New scientific research indicates that the vast ice sheet covering west Antarctica collapsed into the sea hundreds of thousands of years ago - a discovery that could have major implications for the planet.
The precedent of a collapse could mean that west Antarctica will again release its icy contents, raising ocean levels worldwide and submerging coastal areas where as much as one-half of the world's population lives.
"It's of fantastic significance," says Ian Whillans of Ohio State University in Columbus, Ohio. "It means that the whole darn ball of wax went away and reformed."
Occurring over hundreds or thousands of years, the collapse probably involved a slow release of ice from the west Antarctic ice sheet into the sea. And some scientists believe this could be happening again right now.
Reed Scherer, a professor at Uppsala University in Sweden and principal investigator of the study that is published today in the journal Science, speculates that the collapse may have occurred 400,000 years ago. Because of its warm temperatures, scientists have likened that time - an interglacial period - to the current interglacial period, which began roughly 10,000 years ago.
Evidence that the current ice sheet is slowly disappearing comes from an annual 2 millimeter rise in sea level, says Bob Bindschadler of NASA's Goddard Space Flight Center in Greenbelt, Md. Natural temperature increases, global warming, and glacial melting account for 1 millimeter and scientists speculate that the other may come from Antarctica.
But because scientists don't understand the dynamics of the west Antarctic ice sheet or the influence of factors such as global warming and climate fluctuations, they cannot make predictions about Antarctica's future. Dr. Whillans says researchers need to identify the causes behind the ice sheet's demise and subsequent re-formation before they can know whether another collapse is imminent.
Dr. Scherer's findings are the first conclusive evidence of a west Antarctic collapse. Previously, many scientists had thought that once it was formed, west Antarctica's ice sheet remained more or less stable throughout its 8 million to 10 million year history. But Scherer's discovery of the remains of 600,000-year-old diatoms - single-celled animals with shells made of opal - far below the ice sheet's surface, brought that theory into question.
Diatoms require water and sunlight to live, thus those recovered must have lived at a time when the ice sheet did not exist. Considering that much of the land beneath the west Antarctic ice sheet is actually below sea level, evidence of 600,000-year-old diatoms suggests that the sheet must have dissolved - leaving open ocean - then re-formed.
The key determinant in whether the west Antarctic ice sheet will collapse again, say scientists, is the balance between the amount of snow accumulating on the continent and the amount of ice lost to the sea. Ice streams, rivers of ice moving toward the ocean at speeds up to 1,000 meters per year, are a critical part of that system.
Two studies in this week's publication of Nature find that the structure of rock underneath the ice sheets may control where ice streams form and how they drain ice into the ocean. Both groups found that the ice stream they studied formed only over sedimentary rock, a type of young, loose rock. When combined with water, sedimentary rock acts like a lubricant, allowing ice to slide instead of holding it in place.
To see if all ice streams follow a pattern of traveling and expanding along sedimentary rock, scientists need to carry out additional studies. Such investigations will help scientists in making predictions about how the ice streams, and hence west Antarctica, may change over time.
"There are three important things to understand about west Antarctica," says Don Blankenship of University of Texas, who participated in both ice stream studies. "Whether it disappears during interglacial periods, what triggers the disappearance, and how long it will take to disappear." All three, says Dr. Blankenship, will require understanding ice streams.