Summary
When seawater freezes, sea ice forms. Though seemingly straightforward, the formation, growth, and decay of sea ice involve complexities. Sea ice, its extent, and its physical properties are of paramount importance to the climate system. This is in large part due to its vast coverage at the poles. The annual cycles of Arctic and Antarctic sea ice are among the largest annual physical changes on the surface of the Earth. Sea ice provides distinct habitats for microalgae, which are influenced directly by their physical properties. Collectively, Arctic and Antarctic sea ice is one of the largest biomes on Earth and are increasingly recognized as important components of the global biogeochemical system. This chapter covers some fundamentals but focuses on describing sea-ice properties by ice type rather than by age (i.e., how the properties differ for stationary landfast ice, drifting pack ice, and marginal ice zone, which is influenced by ocean swell and waves). This approach was chosen due to the fundamental physical changes imparted by their associated processes. We lay out the geographic distribution of each ice type. From an ice-type perspective, a detailed and in-depth overview of the seasonal evolution of the sea-ice landscape, the typical conditions for each season, and their influence on the evolution and resultant processes of the three ice types is provided. We also describe the processes occurring at the physical interfaces within and bordering the ice and their impacts on ice properties, including the ice-snow interface, the ice-atmosphere interface, internal ice processes, the ice-ocean interface, and the ice edge. This chapter also includes a discussion of trends in sea-ice extent by ice type. In the final section, we identify knowledge gaps to improve our understanding of sea-ice properties from both observational and modeling points of view for further research.