Editing Cryosphere (section)

=== Sea ice ===
{{Main|Sea ice}}
[[File:Arctic ice.jpg|thumb|Broken pieces of Arctic sea ice with a snow cover]]
[[File:Seaice.jpg|thumb|Satellite image of sea ice forming near [[St. Matthew Island]] in the Bering Sea]]
[[Sea ice]] covers much of the polar oceans and forms by freezing of sea water. [[Satellite]] data since the early 1970s reveal considerable seasonal, regional, and interannual variability in the sea ice covers of both hemispheres. Seasonally, sea-ice extent in the [[Southern Hemisphere]] varies by a factor of 5, from a minimum of 3–4 million km<sup>2</sup> in February to a maximum of 17–20 million km<sup>2</sup> in September.<ref name="zwally">Zwally, H. J., J. C. Comiso, C. L. Parkinson, W. J. Campbell, F. D. Carsey, and P. Gloersen, 1983: Antarctic Sea Ice, 1973–1976: Satellite Passive-Microwave Observations. NASA SP-459, National Aeronautics and Space Administration, Washington, D.C., 206 pp.</ref><ref name="gloersen">Gloersen, P., W. J. Campbell, D. J. Cavalieri, J. C. Comiso, C. L. Parkinson, and H. J. Zwally, 1992: Arctic and Antarctic Sea Ice, 1978–1987: Satellite Passive-Microwave Observations and Analysis. NASA SP-511, National Aeronautics and Space Administration, Washington, D.C., 290 pp.</ref> The seasonal variation is much less in the Northern Hemisphere where the confined nature and high latitudes of the [[Arctic Ocean]] result in a much larger perennial ice cover, and the surrounding land limits the equatorward extent of wintertime ice. Thus, the seasonal variability in [[Northern Hemisphere]] ice extent varies by only a factor of 2, from a minimum of 7–9 million km<sup>2</sup> in September to a maximum of 14–16 million km<sup>2</sup> in March.<ref name="gloersen" /><ref name="parkinson">Parkinson, C. L., J. C. Comiso, H. J. Zwally, D. J. Cavalieri, P. Gloersen, and W. J. Campbell, 1987: Arctic Sea Ice, 1973–1976: Satellite Passive-Microwave Observations, NASA SP-489, National Aeronautics and Space Administration, Washington, D.C., 296 pp.</ref>

The ice cover exhibits much greater regional-scale interannual variability than it does hemispherical. For instance, in the region of the [[Sea of Okhotsk]] and [[Japan]], maximum ice extent decreased from 1.3 million km<sup>2</sup> in 1983 to 0.85 million km<sup>2</sup> in 1984, a decrease of 35%, before rebounding the following year to 1.2 million km<sup>2</sup>.<ref name="gloersen" /> The regional fluctuations in both hemispheres are such that for any several-year period of the [[satellite]] record some regions exhibit decreasing ice coverage while others exhibit increasing ice cover.<ref name="parkinson1995">Parkinson, C. L., 1995: Recent sea-ice advances in Baffin Bay/Davis Strait and retreats in the Bellinshausen Sea. Annals of Glaciology, 21, 348–352.</ref>