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A huge flood triggered by the rapid draining of a lake beneath the Greenland ice sheet occurred with such force that it fractured the ice above and burst out across its surface.

This phenomenon, observed for the first time in Greenland and detailed in research published in Nature Geoscience, sheds new light on the destructive potential of meltwater stored beneath the ice sheet.

It reveals how, under extreme conditions, water flooding underneath the ice can force its way upwards through the ice and escape at the ice sheet surface.

This phenomenon is not considered by numerical models that aim to predict the future evolution of the Greenland ice sheet, and this new work raises questions about whether this type of mechanism deserves greater attention in the future.

The international team of researchers led by scientists at Lancaster University's Center of Excellence in Environmental Data Science and the UK Center for Polar Observation and Modeling, studied a previously undetected lake beneath the ice sheet (known as a subglacial lake) in a remote region of northern Greenland, using state-of-the-art satellite data and numerical models.

Using detailed three-dimensional representations of the ice sheet surface from the ArcticDEM project, alongside data from a number of European Space Agency (ESA) and NASA satellite missions, they monitored the sudden drainage of this lake.

The researchers discovered that over a period of 10 days in summer 2014, an 85-meter-deep crater appeared across a 2 km² area on the ice surface, as 90 million cubic meters of water flooded out of the underlying lake.

This roughly equates to nine hours of water gushing over the Niagara Falls during its peak season, and represents one of the largest Greenland subglacial floods in recorded history.

However, what the researchers found further downstream was even more surprising.

In a region of previously unblemished ice, they observed the sudden appearance of an area the size of around 54 football pitches (385,000 square meters) of fractured and distorted ice, comprising deep cracks and 25 m-high uprooted ice blocks, together with a freshly water-scoured ice surface around twice the size of New York's Central Park (six square kilometers).

Lead author Dr. Jade Bowling, who led this work as part of her Ph.D. at Lancaster University, said, "When we first saw this, because it was so unexpected, we thought there was an issue with our data. However, as we went deeper into our analysis, it became clear that what we were observing was the aftermath of a huge flood of water escaping from underneath the ice.

"The existence of subglacial lakes beneath the Greenland ice sheet is still a relatively recent discovery, and—as our study shows—there is still much we don't know about how they evolve and how they can impact on the ice sheet system.

"Importantly, our work demonstrates the need to better understand how often they drain, and, critically, what the consequences are for the surrounding ice sheet."

Although it had been previously assumed that meltwater flows from the surface to the base of the ice sheet, and then onwards to the ocean, this research provides clear evidence that water can also travel upwards, in the opposite direction.

It also surprised the scientists to find that the flood occurred in a region where models predicted that the ice was frozen at the bed, leading the researchers to propose a mechanism whereby pressure-driven fracturing of ice along the ice bed created a pathway for the water to then flow.

These mechanisms are not considered by the models that aim to simulate how the ice sheet might evolve in the future, as Earth's climate warms and the ice sheets experience increasing rates of melting.

As such, these discoveries highlight the complexity of water flow, and the need to better understand how the ice sheet responds to extreme inputs of meltwater; something that is likely to become more common as our climate warms, and surface melting intensifies and expands into new areas.

Prof. Mal McMillan, co-director of the Center of Excellence in Environmental Data Science at Lancaster University, and co-director of science at the UK Center for Polar Observation and Modeling, said, "This research demonstrates the unique value of long-term satellite measurements of Earth's polar ice sheets, which—due to their vast size—would otherwise be impossible to monitor.

"Satellites represent an essential tool for monitoring the impacts of climate change, and provide critical information to build realistic models of how our planet may change in the future. This is something that all of us depend upon for building societal resilience and mitigating the impacts of climate change."

Dr. Amber Leeson, reader in glaciology at Lancaster University and an expert in ice sheet hydrology, observed, "What we have found in this study surprised us in many ways. It has taught us new and unexpected things about the way that ice sheets can respond to extreme inputs of surface meltwater, and emphasized the need to better understand the ice sheet's complex hydrological system, both now and in the future.

"Given the control that subglacial hydrology has on the dynamics of the ice sheet, it is critical that we continue to improve our understanding of these hidden, and poorly understood, hydrological processes, and these satellite observations are key to that."