Each year, approximately 170 billion tons of ice melts from the Greenland Ice Sheet, yet the exact processes behind this melting remain unclear. Previous studies have indicated that surface meltwater accumulates on glaciers due to warm air and sunlight, flowing through cracks in the glacier and out to sea.
However, some scientists argue that this model oversimplifies the situation by assuming that water only flows downward, neglecting the impact of temperature on water movement. They suggest that such simplified models are inadequate for accurately predicting future ice sheet behavior.
To address this, a group of researchers investigated how subglacial lakes beneath the ice sheet control water movement across Greenland. They monitored a newly discovered subglacial lake using high-resolution satellite images from 2012 to 2019, along with 3D surface maps. Their focus was on the period between July 22 and August 1, 2014, during a suspected drainage event of the subglacial lake, similar to a water balloon bursting under pressure.
They analyzed images from the Greenland Ice Sheet, including data from Landsat-8, 3D surface maps from Polar Geospatial Center, and information from ICESat and ICESat-2. They identified a 2-square-kilometer (0.8-square-mile) area of ice that had risen 10 to 15 meters (about 30 to 50 feet) in height, forming a dome on the ice sheet surface. This dome likely developed as a large lake formed underneath the ice, pushing it upwards.
The researchers noted that the dome began to collapse on July 22, 2014, falling 85 meters (about 280 feet) over the next 10 days to form a basin. From the dome’s dimensions, they estimated that about 90 million cubic meters (or roughly 3 billion cubic feet) of water drained from the lake, averaging 100 cubic meters (around 3,500 cubic feet) per second—equivalent to draining 36,000 Olympic swimming pools at a rate of one pool every 25 seconds.
Additionally, they discovered a 40-meter-high (130-foot) block of ice that was displaced about 1 kilometer (0.6 miles) downstream during the collapse, alongside 6 square kilometers (about 2 square miles) of smooth ice. These features likely formed when water surged through the ice, flowed over the surface, and re-entered the ice sheet.
The researchers then utilized data from Landsat 5, Landsat 9, National Snow and Ice Data Center, and United States Geological Survey to show that the drainage event also influenced the surrounding environment. Once the water re-entered the ice sheet, it flowed downstream beneath Harding Glacier. This rapid influx of water lowered the pressure at the glacier’s base, slowing its movement and causing 500 to 600 meters (approximately 1,600 to 2,000 feet) of ice to shear off its edge.
Researchers hypothesized that as the ice sheet froze, subglacial water rose to the surface instead of sinking into the bedrock. To explore this, they employed a computer-generated thermal model to simulate the temperature at the base of the ice sheet, entering various anticipated rock temperatures and ice thicknesses. The results showed that all simulations maintained the base temperature below -5°C (23°F). At such frigid temperatures, the ice would freeze to the bedrock before subglacial water could flow out, necessitating upward movement of the water.
These findings led researchers to develop a new conceptual model for meltwater movement in glaciers. Initially, surface ice melts and flows into the subglacial lake. As the meltwater collects, pressure at the ice sheet’s base builds, creating a dome. A drainage event results in the dome’s collapse. As water descends through the glacier, the ice freezes to bedrock, preventing water from reaching the glacier’s bottom, causing it to move upward, break the surface, and eventually re-enter the glacier and flow toward the ocean.
In conclusion, the interconnected nature of water movement above, through, and below glaciers can weaken ice sheet structures and alter glacier dynamics downstream. This study underscores the importance of considering the processes contributing to glacier ice loss.
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Source: sciworthy.com
