Cold math, hot topic: Sea ice thermal conductivity

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

The authors of a brand new paper published within the Proceedings of the Royal Society A on 28 August, offer latest insights into how heat travels through sea ice, a vital think about regulating Earth’s polar climate.

Dr Noa Kraitzman, Senior Lecturer in Applied Mathematics at Macquarie University and lead creator of the study, says the research addresses a key gap in current climate modelling.

“Sea ice covers about 15 per cent of the ocean’s surface throughout the coldest season when it’s at its overwhelming majority,” Dr Kraitzman says. “It’s a skinny layer that separates the atmosphere and the ocean and is answerable for heat transfer between the 2.”

Sea ice acts as an insulating blanket on the ocean, reflecting sunlight and moderating heat exchange. As global temperatures rise, understanding how sea ice behaves will turn into increasingly necessary for predicting climate change.

The study focuses on the thermal conductivity of sea ice, a critical parameter utilized in many global climate models. The movement of liquid brine inside sea ice, which might potentially increase its heat transport, was not accounted for in previous models.

Dr Kraitzman says the unique structure of sea ice, together with its sensitive dependence on temperature and salinity, means it’s difficult to measure and predict its properties, specifically its thermal conductivity.

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

“Because the atmosphere above the ocean becomes extremely cold, below minus 30 degrees Celsius, while the ocean water stays at about minus two degrees, this creates a big temperature difference, and the water freezes from the highest down.

“Because the water freezes rapidly, it pushes out the salt, creating an ice matrix of purely frozen water which captures air bubbles and pockets of very salty water, called brine inclusions, surrounded by nearly pure ice.”

These dense brine inclusions are heavier than the fresh ocean water which ends up in convective flow throughout the ice, creating big ‘chimneys’ where liquid salt flows out.

The research builds on earlier field work by Trodahl in 1999, which first suggested that fluid flow inside sea ice might enhance its thermal conductivity. Dr Kraitzman’s team has now provided mathematical proof of this phenomenon.

“Our mathematics definitely shows that such an enhancement must be expected once convective flow throughout the sea ice begins,” Dr Kraitzman says.

The model also offers a solution to relate the ocean ice’s thermal properties to its temperature and salt content, allowing theoretical results to be compared with measurements.

Specifically, it provides a tool to be utilized in large-scale climate models, potentially resulting in more accurate predictions of future conditions within the polar regions.

Sea ice within the Arctic has been declining rapidly in recent a long time. This lack of ice can result in a feedback loop: as more dark ocean water is exposed, it absorbs more sunlight, resulting in further warming and ice loss.

The lack of sea ice can affect weather patterns, ocean circulation, and marine ecosystems far beyond the polar regions.

Dr Kraitzman says understanding the thermal conductivity of sea ice is essential for predicting its future.

The researchers note that while their model provides a theoretical framework, more experimental work is required to integrate these findings into large-scale climate models.