Sea ice regulates heat exchange between the atmosphere and the ocean in the polar regions. The thermal conductivity of ice plays an important role; it is a key parameter in climate models. However, it is difficult to calculate due to the special microstructure of ice, its sensitivity to temperature, and salinity. In the new study, scientists applied a number of mathematical methods to calculate thermal conductivity. In the future, this will help improve climate models and find applications in other areas.

In recent years, sea ice has become a focus of climatologists. As it has become clear that average annual temperatures on the planet are rising and that warming is occurring faster in the polar regions than elsewhere, the system “atmosphere – ocean water – sea ice” has become particularly important. It is important for humanity to understand how it works.

Sea ice is an insulating blanket that separates the ocean from the atmosphere. It reflects sunlight and controls the heat exchange between air and water. Sea ice participates in a feedback system: the larger its area, the more solar radiation is reflected and the lower the air temperature. In recent years, the area of ​​sea ice has decreased significantly, which has led to an increase in the greenhouse effect with all its consequences.

“Sea ice covers about 15 percent of the ocean surface during cold seasons and at high altitudes. It is a thin layer that sits at the boundary between the atmosphere and the ocean and affects the heat exchange between them,” explains Noah Kreitzman, senior lecturer in applied mathematics at Macquarie University, Australia, who led the study published in the journal Proceedings of the Royal Society A.

According to Kreitzman, the structure of sea ice, especially its high sensitivity to temperature and salinity, makes it extremely difficult to measure and model its properties, including thermal conductivity. When the air temperature in the ocean drops below minus 30 degrees, the sea water temperature is still minus two degrees. The researcher emphasized that this creates a large temperature difference: Water starts to freeze from the top down. The process is fast, the salt is displaced. What remains is a matrix of pure water ice with air bubbles and pockets of a very salty solution (brine).

These dense drops of brine are heavier than fresh ocean water. Convection occurs within the ice due to the warp, creating large pores through which brines circulate. The movement of liquid brines through sea ice could theoretically increase heat transfer as temperatures rise. This was first suggested by Joe Trodahl of Victoria University in Wellington (New Zealand), who experimentally measured the thermal conductivity of natural sea ice in Antarctica during the 1999 field season. It has now been mathematically proven.

Scientists have modernized the transport equation for a porous composite material with circulating brine, represented by sea ice. They have shown that convective flows within the ice can increase the effective thermal conductivity by two to three times. This refers to the warmer and more permeable lower part of the layer. In winter this is below 10 centimeters, while in summer it can affect the entire thickness. The authors of the study aim to validate their results with field data and incorporate them into climate models.