When the facility opened in 1993, it took six months for the air conditioner to bring the temperature down to the frigid requirements. Proper temperature is not the only key to preserving the ice cores. Cravens and his staff also worry about sublimation, the direct transition of solid ice to gaseous water vapor, skipping the liquid phase. Low air humidity encourages sublimation. That's what happens when ice cubes shrink in your refrigerator, Cravens says.
Sublimation can whittle away an ice core.
To combat it, the lab wraps all cores in plastic to protect them from the dry air in the freezer. The crystals, originally tiny, were squeezed outwards by the immense pressure of the icepack on top of them, he says. 300 years of ice may be squeezed into a meter. Ice at the edge may be distorted by this slow tumult, so scientists try to drill near the center, where deformation has been least severe, he says. For a core to be useful, annual deposition layers must be discernible and in their original order, not flipped or twisted.