Antarctica's glacial border migrates for miles with the tide.
"We typically think of ice sheet change as being very slow, taking place over decades, centuries, or even millennia. But our findings highlight that some processes are operating over minutes to hours that may have significant impacts," Bryony Freer, Lead author of the British Antarctic Survey study and glaciologist at the Centre for Satellite Data in Environmental Science at the University of Leeds.
Antarcica's grounding lines are the boundary point where the land ice meets the floating ice shelf. The Ronne ice shelf is the larger area of the Ronne-Filchner ice shelf, the second largest in Antarctica; only the Ross ice shelf is larger. British Antarctic Survey glaciologists have been monitoring a 170-mile-long chunk of the ice shelf in the far South of the Ronne shelf located in the warming Weddell Sea for five years. Using lasers from the ICESat 2 satellite that can measure the ice height up to mere inches, the team determined how the floating Ronne ice shelf rose and fell with the tides. That information was used to calculate the changing position of the grounding line.
From the British Antarctic Survey:
The 15 km (six mile) shift in the grounding line position between high and low tide described in the new paper is the one of the largest observed anywhere in Antarctica. It shows the grounding line can move at more than 30 km ( 18.64 miles) per hour, flushing ocean water several kilometres further inland under the ice sheet.
This exposure to sea water could help the ice melt more quickly from below. In less stable Antarctic regions, such as the Thwaites Glacier, this process is known to have driven long-term historic grounding line retreat.
Grounding line movement depends on the tidal range, the shape of the seafloor and the strength of the ice. The new study found the grounding line in some regions moved inland much faster during a rising tide than it later returned as the tide dropped — a particularly exciting finding according to the researchers. This is because it suggests that sea water may become trapped under the ice as the grounding line readvances and so takes longer to be flushed out, perhaps increasing the rate at which the ice sheet melts from below.
“It’s vital that we improve both our observations and modelling of these tidal processes, to better understand how they operate and work out the likely implications for long-term ice sheet change,” Freer says.
For further reading, Irreversible ocean warming threatens the Filchner-Ronne Ice Shelf
The warming and thinning of the Weddell Sea have been covered on this site a few times, most recently by FishOutofWater.
The British Antarctic Survey on the missing sea ice in 2023.
“Wind patterns, storms, ocean currents and air and ocean temperatures all affect how much of the sea around Antarctica is covered by ice, and they often push and pull in different directions. This means it can be hard to link the behaviour of Antarctic sea ice in any particular year, or over several years, to just one factor. “Before 2015, contrasting trends in sea ice growth in different regions of the vast continent mostly counterbalanced each other. What’s remarkable about 2023 is that these regional differences are largely absent.” Dr. Caroline Holmes, BACAntarctic sea ice has been quite stable in its average extent over that period – until 2016, when it began to decline. Since 2016, there have been seen several record summer lows, with Antarctic summers 2021/22 and 2022/23 setting new sea ice minimas.
At the start of August 2023, the depths of Antarctic winter, deviation from all previous records has intensified. As of August 2023, the sea ice extent is almost 2.4 million km2 lower than the 1979-2022 average – a missing area around ten times the size of the UK.
However, the spinning up of the winds around Antarctica causes the ocean currents to spin up as well. One of the impacts of low pressure areas is that they spin up cold water from below as they spin water out of the region of low pressure. It’s a process that ocean scientists call Ekman upwelling. The whole southern ocean south of the roaring fifties is a low pressure area when averaged over months or years. Therefor the tightening of the winds and currents around Antarctica has led to increased rates of upwelling of water around Antarctica.
Rapid warming of the upper 300m — one thousand feet — of the southern ocean has been observed around since 2016. Antarctic sea ice has been in precipitous decline since 2016.
The intermediate water that is welling up under the sea ice is a mixture of relatively warm water that originated in the north Atlantic’s Atlantic Meridional Overturning Circulation (AMOC) with a melange of water mixed by the eddies and currents ringing Antarctica. The rising up of these waters is heating the southern ocean and Antarctic sea ice from below.
There is good news in this. The rising up of these waters that originated in the far north Atlantic completes the upper ocean overturning circulation that helps to warm Europe. This completion of the cycle helps prevent the collapse of the “Gulf Stream” and the AMOC because it prevents the stagnation of the southern half of the circulation.
However, the bad news is that this warms the waters around Antarctica, melting both glaciers and sea ice.
He addresses the implications of this combination of winds and the missing sea ice.
The implications are staggering. The Australian scientists coldly discussed the impacts. They include the decline of the formation of the coldest water in the world oceans — Antarctic bottom water. They include increased melting from below of Antarctica’s glaciers and subsequent sea level rise. They include the collapse of colonies of penguins that depend on sea ice. They include the stagnation of the waters in the deepest parts of the ocean, sequestering nutrients from marine life.
Speaking of penguins, as many of you know, there was a mass mortality of Emperor Penguin chicks due to sea ice loss, where hatchlings drowned or froze to death. In those impacted breeding colonies, the entire generation was lost. They had not yet grown their waterproof feathers and quickly drowned or froze to death once wet. The bad news was it was believed that 90% of the total population would die as Penguin nurseries were exposed to increased melt and rainfall.
There is good news for the species, at least temporarily, for the next few decades. Previously, it was thought that Emperors only returned to the colonies where they were hatched. It is now believed that the Emperors are more adaptable and flexible in relocating the nurseries if needed.
Sara Labrousse of the French Polar Institute writes in the Conversation:
A study based on satellite images shows that sea ice broke out early in Antarctica’s Bellingshausen Sea in 2022, potentially resulting in breeding failures across several Emperor penguin colonies in that region.
Our research shows Emperors form colonies in surprisingly diverse environmental conditions that vary depending on location around the continent. Within each of these regions, there is little difference between where birds make their homes and other sites, suggesting they could shift if they had to. This provides a ray of hope in an otherwise bleak outlook.
Emperor penguins may be the only birds to rarely set foot on land. They are unique among penguin species in that they breed on sea ice during the harsh Antarctic winter.
An extreme heatwave took place in East Antarctica in March 2022, which registered the most anomalous temperatures above local climatology ever recorded. The heatwave resulted from a highly unusual weather pattern which produced strong northerly winds and imported warm and moist air from Australia. Weather forecast models skillfully predicted the heatwave up to 8 days in advance. While the heatwave took place soon after the record sea ice minimum of February, Southern Ocean sea surface temperature anomalies had a minimal impact on the magnitude of the heatwave. We have found that a widely used climate model cannot simulate heatwaves of this magnitude, but when the model's winds in the free atmosphere are nudged toward observations, the model can simulate a heatwave closer to observations, suggesting model improvements in atmospheric circulation variability would lead to better heatwave simulation. To address the impact of climate change, we have re-run the model simulations, nudging to the same winds but under past and future anthropogenic forcing. We find that the heatwave was made 2°C warmer by climate change, and future end of century heatwaves to be 5–6°C warmer, suggesting the possibility of near-melting temperatures over the East Antarctic ice cap during extreme heatwaves.