New experiments with ice blocks have revealed that icebergs melt faster on their sides. The discovery paves the way for better models of melting that consider the varied shapes of icebergs. The research could also improve our understanding of the role of iceberg melting in climate change.
Icebergs are a major source of freshwater flowing into some parts of the oceans; the Greenland ice sheet alone releases about 550 Gt of icebergs per year. This affects water salinity, which in turn affects water circulation and the global climate. The production of icebergs by both the Greenland and Antarctic ice sheets has been increasing significantly because of climate change so understanding the melting rate of icebergs is crucial for predicting changes in the oceanic heat flux.
Icebergs come in various shapes and sizes, with the largest recorded iceberg spanning almost 300 km in length and 40 km in width. They melt due to solar radiation, subsurface interaction with seawater and breaking into smaller pieces. Models used to predict the melting of icebergs were initially developed in the 1970s to study the possibility of towing icebergs to arid regions to provide an economical source of freshwater. These early models ignored the shape of icebergs and assumed constant water movement – and both assumptions have been carried through subsequent research.
Irregular melting
Icebergs are classified by an “aspect ratio” – the ratio of their length to submerged depth. New research by scientists in Australia, New Zealand, the US, and France led by Eric Hester, a PhD student of Applied Mathematics at the University of Sydney, has shown that the melting rate strongly depends on an iceberg’s geometry.
In their small-scale experimental simulation, the team submerged large rectangular ice blocks containing blue dye into a tank with circulating salt water and left them to melt for 10 min. The ice blocks were then weighed and photographed to assess the melting of each side.
“We put dye in the ice to observe where the melting occurs. We observed that at the front, the ice started to slope and melted three times faster. The bottom started to melt preferentially in the middle,” Hester tells Physics World.
Varying water velocity
In calculating melting rates, the team held a depth of 3 cm and varied the water velocities. At the highest water flow of 3.5 cm/s, the melting of the sides was roughly twice as fast as the base. If an iceberg moves in an ocean, the front face could be melting up to three times faster than predicted by old models.
Considering the varying size and geometry of real icebergs, their aspect ratio can affect overall melting by changing the areas exposed to water, suggesting that wider icebergs melt slowly, whereas smaller icebergs could melt up to 50% faster because they have more surface area on their sides.
Following their experimental findings, the team compared their results against numerical simulations assessing the flow dynamics.
“We developed a mathematical model to simulate the flow of warm salt water around the melting iceberg,” explains Hester.
Considering different parameters, such as temperature, salinity, and pushing forces, the team observed the same melting behaviour as in the experiment; the front of the ice block was melting the fastest, with a middle part melting faster on the base.
Difficult to observe in the field
According to Ellyn Enderlin of the Geoscience Department at Boise State University, who was not involved in this study, “Iceberg melting is something that cannot really be observed in the field – at best we can use sonar to map iceberg geometries at discrete time steps and back-out melting from the change in geometry – so these experiments tell us a lot about spatial variations in iceberg melting and how both water shear and iceberg geometry influence iceberg melt rates”.
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“The revised parameterizations that [Hester and colleagues] present could be implemented into numerical models that include iceberg melting as a freshwater flux, allowing us to more accurately account for this important source of freshwater and assess the influence of variations in iceberg melt fluxes on fjord water masses. Given the strong dependence of melt rates on iceberg geometry and the changes in iceberg geometry that often accompany rapid glacier change, it is important that the influence of geometry is accounted for in the model parameterizations,” says Enderlin.
Apart from predicting climate change impacts of melting icebergs, these results can be extended to modelling melting glaciers. In addition, improved ice melting models could be used to understand the dynamics of extraterrestrial ice sheets, such as those on Saturn’s moon Enceladus or Jupiter’s moon Europa.
The research is described in Physical Review Fluids.
