Icebergs, massive fragments of freshwater ice broken from glaciers or ice shelves, appear uniformly cold to the casual observer. The reality is that the temperature within an iceberg varies dramatically from its ancient core to its exposed surface, creating a dynamic thermal gradient. Understanding this temperature difference is important because it governs how the iceberg interacts with the ocean and atmosphere and determines its longevity as it drifts through the sea. The distinct temperature zones within the ice mass reveal the physics of its formation and its eventual melting.
The Stable Core Temperature of Glacial Ice
The deepest section of an iceberg, often submerged far below the waterline, holds a remarkably stable and frigid temperature known as the cold core. This internal temperature is a remnant of the extreme conditions under which the ice formed, deep within the parent glacier or ice sheet over centuries. The core temperature is typically found in the range of \(-15^\circ \text{C}\) to \(-20^\circ \text{C}\) (\(5^\circ \text{F}\) to \(-4^\circ \text{F}\)), which is significantly colder than the freezing point of freshwater.
This low, stable temperature is maintained because the immense mass of ice acts as a powerful insulator, effectively inhibiting heat transfer from the surrounding environment. The ice deep inside the glacier is compressed by the weight of thousands of feet of snow and ice above it, which leads to a dense structure. This internal coldness is a defining feature of the iceberg’s mechanical strength, providing the structural integrity that allows the massive body of ice to survive for extended periods.
Factors Influencing Surface Ice Temperature
In sharp contrast to the stable core, the temperature of the iceberg’s exposed surface is highly variable and constantly fluctuating. The surface temperature is not insulated and is directly influenced by external atmospheric and solar conditions. While the cold core remains far below freezing, the surface ice can warm up considerably, sometimes approaching the melting point of \(0^\circ \text{C}\) (\(32^\circ \text{F}\)).
The diurnal cycle causes fluctuations in surface temperature, as do variations in ambient air temperature and wind. Solar radiation contributes to warming the exposed ice, which leads to surface melting and the formation of melt ponds. If meltwater percolates downward and refreezes at depth, it releases latent heat, further warming the upper section of the iceberg and weakening its mechanical structure.
The Temperature Difference Between Icebergs and Seawater
The thermal interaction between the iceberg and the ocean represents the final stage of its life cycle, characterized by a complex exchange of heat. The surrounding seawater in polar regions is typically cold, often near its freezing point of approximately \(-1.8^\circ \text{C}\) (\(28.8^\circ \text{F}\)) due to salinity. Although this water is below the freezing point of the iceberg’s freshwater, it is still significantly warmer than the core’s \(-15^\circ \text{C}\) to \(-20^\circ \text{C}\).
This temperature difference drives the process of melting, where heat transfers from the warmer seawater to the colder ice. Icebergs do not melt instantaneously because of the immense energy required to change the state of ice to water, a property known as the latent heat of fusion. This latent heat must be supplied to the ice without causing a temperature increase, meaning a large amount of energy is absorbed by the ice before the phase change is complete.
As the submerged ice melts, it releases cold, fresh water into the ocean, which is less dense than the surrounding salt water. This layer of fresh meltwater spreads along the surface and can act as a temporary insulating layer, reducing the direct contact between the warmer, saltier water and the ice. The rate of melting ultimately depends on the thermal forcing of the ocean.