How icebergs ‘really’ melt?

Icebergs are melting faster than current models describe, according to a new study at the University of Sydney. The researchers have proposed a new model to more accurately represent the melting speed of icebergs into oceans.

• Today, several million icebergs are floating on the oceans of all sizes and shapes. They are formed in the Arctic and Antarctic and, once detached, follow the currents (ocean) until they melt and disappear.
• Iceberg melt is responsible for about half the freshwater entering the ocean from the Antarctic and Greenland ice sheets.
• However, current models wrongly assume icebergs melt uniformly.
• Thus, accurately modeling how it enters is important for understanding the potential impact on ocean circulation.

Understanding the geography of Icebergs

• Icebergs are pieces of ice that formed on land and float in an ocean or lake.
• Icebergs come in all shapes and sizes, from ice-cube-sized chunks to ice islands the size of a small country.
  • The term "iceberg" refers to chunks of ice larger than 5 meters (16 feet) across.
  • Smaller icebergs are known as bergy bitsand growlers. They can be especially dangerous for ships because they are harder to spot.
• The North Atlantic and the cold waters surrounding Antarctica are home to most of the icebergs on Earth.

How do they form? Where do they go?

• Icebergs calve from glaciers when direct sunlight or rising air temperature causes the glacier's surface ice to become more brittle.
• Once an iceberg has calved off a glacier it is already floating and, by the principle known since Archimedes, raises the oceans' volume.  

• Icebergs travel with ocean currents, sometimes smashing up against the shore or getting caught in shallow waters.
• When an iceberg reaches warm waters, the new climate attacks it from all sides.
• On the iceberg surface, warm air melts snow and ice into pools called melt pondsthat can trickle through the iceberg and widen cracks.
• At the same time, warm water laps at the iceberg edges, melting the ice and causing chunks of ice to break off.
• On the underside, warmer waters melt the iceberg from the bottom up.
• Impacts: Icebergs have two main impacts on climate
  • Iceberg production affects the mass balance of the parent ice sheets
  • melting icebergs influence both ocean structure and global sea level

What were the assumptions made in the current model?

• Current models, which are incorporated into the methodology used by the Intergovernmental Panel on Climate Change, assume that icebergs melt uniformly in ocean currents.
  • The IPCC is the gold standard for climate science.
• However, this recent research has shown that icebergs do not melt uniformly and melt at different speeds depending on their shape.

What is the proposed method?

The results, published in Physical Review Fluids, have implications for oceanographers and climate scientists.

• The new research proposes a very simple model that accounts for iceberg shape, as a prototype for an improved model of iceberg melting.
• To test these models, the researchers developed the first realistic small-scale simulations of melting ice in saltwater.
• The old models assumed that stationary icebergs didn't melt at all, whereas the latest experiments show melting of about a millimeter every minute.
• In icebergs moving in oceans, the melting on the base can be up to 30 percent faster than in old models.
• The research shows that iceberg shape is important. Given that the sides melt faster, wide icebergs melt more slowly but smaller, narrower icebergs melt faster.
• The sides of icebergs melt about twice as fast as their base.
• For icebergs that are moving in the ocean, melting at the front can be three or four times faster than what the old models predicted.

Conclusion

While icebergs are only one part of the global climate system, the improved model provides scientists with a dial that they can tune to better capture the reality of Earth's changing climate.

These methods can also be applied to many other systems, including glaciers melting or the melting of frozen, saline sea ice. Moreover, these methods could be used by astrobiologists to better understand ice moons like Saturn's Enceladus, a candidate for finding life elsewhere in the Solar System.