Monday, April 15, 2013

Antarctic Peninsula: nonlinear intensification of melt unprecedented in last 1000 years

This study analysing an Antarctic Peninsula ice core from James Ross island has determined that there has been a ten fold increase in melt intensity over the last 600 years. I reported recently that the Antarctic Peninsula summer melt season prolonged by global warming according to research from the British Antarctic Survey.

"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." concludes the study.

The information comes from a 364 meter ice core drilled by a joint UK and French research team on James Ross Island, near the northern tip of the Antarctic Peninsula. Analysis of the ice core can reveal past temperature changes as well as revealing past extent of ice melt events.

Lead author Dr Nerilie Abram of The Australian National University and British Antarctic Survey (BAS) said,

"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 1000 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."

The study is the first time that levels of ice melt on the Antarctic Peninsula have been demonstrated to be particularly sensitive to increasing temperature during the 20th Century.

"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," said Dr Abram.

Study co-author Dr Robert Mulvaney from the British Antarctic Survey who lead the drilling expedition explained:

"Having a record of previous melt intensity for the Peninsula is particularly important because of the glacier retreat and ice shelf loss we are now seeing in the area. 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."

While warm ocean currents are driving ice shelf melting around much of the coast of Antarctica, this study argues that on the Antarctic Peninsula, ice-shelf instability and loss has instead been linked to an increase in surface melting due to higher air temperatures in summer. These higher temperatures almost certainly are driven by human greenhouse gas emissions driving global warming. According to data between 2000 and 2009 Antarctic Peninsula melt accounted for more than 50% of the total Antarctic surface melt intensity. Regional climate modelling of surface meltwater production from 1979 - 2010 supports this data.

So what are the implications of the study? Dr Abram concluded:

"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."

Climatologist Eric Stieg, who lead a different ice core study for West Antarctica, suggests the dramatic melting and ice loss on the Antarctic Peninsula is unprecedented in the last 1000 years and can likely be attributed to global warming. This is in contrast to recent changes in the climate and ice thinning in the West Antarctic Ice Sheet which his study argues are still just within the bounds of natural variability. The two ice core studies show the complexity of assessing the climate changes in Antarctica.

You can read Eric Steig's article when the initial Ross Island ice core results were published in August 2012: Antarctic Peninsula warming: natural variability or "global warming"?