August 23, 2012
Antarctic Ice Shelf Collapse Has Been Occurring For 600 Years, Manmade Climate Change Is Now Adding To Problem
Lawrence LeBlond for redOrbit.com - Your Universe Online
The Antarctic Peninsula has been continually shrinking for centuries, since long before the Industrial Revolution, according to an international team of researchers. However, rapid warming over the past 100 years has been unusual and, if it continues, the ice shelf could be on par for a complete collapse.
Temperatures in the Antarctic Peninsula began rising around 600 years ago, occurring naturally. This was long before manmade influences on the climate further increased them, scientists said on Wednesday, explaining the recent collapses of vast areas of the ice shelf.
Robert Mulvaney of the British Antarctic Survey, lead researcher on the project, said the evidence comes from long-term ice-core climate records. The team have been reconstructing ancient temperatures to gain a better understanding of the rapid warming that is occurring at a much faster rate in the Antarctic than anywhere else in the Southern Hemisphere.
The results of the British, Australian and French study adds a new dimension to the understanding of the climate change of the Antarctic Peninsula and the likely causes of the collapse of its ice shelves. Ice cores were collected continuously through the James Ross Island ice cap, providing researchers with a history of temperature changes since the last ice age.
The most current ice core drilled has been studied heavily over the past four years in laboratories in Europe. It shows that after several centuries of cooling, temperatures began a steady climb of about 0.36 degrees Fahrenheit per century for 500 years. The warming increased exponentially, reaching an unusually high, though not unprecedented, rate during the last 100 years of about 2.7 F, according to the report published in the journal Nature.
The results of the study “are consistent with a more rapid human-induced warming on top of a slower natural warming,” Mulvaney told AFP. “If the warming continues, then ice shelves further south in the peninsula that are believed to have been stable since the last ice age may also break up in the next few decades.”
Ice shelves are floating masses of ice, attached to the Antarctic coast, and are formed by flowing glaciers. Several major ice shelves have broken away from the peninsula due to an upwelling of warmer water from the Southern Ocean.
One major ice shelf -- the Larsen B ice shelf -- has already collapsed and experts are concerned that ice shelves further south on the peninsula, and even on the Antarctic mainland, are now weakening in the warming temperatures.
“If warming continues in this region ... then temperatures will soon exceed the stable conditions that persisted in the eastern Antarctic Peninsula for most of the Holocene,” note the researchers.
Bianca Nogrady of ABC Science reports that the ice cores in the BAS study show that natural warming over a number of centuries on the eastern Antarctic Peninsula left ice shelves in this area vulnerable to collapse and further warming could cause ice shelf instability to encroach further southwards along the peninsula.
The Antarctic Peninsula is one of the most rapidly warming regions on Earth -- records since 1958 suggest warming of around 6.3°F per century.
However, long-term climate data has been largely unavailable because of the logistical difficulties associated with finding good ice-core sites, according to paleoclimatologist and study coauthor Dr. Nerilie Abram of the Australian National University.
With the latest ice core samples, researchers are getting a better understanding of how much of the ice shelf collapse is naturally occurring, and how much is caused by manmade climate change.
“The centuries of ongoing warming have meant that marginal ice shelves on the northern Peninsula were poised for the succession of collapses that we have witnessed over the last two decades. And if this rapid warming that we are now seeing continues, we can expect that ice shelves further south along the Peninsula that have been stable for thousands of years will also become vulnerable,” said Abram.
Burning of fossil fuels since the Industrial Revolution has emitted heat-trapping greenhouse gases, which have raised temperatures, causing floods, droughts and rising sea levels as ice melts, according to a United Nations (UN) panel of scientists.
“What we are seeing is consistent with a human-induced warming, on top of a natural one,” Mulvaney told Reuters´ Alister Doyle. He cautioned that the study only refers to one small part of Antarctica.
The loss of floating ice shelves does not itself raise sea levels, noted the researchers. These ice shelves already make up part of the ocean. But glaciers on land can start sliding toward the sea at a faster rate, adding more water when the shelves holding them back vanish. If all the snow and ice on Antarctica´s peninsulas were to disappear into the sea, it would be enough to raise worldwide sea levels by about 200 feet. Even a small thaw would threaten low-lying nations and cities.
In an accompanying editorial, glaciologist Dr. Eric Steig of the University of Washington, said it is possible to propose the 'null-hypothesis' that the warming on the Antarctic Peninsula is happening independent of recent global warming trends, but that this is highly unlikely.
“The rate of recent warming at James Ross Island is highly unusual, falling within the uppermost 0.3 per cent of all century-scale temperature trends of the past two millennia, which would compel us to reject the null hypothesis with confidence,” Steig wrote in the editorial in Nature.
“The best data from Antarctica is on the peninsula – that's where we see the largest changes occurring. But while there's not much data from west Antarctica, what data there actually looks a lot like what's happening on the peninsula,” he added.
This research was funded by the Natural Environment Research Council (NERC).