Land properly speaking no longer exists, nor sea nor air, but a mixture of these things, like a marine lung, in which earth and water and all things are in suspension. —Pytheas of Massilia, On The Ocean; Quoted from Barry Cunliffe, The Extraordinary Voyage of Pytheas the Greek

Pytheas is believed to be the first Western Explorer to document sea ice, encountering it at some undetermined destination ("Thule") 7 days' sail north of Britain. Pytheas was an astute observer; among other things, he was the first to document the effects of the phase of the moon on tides. While his portrayal of sea ice could be construed as the description of jellyfish, Py-theas was probably trying to find words for thin sheets of young ice. Pytheas made other references to "mare concretum"—the frozen ocean—and he described the midnight sun, so his account is plausible. Sea ice must have been a difficult notion for someone that hailed from the Mediterranean. The original writings of Py-theas no longer exist, so we rely on the interpretations of Pliny, Strabo, and Diodorus. of course, at this time the Sami and Dorset people were well established on the Arctic coastline, and the Tuniit, predecessors of the Inuit, had been living with the rhythms of the sea ice for more than two millennia, with travel, migration, and hunting tailored around the seasons and moods of the frozen ocean.

This chapter provides a brief introduction to the geography and physics of sea ice. The seasonal flood and ebb of sea ice parallels the advance and retreat of snow and ice over the land surface, with many similarities to the thermodynamics of freshwater ice described in chapter 4. Cold air temperatures drive accretion from below, freezing the seawater. Salinity effects and ocean currents complicate matters in sea ice, however, and the scale of sea ice and its resultant climatic influences are also global in scope. Sea ice has been recognized as a central component in Earth's climate dynamics for several decades now, and most global climate models include a reasonably sophisticated treatment of sea ice.

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