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The expected response of the Earth's cryosphere to global warming is a critical open research area in which model certainty is still unsatisfactory On the other hand, past climate history can teach us quite a bit. If one had returned from a vacation to find that the freezer door was left partially open and ice had collected in it, one would have learned an important climate lesson: ice does not form when conditions are merely cold, but when there is a supply of warm, moist air to a sufficiently cold environment.

While it is good practice to close one's freezer door and not experience this climate lesson personally, numerous observations and model results demonstrate that the large ice sheets that developed during past ice ages grew when the climate was relatively warm, and therefore moist. One must conclude that statements such as “…glaciers grow when climate is cold, not warm and moist” [Rahmstorf, 2002] contradicts a large body of scientific evidence, observations, and dynamical arguments. We now understand that the ocean also plays an important role in this feedback due to the role of sea ice in glacial dynamics.