An ice shove (also known as fast ice, an ice surge, ice push, ice heave, shoreline ice pileup, ice piling, ice thrust, ice tsunami, ice ride-up, or ivu in Iñupiat) is a surge of ice from an ocean or large lake onto the shore. Ice shoves are caused by ocean currents, strong winds, or temperature differences pushing ice onto the shore, creating piles up to 12 metres (40 feet) high. Ice shoves can be caused by temperature fluctuations, wind action, or changing water levels and can cause devastation to coastal Arctic communities. Cyclical climate change will also play a role in the formation and frequency of ice shove events; a rise in global temperatures leads to more open water to facilitate ice movement. Low pressure systems will destabilize ice sheets and send them shoreward. Also referred to as "landfast ice", it is an essential component to the coastal sea ice system, including the sediment dynamics. Arctic peoples utilize these ice shoves to travel and hunt. Ringed seals, an important prey for polar bears, are specifically adapted to maintain breathing holes in ice shoves, which lack the same openings usually used by marine mammals in drifting ice packs. The mere fact that the Ringed seal is uniquely adapted to utilizing ice shoves for breathing holes, and that polar bears have adapted to this behaviour for hunting, as well as the fact that the Iñupiat have a distinct term for the phenomena, indicates that ice shoves are a regular and continuing phenomena in the Arctic.
Causes
Temperature fluctuations
When temperatures decrease, ice contracts and forms stress fractures; water then seeps into these tension cracks and freezes. When temperatures rise, the ice sheet expands. This sequence of events occurs cyclically until the total ice sheet has expanded considerably. If this ice sheet is in contact with a shoreline, it can exert considerable force on the land, causing the displacement of shore material. When temperature fluctuations are drastic enough, the ice sheet contraction pulls far enough from shore to form a lane of water; for example, a drop from 0 °C to -20 °C results in an 11% volume decrease of a 1.5-km ice sheet. This lane of water subsequently freezes. When temperatures rise at sufficient rates (~1 °C/hr for upwards of 5 hours), the ice sheet expands onto land. The physical composition of the ice itself is also important; ice that has formed from water-soaked snow, known as white ice, hinders the process of thermal ice expansion because its albedo is higher than other forms of ice, resulting in lower thermal conductivity. In order for conditions to facilitate thermal expansion of ice and in turn ice shoves, the ice needs to be susceptible to temperature change, making black ice more suitable for the formation of ice shoves.
Wind action Because land heats faster than ice and transfers heat to the adjacent ice, the ice nearest to the shore melts first when temperatures rise. Water then exists between the ice sheet and the shoreline, facilitating the movement of ice sheets when wind acts on them. An open channel of water allows for reduced resistive forces on the ice sheet, increasing the likelihood that an ice shove event can occur. The direction of the wind ultimately directs the motion of the ice shove. The effectiveness of wind as a driving force for ice movement relies on a multitude of factors including the size and shape of the body of water and wind strength. Large, wide-open bodies of water have a greater surface area for wind to act upon compared to smaller, sheltered bodies of water. Persistent, high-speed winds apply more force than slower wind gusts, making them optimal for driving ice sheets ashore.
Fluctuating water levels Falling water levels cause a bending force between ice that has already breached the shore and the ice floating immediately offshore. This bending force causes cracks in the ice where water can seep in and freeze. When water levels rise again, the ice experiences compression forces that subsequently push it onto land. This mechanism is comparable to the thermal expansion process described above.
Effects on arctic communities Arctic communities can be affected by ice shoves. Ice shoves commonly occur along the Chukchi Sea including in Wainwright, Alaska and Barrow, Alaska. Studies have shown that the formation of landfast ice is starting to form later and breakup earlier in the Chukchi and Beaufort seas. More open water days increase the likelihood of destructive coastal events like ice shoves in these regions. Some have described them as 'ice tsunamis', but the phenomenon works like an iceberg. Witnesses have described the shove's sound as being like that of a train or thunder. Ice shoves can damage buildings and plants that are near to the body of water.
Arctic ice and climate change
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