There are clues to the behavior in the near future of a warming Greenland, and possibly even a warming Antarctica, buried under the North Sea.
Look below its muddy bottoms and you will find huge valleys. Hundreds of them.
They were carved by rivers of meltwater flowing beneath the ice sheet that covered northern Europe near the end of Earth’s last great cold phase.
We’ve known about these valleys (often called “tunnel valleys” because they’ve been incised beneath the ice sheet) for some time, but it’s only in recent years that their true scale has become apparent.
Modern seismic data (sound waves) collected by oil and gas prospectors has brought new clarity that has allowed scientists to investigate hidden features in greater detail.
In places, the channels are more like canyons – up to 150 km long, 6 km wide and 500 m deep.
The new advance is to describe the erosional processes involved in cutting the valleys, the rate at which this happened and the amount of water that could be drained by them.
We are talking about discharges of 900 cubic meters per second in the largest channels. To put this into context, English rivers such as the Thames, Severn and Trent could only exceed 100 m3/s in a flood.
Researchers from the British Antarctic Survey (BAS) worked with a team led by the University of Sheffield to decipher the development of the valleys as the northern ice sheet advanced and then retreated, around 27,000 to 19,000 years ago.
The Sheffield team spent a decade determining the precise extent of the ice in a remarkable project called Britice-Chrono.
The BAS team was able to use this information, along with high-resolution seismic imagery, to model channel evolution.
“We started to see these smaller channels at the base of the very large tunnel valleys, which seemed to meander and migrate over time,” explained BAS PhD student James Kirkham.
“We calculated, basically, that when you force all that water through these little channels, they can be extremely erosive. You end up forming the larger channels in just a few hundred years. It’s really, really fast. on ice sheet timescales.
Spilling large volumes of water under an ice cap can have implications for its stability. If the water is spread out, it can lubricate the flow, potentially helping the sheet collapse – with all that means for sea level rise.
On the other hand, if this water can be expelled quickly in discreet channels, it could allow the ice sheet to sit more firmly on the bedrock, to stabilize it.
These are the scenarios currently being considered for the future of the Greenland Ice Sheet.
Already, it is experiencing large-scale surface melting in the summer.
Water accumulates in many ponds before flowing to the bed and escaping under the ice to the ocean.
If, in the decades to come, similar valleys are carved under Greenland as those that were incised under the ancient ice sheet of northern Europe, how will this affect the future stability of Greenland?
“There is debate about whether these rapidly forming channels will accelerate or stabilize ice retreat in a warming world. What we do know, however, is that currently these processes are not are not integrated into our ice sheet models at all,” BAS said. geophysicist Dr Kelly Hogan.
“Here we are looking at a snapshot of what could happen in Greenland as it gets hotter and hotter. Will the ice go much faster or will it slow down? We have to put what we have learned in the North Sea in models to see what that does to ice dynamics.”
Circumstances in Antarctica, the largest ice cap on Earth, are somewhat different from those in Greenland. Certainly, in the short and medium term.
Losses of ice in the polar south are largely due to incursions of warm ocean waters at the margins of the ice sheet. Warmer air causing melting and pooling on the surface, as seen in Greenland, is evident in a few places but is less of a factor. That could change, of course, if temperatures continue to rise globally.
Kirkham, Hogan and others have published their investigations of the North Sea Tunnel Valleys in Quaternary Science Reviews.
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