Continual retreating of West Antarctica’s enormous glaciers has apparently been posing an “unstoppable” threat by raising global sea levels but the problem dates back to mid-1940s, a new study suggested.

An international team of researchers from the United States, Denmark, German, and Britain found in the new study that Pine Island Glacier and its next door neighbor Thwaites are increasingly retreating because of “un-grounding.”

The Pine Island Glacier Glaciers is resting upon a nearly 800 meter deep undersea ridge, but warmer ocean water is “un-grounding” it, like most of other glaciers.

The 1940s were hotter due to a particular reason -- a strong and longer Pacific El Nino event that spanned from 1939 to 1942. This long-lasting El Nino affected the circulation of the atmosphere all the way down in Antarctica, allowing hotter deep water or “circumpolar deep water” to move in and un-grounding the glaciers.

Lead researcher James Smith, a glaciologist with the British Antarctic Survey, added, “The 1940s is potentially a few years before the really big spike in anthropogenic-forced warming but it’s certainly within the realms of human-induced change.”

Melting of the glaciers could eventually turn the West Antarctic ice sheet into water, increasing global sea levels by up to ten feet. Pine Island Glacier, one of the most threatening glaciers in the West Antarctica region, has been dumping roughly 50 billion tones of ice into the oceans per annum for years. It alone could ultimately increase sea levels by nearly two feet.