- The southward march in the breakup of ice shelves down the Antarctic Peninsula toward the South Pole is consistent with the global direction of climate change.
- Air temperatures in the Antarctic Peninsula region have been warming dramatically since the 1950s, and the very latest temperature readings reinforce that trend.
- In the wake of this most recent retreat, the extent of Larsen C is now the smallest on record and probably the smallest since the last interglacial period 115,000 years ago.
- The Antarctic Peninsula exhibits an extraordinarily large range of natural variation.
- Calving of icebergs is part of the natural ebb and flow of ice shelves along the Antarctic Peninsula.
- Some key questions about the conditions for this calving event remain unanswered, hampered in part by the lack of direct observations, including why the ice shelf broke along this particular line and retreated as far back as it did, as opposed to continuing its rebound from the last major calving event.
- Ocean temperatures under the Larsen C ice shelf are largely unknown.
- The Larsen C shelf has experienced a stretch of thinning from 1994 to 2009 and then thickening thereafter.
- After completion of the current calving, the geometry of the remaining ice shelf is projected to be unstable and may be prone to runaway collapse over the coming years.
- Long-term trends driven by climate change, including changing winds and warming atmospheric temperatures, may have added to the vulnerability of the ice sheet.
- In particular, climate change may be contributing to the potential retreat and collapse of the Larsen C ice shelf through wind-driven loss of snow cover, thinning, ponding of meltwater, and hydrofracturing.
- A collapse of the Larsen C ice shelf may release blocked glaciers, contributing 1 centimeter to sea level rise.
…what we see on the Antarctic Peninsula is a place where the air temperatures are rising faster than almost anywhere else on the planet. So, it would be inconceivable that it didn’t have an accelerating affect on disintegration of ice shelves in general on the Antarctic Peninsula. And we see this limit of viability of ice shelves moving further and further south. In the case of Larsen C, this is the point at which the rift itself, we can’t say that’s been caused by climate change, it’s a rift that’s been there for decades, it might just be that it was always going to go. But I would say that it’s likely that climate warming will have helped that…”
- Dr. Adrian Luckman, Swansea University, MIDAS Project
Larsen C ice shelf: At the cross roads
The collapse of several ice shelves in sequence going down the eastern side of the Antarctic Peninsula (AP) occurred after decades of observed surface warming. And the direction of the breakup has progressed poleward toward the largest AP ice shelf, Larsen C, suggesting a southerly migration of the climatic limit for ice shelves on the AP.
Some of these losses are unusual over the last 10,000 years and have been widely attributed to atmospheric and oceanic changes. As of 2010, 18% of the overall ice shelf area on the AP had been lost, roughly 11,000 square miles. The extent of Larsen C is now the smallest on record and probably the smallest since the last interglacial period 115,000 years ago.
Larsen now straddles the temperature line considered as the practical limit for the viability of ice shelves in the Antarctic. And satellite images show that Larsen C has receded since the 1980s.
Dramatic long-term regional warming on the Peninsula has continued into 2017,  and the larger Larsen C ice shelf may be responding to this long-term warming trend, particularly in the emergence of melt-water ponds further back from the current rift. The role of this warming, if any, in the current calving event is unknown.
A considerable body of evidence is emerging pointing to warm winds, known as foehns, increasingly sweeping across the Larsen ice shelf. The increase in foehn winds has been linked to a larger increase in westerly winds circling the Antarctic, an increase driven by climate change.
In addition to sweeping away accumulating snow and thinning the glacier, these warm winds create melt ponds, the dark surfaces of which absorb additional heat from the sun — radiation that would have largely been reflected off by the frozen surface of the ice sheet. Extensive ponds have now been observed in one small section of Larsen C — far from current calving event. Patterns of ice in that area indicate conditions approaching the conditions observed in Larsen B immediately prior to its collapse. One less-favored theory holds that these conditions indicate hydrofacturing, which would indicate triggering for collapse. However, to date, ponds on Larsen C are not nearly as widespread as had been observed on Larsen B prior to its collapse.
The current calving and subsequent loss of roughly 12 percent of the ice shelf is likely to add heat to the local and regional area. Solar radiation during Austral Summer will be absorbed by the part of the ocean surface previously covered by ice; this will go on until new, sufficiently thick ice forms. The resultant warming will contribute to the overall atmospheric- and oceanic-driven thinning.
Dr. Kelly Brunt of NASA notes, "We know from Larsen B that when a whole ice shelf disintegrates, you’re left with these glaciers that feed it with no buttressing. With Larsen C we won’t lose the whole shelf, but we’ll lose a significant amount."
Dr. Christopher Shuman, also with NASA, notes, “although the Larsen C could re-advance, my best guess is that this initial large loss of shelf area will be followed in the years to come by much more retreat of the shelf ice in the Larsen C area just as happened after the initial losses in the Larsen A (late 1980s) and Larsen B (mid 1990s).”
The eastern side of the AP has a significantly different climate than the western side, where most of the Peninsula air surface temperature records are available. The data for the region as a whole reflects a clear long-term warming trend imposed upon considerable variability. Some studies have reported detecting a recent trend change from warming to cooling. However, additional statistical analysis has affirmed the continuation of the long-term trend. In addition, newly available data for 2016 and 2017 are in line with that long-term trend.
Measurement of the ice shelf reported a stretch of thinning from 1994 to 2009 and analysis indicated this was due in large part to ocean melting from below. However more recent data in the same time series reports thickening of the ice shelf since 2009. The oceanographic setting of Larsen C makes it difficult to get warm water under the ice shelf, so that the rapid melting measured under Amundsen Sea ice shelves is thought to be unlikely to occur under Larsen C.
Key Unanswered Questions
One key unanswered question is why was the current rift able cut through Joerg Peninsula suture zone that to date had stopped all other rifts? And did climate change play a role? This strip of ice currently stabilizes many other rifts and has a critically important role in holding the shelf together.
Also unknown is why the ice shelf is breaking at this time and in this rearward location, as opposed to continuing to grow back to its former extant just prior to the last major calving event.
The Big Picture
Given the large range of natural variation in the region, the lack of data in key areas (e.g. under-shelf water conditions), as well as conflicting and shifting short-term trends, it is impossible to gauge the overall impact of climate change by identifying and adding up the effects of each individual component of change.
The most illuminating metric for understanding the recent calving event may be the longest and largest-scale event that integrates the effects of all the individual factors and reflects their sum: the southward progression of ice-sheet breakup down the AP toward the South Pole, a progression suggesting a southerly migration of the climatic limit for Antarctic ice shelves.