Ice core study shows rapid pace of change along Antarctic Peninsula
By Bob Berwyn
FRISCO — Careful study of a 1,200-foot long ice core sample spanning 1,000 years suggests that summer ice melt in parts of the Antarctic Peninsula region has intensified almost tenfold. About 5 percent of the annual snowfall has been melting in recent years, compared with only about 0.5 percent during the coolest phase (about 600 years ago) of that 1,000-year span.
“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,” said Dr. Nerilie Abram, a climate researcher at Australian National University who studied the ice core from James Ross Island.
Most of the increased melting occurred during the past half-century, corresponding with the era of increasing greenhouse gas emissions and a remarkable warmup around the peninsula and some other parts of Antarctica. Borehole temperature estimates from the West Antarctic Ice Sheet also indicate rapid acceleration of West Antarctic warming during the past two decades.
The Antarctic Peninsula stretches north from the continent, toward the tip of South America. Temperatures in the region are warming faster than most other areas — up to 2.8 degrees Celsius in the past 50 years, comparable to some of the fastest-warming parts of the Arctic. Scientists attribute the warmup to increased advection of warm ocean air masses, at least partly driven by ozone depletion.
The study suggests that the climate in the Antarctic Peninsula region is reaching a threshold — 32 degrees Fahrenheit — at which summer melting will be much more common, as temperatures climb — and remain — above freezing more frequently during the summer, Abram said. Once that threshold is reached, additional energy input into the ice goes into melting rather than warming the ice.
The findings show how melting can increase relatively quickly once the threshold is reached, and increase more rapidly than the temperature trend itself from that point forward, because the energy is now going into melting rather than warming ice, said British Antarctic Survey scientist Dr.Robert Mulvaney.
The mean annual temperature variations in the region aren’t unprecedented in the context of long-term climate variability, but the strong summer component of twentieth-century warming on the Antarctic Peninsula may be the key in driving the unprecedented levels of melt in recent decades, Nerilie said.
Natural climate variability?
Whether the observed changes are part of a longer as-yet unknown cycle of natural variability isn’t clear, but the question always looms large for ice core researchers, Mulvaney said.
“In fact, I always introduce my talks about climate change by asking whether what we see in recent decades is part of a natural cycle,” Mulvaney said. “I guess the problem is that we won’t know if we are in a natural upswing in the climate cycle until we’re well into a natural downturn. What we can observe from the James Ross melt record is that nowhere in the last 1,000 years has the level of melt reached what we see in the last few decades.”
Melting of the ice sheets affects the stability of Antarctic ice shelves and glaciers, contributing to collapses and the acceleration of glacier ice loss in the region.
“Often the loss of ice shelves is dramatic — they break up and are lost in a matter of weeks. The most likely cause of the rapidity of break-up is surface melt water penetrating deep into the ice shelves, and causing weaknesses and cracks in the ice shelves,” said Mulvaney, who led the ice core expedition. “Hence, knowledge of how much surface melt is changing may help us understand the mechanism for ice loss.”
The ice core was drilled in 2008 by a joint UK-French team on James Ross Island, near the northern tip of the Antarctic Peninsula. The initial goal was to establish a temperature record for the area, but the researchers found that bubble-free layers in the ice core could also give a unique and unexpected insight into ice melt in the region.
By measuring the thickness of melt layers the scientists were able to examine how the history of melting compared with changes in temperature at the ice core site over the last 1,000 years. The layers indicating melting are clearly visible as bubble-free, but the researchers quantified them by using a line scanner.
“Effectively, light was shone through the core from one side of the slab, and the level of transmitted light recorded on the other side. This then allowed us to image the ice layers, and define a mathematical routine to analyze the line scan data, setting criteria when the ice was bubble-free,” Mulvaney explained.
“We found that the coolest conditions on the Antarctic Peninsula and the lowest amount of summer melt occurred around 600 years ago,” Abram said. “At that time, temperatures were about 1.6 degrees Celsius lower lower than those recorded in the late 20th Century. The amount of annual snowfall that melted and refroze was about 0.5 percent. Today, we see almost ten times as much (5 percent) of the annual snowfall melting each year,” she said.
“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,” she said.
“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 added.
In other parts of Antarctica, such as the West Antarctic Ice Sheet, the picture is more complex and it is not yet clear that the levels of recent ice melt and glacier loss are exceptional or caused by human-driven climate changes.
“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 degrees Celsius, such as along the Antarctic Peninsula, and this has important implications for ice instability and sea level rise in a warming climate,” Abram said.
Implications for sea level rise
“This region of Antarctica has seen the loss of thousands of square kilometres of ice shelves in the last few decades … These ice shelves are composed of ice hundreds of meters thick that has flowed off the land-based ice caps,” Mulvaney said.
“While satellite and meteorological observations tell us that the melt season has lengthened over recent decades, the problem with modern records is that we have no longer term context. Ice core records enable us to ask the question: Is the recent change in melting unusual, or part of a natural climate cycle,” Mulvaney said. “Scientists think that the warming observed in the Antarctic Peninsula (about 3 degrees Celsius in the last 50 years, and one of the most rapidly warming regions on earth), plus the increased surface melting, have combined to make the ice shelves unstable.
“However, does this predict sufficient loss of ice to impact sea-level? Not really — we can look at the surface melt and local temperature, and know that the ice shelf is unstable and vulnerable to collapse, but the actual break-up tends to take us by surprise. Sea-level rise does not change when an ice shelf collapses — the ice is already floating and displacing sea water. Where sea-level is impacted is due to the more rapid loss of land-based ice caps flowing towards the sea in glaciers. Once the buttressing effect of the ice shelves is removed, glaciers tend to flow faster, bringing ice to the sea that does add to sea-level,” he said.
This research was funded by the Natural Environment Research Council. Dr. Abram is an Australian Research Council Queen Elizabeth II Fellow. It was published this week in the journal Nature Geoscience.