Antarctic sea ice #1: physical role and function

Sea ice – comprising frozen seawater in the form of both moving pack ice and stationary coastal landfast ice (fast ice) – is of major climatic, ecological and societal importance in that it:

• forms a bright surface that strongly reflects incoming solar radiation (the albedo effect) to moderate the equator-to-pole temperature gradient, which in turn influences large-scale atmospheric and oceanic circulation;
• strongly influences ocean-atmosphere interactions by forming an insulative substrate and physical barrier that reduces and/or modifies air-sea exchanges of heat and water vapour and the transfer of momentum (wind energy) to mix the upper ocean;
• is a major contributor to global ocean (thermohaline) circulation through the production of dense (cold and salty) water during its formation, leading in places to the generation of Antarctic Bottom Water;
• modulates the ocean freshwater budget, and regulates the properties and structure of the atmosphere and ocean (including upper-ocean stratification and deep-ocean ventilation);
• interacts with floating ice-sheet margins (including ice shelves) and influences ice-shelf basal melt and stability and iceberg calving;
• is a major habitat and crucial component of the marine ecosystem (see Antarctic Sea Ice #2); and
• forms a biogeochemically-active substrate that plays a key role in the atmosphere-ocean exchange, storage and cycling of climate-relevant gases, including carbon dioxide (CO₂), dimethyl sulphide (DMS) and methane (CH₄) (see Antarctic Sea Ice #2).

Each year around Antarctica, sea ice transforms a vast area of the surface of the Southern Ocean, covering up to 19-20 million km² at its maximum extent in September (~4% of Earth’s surface) before diminishing to 2-4 million km² in February. This remarkable annual cycle has an immense influence on the Southern Ocean environment and beyond. The sea ice also accumulates snowfall, which substantially influences its physical and optical properties, its growth and decay, and its interactions with other parts of the ice-ocean-atmosphere system.

Improved knowledge of Antarctic sea ice characteristics and ice-ocean-atmosphere-biosphere processes, interactions, and feedbacks is required to develop and improve Earth System models. Such knowledge is crucial to reducing current uncertainties in those models and to improve confidence in projections of the Antarctic sea-ice system over the coming decades and beyond (see Antarctic Sea Ice #3), including its impacts and coupled feedbacks. Improved sea-ice forecasting capability is also required to support safe and efficient shipping and logistical activities around the Antarctic continent.