp rise in subglacial waters

Two Georgia Tech researchers, Alex Robel and Shi Joyce Sim, have collaborated on a new model for how water moves under glaciers. The new theory shows that up to twice the amount of subglacial water that was originally predicted might be draining into the ocean -- potentially increasing glacial melt, sea level rise, and biological disturbances.

The paper, published in Science Advances, "Contemporary Ice Sheet Thinning Drives Subglacial Groundwater Exfiltration with Potential Feedbacks on Glacier Flow," is co-authored by Colin Meyer (Dartmouth), Matthew Siegfried (Colorado School of Mines), and Chloe Gustafson (USGS).

While there are pre-existing methods to understand subglacial flow, these techniques involve time-consuming computations. In contrast, Robel and Sim developed a simple equation, which can predict how fast exfiltration, the discharge of groundwater from aquifers under ice sheets, using satellite measurements of Antarctica from the last two decades.

"In mathematical parlance, you would say we have a closed form solution," explains Robel, an assistant professor in the School of Earth and Atmospheric Sciences. "Previously, people would run a hydromechanical model, which would have to be applied at every point under Antarctica, and then run forward over a long time period." Since the researchers' new theory is a mathematically simple equation, rather than a model, "the entirety of our prediction can be done in a fraction of a second on a laptop," Robel says.

Robel adds that while there is precedence for developing these kinds of theories for similar kinds of models, this theory is specific in that it is for the particular boundary conditions and other conditions that exist underneath ice sheets. "This is, to our knowledge, the first mathematically simple theory which describes the exfiltration and infiltration underneath ice sheets."

"It's really nice whenever you can get a very simple model to describe a process -- and then be able to predict what might happen, especially using the rich data that we have today. It's incredible" adds Sim, a research scientist in the School of Earth and Atmospheric Sciences. "Seeing the results was pretty surprising."

One of the main arguments in the paper underscores the potentially large source of subglacial water -- possibly up to double the amount previously thought -- that could be affecting how quickly glacial ice flows and how quickly the ice melts at its base. Robel and Sim hope that the predictions made possible by this theory can be incorporated into ice sheet models that scientists use to predict future ice sheet change and sea level rise.

A dangerous feedback cycle

Aquifers are underground areas of porous rock or sediment rich in groundwater. "If you take weight off aquifers like there are under large parts of Antarctica, water will start flowing out of the sediment," Robel explains, referencing a diagram Sim created. While this process, known as exfiltration, has been studied previously, focus has been on the long time scales of interglacial cycles, which cover tens of thousands of years.

There has been less work on modern ice sheets, especially on how quickly exfiltration might be occurring under the thinning parts of the current-day Antarctic ice sheet. However, using recent satellite data and their new theory, the team has been able to predict what exfiltration might look like under those modern ice sheets.

"There's a wide range of possible predictions," Robel explains. "But within that range of predictions there is the very real possibility that groundwater may be flowing out of the aquifer at a speed that would make it a majority, or close to a majority of the water that is underneath the ice sheet."

If those parameters are correct, that would mean there's twice as much water coming into the subglacial interface than previous estimates assumed.

Ice sheets act like a blanket, sitting over the warm earth and trapping heat on the bottom, away from Antarctica's cold atmosphere -- and this means that the warmest place in the Antarctic ice sheet is at the bottom of a sheet, not on the surface. As an ice sheet thins, the warmer underground water can exfiltrate more readily, and this heat gradient can accelerate the melting that an ice sheet experiences.

"When the atmosphere warms up, it takes tens of thousands of years for that signal to diffuse through an ice sheet of the size of the thickness of the Antarctic ice sheet," Robel explains. "But this process of exfiltration is a response to the already-ongoing thinning of the ice sheet, and it's an immediate response right now."

Broad implications

Beyond sea level rise, this additional exfiltration and melt has other implications. Some of the places of richest marine productivity in the world occur off the coast of Antarctica, and being able to better predict exfiltration and melt could help marine biologists better understand where marine productivity is occurring, and how it might change in the future.

Robel also hopes this work will open the doorway to more collaborations with groundwater hydrologists who may be able to apply their expertise to ice sheet dynamics, while Sim underscores the need for more fieldwork.

"Getting the experimentalists and observationalists interested in trying to help us better constrain some of the properties of these water-laden sediments -- that would be very helpful," Sim says. "That's our largest unknown at this point, and it heavily influences the results."

"It's really interesting how there's a potential to draw heat from deeper in the system," she adds. "There's quite a lot of water that could be drawing more heat out, and I think that there's a heat budget there that could be interesting to look at."

Read more at Science Daily

The Greenland Ice Sheet is close to a melting point of no return

The Greenland Ice Sheet covers 1.7 million square kilometers (660,200 square miles) in the Arctic. If it melts entirely, global sea level would rise about 7 meters (23 feet), but scientists aren't sure how quickly the ice sheet could melt. Modeling tipping points, which are critical thresholds where a system behavior irreversibly changes, helps researchers find out when that melt might occur.

Based in part on carbon emissions, a new study using simulations identified two tipping points for the Greenland Ice Sheet: releasing 1000 gigatons of carbon into the atmosphere will cause the southern portion of the ice sheet to melt; about 2500 gigatons of carbon means permanent loss of nearly the entire ice sheet.

Having emitted about 500 gigatons of carbon, we're about halfway to the first tipping point.

"The first tipping point is not far from today's climate conditions, so we're in danger of crossing it," said Dennis Höning, a climate scientist at the Potsdam Institute for Climate Impact Research who led the study. "Once we start sliding, we will fall off this cliff and cannot climb back up."

The study was published in AGU's journal Geophysical Research Letters, which publishes short-format, high-impact research spanning the Earth and space sciences.

The Greenland Ice Sheet is already melting; between 2003 and 2016, it lost about 255 gigatons (billions of tons) of ice each year. Much of the melt to date has been in the southern part of the ice sheet. Air and water temperature, ocean currents, precipitation and other factors all determine how quickly the ice sheet melts and where it loses ice.

The complexity of how those factors influence each other, along with the long timescales scientists need to consider for melting an ice sheet of this size, make it difficult to predict how the ice sheet will respond to different climate and carbon emissions scenarios.

Previous research identified global warming of between 1 degree to 3 degrees Celsius (1.8 to 5.4 degrees Fahrenheit) as the threshold beyond which the Greenland Ice Sheet will melt irreversibly.

To more comprehensively model how the ice sheet's response to climate could evolve over time, Höning's new study for the first time used a complex model of the whole Earth system, which includes all the key climate feedback processes, paired with a model of ice sheet behavior. They first used simulations with constant temperatures to find equilibrium states of the ice sheet, or points where ice loss equaled ice gain. Then they ran a set of 20,000-year-long simulations with carbon emissions ranging from 0 to 4000 gigatons of carbon.

From among those simulations, the researchers derived the 1000-gigaton carbon tipping point for the melting of the southern portion of the ice sheet and the even more perilous 2,500-gigaton carbon tipping point for the disappearance of nearly the entire ice sheet.

As the ice sheet melts, its surface will be at ever-lower elevations, exposed to warmer air temperatures. Warmer air temperatures accelerate melt, making it drop and warm further. Global air temperatures have to remain elevated for hundreds of years or even longer for this feedback loop to become effective; a quick blip of 2 degrees Celsius (3.6 degrees Fahrenheit) wouldn't trigger it, Höning said. But once the ice crosses the threshold, it would inevitably continue to melt. Even if atmospheric carbon dioxide were reduced to pre-industrial levels, it wouldn't be enough to allow the ice sheet to regrow substantially.

Read more at Science Daily

3000+ billion tons of ice lost from Antarctic Ice Sheet over 25 years

Scientists have calculated that the fastest changing Antarctic region - the Amundsen Sea Embayment - has lost more than 3,000 billion tonnes of ice over a 25-year period.

If all the lost ice was piled on London, it would stand over 2 km tall - or 7.4 times the height of the Shard. If it were to cover Manhattan, it would stand at 61 km - or 137 Empire State Buildings placed on top of one another. 

Twenty major glaciers form the Amundsen Sea Embayment in West Antarctica, which is more than four times the size of the UK, and they play a key role in contributing to the level of the world's oceans.  

So much water is held in the snow and ice, that if it were to all to drain into the sea, global sea levels could increase by more than one metre.  

The research, led by Dr Benjamin Davison at the University of Leeds, calculated the "mass balance" of the Amundsen Sea Embayment. This describes the balance between mass of snow and ice gain due to snowfall and mass lost through calving, where icebergs form at the end of a glacier and drift out to sea.

When calving happens faster than the ice is replaced by snowfall, then the Embayment loses mass overall and contributes to global sea level rise. Similarly, when snowfall supply drops, the Embayment can lose mass overall and contribute to sea level rise.

The results show that West Antarctica saw a net decline of 3,331 billion tonnes of ice between 1996 and 2021, contributing over nine millimetres to global sea levels.  Changes in ocean temperature and currents are thought to have been the most important factors driving the loss of ice. 

Dr Davison, a Research Fellow at the Institute for Climate and Atmospheric Science at Leeds, said: "The 20 glaciers in West Antarctica have lost an awful lot of ice over the last quarter of a century and there is no sign that the process is going to reverse anytime soon although there were periods where the rate of mass loss did ease slightly. 

"Scientists are monitoring what is happening in the Amundsen Sea Embayment because of the crucial role it plays in sea-level rise. If ocean levels were to rise significantly in future years, there are communities around the world who would experience extreme flooding." 

The research has been published in the scientific journal Nature Communications.

Extreme snowfall events

Using climate models that show how air currents move around the world, the scientists identified that the Amundsen Sea Embayment had experienced several extreme snowfall events over the 25-year study period. 

These would have resulted in periods of heavy snowfall and periods of very little snowfall or a "snow drought."  

The researchers factored these extreme events into their calculations. Surprisingly, they found that these events contributed up to half of the ice change at certain times, and therefore played a key role in the contribution the Amundsen Sea Embayment was making to sea level rise during certain time periods.  

For example, between 2009 and 2013, the models revealed a period of a persistant snow drought. The lack of snowfall starved the ice sheet and caused it to lose ice, therefore contributing about 25% more to sea level rise than in years of average snowfall. 

In contrast, during the winters of 2019 and 2020 there was very heavy snowfall. The scientists estimated that this heavy snowfall mitigated the sea level contribution from the Amundsen Sea Embayment, reducing it to about half of what it would have been in an average year.  

Dr Davison said: "Changes in ocean temperature and circulation appear to be driving the long-term, large-scale changes in West Antarctica ice sheet mass.  We absolutely need to research those more because they are likely to control the overall sea level contribution from West Antarctica.  

"However, we were really surprised to see just how much periods of extremely low or high snowfall could affect the ice sheet over two to five-year periods -- so much so that we think they could play an important, albeit secondary role, in controlling rates of West Antarctic ice loss." 

Dr Pierre Dutrieux, a scientist at the British Antarctic Survey and co-author of the study, added: "Ocean temperature changes and glacial dynamics appear strongly connected in this part of the world, but this work highlights the large variability and unexpected processes by which snowfall also plays a direct role in modulating glacier mass."

New glacier named

The ice loss from the region over the past 25 years has seen the retreat of the Pine Island Glacier,  also known as PIG.

As it retreated, one of its tributary glaciers became detached from the main glacier and rapidly accelerated. As a result, the tributary glacier has now been named by the UK Antarctic Place-names Committee, Piglet Glacier, so that it can be unambiguously located and identified by future studies.  

Dr Anna Hogg, one of the authors of the paper and Associate Professor at the Institute of Climate and Atmospheric Science at Leeds, said: "As well as shedding new light on the role of extreme snowfall variability on ice sheet mass changes, this research also provides new estimates of how quickly this important region of Antarctica is contributing to sea level rise.  

"Satellite observations have showed that the newly named Piglet Glacier accelerated its ice speed by 40%, as the larger PIG retreated to its smallest extent since records began."

Read more at Science Daily

Global warming reaches central Greenland

At high elevations of the Greenland Ice Sheet, the years 2001 to 2011 were 1.5 °C warmer than in the 20th century and represent the warmest decade in the last thousand years.

A temperature reconstruction from ice cores of the past 1,000 years reveals that today's warming in central-north Greenland is surprisingly pronounced. The most recent decade surveyed in a study, the years 2001 to 2011, was the warmest in the past 1,000 years, and the region is now 1.5 °C warmer than during the 20th century, as researchers led by the Alfred Wegener Institute just report in the journal Nature. Using a set of ice cores unprecedented in length and quality, they reconstructed past temperatures in central-north Greenland and melting rates of the ice sheet.

The Greenland Ice Sheet plays a pivotal part in the global climate system. With enormous amounts of water stored in the ice (about 3 million cubic kilometres), melt and resulting sea-level rise is considered a potential tipping point. For unmitigated global emissions rates ('business as usual'), the ice sheet is projected to contribute up to 50 centimetres to global mean sea-level by 2100. Weather stations along the coast have been recording rising temperatures for many years. But the influence of global warming on the up to 3,000 m elevated parts of the ice sheet have remained unclear to due to the lack of long-term observations. In a study now published in Nature, experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) present clear evidence that effects of global warming have reached the remote, high-elevation areas of central-north Greenland.

"The time series we recovered from ice cores now continuously covers more than 1,000 years, from year 1000 to 2011. This data shows that the warming in 2001 to 2011 clearly differs from natural variations during the past 1,000 years. Although grimly expected in the light of global warming, we were surprised by how evident this difference really was," says AWI glaciologist Dr Maria Hörhold, lead author of the study. Together with colleagues from AWI and the University of Copenhagen's Niels Bohr Institute, she analysed the isotope composition in shallow ice cores gathered in central-north Greenland during dedicated AWI expeditions.

Previous ice cores obtained at co-located sites starting in the 1990s, did not indicate clear warming in central-north Greenland, despite rising global mean temperatures. Part of the reason is substantial natural climate variability in the region.

The AWI researchers have now extended the previous datasets up to winter 2011/2012 by a dedicated redrilling effort, recovering time series unprecedented length and quality. The temperatures were reconstructed by using consistently one single method for the entire record in the lab: measuring concentrations of stable oxygen isotopes within the ice, which vary with the temperatures prevailing at times of ice formation. Previous studies had to draw on a range of different climate archives and combine results to reconstruct temperature, introducing much larger uncertainties in the assessment of natural variability.

In addition to the temperature, the team reconstructed the melt production of the ice sheet. Melting has increased substantially in Greenland since the 2000s and now significantly contributes to global sea-level rise. "We were amazed to see how closely temperatures inland are connected to Greenland-wide meltwater drainage -- which, after all, occurs in low-elevation areas along the rim of the ice sheet near the coast," says Maria Hörhold.

In order to quantify this connection between temperatures in high-elevation parts and melting along the edges of the ice sheet, the authors used data from a regional climate model for the years 1871 to 2011 and satellite observations of ice-mass changes for the years 2002 to 2021 from the GRACE/GRACE-FO gravimetry missions. This allowed them to convert the temperature variations identified in the ice cores into melting rates and provide estimates for the past 1,000 years. This represents an important dataset for climate research: better understanding of the melt dynamics of the ice sheet in the past improves projections of related future sea-level rise; reduced uncertainties in projections is one step to help optimize adaptation measures.

Read more at Science Daily

Ancient ice age valleys offer clues to future ice sheet change

Deep valleys buried under the seafloor of the North Sea record how the ancient ice sheets that used to cover the UK and Europe expelled water to stop themselves from collapsing.

A new study published this week has surprised the research team, who discovered that the valleys took just hundreds of years to form as they transported vast amounts of meltwater away from under the ice and out into the sea.

This new understanding of when the vast ice sheets melted 20,000 years ago has implications for how glaciers may respond to climate warming today. The study is published in the journal Quaternary Science Reviews.

Tunnel valleys are enormous channels, sometimes up to 150km long, 6km wide and 500m deep (each several times larger than Loch Ness), that drain water from beneath melting ice sheets. There are thousands buried beneath the seafloor of the North Sea that record the melting of ice sheets that have covered the UK and Western Europe over the last two million years.

Lead author James Kirkham, from British Antarctic Survey (BAS) and the University of Cambridge, says:

"This is an exciting discovery. We know that these spectacular valleys are carved out during the death throes of ice sheets. By using a combination of state-of-the-art subsurface imaging techniques and a computer model, we have learnt that tunnel valleys can be eroded rapidly beneath ice sheets experiencing extreme warmth."

The team analysed 'jaw-droppingly detailed' seismic images that provide a 3D scan of the Earth's buried layers. Informed by delicate clues discovered within the valleys, the authors performed a series of computer modelling experiments to simulate valley development, and test how quickly they formed as the last ice sheet to cover the UK melted away at the end of the most recent ice age about 20,000 years ago.

The research suggests that this process is quick by geological timescales, with the melting ice forming giant tunnel valleys within hundreds of years, expelling water that could otherwise accelerate rates of ice loss.

Traditionally, the drainage of water from beneath ice sheets is thought to stabilise ice flow, a process that could potentially buffer modern ice sheets from collapse in a warming climate. But while inspecting the detailed seismic scans, the authors began to find tell-tale signatures of both stagnant and rapid ice movement within the valleys, complicating the picture of how these rapidly forming channels might affect future ice sheet behaviour.

What is certain, is that the surprisingly fast rate at which these tunnels form means that scientists need to start considering their effects in models of how today's ice sheets will evolve in the coming decades to centuries.

There are no modern analogues for this rapid process, but these ancient valleys, now buried hundreds of metres beneath the muds of the North Sea seafloor, record a mechanism for how ice sheets respond to extreme warmth that is missing from present-day ice sheet models. Such models do not currently resolve fine-scale water drainage processes, despite them appearing to be an important control on future ice loss rates and ultimately sea level rise.

James Kirkham continues: "The pace at which these giant channels can form means that they are an important, yet currently ignored, mechanism that may potentially help to stabilise ice sheets in a warming world. As climate change continues to drive the retreat of the modern-day Greenland and Antarctic ice sheets at ever increasing rates, our results call for renewed investigation of how tunnel valleys may help to stabilise contemporary ice losses, and therefore sea level rise, if they switch on beneath the Earth's ice sheets in the future."

Dr Kelly Hogan, co-author and a geophysicist at BAS, says:

"We have been observing these huge meltwater channels from areas covered by ice sheets in the past for more than a century but we did not really understand how they formed. Our results show, for the first time, that the most important mechanism is probably summer melting at the ice surface that makes its way to the bed through cracks or chimneys-like conduits and then flows under the pressure of the ice sheet to cut the channels. Surface melting is already hugely important for the Greenland Ice Sheet today, and this process of water transport through the system will only increase as our climate warms. The crucial question now is will this "extra" meltwater flow in channels cause our ice sheets to flow more quickly, or more slowly, into the sea."

Read more at Science Daily

Faster in the Past: New seafloor images of West Antarctic Ice Sheet upend understanding of Thwaites Glacier retreat

The Thwaites Glacier in West Antarctica -- about the size of Florida -- has been an elephant in the room for scientists trying to make global sea level rise predictions.

This massive ice stream is already in a phase of fast retreat (a "collapse" when viewed on geological timescales) leading to widespread concern about exactly how much, or how fast, it may give up its ice to the ocean.

The potential impact of Thwaites' retreat is spine-chilling: a total loss of the glacier and surrounding icy basins could raise sea level from three to 10 feet.

A new study in Nature Geoscience led by marine geophysicist Alastair Graham at the University of South Florida's College of Marine Science adds cause for concern. For the first time, scientists mapped in high-resolution a critical area of the seafloor in front of the glacier that gives them a window into how fast Thwaites retreated and moved in the past.

The stunning imagery shows geologic features that are new to science, and also provides a kind of crystal ball to see into Thwaites' future. In people and ice sheets alike, past behavior is key to understanding future behavior.

The team documented more than 160 parallel ridges that were created, like a footprint, as the glacier's leading edge retreated and bobbed up and down with the daily tides.

"It's as if you are looking at a tide gauge on the seafloor," Graham said. "It really blows my mind how beautiful the data are."

Beauty aside, what's alarming is that the rate of Thwaites' retreat that scientists have documented more recently are small compared to the fastest rates of change in its past, said Graham.

To understand Thwaites' past retreat, the team analyzed the rib-like formations submerged 700 meters (just under half a mile) beneath the polar ocean and factored in the tidal cycle for the region, as predicted by computer models, to show that one rib must have been formed every single day.

At some point in the last 200 years, over a duration of less than six months, the front of the glacier lost contact with a seabed ridge and retreated at a rate of more than 2.1 kilometers per year (1.3 miles per year) -- twice the rate documented using satellites between 2011 and 2019.

"Our results suggest that pulses of very rapid retreat have occurred at Thwaites Glacier in the last two centuries, and possibly as recently as the mid-20th Century," Graham said.

"Thwaites is really holding on today by its fingernails, and we should expect to see big changes over small timescales in the future-even from one year to the next-once the glacier retreats beyond a shallow ridge in its bed," said marine geophysicist and study co-author Robert Larter from the British Antarctic Survey.

To collect the imagery and supporting geophysical data, the team, which included scientists from the United States, the United Kingdom and Sweden, launched a state-of-the-art orange robotic vehicle loaded with imaging sensors called 'Rán'from the R/V Nathaniel B. Palmer during an expedition in 2019.

Rán, operated by scientists at the University of Gothenburg in Sweden, embarked on a 20-hour mission that was as risky as it was serendipitous, Graham said. It mapped an area of the seabed in front of the glacier about the size of Houston -- and did so in extreme conditions during an unusual summer notable for its lack of sea ice.

This allowed scientists to access the glacier front for the first time in history.

"This was a pioneering study of the ocean floor, made possible by recent technological advancements in autonomous ocean mapping and a bold decision by the Wallenberg foundation to invest into this research infrastructure," said Anna Wåhlin, a physical oceanographer from the University of Gothenburg who deployed Rán at Thwaites. "The images Ran collected give us vital insights into the processes happening at the critical junction between the glacier and the ocean today."

"It was truly a once in a lifetime mission," said Graham, who said the team would like to sample the seabed sediments directly so they can more accurately date the ridge-like features.

"But the ice closed in on us pretty quickly and we had to leave before we could do that on this expedition," he said.

While many questions remain, one thing's for sure: It used to be that scientists thought of the Antarctic ice sheets as sluggish and slow to respond, but that's simply not true, said Graham.

"Just a small kick to Thwaites could lead to a big response," he said.

According to the United Nations, roughly 40 percent of the human population lives within 60 miles of the coast.

"This study is part of a cross-disciplinary collective effort to understand the Thwaites Glacier system better," said Tom Frazer, dean of the USF College of Marine Science, "and just because it's out of sight, we can't have Thwaites out of mind. This study is an important step forward in providing essential information to inform global planning efforts."

Read more at Science Daily

Scientists shine new light on role of Earth's orbit in the fate of ancient ice sheets

Scientists have finally put to bed a long-standing question over the role of Earth's orbit in driving global ice age cycles.

In a new study published today in the journal Science, the team from Cardiff University has been able to pinpoint exactly how the tilting and wobbling of the Earth as it orbits around the Sun has influenced the melting of ice sheets in the Northern Hemisphere over the past 2 million years or so.

Scientists have long been aware that the waxing and waning of massive Northern Hemisphere ice sheets results from changes in the geometry of Earth's orbit around the Sun.

There are two aspects of the Earth's geometry that can influence the melting of ice sheets: obliquity and precession.

Obliquity is the angle of the Earth's tilt as it travels around the Sun and is the reason why we have different seasons.

Precession is how the Earth wobbles as it rotates, much like a slightly off-centre spinning top. The angle of this wobble means that sometimes the Northern Hemisphere is closest to the Sun and other times the Southern Hemisphere is closest, meaning that roughly every 10,000 years one hemisphere will have warmer summers compared to the other, before it switches.

Scientists have determined that over the past million years or so, the combined effects of obliquity and precession on the waxing and waning of Northern Hemisphere ice sheets has resulted, through complicated interactions within the climate system, in ice age cycles lasting approximately 100 thousand years.

However, before 1 million years ago, in a period known as the early Pleistocene, the duration of ice age cycles was controlled only by obliquity and these ice age cycles were almost exactly 41,000 years long.

For decades, scientists have been puzzled as to why precession did not play a more important part in driving ice age cycles during this period.

In their new study, the Cardiff University team reveal new evidence suggesting that precession did actually play a role during the early Pleistocene.

Their results show that more intense summers, driven by precession, have always caused Northern Hemisphere ice sheets to melt, but before 1 million years ago, these events were less devastating and did not lead to the complete collapse of ice sheets.

Lead author of the study Professor Stephen Barker, from Cardiff University's School of Earth and Environmental Sciences, said: "Early Pleistocene ice sheets in the northern hemisphere were smaller than their more recent counterparts, and limited to higher latitudes where the effects of obliquity dominate over precession. This probably explains why it has taken so long for us to find evidence of precession forcing during early Pleistocene.

"These findings are the culmination of a major effort, involving more than 12 years of painstaking work in the laboratory to process nearly 10,000 samples and the development of a range of new analytical approaches. Thanks to this we can finally put to rest a long-standing problem in paleoclimatology and ultimately contribute to a better understanding of Earth's climate system.

Read more at Science Daily

Rapid glacial advance reconstructed during the time of Norse occupation in Greenland

The Greenland Ice Sheet is the second largest ice body in the world, and it has the potential to contribute significantly to global sea-level rise in a warming global climate. Understanding the long-term record of the Greenland Ice Sheet, including both records of glacial advance and retreat, is critical in validating approaches that model future ice-sheet scenarios. However, this reconstruction can be extremely challenging. A new study published Thursday in the journal Geology reconstructed the advance of one of the largest tidewater glaciers in Greenland to provide a better understanding of long-term glacial dynamics.

"In the news, we're very used to hearing about glacial retreat, and that's because in a warming climate scenario -- which is what we're in at the moment -- we generally document ice masses retreating. However, we also want to understand how glaciers react if there is a climate cooling and subsequent advance. To do this, we need to reconstruct glacier geometry from the past," said Danni Pearce, co-lead author of the study.

An interdisciplinary team of researchers studied the advance of Kangiata Nunaata Sermia (KNS) -- the largest tidewater glacier in southwest Greenland -- during a period of cooling when the Norse had settlements in Greenland. Differing from glaciers that are strictly on land, tidewater glaciers extend and flow all the way to the ocean or a sea, where they can then calve and break up into icebergs.

Reconstructing the advance of glaciers can be exceptionally difficult, because the glacier typically destroys or reworks everything in its path as it advances forward. The research team undertook multiple field seasons in Greenland, traveling on foot to remote sites -- many of which hadn't been visited since the 1930s -- to try and uncover the record of KNS advance.

"When we went out into the field, we had absolutely no idea whether the evidence would be there or not, so I was incredibly nervous. Though we did a huge amount of planning beforehand, until you go out into the field you don't know what you're going to find," said James Lea, the other co-lead author of the study.

By traveling on foot, the research team was able to more closely examine and explore sites that otherwise may have been missed if traveling by helicopter. The team's planning paid off, and the sedimentary sequences they studied and sampled held the clues they were looking for to date and track the advance of the glacier.

The research team found that during the twelfth and thirteenth centuries CE, KNS advanced at least 15 km, at a rate of ~115 m/yr. This rate of advance is comparable to modern rates of glacial retreat observed over the past ~200 years, indicating that when climate is cooler glaciers can advance equally as fast as they are currently retreating. The glacier reached its maximum extent by 1761 CE during the Little Ice Age, culminating in a total advance of ~20 km. Since then, KNS has retreated ~23 km to its present position.

The period when the glacier was advancing coincided with when the Norse were present in Greenland. Prior to its maximum extent during the Little Ice Age, the researchers found that KNS advanced to a location within only 5 km of a Norse farmstead.

"Even though KNS was rapidly coming down the fjord, it did not seem to affect the Norse, which we found really unusual," said Pearce. "So the team started to think about the surrounding environment and the amount of iceberg production in the fjord during that time. At the moment, the fjord is completely filled with icebergs, making boat access challenging, and we know from historical record that it has been like this for the last 200 years while the glacier has been retreating. However, for KNS to advance at 115 m/yr, it needed to hang onto its ice and could not have been producing a lot of icebergs. So we actually think that the fjord would have looked very different with few icebergs, which allowed the Norse far more easy access to this site for farming, hunting, and fishing."

In the 1930s, archaeologists who visited the site hypothesized that conditions in the fjord must have been different from the present day in order for the Norse to have occupied the site, and this current research study provides data to support these long-held ideas.

"So we have this counterintuitive notion that climate cooling and glacier advance might have actually helped the Norse in this specific circumstance and allowed them to navigate more of the fjord more easily," said Lea.

The Norse left Greenland during the fifteenth century CE, and these results are consistent with the idea that a cooling climate was likely not the cause of their exodus; rather, a combination of economic factors likely led the Norse to abandon Greenland.

The results from this research reconstructing rapid glacial advance are also shown to be consistent with the ways ice sheet models work, which brings confidence to the projections from these models. Having accurate models and projections are crucial in understanding and preparing for future scenarios of continued retreat of the Greenland Ice Sheet and associated sea-level rise.

"Melt from Greenland not only impacts sea-level change but also the ecology around the ice sheets, fisheries, the biological productivity of the oceans -- how much algae is growing. And also because the types of glaciers we're looking at produce icebergs these can cause hazards to shipping and trade, especially if the Northwest Passage opens up as it is expected to," said James Lea.

Read more at Science Daily

Ice sheet retreat and forest expansion turned ancient subtropical drylands into oases

As human-caused greenhouse gas emissions continue to rise beyond limits for what our species has experienced, researchers are looking to a mystery in the past to answer questions about what may lay ahead.

This work, published today in Nature Communications by an international team of scientists, is part of a project called the 2nd Pliocene Model Intercomparison Project, or PlioMIP2.

The team focused on the climate of the Pliocene, over 3 million years ago, the last time Earth has seen concentrations of over 400 PPM CO2 in the atmosphere, similar to today's concentrations. The Pliocene prompts a long-standing question, says UConn Department of Geosciences researcher and lead author Ran Feng: despite the similarity to the present-day, why were dry areas like the Sahel in Africa and Northern China much wetter and greener in the Pliocene than they are today?

The Pliocene was warmer than present-day conditions by 2 to 3°C, and everything we know about the physics of the climate system suggests the Pliocene should have been drier in the subtropics, says co-author Tripti Bhattacharya, Thonis Family Professor of Earth and Environmental Sciences at Syracuse University.

"Our paper was motivated by a desire to understand this apparent discrepancy and see whether there are processes that can account for wetter Pliocene subtropics," Bhattacharya says.

The answer, the researchers found, is more complex than simply looking at CO2.

Evidence from the geologic record -- which includes a wide variety of sedimentary and paleobotanical indicators of past climate -- show that the Sahel and subtropical Eurasian regions were once home to lusher landscapes with drastically different hydroclimates. Along with proxy data, the team utilized a suite of the latest state-of-the-art model simulations to identify the factors responsible for subtropical rainfall changes in the Pliocene.

Previous studies suggest the only explanation for the Pliocene discrepancy was that there must be some mechanism unaccounted for in models to explain the Pliocene. However, to their surprise, the researchers found that current generation models perform well at simulating wet conditions on Pliocene subtropical continents.

"We discovered the hydroclimate in the dry areas like the Sahel and subtropical East Asia get much wetter when we prescribed vegetation and ice sheet changes in the Pliocene simulations," says Feng.

Feng explains this work is providing a new perspective when studying hydrological cycle responses to CO2 changes: long-term changes in terrestrial conditions like the shifting range of the biomes and the ice sheets are important.

"Continental greening and ice sheet retreat have profound impacts on the surface temperature through lowering the surface albedo -- the ability of the Earth's surface to reflect sunlight back to space -- and a profound effect on the hydrological cycle through allowing for greater evaporation and altering paths of moisture transport. In the long run, there's much bigger change in hydrological cycle, compared to what we are anticipating today," says Feng. "Currently, few of these changes is considered when predicting climate conditions for the next 10 years, or next 50 years."

This is cause for concern, says Feng, because changes in the Earth system's hydrological cycle will mean places already receiving excessive amounts of summer rainfall such as Southeastern Asia, Northern India, and West Africa, are going to see even more summer rainfall as continental greening increases and the ice sheets continue to recede.

Additionally, this work redefines the way we see the Pliocene climate, says Bhattacharya. "The other nice takeaway is that the Pliocene does not really challenge our fundamental understanding of the physics of climate. Our study suggests that we do not need exotic physical mechanisms to explain the Pliocene. Rather, we can explain regional patterns of change in aridity by including earth system feedbacks in models and considering the relationship between earth system sensitivity and rainfall changes. This ultimately increases our confidence that models do a good job at simulating the past and can be trusted to provide reliable projections of future climate."

Read more at Science Daily

Tilting of Earth’s crust governed the flow of ancient megafloods

As ice sheets began melting at the end of the last ice age, a series of cataclysmic floods called the Missoula megafloods scoured the landscape of eastern Washington, carving long, deep channels and towering cliffs through an area now known as the Channeled Scablands. They were among the largest known floods in Earth’s history, and geologists struggling to reconstruct them have now identified a crucial factor governing their flows.

In a study published February 14 in Proceedings of the National Academy of Sciences, researchers showed how the changing weight of the ice sheets would have caused the entire landscape to tilt, changing the course of the megafloods.

“People have been looking at high water marks and trying to reconstruct the size of these floods, but all of the estimates are based on looking at the present-day topography,” said lead author Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz. “This paper shows that the ice age topography would have been different over broad scales due to the deformation of Earth’s crust by the weight of the ice sheets.”

During the height of the last ice age, vast ice sheets covered much of North America. They began to melt after about 20,000 years ago, and the Missoula megafloods occurred between 18,000 and 15,500 years ago. Pico’s team studied how the changing weight of the ice sheets during this period would have tilted the topography of eastern Washington, changing how much water would flow into different channels during the floods.

Glacial Lake Missoula formed in western Montana when a lobe of the Cordilleran ice sheet dammed the Clark Fork valley in the Idaho panhandle and melt water built up behind the dam. Eventually the water got so deep that the ice dam began to float, resulting in a glacial outburst flood. After enough water had been released, the ice dam resettled and the lake refilled. This process is thought to have been repeated dozens of times over a period of several thousand years.

Downstream from glacial Lake Missoula, the Columbia River was dammed by another ice lobe, forming glacial Lake Columbia. When Lake Missoula’s outburst floods poured into Lake Columbia, the water spilled over to the south onto the eastern Washington plateau, eroding the landscape and creating the Channeled Scablands.

During this period, the deformation of the Earth’s crust in response to the growing and shrinking of ice sheets would have changed the elevation of the topography by hundreds of meters, Pico said. Her team incorporated these changes into flood models to investigate how the tilting of the landscape would have changed the routing of the megafloods and their erosional power in different channels.

“We used flood models to predict the velocity of the water and the erosional power in each channel, and compared that to what would be needed to erode basalt, the type of rock on that landscape,” Pico said.

They focused on two major channel systems, the Cheney-Palouse and Telford-Crab Creek tracts. Their results showed that earlier floods would have eroded both tracts, but that in later floods the flow would have been concentrated in the Telford-Crab Creek system.

“As the landscape tilted, it affected both where the water overflowed out of Lake Columbia and how water flowed in the channels, but the most important effect was on the spillover into those two tracts,” Pico said. “What’s intriguing is that the topography isn’t static, so we can’t just look at the topography of today to reconstruct the past.”

The findings provide a new perspective on this fascinating landscape, she said. Steep canyons hundreds of feet deep, dry falls, and giant potholes and ripple marks are among the many remarkable features etched into the landscape by the massive floods.

“When you are there in person, it’s crazy to think about the scale of the floods needed to carve those canyons, which are now dry,” Pico said. “There are also huge dry waterfalls—it’s a very striking landscape.”

She also noted that the oral histories of Native American tribes in this region include references to massive floods. “Scientists were not the first people to look at this,” Pico said. “People may even have been there to witness these floods.”

Read more at Science Daily

Antarctic ice-sheet destabilized within a decade

After the natural warming that followed the last Ice Age, there were repeated periods when masses of icebergs broke off from Antarctica into the Southern Ocean. A new data-model study led by the University of Bonn (Germany) now shows that it took only a decade to initiate this tipping point in the climate system, and that ice mass loss then continued for many centuries. Accompanying modeling studies suggest that today's accelerating Antarctic ice mass loss also represents such a tipping point, which could lead to irreversible and long-lasting ice retreat and global sea level rise. The study has now been published in the journal Nature Communications.

To understand what the consequences of current and future human-induced climate warming may be, it helps to take a look at the past: how did sea-level changes look like during times of natural climate warming? In a recent study, an international research team led by Dr. Michael Weber from the Institute of Geosciences at the University of Bonn investigated this question. In doing so, they focused on the Antarctic Ice Sheet as the largest remaining ice sheet on Earth.

There, they searched for evidence of icebergs that broke off the Antarctic continent, floated in the surrounding ocean and melted down in the major gateway to lower latitudes called "Iceberg Alley." In the process, the icebergs released encapsulated debris that accumulated on the ocean floor. The team took sediment cores from the deep ocean in 3.5 km water depth from the area, dated the natural climate archive and counted the ice-rafted debris.

The scientists identified eight phased with high amounts of debris which they interpret as retreat phases of the Antarctic Ice Sheet after the Last Glacial Maximum about 19,000 to 9,000 years ago, when climate warmed and Antarctica shed masses of icebergs repeatedly into the ocean. The result of the new data-model study: each such phase destabilized the ice sheet within a decade and contributed to global sea-level rise for centuries to a millennium. The subsequent re-stabilization was equally rapidly within a decade.

The research team found three other independent pieces of evidence for such post-glacial tipping points: Model experiments showing the melting of the entire Antarctic ice sheet, a West Antarctic ice core documenting ice-sheet elevation draw-down and drill cores revealing a step-wise ice-sheet retreat across the Ross Sea shelf.

Today's ice mass loss could be start of long-lasting period

The results are also relevant for ice retreat observed today: "Our findings are consistent with a growing body of evidence suggesting the acceleration of Antarctic ice-mass loss in recent decades may mark the begin of a self-sustaining and irreversible period of ice sheet retreat and substantial global sea level rise," says study leader Dr. Michael Weber from the University of Bonn.

Combining the sediment record with computer models of ice sheet behaviour the team showed that each episode of increased iceberg calving reflected increased loss of ice from the interior of the ice sheet, not just changes in the already-floating ice shelves. "We found that iceberg calving events on multi-year time scales were synchronous with discharge of grounded ice from the Antarctic Ice Sheet," said Prof. Nick Golledge from the University of Wellington (New Zealand), who led the ice-sheet modelling.

Dr. Zoë Thomas, a co-author of the study from the University of New South Wales in Sydney, Australia, then applied statistical methods to the model outputs to see if early warning signs could be detected for tipping points in the ice sheet system. Her analyses confirmed that tipping points did indeed exist. "If it just takes one decade to tip a system like this, that's actually quite scary because if the Antarctic Ice Sheet behaves in future like it did in the past, we must be experiencing the tipping right now," Thomas said.

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Greenland’s groundwater changes with thinning ice sheet

For more than a decade, a team of University of Montana researchers and students have studied the dynamics of the Greenland Ice Sheet as it responds to a warming climate. Department of Geosciences researchers Toby Meierbachtol and Joel Harper said water has always been central to their research.

"The water from melting of the ice can run off the surface to the ocean and contribute to sea level rise, it can refreeze in place and actually warm the ice, and it can even reach the bottom of the ice sheet and act as a sort of lubricant to make the ice slide quickly over its bed," Meierbachtol said. "The importance of water in controlling the response of Greenland to warming is hard to overstate."

But while much of their focus has been on the importance of water in controlling processes occurring on the ice sheet, their most recent research findings have flipped the order of their thinking.

As outlined in their recent article in Nature Geoscience, Meierbachtol, Harper and an international team of researchers discovered that changes to the ice sheet have an immediate impact on the groundwater underlying the Greenland island, an area larger than the state of Alaska.

"We have been focused on water's impacts on ice sheet change," said Harper. "But our most recent findings show that changes in the ice sheet have a real impact on Arctic hydrology -- specifically the massive groundwater system extending under the ice sheet."

This latest revelation occurred thanks to a marriage of drilling techniques, with international collaborators boring an angled hole 650 meters through bedrock underneath a Greenland glacier to measure groundwater conditions deep under the ice sheet. Meanwhile, UM and University of Wyoming researchers drilled 32 holes from atop the glacier, through nearly a kilometer of ice, to measure water conditions at the interface between ice and bedrock, which forms an important boundary controlling groundwater flow below.

The system that UM has perfected over the years involves drilling with a combination of very hot water under high pressure typically for 12 or more hours at a time.

"We practice and rehearse to make the operation flow smoothly," Harper said, noting they always include one to two undergraduate students on an expedition. "Everyone on the team has an important and specific role to fill."

After drilling the team installs sensors in the ice column and at the ice sheet bed to measure ice dynamics and water conditions as water flows under the ice to margin. Time is always of the essence because the cold ice freezes the hole shut in as little as two hours.

The dual drilling approach facilitated the first-ever measurements of groundwater response to a changing ice sheet, and the eight-year data record yielded some unexpected results.

"By studying areas that were covered by ice 10,000 years ago during the last ice age, the field has known that the huge mass and vast amounts of water from melting ice can impact the underlying groundwater," Meierbachtol said, "but the paradigm has been that the groundwater response to ice sheet change is long: thousands of years. What we've shown here is that the groundwater response to Greenland's change is immediate."

This new understanding could have important downstream implications for how Greenland's thinning impacts the Arctic, Harper said. The thinning ice could reduce the rate of groundwater flow to the ocean, changing the water temperature and salinity balance that is important for ocean circulation patterns.

"In thinking about the complex feedbacks that occur from Greenland's ongoing change, we as a field have really neglected the groundwater component because we thought it was more or less dormant over the decade to century timescales that are important for us as a society," Harper said. "But now we recognize that the groundwater system actually changes quite rapidly, and there are some compelling reasons for why this could really matter for the broader Arctic."

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Greenland ice sheet faces irreversible melting

 

Illustration of northern Canada and Greenland from space.

In a study published this week in The Cryosphere, researchers from the National Centre for Atmospheric Science and University of Reading demonstrate how climate change could lead to irreversible sea level rise as temperatures continue to rise and the Greenland ice sheet continues to decline.

The massive ice sheet faces a point of no return, beyond which it will no longer fully regrow, permanently changing sea levels around the world.

The Greenland ice sheet is seven times the area of the UK, and stores a large amount of the Earth's frozen water. At current rates of melting, it contributes almost 1mm to sea level per year, and accounts for around a quarter of total sea level rise.

Since 2003, despite seasonal periods of growth, Greenland's ice sheet has lost three and a half trillion tonnes of ice.

Rising sea levels are one of the most severe effects of climate change, threatening coastal areas around the world, and putting millions of people who live in low-lying areas at risk. Bangladesh, Florida, and eastern England are among many areas known to be particularly vulnerable.

Under scenarios in which global warming goes beyond 2°C, the Paris Agreement target, we should expect significant ice loss and several metres of global sea level rise to persist for tens of thousands of years, according to the new research. The warmer the climate, the greater the sea-level rise.

In addition, even if temperatures later return to current levels, scientists have shown that the Greenland ice sheet will never fully regrow once it melts beyond a critical point. After that point, sea levels would permanently remain two meters higher than now, regardless of other factors contributing to sea level rise.

This is because the ice sheet is so large that it has a substantial impact on its local climate, and as it declines, Greenland would experience warmer temperatures and less snowfall.

Once the ice-sheet retreats from the Northern part of the island, the area would remain ice-free.

To avoid the irreversible sea level rise the melting would cause, scientists say that climate change must be reversed before the ice sheet has declined to the threshold mass, which would be reached in about 600 years at the highest rate of mass loss within the likely range of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

Professor Jonathan Gregory, Climate Scientist from the National Centre for Atmospheric Science and University of Reading, said: "Our experiments underline the importance of mitigating global temperature rise. To avoid partially irreversible loss of the ice sheet, climate change must be reversed -- not just stabilised -- before we reach the critical point where the ice sheet has declined too far."

To study the ice-sheet, scientists from the National Centre for Atmospheric Science simulated the effects of Greenland ice sheet melting under a range of possible temperature rises, ranging from minimal warming to worst-case scenarios.

Under all future climates like the present or warmer, the ice-sheet declined in size and contributed to some degree of sea-level rise.

Importantly, there were scenarios in which the ice sheet melting could be reversed. But, they rely on actions to counteract global warming before it's too late.

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