Bursting bubbles increase ice melt

Glacial ice melts surprisingly quickly when it comes into contact with water. © Oregon State University

Why are glaciers flowing into the sea melting faster than models predict? Apparently, one important factor has not been taken into account so far, according to a study: the experiments show that the pressurized bubbles in the glacier ice intensively intensify the thawing process in the water when they burst. This aspect should now inform forecasts of sea-level-raising retreat from the world's massive tidal glaciers, the scientists say.

Ever warmer water washes around the ice masses that protrude from the land masses near the poles into the sea: climate change is leading to a strong retreat of the so-called tidal glaciers in Greenland, the Antarctic Peninsula and other glaciated regions of the world. The melting effect of the increasing heat is clear - but the extent seems surprising: The actual melting rates of the glaciers that flow into the sea are higher than predicted by models of the process based on previous data. However, they are mostly based on the results of investigations into the melting processes of laboratory ice in the water, which does not exactly correspond to the characteristics of glacial ice.

Glacial ice is special

Scientists led by Meagan Wengrove from Oregon State University in Corvallis have now devoted themselves to this aspect. As they explain, the key point is that glacial ice is not formed by the freezing of water, but by the compaction of snow. When the flakes are stacked on top of each other, air is trapped between the crystals, from which small bubbles ultimately form. Glacial ice typically has around 200 of these tiny structures per cubic centimeter. As the layers grow deeper and deeper into the glacier, they become increasingly squashed, which causes them to build up pressure. Actually, all of this was already known and also that the charged bubbles are clearly noticeable when they come into contact with water: they produce loud popping noises when they burst when they reach the boundary surfaces.

Sparkling enhanced ice melt

But so far no one has looked into the possible role of these structures in ice melting. "It wasn't until we started talking about the physics of the process that we realized that there's potentially a lot more to these bubbles than just making noise underwater," says Wengrove. To investigate the suspicion, the researchers carried out laboratory studies of the physical processes involved in melting different forms of ice and examined what effects this has on melting rates. Glacier ice with its “pressure bubbles” was also used. “There can be very high pressure in the tiny bubbles,” says co-author Erin Pettit from Oregon State University. “Sometimes it reaches up to 20 atmospheres – 20 times the normal pressure at sea level.”

As the scientists report, their results now document the presumably significant importance of the bubbles in the melting processes. Accordingly, these cavities in glacier ice can even lead to a melting rate that is approximately twice as fast as bubble-free ice. As the researchers explain, in addition to buoyancy effects, this is primarily due to the physical effects of the release of the gas: "The explosive bursting of the bubbles gives the icy boundary layer additional energy when it melts," says Wengrove. Ultimately, the researchers conclude that the bubbles could be largely responsible for the difference between the observed and predicted melting rates of tidal glaciers.

According to them, this previously overlooked aspect should now be integrated into the models of the future development of tidal glaciers. A more accurate characterization of the way ice melts can lead to better predictions of how quickly the ice will retreat as climate change continues. In turn, this information is of great importance, because the melting of the huge ice masses of the polar tidal glaciers can have a significant impact on global sea level rise. "The small bubbles could thus play a major role in understanding critical future scenarios," concludes Pettit.

Source: Oregon State University

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