by Joe Romm, Climate Progress, April 15, 2013
Scientist: “The Antarctic Peninsula has warmed to a level where even small increases in temperature can now lead to a big increase in summer ice melt.”
Credit: British Antarctic Survey
A new study finds “a nearly tenfold increase in melt intensity” on the Antarctic Peninsula in the last few hundred years.
Here’s the most worrisome news from this 1,000-year reconstruction of ”ice-melt intensity and mean temperature” published in Nature Geoscience:
The warming has occurred in progressive phases since about AD 1460, but intensification of melt is nonlinear, and has largely occurred since the mid-twentieth century. Summer melting is now at a level that is unprecedented over the past 1,000 years. We conclude that ice on the Antarctic Peninsula is now particularly susceptible to rapid increases in melting and loss in response to relatively small increases in mean temperature.
In short, while some mistakenly assert the climate is less sensitive than we thought, the fact is that polar ice loss is accelerating far beyond what the models had projected even a few years ago, and the whole region appears even more sensitive than previously thought.
It was just 2006 when Penn State climatologist Richard Alley explained that observations had indicated the great ice sheets appear to be shrinking “100 years ahead of schedule.”
NASA’s Jet Propulsion Laboratory reported in 2011 that polar ice sheet mass loss is speeding up and is on pace for 1 foot sea level rise by 2050.
What’s happening at the South Pole, which has 90% of the world’s ice? In 2009, we learnedthat Antarctica’s Pine Island Glacier has been thinning 4 times faster than it was 10 years ago: “Nothing in the natural world is lost at an accelerating exponential rate like this glacier.” The same year Nature reported that “Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized.”
As for the cause, we know that “deep ocean heat is rapidly melting Antarctic ice.” And we knew that these warm ocean currents melting Antarctica were so intense that, seawater appears to “boil on the surface like a kettle on the stove.” Last April, researchers reported in Nature that this melting from below “May Already Have Triggered A Period of Unstable Glacier Retreat.” We learned in December that West Antarctica is warming three times faster than global average.
The key point is that the West Antarctic ice sheet (WAIS) is inherently far less stable than the Greenland ice sheet because most of it is grounded far below sea level. As I wrote in the “high water” part of my book:
Perhaps the most important, and worrisome, fact about the WAIS is that it is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. One 2004 NASA-led study found that most of the glaciers they were studying “flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way”….The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS (see, for instance, here).
Right next to WAIS is the Antarctic Peninsula, which, a 2012 study noted, “is grounded on bedrock substantially above sea level.” So it isn’t unstable — it’s just warming and melting faster and faster.
As the British Antarctic Survey (BAS) news release for this latest study reports:
This is the first time it has been demonstrated that levels of ice melt on the Antarctic Peninsula have been particularly sensitive to increasing temperature during the 20th Century.
Co-author Dr Robert Mulvaney from BAS, who led the ice core drilling expedition, points out, “Summer ice melt is a key process that is thought to have weakened ice shelves along the Antarctic Peninsula leading to a succession of dramatic collapses, as well as speeding up glacier ice loss across the region over the last 50 years.”
Lead author Dr. Nerilie Abram of BAS and the Australian National University says, “What that means is that the Antarctic Peninsula has warmed to a level where even small increases in temperature can now lead to a big increase in summer ice melt.” She explains:
“We found that the coolest conditions on the Antarctic Peninsula and the lowest amount of summer melt occurred around 600 years ago. At that time temperatures were around 1.6 °C lower than those recorded in the late 20th century and the amount of annual snowfall that melted and refroze was about 0.5%. Today, we see almost ten times as much (5%) of the annual snowfall melting each year.“Summer melting at the ice core site today is now at a level that is higher than at any other time over the last 1,000 years. And whilst temperatures at this site increased gradually in phases over many hundreds of years, most of the intensification of melting has happened since the mid-20th century.”
Abram concludes, “This new ice core record shows that even small changes in temperature can result in large increases in the amount of melting in places where summer temperatures are near to 0 °C, such as along the Antarctic Peninsula, and this has important implications for ice instability and sea level rise in a warming climate.”
