CORVALLIS, Ore. - The rapid structural breakdown of some important parts of the ice sheets on Greenland and Antarctica is possible and has happened in the distant past, and some "startling changes" on the margin of these ice masses has been observed in recent years, raising disturbing concerns about sea level rise.
In a new report to be released Friday in the journal Science, researchers from Oregon State University and four other institutions in the U.S. and Europe outline dynamic mechanisms of glacial change that appear to be under way. These changes could significantly speed up the melting of major ice sheets, and have not been considered in current projections for sea level rise.
A possibility, scientists say, is that the melting and collapse of floating ice shelves near the coasts of Greenland and Antarctica will continue and in the process destabilize the ice sheets behind them. This could cause a much more rapid flow of ice to the sea and lead to melting events that transcend those now anticipated due to global warming. Based on this, the researchers say that current projections of sea level rise should be considered an expected minimum; the levels could be much higher and happen more quickly.
"Most of the sea level rise we're now expecting in the next 200 years is due to thermal expansion of water, not the overall loss of ice from Greenland and Antarctica," said Peter Clark, a professor of geosciences at OSU. "But recent events we've studied with improved observational systems and computer modeling suggest there may be much more going on.
"We may be more vulnerable to sea level rise than we thought and it may be more rapid than we have anticipated. This is an issue we should take very seriously." Although they are learning a great deal more about the mechanisms that may lead to more rapid glacial collapse, the scientists cannot yet predict with certainty whether or how fast it might happen, or what the resulting sea level rise may be.
In one event about 14,600 years ago, Earth's sea level rose about 70 feet in less than 500 years - 20 times faster than the current rate of sea level rise. However, climatic conditions then may have been considerably different than today, and that event may not provide an exact analog to what we might expect from current glacial melting events, Clark said. Nevertheless, that event illustrates the potential for existing ice sheets to cause sea level to rise rapidly, he said.
Current projections in climate models suggest that global warming will cause some melting of glacial ice in Greenland in the next century or two, but that may be largely offset by increased precipitation and glacial buildup in Antarctica. During that period, thermal warming of the Earth's oceans is expected to increase sea level by about one-half meter, Clark said. The breakdown of glaciers currently being studied could double the sea level rise to a full meter, he said.
More problems are anticipated later. The Greenland ice sheet may disappear within about 1,000 years, raising Earth's sea level by about 20 feet, and the glacial breakdown mechanisms being studied could speed that up considerably.
What has caught the attention of scientists in recent years is the rapid collapse of some glaciers near the coasts of Greenland and Antarctica.
Jakobshavn glacier in Greenland nearly doubled its flow speed in the past decade. Along the Antarctic Peninsula, warming over the past few decades has caused retreat or near-total loss of several ice shelves, some of which had existed for thousands of years - and surface melting cannot explain most of the losses. In 2002 the Larsen B Ice Shelf in Antarctica collapsed, and major tributary glaciers entering the former ice shelf began to move 2-8 times faster than they had previously. Also in Antarctica, large glaciers feeding the Amundsen Coast thinned and accelerated by up to 26 percent over the last three decades, with repercussions more than 120 miles inland.
It's become clear, Clark said, that the West Antarctic Ice Sheet, much of which sits on land that's actually below sea level, is one of the most vulnerable in the world to these types of rapid breakdowns. If it were to melt, that would add another 20 feet to global sea levels.
In future modeling of potential sea level rise, the researchers said in their report, it's essential that the mechanisms for breakup of major ice sheets in Greenland and Antarctica be more carefully considered in the projections. If these mechanisms continue and prove to be significant, sea level projections will have to be revised upward, the scientists said.
Other collaborators on this study were from Pennsylvania State University, the University of Washington, and institutes or universities in Germany and Belgium.
"The events that have happened so far are pretty small, compared to what we're concerned about," Clark said. "The real problem would occur if these smaller glacial breakups trigger larger ones, and rising sea levels by themselves might cause a feedback mechanism which would further speed up the process."