Applied theory offers new insights into sea ice thermal conductivity

A new applied mathematical theory could improve our understanding of how sea ice affects global climate, potentially improving the accuracy of climate predictions.

The authors of a new article published in the Proceedings of the Royal Society A: Mathematical and Physical Sciencesoffer new insights into how heat moves through sea ice, a crucial factor in regulating Earth’s polar climate.

Dr Noa Kraitzman, a lecturer in applied mathematics at Macquarie University and lead author of the study, says the research fills a key gap in current climate modelling.

“Sea ice covers about 15% of the ocean surface during the coldest season, when it is most abundant,” says Dr. Kraitzman. “It’s a thin layer that separates the atmosphere from the ocean and is responsible for transferring heat between the two.”

Sea ice acts as an insulating blanket over the ocean, reflecting sunlight and moderating heat exchange. As global temperatures rise, understanding sea ice behavior becomes increasingly important for predicting climate change.

The study focuses on the thermal conductivity of sea ice, a key parameter used in many global climate models. The movement of liquid brine in sea ice, which can potentially increase its heat transport, was not considered in previous models.

According to Dr Kraitzman, the unique structure of sea ice, as well as its sensitive dependence on temperature and salinity, means that it is difficult to measure and predict its properties, particularly its thermal conductivity.

“When you look at sea ice on a small scale, what makes it interesting is its complex structure, because it’s made up of ice, air bubbles and brine inclusions.

“As the atmosphere above the ocean becomes extremely cold, below minus 30 degrees Celsius, while the ocean water remains at about minus two degrees, this creates a large temperature difference and the water freezes from top to bottom.

“As the water rapidly freezes, it expels the salt, creating a matrix of pure frozen water ice that traps air bubbles and pockets of very salty water, called brine inclusions, surrounded by nearly pure ice.”

These dense brine inclusions are heavier than fresh ocean water, causing convective flow within the ice, creating large “chimneys” through which liquid salt flows.

This study builds on previous work by Trodahl in 1999, which first suggested that fluid flow in sea ice could enhance its thermal conductivity. Dr Kraitzman’s team has now provided mathematical proof of this phenomenon.

“Our calculations clearly show that such an improvement should be expected once convective flow within the sea ice begins,” says Dr. Kraitzman.

The model also provides a way to relate the thermal properties of sea ice to its temperature and salt content, allowing theoretical results to be compared with measurements.

Specifically, it provides a tool for use in large-scale climate models, potentially leading to more accurate predictions of future conditions in polar regions.

Arctic sea ice has been rapidly declining in recent decades. This ice loss can cause a feedback loop: As more dark ocean water is exposed, it absorbs more sunlight, leading to further warming and ice loss.

Sea ice loss can affect weather, ocean circulation and marine ecosystems far beyond the polar regions.

Dr. Kraitzman says understanding the thermal conductivity of sea ice is important to predicting its future.

The researchers stress that while their model provides a theoretical framework, further experimental work is needed to integrate these results into large-scale climate models.

The study was conducted by mathematicians from Macquarie University in Australia, the University of Utah and Dartmouth College in New Hampshire, USA.