Crazy: on a microscopic level, carbon dioxide weakens the atomic strength of water molecules — ie weaker ice.
"Buehler and his co-author, Zhao Qin, used computer simulations at the atomic level to evaluate the dynamics of ice strength in the presence of various concentrations of carbon dioxide. They found that the gas diminishes the strength of ice by interfering with the hydrogen bonds that hold together the water molecules in an ice crystal. Specifically, at the atomic level, the carbon dioxide competes with the bonded water molecules and, at high enough concentrations, displaces them from the bonds and takes their place.
The carbon dioxide molecules start infiltrating a piece of ice at an outer edge, then slowly split it apart by migrating inward as a crack forms. In doing so, they also attract water molecules outward to the edge by forming bonds with the water molecules’ hydrogen atoms, leaving broken bonds within the crystalline structure and decreasing the ice’s strength overall. The simulations showed that ice that has been infiltrated with carbon dioxide to the point that the gas occupies two percent of its volume is roughly 38 percent less strong.”
(via Bad News Chemistry: Carbon Dioxide Makes Ice Weaker | Surprising Science)